title:
Spectroscopic Constraints on the Stellar Population of Elliptical
  Galaxies in the Coma Cluster

abstract:
  Near-IR spectra for a sample of 31 elliptical galaxies in the Coma cluster
are obtained. The galaxies are selected to be ellipticals (no lenticulars),
with a large spatial distribution, covering both the core and outskirt of the
cluster (ie. corresponding to regions with large density contrasts).
Spectroscopic CO (2.3 micron) absorption indices, measuring contribution from
intermediate-age red giant and supergiant stars to the near-IR light of the
ellipticals, are then estimated.
  It is found that the strength of spectroscopic CO features in elliptical
galaxies increases from the core (r < 0.2 deg) to the outskirts (r > 0.2 deg)
of the Coma cluster. Using the Mg2 strengths, it is shown that the observed
effect is not due to metallicity and is mostly caused by the presence of a
younger population (giant and supergiant stars) in ellipticals in outskirts
(low density region) of the cluster.
  Using the spectroscopic CO features, the origin of the scatter on the near-IR
Fundamental Plane of elliptical galaxies is studied. Correcting this relation
for contributions from the red giant and supergiant stars, the rms scatter
reduces from 0.077dex to 0.073dex. Although measurable, the contribution from
these intermediate-age stars to the scatter on the near-IR Fundamental Plane of
ellipticals is only marginal.
  A relation is found between the CO and V-K colours of ellipticals with a
slope 0.036 +/- 0.016. This is studied using stellar synthesis models.


title: Fluid Dark Matter

abstract:
  Dark matter modeled as a classical scalar field that interacts only with
gravity and with itself by a potential that is close to quartic at large field
values and approaches a quadratic form when the field is small would be
gravitationally produced by inflation and at the present epoch could act like
an ideal fluid with pressure that is a function only of the mass density. This
could have observationally interesting effects on the core radii and solid body
rotation of dark matter halos and on the low mass end of the primeval mass
fluctuation power spectrum.


title:
Gravitational potential and energy of homogeneous rectangular
  parallelepiped

abstract:
  Gravitational potential and gravitational energy are presented in analytical
form for homogeneous rectangular parallelepiped.


title:
Limits on Hot Intracluster Gas Contributions to the Tenerife Temperature
  Anisotropy Map

abstract:
  We limit the contribution of the hot intracluster gas, by means of the
Sunyaev-Zel'dovich effect, to the temperature anisotropies measured by the
Tenerife experiment. The data is cross-correlated with maps generated from the
ACO cluster catalogue, the ROSAT PSPC catalogue of clusters of galaxies, a
catalogue of superclusters and the HEAO 1 A-1 map of X-ray sources. There is no
evidence of contamination by such sources at an rms level of $\sim 8\mu$K at
99% confidence level at $5^o$ angular resolution. We place an upper limit on
the mean Comptonization parameter of $ y \le 1.5\times 10^{-6}$ at the same
level of confidence. These limits are slightly more restrictive than those
previously found by a similar analysis on the COBE/DMR data and indicate that
most of the signal measured by Tenerife is cosmological.


title: Reconstructing the Star Formation History of the Galaxy

abstract:
  The evolution of the star formation rate in the Galaxy is one of the key
ingredients quantifying the formation and determining the chemical and
luminosity evolution of galaxies. Many complementary methods exist to infer the
star formation history of the components of the Galaxy, from indirect methods
for analysis of low-precision data, to new exact analytic methods for analysis
of sufficiently high quality data. We summarise available general constraints
on star formation histories, showing that derived star formation rates are in
general comparable to those seen today. We then show how colour-magnitude
diagrams of volume- and absolute magnitude-limited samples of the solar
neighbourhood observed by Hipparcos may be analysed, using variational calculus
techniques, to reconstruct the local star formation history. The remarkable
accuracy of the data coupled to our maximum-likelihood variational method
allows objective quantification of the local star formation history with a time
resolution of ~ 50 Myr. Over the past 3Gyr, the solar neighbourhood star
formation rate has varied by a factor of ~ 4, with characteristic timescale
about 0.5Gyr, possibly triggered by interactions with spiral arms.


title: Tidal Disruption of a Solar Type Star by a Super-Massive Black Hole

abstract:
  We study the long term evolution of a solar type star that is being disrupted
by a super massive (10^6 solar mass) black hole. The evolution is followed from
the disruption event, which turns the star into a long thin stream of gas, to
the point where some of this gas returns to pericenter and begins its second
orbit around the black hole. Following the evolution for this long allows us to
determine the amount of mass that is accreted by the black hole. We find that
approximately 75% of the returning mass is not accreted but instead becomes
unbound, following the large compression characterizing the return to
pericenter. The impact of a tidal disruption on the surrounding gas may
therefore be like that of two consecutive supernove-type events.


title:
Detection of correlated galaxy ellipticities on CFHT data: first
  evidence for gravitational lensing by large-scale structures

abstract:
  We report the detection of a significant (5.5 sigma) excess of correlations
between galaxy ellipticities at scales ranging from 0.5 to 3.5 arc-minutes.
This detection of a gravitational lensing signal by large-scale structure was
made using a composite high quality imaging survey of 6300 arcmin^2 obtained at
the Canada France Hawaii Telescope (CFHT) with the UH8K and CFH12K panoramic
CCD cameras. The amplitude of the excess correlation is 2.2\pm 0.2 % at 1
arcmin scale, in agreement with theoretical predictions of the lensing effect
induced by large-scale structure.We provide a quantitative analysis of
systematics which could contribute to the signal and show that the net effect
is small and can be corrected for. We show that the measured ellipticity
correlations behave as expected for a gravitational shear signal. The
relatively small size of our survey precludes tight constraints on cosmological
models. However the data are in favor of cluster normalized cosmological
models, and marginally reject Cold Dark Matter models with (Omega=0.3,
sigma_8<0.6) or (Omega=1, sigma_8=1). The detection of cosmic shear
demonstrates the technical feasibility of using weak lensing surveys to measure
dark matter clustering and the potential for cosmological parameter
measurements, in particular with upcoming wide field CCD cameras.


title: Radio Supernovae and GRB 980425

abstract:
  Study of radio supernovae (RSNe) over the past 20 years includes two dozen
detected objects and more than 100 upper limits. From this work we are able to
identify classes of radio properties, demonstrate conformance to and deviations
from existing models, estimate the density and structure of the circumstellar
material and, by inference, the evolution of the presupernova stellar wind, and
reveal the last stages of stellar evolution before explosion. It is also
possible to detect ionized hydrogen along the line of sight, to demonstrate
binary properties of the stellar system, and to show clumpiness of the
circumstellar material. More speculatively, it may be possible to provide
distance estimates to radio supernovae.
  The interesting and unusual radio supernova SN 1998bw, which is thought to be
related to the gamma-ray burst GRB 980425, is discussed in particular detail.
Its radio properties are compared and contrasted with those of other known
RSNe.


title: Boltzmann equations for neutrinos with flavor mixings

abstract:
  With a view of applications to the simulations of supernova explosion and
proto neutron star cooling, we derive the Boltzmann equations for the neutrino
transport with the flavor mixing based on the real time formalism of the
nonequilibrium field theory and the gradient expansion of the Green function.
The relativistic kinematics is properly taken into account. The advection terms
are derived in the mean field approximation for the neutrino self-energy whiles
the collision terms are obtained in the Born approximation. The resulting
equations take the familiar form of the Boltzmann equation with corrections due
to the mixing both in the advection part and in the collision part. These
corrections are essentially the same as those derived by Sirera et al. for the
advection terms and those by Raffelt et al. for the collision terms,
respectively, though the formalism employed here is different from theirs. The
derived equations will be easily implemented in numerical codes employed in the
simulations of supernova explosions and proto neutron star cooling.


title:
The evolution of helium white dwarfs: III. On the ages of millisecond
  pulsar systems

abstract:
  We employed recently computed evolutionary white-dwarf models with helium
cores, supplemented by heavier models with carbon-oxygen cores, in order to
investigate the ages of millisecond pulsar systems based on the cooling
properties of the compact companions. Contrary to the behaviour of more massive
white dwarfs, the evolutionary speed of low-mass white-dwarf models is
substantially slowed down by ongoing hydrogen burning. By comparing the cooling
ages of these models with the spin-down ages of the pulsars for those systems
for which reasonable information about the compact companions is available, we
found good correspondence between both ages. Based on these models any
revisions concerning the temporal evolution of millisecond pulsars do not
appear to be necessary.


title: Fractal Cosmology in an Open Universe

abstract:
  The clustering of galaxies is well characterized by fractal properties, with
the presence of an eventual cross-over to homogeneity still a matter of
considerable debate. In this letter we discuss the cosmological implications of
a fractal distribution of matter, with a possible cross-over to homogeneity at
an undetermined scale R_{homo}. Contrary to what is generally assumed, we show
that, even when R_{homo} -> \infty, this possibility can be treated
consistently within the framework of the expanding universe solutions of
Friedmann. The fractal is a perturbation to an open cosmology in which the
leading homogeneous component is the cosmic background radiation (CBR). This
cosmology, inspired by the observed galaxy distributions, provides a simple
explanation for the recent data which indicate the absence of deceleration in
the expansion (q_o \approx 0). Correspondingly the `age problem' is also
resolved. Further we show that the model can be extended back from the
curvature dominated arbitrarily deep into the radiation dominated era, and we
discuss qualitatively the modifications to the physics of the anisotropy of the
CBR, nucleosynthesis and structure formation.


title: Time Lags in Compact Objects: Constraints on the Emission Models

abstract:
  Accreting black holes and neutron stars in their hard (low) state show not
only very similar X/gamma-ray spectra but also that the behaviour of their
light curves is quite similar which can be quantified as having similar
power-density spectra and Fourier-frequency-dependent time/phase lags. Taken
together this argues for a common mechanism of the X/gamma-ray production in
these objects. This mechanism is probably a property of the accretion flow only
since it does not depend on the nature of the compact object. In this paper, I
review the observational data paying most attention to the properties of the
temporal variability such as the time/phase lags that hopefully can help us to
discriminate between different theoretical models. I also discuss the models
developed to account for the basic observational facts. Particularly, I show
that the commonly used Compton cloud models with constant temperature cannot
explain variable sources without violating the energy conservation law.
Alternative models where time lags are related to the spectral evolution during
X-ray flares are discussed and compared with observations. Compton reflection
from the outer edge of the accretion disc is shown to markedly affect the time
lag Fourier spectrum.


title: The properties of cooling flows in X-ray luminous clusters of galaxies

abstract:
  We discuss the X-ray properties of the cooling flows in a sample of thirty
highly X-ray luminous clusters of galaxies observed with the ASCA and ROSAT
satellites. We demonstrate the need for multiphase models to consistently
explain the spectral and imaging X-ray data for the clusters. The mass
deposition rates of the cooling flows, independently determined from the ASCA
spectra and ROSAT images, exhibit good agreement and exceed 1000 solar masses
per year in the largest systems. We confirm the presence of intrinsic X-ray
absorption in the clusters using a variety of spectral models. The measured
equivalent hydrogen column densities of absorbing material are sensitive to the
spectral models used in the analysis, varying from average values of a few
10^{20} atom/cm^2 for a simple isothermal emission model, to a few 10^{21}
atom/cm^2 using our preferred cooling flow models, assuming in each case that
the absorber lies in a uniform foreground screen. The true intrinsic column
densities are likely to be even higher if the absorbing medium is distributed
throughout the clusters. We summarize the constraints on the nature of the
X-ray absorber from observations in other wavebands. A significant part of the
X-ray absorption may be due to dust.


title:
HST Photometry and Keck Spectroscopy of the Rich Cluster MS1054-03:
  Morphologies, Butcher-Oemler Effect and the Color-Magnitude Relation at
  z=0.83

abstract:
  We present a study of 81 I selected, spectroscopically-confirmed members of
the X-ray cluster MS1054-03 at z=0.83. Redshifts and spectral types were
determined from Keck spectroscopy. Morphologies and accurate colors were
determined from a large mosaic of HST WFPC2 images in F606W and F814W.
Early-type galaxies constitute only 44% of this galaxy population. Thirty-nine
percent are spiral galaxies, and 17% are mergers. The early-type galaxies
follow a tight and well-defined color-magnitude relation, with the exception of
a few outliers. The observed scatter is 0.029 +- 0.005 magnitudes in restframe
U-B. Most of the mergers lie close to the CM relation defined by the early-type
galaxies. They are bluer by only 0.07 +- 0.02 magnitudes, and the scatter in
their colors is 0.07 +- 0.04 magnitudes. Spiral galaxies in MS1054-03 exhibit a
large range in their colors. The bluest spiral galaxies are 0.7 magnitudes
bluer than the early-type galaxies, but the majority is within +- 0.2
magnitudes of the early-type galaxy sequence. The red colors of the mergers and
the majority of the spiral galaxies are reflected in the fairly low
Butcher-Oemler blue fraction of MS1054-03: f_B=0.22 +- 0.05. The slope and
scatter of the CM relation of early-type galaxies are roughly constant with
redshift, confirming previous studies that were based on ground-based color
measurements and very limited membership information. However, the scatter in
the combined sample of early-type galaxies and mergers is twice as high as the
scatter of the early-type galaxies alone. This is a direct demonstration of the
``progenitor bias'': high redshift early-type galaxies seem to form a
homogeneous, old population because the progenitors of the youngest present-day
early-type galaxies are not included in the sample.


title: The Rest-Frame Optical Spectrum of MS 1512-cB58

abstract:
  Moderate resolution, near-IR spectroscopy of MS1512-cB58 is presented,
obtained during commissioning of the the Near IR Spectrometer (NIRSPEC) on the
Keck II telescope. The strong lensing of this z=2.72 galaxy by the foreground
cluster MS1512+36 makes it the best candidate for detailed study of the
rest-frame optical properties of Lyman Break Galaxies.
  A redshift of z=2.7290+/-0.0007 is inferred from the emission lines, in
contrast to the z=2.7233 calculated from UV observations of interstellar
absorption lines. Using the Balmer line ratios, we find an extinction of
E(B-V)=0.27. Using the line strengths, we infer an SFR=620+/-18 Msun/yr
(H_0=75, q_0=0.1, Lambda =0), a factor of 2 higher than that measured from
narrow-band imaging observations of the galaxy, but a factor of almost 4 lower
than the SFR inferred from the UV continuum luminosity. The width of the Balmer
lines yields a mass of M_vir=1.2x10^10 Msun. We find that the oxygen abundance
is 1/3 solar, in good agreement with other estimates of the metallicity.
However, we infer a high nitrogen abundance, which may argue for the presence
of an older stellar population.


title: Observational Constraints to the Evolution of Massive Stars

abstract:
  We consider some aspects of the evolution of massive stars which can only be
elucidated by means of "indirect" observations, i.e. measurements of the
effects of massive stars on their environments. We discuss in detail the early
evolution of massive stars formed in high metallicity regions as inferred from
studies of HII regions in external galaxies.


title:
The MACHO project: Microlensing Optical Depth towards the Galactic Bulge
  from Difference Image Analysis

abstract:
  We present the microlensing optical depth towards the Galactic bulge based on
the detection of 99 events found in our Difference Image Analysis (DIA) survey.
This analysis encompasses three years of data, covering ~ 17 million stars in ~
4 deg^2, to a source star baseline magnitude limit of V = 23. The DIA technique
improves the quality of photometry in crowded fields, and allows us to detect
more microlensing events with faint source stars. We find this method increases
the number of detection events by 85% compared with the standard analysis
technique. DIA light curves of the events are presented and the microlensing
fit parameters are given. The total microlensing optical depth is estimated to
be tau_(total)= 2.43^(+0.39/-0.38) x 10^(-6) averaged over 8 fields centered at
l=2.68 and b=-3.35. For the bulge component we find
tau_(bulge)=3.23^(+0.52/-0.50) x 10^(-6) assuming a 25% stellar contribution
from disk sources. These optical depths are in good agreement with the past
determinations of the MACHO Alcock et al. (1997) and OGLE Udalski et al. (1994)
groups, and are higher than predicted by contemporary Galactic models. We show
that our observed event timescale distribution is consistent with the
distribution expected from normal mass stars, if we adopt the stellar mass
function of Scalo (1986) as our lens mass function. However, we note that as
there is still disagreement about the exact form of the stellar mass function,
there is uncertainty in this conclusion. Based on our event timescale
distribution we find no evidence for the existence of a large population of
brown dwarfs in the direction of the Galactic bulge.


title:
Aluminum Abundances, Deep Mixing and the Blue-Tail Second-Parameter
  Effect in the Globular Clusters M 3 and M 13

abstract:
  We explore the relatiohsip between deep mixing on the red-giant branch and
the second parameter that gives rise to the extended blue tail seen on the
horizontal branch of the globular cluster M 13. We use the [Al/Fe] values of 96
RGB stars and the [Na/Fe] values of 119 RGB stars in M 13 to follow the
evolution of mixing in this cluster. The data are compared to an
"instantaneous" mixing algorithm to predict the effects of deep mixing on the
HB morphology. In addition, we present new data for six giants in the cluster M
3 and three in M 13. Our final results are consistent with deep mixing as a
blue-tail second parameter. We also examine the question of overproducing
sodium during deep mixing and suggest ways around this problem and
observational tests that center around the s-process elements Sr, Y and Zr.


title: The Detection of Multimodal Oscillations on Alpha UMa

abstract:
  We have used the star camera on the WIRE satellite to observe the K0 III star
Alpha UMa, and we report the apparent detection of 10 oscillation modes. The
lowest frequency mode is at 1.82 microhertz, and appears to be the fundamental
mode. The mean spacing between the mode frequencies is 2.94 microhertz, which
implies that all detected modes are radial. The mode frequencies are consistent
with the physical parameters of a K0 III star, if we assume that only radial
modes are excited. Mode amplitudes are 100 -- 400 micromagnitudes, which is
consistent with the scaling relation of Kjeldsen & Beddinge (1995).


title: Metallicity Evolution in the Early Universe

abstract:
  Observations of the damped Lya systems provide direct measurements on the
chemical enrichment history of neutral gas in the early universe. In this
Letter, we present new measurements for four damped Lya systems at high
redshift. Combining these data with [Fe/H] values culled from the literature,
we investigate the metallicity evolution of the universe from z~1.5-4.5.
Contrary to our expectations and the predictions of essentially every chemical
evolution model, the N(HI)-weighted mean [Fe/H] metallicity exhibits minimal
evolution over this epoch. For the individual systems, we report tentative
evidence for an evolution in the unweighted [Fe/H] mean and the scatter in
[Fe/H] with the higher redshift systems showing lower scatter and lower typical
[Fe/H] values. We also note that no damped Lya system has [Fe/H] < -2.7 dex.
Finally, we discuss the potential impact of small number statistics and dust on
our conclusions and consider the implications of these results on chemical
evolution in the early universe.


title:
The Future of Radio Astronomy: Options for Dealing with Human Generated
  Interference

abstract:
  Radio astronomy provides a unique window on the universe, allowing us to
study: non-thermal processes (galactic nuclei, quasars, pulsars) at the highest
angular resolution using VLBI, with low opacity. It is the most interesting
wave band for SETI searches. To date it has yielded 3 Nobel prizes (microwave
background, pulsars, gravitational radiation). There are both exciting
possibilities and substantial challenges for radio astronomy to remain at the
cutting edge over the next 3 decades. New instruments like ALMA and the SKA
will open up new science if the challenge of dealing human generated
interference can be met. We summarise some of the issues and technological
developments that will be essential to the future success of radio astronomy.


title: Radio Frequency Interference

abstract:
  We describe the nature of the interference challenges facing radio astronomy
in the next decade. These challenges will not be solved by regulation only,
negotiation and mitigation will become vital. There is no silver bullet for
mitigating against interference. A successful mitigation approach is most
likely to be a hierarchical or progressive approach throughout the telescope
and signal conditioning and processing systems. We summarise some of the
approaches, including adaptive systems.


title: Cancellation of GLONASS signals from Radio Astronomy Data

abstract:
  Astronomers use the 1612 MHz OH spectral line emission as a unique window on
properties of evolved stars, galactic dynamics, and putative proto-planetary
disk systems around young stars. In recent years, experiments using this OH
line have become more difficult because radio telescopes are very sensitive to
transmissions from the GLONASS satellite system. The weak astronomical signals
are often undetectable in the presence of these unwanted human generated
signals. In this paper we demonstrate that GLONASS narrow band signals may be
removed using digital signal processing in a manner that is robust and
non-toxic to the weak astronomy signals, without using a reference antenna. We
present results using real astronomy data and outline the steps required to
implement useful systems on radio telescopes.


title: A Broad 22 Microns Emission Feature in the Carina Nebula H II Region

abstract:
  We report the detection of a broad 22 microns emission feature in the Carina
nebula H II region by the Infrared Space Observatory (ISO) Short Wavelength
Spectrometer. The feature shape is similar to that of the 22 microns emission
feature of newly synthesized dust observed in the Cassiopeia A supernova
remnant. This finding suggests that both of the features are arising from the
same carrier, and that supernovae are probably the dominant production source
of this new interstellar grain. A similar broad emission dust feature is also
found in the spectra of two starburst galaxies from the ISO archival data. This
new dust grain could be an abundant component of interstellar grains and can be
used to trace the supernova rate or star formation rate in external galaxies.
The existence of the broad 22 microns emission feature complicates the dust
model for starburst galaxies and must be taken into account correctly in the
derivation of dust color temperature. Mg protosilicate has been suggested as
the carrier of the 22 microns emission dust feature observed in Cassiopeia A.
The present results provide useful information in studies on chemical
composition and emission mechanism of the carrier.


title:
X-ray observation of the super-luminal quasar 4C+73.18 (1928+738) by
  ASCA

abstract:
  The results of an X-ray observation of the nearby core-dominated, super-
luminal quasar 4C+73.18 (1928+738) with ASCA are reported. Variations of the
1-10keV flux on time-scales of the order of 10 hours were found while the
0.5-1keV flux remained almost unchanged. The quasar shows a curved spectrum in
the 0.8-10keV band, which can be modeled as a power law plus a soft component.
Both, the spectrum and the uncorrelated flux variations indicate the presence
of a soft X-ray excess, which might be the high-energy extension of the intense
UV bump emission found in this quasar. The hard X-ray emission is dominated by
the flat power law (photon index 1.1-1.5). The results are consistent with
models in which the hard X-rays are emitted from the relativistic jet. An iron
line emission as reported in a previous Ginga observation (Lawson & Turner
1997) was not detected. We suggest that the line detected by Ginga might be not
associated with the quasar. There are indications for a tentative weak emission
line-like feature around 1.25keV (1.63keV in the quasar frame) in both, the SIS
and GIS spectra. The energy is surprisingly close to that of an unexplained
line found in the flat-spectrum quasar PKS0637-752 by Yaqoob et al. (1998).


title: Optical and X-ray analysis of the cluster of galaxies Abell 496

abstract:
  We present a detailed analysis of the cluster of galaxies \a4. The optical
data include a redshift catalogue of 466 galaxies, out of which 274 belong to
the main cluster and a CCD photometric catalogue in a much smaller region, with
239 and 610 galaxies in the V and R bands respectively. The X-ray analysis is
based on an image obtained with the ROSAT PSPC. Besides Abell 496 itself, the
velocity distribution along the line of sight shows the existence of at least
four structures at different redshifts, one of them seeming to be a poor
cluster at a velocity of 30083 km/s. The other of these structures have a too
large spatial extent to be clusters but may be filaments along the line of
sight or other young structures. Various independent methods show that Abell
496 appears to be a quite relaxed cluster, except perhaps for the distribution
of emission line galaxies. These appear to be distributed in two samples
falling on to the main cluster, one from the back (the ELGs concentrated
towards the west) and one from the front (the high velocity ELGs). The bright
part of the galaxy luminosity function, built from the redshift catalogue,
shows a flattening at R$\sim 16$ (M$_{\rm R}\sim -20.5$), and can be accounted
for by a gaussian distribution of bright galaxies and a power law or Schechter
function for faint galaxies. The deeper galaxy counts derived from CCD imaging
show a dip at R$\sim 19.5$ (M$_{\rm R} \sim -17$) which can be modelled
assuming a cut-off in the luminosity function such as that observed in Coma. We
propose a model for the X-ray gas and derive the galaxy, X-ray gas and total
dynamical masses, as well as the baryon fraction in the cluster. \a4 appears as
a relaxed cluster which can be used as a prototype for further studies.


title: CMB Anisotropies: A Decadal Survey

abstract:
  We review the theoretical implications of the past decade of CMB anisotropy
measurements, which culminated in the recent detection of the first feature in
the power spectrum, and discuss the tests available to the next decade of
experiments. The current data already suggest that density perturbations
originated in an inflationary epoch, the universe is spatially flat, and
baryonic dark matter is required. We discuss the underlying assumptions of
these claims and outline the tests required to ensure they are robust. The most
critical test - the presence of a second feature at the predicted location -
should soon be available. Further in the future, secondary anisotropies and
polarization should open new windows to the early and low(er) redshift
universe.


title: Distances to Cepheid Open Clusters Via Optical and K-Band Imaging

abstract:
  We investigate the reddening and Main Sequence fitted distances to eleven
young, Galactic open clusters that contain Cepheids. Each cluster contains or
is associated with at least one Cepheid variable star. Reddening to the
clusters is estimated using the U-B:B-V colours of the OB stars and the
distance modulus to the cluster is estimated via B-V:V and V-K:V
colour-magnitude diagrams. By main-sequence fitting we proceed to calibrate the
Cepheid P-L relation and find M_V=-2.81xlogP-1.33 +/-0.32 and
M_K=-3.44xlogP-2.20 +/-0.29 and a distance modulus to the LMC of 18.55+/-0.32
in the V-band and 18.47+/-0.29 in the K-band giving an overall distance modulus
to the LMC of 18.51+/-0.3.
  In the case of two important clusters we find that the U-B:B-V diagram in
these clusters is not well fitted by the standard Main Sequence line. In one
case, NGC7790, we find that the F stars show a UV excess which if caused by
metallicity would imply Fe/H ~ -1.5; this is anomalously low compared to what
is expected for young open clusters. In a second case, NGC6664, the U-B:B-V
diagram shows too red U-B colours for the F stars which in this case would
imply a higher than solar metallicity. If these effects are due to metallicity
then it would imply that the Cepheid PL(V) and PL(K) zeropoints depend on
metallicity according to delta(M)/delta(Fe/H) ~0.66 in the sense that lower
metallicity Cepheids are intrinsically fainter. Medium-high resolution
spectroscopy for the main-sequence F stars in these two clusters is needed to
determine if metallicity really is the cause or whether some other explanation
applies.


title:
Determination of cosmological parameters from large scale structure
  observations

abstract:
  The possibility of determining cosmological parameters on the basis of a wide
set of observational data including the Abell-ACO cluster power spectrum and
mass function, peculiar velocities of galaxies, the distribution of Ly-$\alpha$
clouds and CMB temperature fluctuations is analyzed. Using a $\chi^2$
minimization method, assuming $\Omega_{\Lambda}+\Omega_m =1$ and no
contribution from gravity waves, we found that a tilted $\Lambda$MDM model with
one sort of massive neutrinos and the parameters $n\approx 1.12$,
$\Omega_m\approx 0.4$ ($\Omega_{\Lambda}\approx 0.6$), $\Omega_{cdm}\approx
0.3$, $\Omega_{\nu}\approx 0.06$, $\Omega_b\approx 0.04$ and $h\approx 0.7$
matches observational data best. $\Omega_{\nu}$ is higher for more species of
massive neutrinos, $\sim 0.1$ for two and $\sim 0.13$ for three species.
$\Omega_m$ raises by $\sim 0.08$ and $\sim 0.15$ respectively.
  Varying only a subset of parameters and fixing the others shows also that the
observational data set used here rules out pure CDM models with $h\ge 0.5$,
scale invariant primordial power spectrum, zero cosmological constant and
spatial curvature at a very high confidence level, $>99.99%$. The corresponding
class of MDM models are ruled out at $\sim 95%$ C.L.
  It is notable also that this data set determines the amplitude of scalar
fluctuations approximately at the same level as COBE four-year data. It
indicates that a possible tensor component in the COBE data cannot be very
substantial.


title: Radially truncated galactic discs

abstract:
  We present the first results of a systematic analysis of radially truncated
exponential discs for four galaxies of a complete sample of disc-dominated
edge-on spiral galaxies. The discs of our sample galaxies are truncated at
similar radii on either side of their centres. With possible the exception of
the disc of ESO 416-G25, it appears that the truncations in our sample galaxies
are closely symmetric, in terms of both their sharpness and the truncation
length. However, the truncations occur over a larger region and not as abruptly
as found in previous studies. We show that the truncated luminosity
distributions of our sample galaxies, if also present in the mass
distributions, comfortably meet the requirements for longevity. The formation
and maintenance of disc truncations are likely closely related to stability
requirements for galactic discs.


title: Do strange stars exist in the Universe?

abstract:
  Definitely, an affirmative answer to this question would have implications of
fundamental importance for astrophysics (a new class of compact stars), and for
the physics of strong interactions (deconfined phase of quark matter, and
strange matter hypothesis). In the present work, we use observational data for
the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 and for the
atoll source 4U 1728-34 to constrain the radius of the underlying compact
stars. Comparing the mass-radius relation of these two compact stars with
theoretical models for both neutron stars and strange stars, we argue that a
strange star model is more consistent with SAX J1808.4-3658 and 4U 1728-34, and
suggest that they are likely strange star candidates.


title: On the particle acceleration near the light surface of radio pulsars

abstract:
  The two-fluid effects on the radial outflow of relativistic electron-positron
plasma are considered. It is shown that for large enough Michel magnetization
parameter (1969) and multiplication parameter, the one-fluid MHD approximation
remains correct in the whole region where |E| < |B|. In the case in which the
longitudinal electric current is smaller than the Goldreich-Julian one, the
acceleration of particles is determined near the light surface where |E| = |B|.
  It is shown that, as in the previously considered cylindrical geometry
(Beskin, Gurevich & Istomin 1983), almost all electromagnetic energy is
transformed into the energy of particles in the narrow boundary layer. The
relative thickness of this layer is shown to be of the order of the inverse of
multiplication parameter.


title: Absorption-Line Probes of Gas and Dust in Galactic Superwinds

abstract:
  We discuss moderate resolution spectra of the NaD absorption-line in a sample
of 32 far-IR-bright starburst galaxies. In 18 cases, the line is produced
primarily by interstellar gas, and in 12 of these it is blueshifted by over 100
km/s relative to the galaxy systemic velocity. The absorption-line profiles in
these outflow sources span the range from near the galaxy systemic velocity to
a maximum blueshift of 400 to 600 km/s. The outflows occur in galaxies
systematically viewed more nearly face-on than the others. We therefore argue
that the absorbing material consists of ambient interstellar gas accelerated
along the minor axis of the galaxy by a hot starburst-driven superwind. The NaD
lines are optically-thick, but indirect arguments imply total Hydrogen column
densities of N_H = few X 10^{21} cm^{-2}. This implies that the superwind is
expelling matter at a rate comparable to the star-formation rate. This
outflowing material is very dusty: we find a strong correlation between the
depth of the NaD profile and the line-of-sight reddening (E(B-V) = 0.3 to 1
over regions several-to-ten kpc in size). The estimated terminal velocities of
superwinds inferred from these data and extant X-ray data are typically 400 to
800 km/s, are independent of the galaxy rotation speed, and are comparable to
(substantially exceed) the escape velocities for $L_*$ (dwarf) galaxies. The
resulting loss of metals can establish the mass-metallicity relation in
spheroids, produce the observed metallicity in the ICM, and enrich a general
IGM to 10$^{-1}$ solar metallicity. If the outflowing dust grains survive their
journey into the IGM, their effect on observations of cosmologically-distant
objects is significant.


title:
Prompt and afterglow emission from the X-ray rich GRB981226 observed
  with BeppoSAX

abstract:
  We discuss observations of the prompt X and gamma-ray emission and X-ray
afterglow from GRB981226. This event has the weakest gamma-ray peak flux
detected with the Gamma-Ray Burst Monitor. It shows an isolated X-ray precursor
and the highest X-ray to gamma-ray fluence ratio measured thus far with the
Wide Field Cameras. The event was followed up with the Narrow Field
Instruments, and the X-ray afterglow was detected up to 10 keV. The afterglow
flux is observed to rise from a level below the sensitivity of the MECS/LECS
telescopes up to a peak flux of (5 +/-1) * 10^(-13) erg cm-2 s-1, in the 2-10
keV energy band. This rise is followed by a decline according to a power law
with index of 1.31 (+0.44,-0.39). We discuss these results in the light of the
current GRB models.


title:
The Final Fate of Coalescing Compact Binaries: From Black Hole to Planet
  Formation

abstract:
  Coalescing compact binaries are thought to be involved in a wide variety of
astrophysical phenomena. In particular, they are important sources of
gravitational radiation for both ground-based and space-based
laser-interferometer detectors, and they may be sources of supernova explosions
or gamma-ray bursts. Mergers of two white dwarfs may produce neutron stars with
peculiar properties, including perhaps millisecond radio pulsars sometimes
accompanied by planets (as observed in PSR 1257+12). According to a widely held
belief, the coalescence of two neutron stars should produce a rapidly rotating
black hole surrounded by an accretion disk or torus, but this is by no means
certain. This review paper focuses on the final hydrodynamic coalescence and
merger of double neutron stars and double white dwarfs, and addresses the
question of the nature of the final merger products.


title:
Spherical, Oscillatory $\alpha^2$-Dynamo Induced by Magnetic Coupling
  Between a Fluid Shell and an Inner Electrically Conducting Core: Relevance to
  the Solar Dynamo

abstract:
  A two-layer spherical $\alpha^2$-dynamo model consisting of an inner
electrically conducting core (magnetic diffusivity $\lambda_i$ and radius
$r_i$) with $\alpha = 0$ surrounded by an electrically conducting spherical
shell (magnetic diffusivity $\lambda_o$ and radius $r_o$) with a constant
$\alpha$ is shown to exhibit oscillatory behavior for values of $\beta =
\lambda_i/\lambda_o$ and $r_i/r_o$ relevant to the solar dynamo. Time-dependent
dynamo solutions require $r_i/r_o \geq 0.55$ and $\beta \leq O(1)$. For the
Sun, $r_i/r_o$ is about 0.8 and $\beta\approx 10^{-3}$. The time scale of the
oscillations matches the 22 year period of the sunspot cycle for $\lambda_0 =
O(10^2 km^2 s^{-1}$). It is unnecessary to hypothesize an $\alpha\omega$-dynamo
to obtain oscillatory dynamo solutions; an $\alpha^2$-dynamo suffices provided
the spherical shell region of dynamo action lies above a large, less
magnetically diffusive core, as is the case for the solar dynamo.


title: The Discovery of an Embedded Cluster of High-Mass Stars Near SGR 1900+14

abstract:
  Deep I-band imaging to approximately I = 26.5 of the soft gamma-ray repeater
SGR 1900+14 region has revealed a compact cluster of massive stars located only
a few arcseconds from the fading radio source thought to be the location of the
SGR (Frail, Kulkarni, & Bloom 1999). This cluster was previously hidden in the
glare of the pair of M5 supergiant stars (whose light was removed by PSF
subtraction) proposed by Vrba et al. (1996) as likely associated with the SGR
1900+14. The cluster has at least 13 members within a cluster radius of
approximately 0.6 pc, based on an estimated distance of 12-15 kpc. It is
remarkably similar to a cluster found associated with SGR 1806-20 (Fuchs et al.
1999). That similar clusters have now been found at or near the positions of
the two best-studied SGRs suggests that young neutron stars, thought to be
responsible for the SGR phenomenon, have their origins in proximate compact
clusters of massive stars.


title: Models of Wave Supported Clumps in Giant Molecular Clouds

abstract:
  We present plane-parallel equilibrium models of molecular clumps that are
supported by Alfven waves damped by the linear process of ion-neutral friction.
We used a WKB approximation to treat the inward propagation of waves and
adopted a realistic ionization structure influenced by dissociation and
ionization due to photons of external origin. The model clumps are larger and
less centrally condensed than those obtained for an assumed ionization
structure, used in some previous studies, that is more appropriate for dark
regions.


title: The Baryonic Tully-Fisher Relation

abstract:
  We explore the Tully-Fisher relation over five decades in stellar mass in
galaxies with circular velocities ranging over 30 < Vc < 300 km/s. We find a
clear break in the optical Tully-Fisher relation: field galaxies with Vc < 90
km/s fall below the relation defined by brighter galaxies. These faint galaxies
are however very gas rich; adding in the gas mass and plotting baryonic disk
mass Md = M* + Mg in place of luminosity restores a single linear relation. The
Tully-Fisher relation thus appears fundamentally to be a relation between
rotation velocity and total baryonic mass of the form Md = A Vc^4.


title:
RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete
  Sample of 1500 Bright X-ray Sources

abstract:
  We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAO VLA
Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightest X-ray
sources (>= 0.1 counts/s or ~1E-12 ergs/cm/cm/s in the 0.1-2.4 keV band) that
are also radio sources (S >= 2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalactic
sky with |b| > 15 degrees. and delta > -40 degrees. The sky density of NVSS
sources is low enough that they can be reliably identified with RBSC sources
having average rms positional uncertainties = 10 arcsec. We used the more
accurate radio positions to make reliable X-ray/radio/optical identifications
down to the POSS plate limits. We obtained optical spectra for many of the
bright identifications lacking published redshifts. The resulting X-ray/radio
sample is unique in its size (N ~ 1500 objects), composition (a mixture of
nearly normal galaxies, Seyfert galaxies, quasars, and clusters), and low
average redshift (<z> ~ 0.1).


title: Galactic Globular Clusters as a test for Very Low-Mass stellar models

abstract:
  We make use of the Next Generation model atmospheres by Allard et al. (1997)
and Hauschildt, Allard & Baron (1999) to compute theoretical models for low and
very low-mass stars for selected metallicities in the range Z= 0.0002 to 0.002.
On this basis, we present theoretical predictions covering the sequence of
H-burning stars as observed in galactic globulars from the faint end of the
Main Sequence up to, and beyond, the cluster Turn Off. The role played by the
new model atmospheres is discussed, showing that present models appear in
excellent agreement with models by Baraffe et al. (1997) as computed on quite
similar physical basis. One finds that the theoretical mass-luminosity
relations based on this updated set of models, are in good agreement with the
empirical data provided by Henry & McCarthy (1993). Comparison with HST
observation discloses that the location in the Color-Magnitude diagram of the
lower Main Sequence in galactic globular clusters appears again in good
agreement with the predicted sensitive dependence of these sequences on the
cluster metallicity.


title: On the helium content of Galactic globular clusters via the R parameter

abstract:
  We estimate the empirical R parameter in 26 Galactic Globular Clusters
covering a wide metallicity range, imaged by WFPC2 on board the HST. The
improved spatial resolution permits a large fraction of the evolved stars to be
measured and permits accurate assessment of radial populaton gradients and
completeness corrections. In order to evaluate both the He abundance and the He
to metal enrichment ratio, we construct a large set of evolutionary models by
adopting similar metallicities and different He contents. We find an absolute
He abundance which is lower than that estimated from spectroscopic measurements
in HII regions and from primordial nucleosynthesis models. This discrepancy
could be removed by adopting a C12O16 nuclear cross section about a factor of
two smaller than the canonical value, although also different assumptions for
mixing processes can introduce systematical effects. The trend in the R
parameter toward solar metallicity is consistent with an upper limit to the He
to metal enrichment ratio of the order of 2.5.


title:
Effects of galaxy mergers on the faint IRAS source counts and the
  background

abstract:
  Binary merging constitutes a complementary mode of galaxy evolution to the
canonical hierarchical clustering theory. This merger-driven evolution not only
influences the galaxy mass distribution function, but may drive the main
galactic activity cycles, such as starbursts and the activity in the nuclei
(AGNs). In this paper, we use galaxy aggregation dynamics together with a
possible merger-driven starburst and AGN phenomena, to study the effects of
evlution of these consequential activities for the faint IRAS source counts and
the infra-red background. We find that the strong evolution of IRAS 60$ \mu m$
source count at flux range of $10 mJy \sim 1 Jy$ is difficult to be explained
only by the merger rate decrease with cosmic time. We need to assume a redshift
dependent infrared burst phase of ultraluminous infrared galaxies (ULIGs) from
the gas rich mergers at high redshift and the gas poor mergers at low redshift.
The background intensity at 60$ \mu m$ which we get from this aggregation
evolution is a lower limit of only $1.9 nW m^{-2} sr^{-1}$, about half of those
estimated by some previous models, but close to the lower end of the range
derived by Malkan & Stecker (1998).


title: Bulges

abstract:
  We model the evolution of the galactic bulge and of the bulges of a selected
sample of external spiral galaxies, via the multiphase multizone evolution
model. We address a few questions concerning the role of the bulges within
galactic evolution schemes and the properties of bulge stellar populations. We
provide solutions to the problems of chemical abundances and spectral indices,
the two main observational constraints to bulge structure.


title: Analysis of Balmer Profiles of early type stars

abstract:
  The spectral energy distribution (SED) of recent star formation regions is
dominated by the more massive and early stars (O and B types). These stars show
large and very significant absorption features, the most prominent being the
recombination lines of H, HeI and HeII. In particular, the shape of their
profiles are very dependent on the luminosity of the star. We have explored the
potential use of high resolution profiles to discriminate between different
luminosity classes and spectral types, by using profiles of the He and Balmer
lines. We have calculated growth curves for each of the lines and their
dependence on gravity and effective temperature. We show some of these
theoretical growth curves and our preliminary conclusions are analyzed and
discussed.


title: Detection of Weak Gravitational Lensing by Large-scale Structure

abstract:
  We report a detection of the coherent distortion of faint galaxies arising
from gravitational lensing by foreground structures. This ``cosmic shear'' is
potentially the most direct measure of the mass power spectrum, as it is
unaffected by poorly-justified assumptions made concerning the biasing of the
distribution. Our detection is based on an initial imaging study of 14
separated 8' x 16' fields observed in good, homogeneous conditions with the
prime focus EEV CCD camera of the 4.2m William Herschel Telescope. We detect an
rms shear of 1.6% in 8' x 8' cells, with a significance of 3.4 sigma. We
carefully justify this detection by quantifying various systematic effects and
carrying out extensive simulations of the recovery of the shear signal from
artificial images defined according to measured instrument characteristics. We
also verify our detection by computing the cross-correlation between the shear
in adjacent cells. Including (gaussian) cosmic variance, we measure the shear
variance to be (0.016)^2 plus/minus (0.012)^2 plus/minus (0.006)^2, where these
1 sigma errors correspond to statistical and systematic uncertainties,
respectively. Our measurements are consistent with the predictions of
cluster-normalised CDM models (within 1 sigma) but a COBE-normalised SCDM model
is ruled out at the 3.0 sigma level. For the currently-favoured Lambda-CDM
model (with Omega_m = 0.3), our measurement provides a normalisation of the
mass power spectrum of sigma_8 = 1.5 plus/minus 0.5, fully consistent with that
derived from cluster abundances. Our result demonstrates that ground-based
telescopes can, with adequate care, be used to constrain the mass power
spectrum on various scales. The present results are limited mainly by cosmic
variance, which can be overcome in the near future with more observations.


title:
Design Considerations for Large Detector Arrays on Submillimeter-wave
  Telescopes

abstract:
  The emerging technology of large (~ 10,000 pixel) submillimeter-wave
bolometer arrays presents a novel optical design problem---how can such arrays
be fed by diffraction-limited telescope optics where the primary mirror is less
than 100,000 wavelengths in diameter? Standard Cassegrain designs for
radiotelescope optics exhibit focal surface curvature so large that detectors
cannot be placed more than 25 beam diameters from the central ray. The problem
is worse for Ritchey-Cretien designs, because these minimize coma while
increasing field curvature. Classical aberrations, including coma, are usually
dominated by diffraction in submillimeter-wave single dish telescopes. The
telescope designer must consider (1) diffraction, (2) aberration, (3) curvature
of field, (4) cross-polarization, (5) internal reflections, (6) the effect of
blockages, (7) means of beam chopping on- and off-source, (8) gravitational and
thermal deformations of the primary mirror, (9) the physical mounting of large
detector packages, and (10) the effect of gravity and (11) vibration on those
detectors. Simultaneous optimization of these considerations in the case of
large detector arrays leads to telescopes that differ considerably from
standard radiotelescope designs. Offset optics provide flexibility for mounting
detectors, while eliminating blockage and internal reflections. Aberrations and
cross-polarization can be the same as on-axis designs having the same diameter
and focal length. Trade-offs include the complication of primary mirror
homology and an increase in overall cost. A dramatic increase in usable field
of view can be achieved using shaped optics. Solutions having one to six
mirrors will be discussed, including a possible six-mirror design for the
proposed South Pole 10m telescope.


title:
Lithium and rotation on the subgiant branch II. Theoretical analysis of
  observations

abstract:
  Lithium abundances and rotation, determined for 120 subgiant stars in Lebre
et al. (1999) are analyzed. To this purpose, the evolutionary status of the
sample as well as the individual masses have been determined using the
HIPPARCOS trigonometric parallax measurements to locate very precisely our
sample stars in the HR diagram. We look at the distributions of A(Li) and Vsini
with mass when stars evolve from the main sequence to the subgiant branch. For
most of the stars in our sample we find good agreement with the dilution
predictions. However, the more massive cool stars with upper limits of Li
abundances show a significant discrepancy with the theoretical predictions,
even if the Non-LTE effects are taken into account. For the rotation behaviour,
our analysis confirms that low mass stars leave the main sequence with a low
rotational rate, while more massive stars are slowed down only when reaching
the subgiant branch. We also checked the connection between the observed
rotation behaviour and the magnetic braking due to the deepening of the
convective envelope. Our results shed new light on the lithium and rotation
discontinuities in the evolved phase.


title:
The rate of cosmic ray showers at large zenith angles: a step towards
  the detection of ultra-high energy neutrinos by the Pierre Auger Observatory

abstract:
  It is anticipated that the Pierre Auger Observatory can be used to detect
cosmic neutrinos of >10^19 eV that arrive at very large zenith angles. However
showers created by neutrino interactions close to the detector must be picked
out against a background of similar events initiated by cosmic ray nuclei. As a
step towards understanding this background, we have made the first detailed
analysis of air showers recorded at Haverah Park (an array which used similar
detectors to those planned for the Auger Observatory) with zenith angles above
60 degs. We find that the differential shower rate from 60 degs to 80 degs. can
be predicted accurately when we adopt the known primary energy spectrum above
10^17 eV and assume the QGSJET model and proton primaries. Details of the
calculation are given.


title: Stellar Populations in the Large Magellanic Cloud from 2MASS

abstract:
  We present a morphological analysis of the feature-rich 2MASS LMC
color-magnitude diagram, identifying Galactic and LMC populations and
estimating the density of LMC populations alone. We also present the projected
spatial distributions of various stellar populations. Major populations are
identified based on matching morphological features of the CMD with expected
positions of known populations, isochrone fits, and analysis of the projected
spatial distributions. The LMC populations along the first-ascent RGB and AGB
are quantified. We find the RGB tip at $K_s=12.3\pm0.1$. Preliminary isochrone
analysis is done for giant populations in the bar and the outer regions of the
Cloud. We find no significant differences in metallicities and ages between the
fields. The observed LMC giant branch is well-fit by published tracks in the
CIT/CTIO system with a distance modulus of $\mu=18.5\pm0.1$, reddening
$E_{B-V}=0.15-0.20$, metallicity $Z=0.004^{+0.002}_{-0.001}$ and age 3-13 Gyr.
Analysis of deep 2MASS engineering data with six times the standard exposure
produces similar estimates.


title: The Mystery of Ultra-High Energy Cosmic Rays

abstract:
  The origin of cosmic rays with energies higher than 10$^{20}$ eV remains a
mystery. Accelerating particles up to these energies is a challenge even for
the most energetic astrophysical objects known. While the isotropy in arrival
directions argues for an extra-galactic origin, the photon-pion production off
the cosmic background radiation limits the sources of such particles to systems
less than 50 Mpc away from us. The combination of large gyroradii, efficient
energy losses, and isotropic arrival directions defies most of the proposed
astrophysical accelerators as well as the more exotic alternatives. I briefly
review theoretical models for the acceleration and propagation of
ultra-high-energy cosmic-rays and discuss the potential for future
observatories to resolve this cosmic mystery.


title:
Detection of weak gravitational lensing distortions of distant galaxies
  by cosmic dark matter at large scales

abstract:
  Most of the matter in the universe is not luminous and can be observed
directly only through its gravitational effect. An emerging technique called
weak gravitational lensing uses background galaxies to reveal the foreground
dark matter distribution on large scales. Light from very distant galaxies
travels to us through many intervening overdensities which gravitationally
distort their apparent shapes. The observed ellipticity pattern of these
distant galaxies thus encodes information about the large-scale structure of
the universe, but attempts to measure this effect have been inconclusive due to
systematic errors. We report the first detection of this ``cosmic shear'' using
145,000 background galaxies to reveal the dark matter distribution on angular
scales up to half a degree in three separate lines of sight. The observed
angular dependence of this effect is consistent with that predicted by two
leading cosmological models, providing new and independent support for these
models.


title: How to Identify Pre-Protostellar Cores

abstract:
  We have observed the HCO+ J=3-2 line toward 17 starless cores selected from
the list of Ward-Thompson et al. (1994). Six of these cores have line
asymmetries indicative of collapse. The excess of blue-skewed profiles over
red-skewed profiles is at least as large as that found in samples of Class 0
and early Class I sources. The observed line profiles have the same narrow
linewidths and small peak temperatures predicted for young sources in
evolutionary models, but the blue/red ratios, like those of older sources, are
higher than models predict. The infall signature also occurs over large scales,
suggesting that these cores have overall inward motions. We have divided these
starless cores into two groups based on the continuum photometry of
Ward-Thompson et al. and our HCO+ data. We find stronger HCO+ emission among
the cores detected in the submillimeter and all the blue-skewed line profiles
are in this group, supporting the suggestion of Ward-Thompson et al. that these
are the pre-protostellar cores.


title:
Accretion Rates onto Massive Black Holes in Four Quiescent Elliptical
  Galaxies

abstract:
  Four quiescent elliptical galaxies were imaged with the NRAO VLA at 8.5 GHz.
Within the context of canonical advection-dominated accretion flows (ADAFs),
these VLA images plus published black hole masses constrain the accretion rates
to be $ <1.6\times10^{-4}$, $ <3.6\times10^{-4}$, $\le7.8\times10^{-4}$, and
$\le7.4\times10^{-4}$ of the Eddington rates. These ADAF accretion rates
derived at 8.5 GHz have important implications for the levels of soft and hard
X-rays expected from these quiescent galaxies.


title:
Slim Disk Model for Soft X-Ray Excess and Variability of Narrow-Line
  Seyfert 1 Galaxies

abstract:
  Narrow-line Seyfert 1 galaxies (NLS1s) exhibit extreme soft X-ray excess and
large variability. We argue that both features can be basically accounted for
by the slim disk model. We assume that a central black-hole mass in NLS1 is
relatively small, $M \sim 10^{5-7}M_\odot$, and that a disk shines nearly at
the Eddington luminosity, $L_{\rm E}$. Then, the disk becomes a slim disk and
exhibits the following distinctive signatures: (1) The disk luminosity
(particularly of X-rays) is insensitive to mass-flow rates, $\dot M$, since the
generated energy is partly carried away to the black hole by trapped photons in
accretion flow. (2) The spectra are multi-color blackbody. The maximum
blackbody temperature is $T_{\rm bb} \simeq 0.2(M/10^5 M_\odot)^{-1/4}$ keV,
and the size of the blackbody emitting region is small, $r_{\rm bb} \lsim 3
r_{\rm S}$ (with $r_{\rm S}$ being Schwarzschild radius) even for a
Schwarzschild black hole. (3) All the ASCA observation data of NLS1s fall onto
the region of $\dot M/(L_{\rm E}/c^2)>10$ (with $L_{\rm E}$ being the Eddington
luminosity) on the ($r_{\rm bb},T_{\rm bb}$) plane, supporting our view that a
slim disk emits soft X-rays at $\sim L_{\rm E}$ in NLS1s. (4) Magnetic energy
can be amplified, at most, up to the equipartition value with the trapped
radiation energy which greatly exceeds radiation energy emitted from the disk.
Hence, energy release by consecutive magnetic reconnection will give rise to
substantial variability in soft X-ray emission.


title: Repulsive Dark Matter

abstract:
  It seems necessary to suppress, at least partially, the formation of
structure on subgalactic scales. As an alternative to warm or collisional dark
matter, I postulate a condensate of massive bosons interacting via a repulsive
interparticle potential, plus gravity. This leads to a minimum lengthscale for
bound objects, and to superfluidity. Galactic dynamics may differ significantly
from that of more generic dark matter in not unwelcome ways, especially in the
core. Such particles can be realized as quanta of a relativistic massive scalar
field with a quartic self-interaction. At high densities, the equation of state
has the same form as that of an ideal relativistic gas despite the
interactions. If the nonrelativistic lengthscale is of order a kiloparsec, then
the energy density in these particles was comparable to that of photons at
early times, but small enough to avoid conflict with primordial
nucleosynthesis.


title: Gamma-loud quasars: a view with BeppoSAX

abstract:
  We present $Beppo$SAX observations of the $\gamma $-ray emitting quasars
0836+710, 1510-089 and 2230+114. All the objects have been detected in the PDS
up to 100 keV and have extremely flat power-law spectra above 2 keV ($\alpha
_x$=0.3--0.5). 0836+710 shows absorption higher than the galactic value and
marginal evidence for the presence of the redshifted 6.4 keV Iron line.
1510-089 shows a spectral break around 1 keV, with the low energy spectrum
steeper ($\alpha_l$=1.6) than the high energy power-law ($\alpha_h$=0.3). The
data are discussed in the light of current Inverse Compton models for the high
energy emission.


title: Ambiguities in fits to the complex X-ray spectra of starburst galaxies

abstract:
  Spectral fits to X-ray data from both NGC 253 and M 82 provide ambiguous
results. The so-called ``best fit'' results depend on the instrument with which
the data were obtained and obviously on the choice of spectral model
composition. We show that different spectral models can be fit equally well to
BeppoSAX data of both galaxies. Metallicities are unreliable in general, with a
strong dependence on the choice of model. Preference to one particular spectral
model can only be given by combining spectroscopic and imaging X-ray data from
all available satellites (ROSAT, ASCA, and BeppoSAX). Based on spectra of NGC
253, we demonstrate that a model consisting of two or more thermal plasma
components plus a hard power law continuum and Fe K alpha line emission can
explain all observations. These model components represent the integral
spectrum of thermal gas and compact sources in starburst galaxies that are most
likely supernova remnants and X-ray binaries. The same model can fit the X-ray
data of M 82, but there the evidence, from ROSAT imaging, of the existence of
compact sources which might represent high-mass X-ray binaries is weaker. This
implies that its hard X-ray emission, which is extended in ROSAT images, might
- if truly diffuse - be dominated by a very hot (several keV energy) thermal
gas component.


title:
Synchrotron and Compton Components and their Variability in BL Lac
  Objects

abstract:
  BL Lacertae objects are extreme extragalactic sources characterized by the
emission of strong and rapidly variable nonthermal radiation over the entire
electromagnetic spectrum. Synchrotron emission followed by inverse Compton
scattering in a relativistic beaming scenario is generally thought to be the
mechanism powering these objects. ...


title:
Evolution of lithium in solar-type stars: clues from intermediate age
  clusters

abstract:
  We present Li abundances for 14 solar-type stars in the intermediate age (~ 2
Gyr) clusters IC 4651 and NGC 3680. The log n(Li) vs. effective temperature
distributions are compared with those of the similar age cluster NGC 752, of
the younger Hyades (600 Myr) and of the older M 67 (4.5 Gyr) and NGC 188 (6-7
Gyr) clusters. Neither IC 4651 nor NGC 3680 show the dispersion in Li which is
observed in M 67. The 2 Gyr clusters have very similar Li vs. effective
temperature distributions; in addition, stars in the upper envelope of the M 67
distribution have the same Li content as stars in the 2 Gyr clusters,
suggesting that either they have not suffered any significant depletion between
2 and 4.5 Gyr or they had a much slower Li depletion. Mechanisms that lead to
Li depletion on the main sequence are discussed in the light of these
observations. None of the existing models seem to reproduce well the observed
features.


title:
ROSAT HRI monitoring of X-ray variability in the NLS1 galaxy PKS
  0558-504

abstract:
  We present the results of ROSAT High Resolution Imager (HRI) observations and
the survey data of the radio-loud Narrow-Line Seyfert 1 galaxy (NLS1) PKS
0558--504. We find strong and persistent X-ray variability on both short and
medium time-scales. The most extreme amplitude variations require a radiative
efficiency exceeding the theoretical maximum for a Schwarzschild black hole,
suggesting the presence of a rotating black hole or the influence of
relativistic beaming effects. The spatial analysis rules out the possibility
that the high luminosity and the strong variability are related to a nearby
source.


title: Magnetic field strength from peaked synchrotron spectra

abstract:
  Key diagnostics of non-thermal plasmas, such as the position of the
synchrotron self-absorption turnover or the Faraday rotation optical depth,
depend crucially on the unobserved low-energy electron spectrum. We investigate
the effect of physically viable assumptions about the low-energy electron
spectrum on the magnetic field strength as inferred from the position of the
peak in the synchrotron spectrum marking the transition from optically thick to
optically thin radiation. To this end the kinetic equations are written down
and stationary solutions are given. In this context the inviability of the
delta function approximation of the synchrotron power spectrum is discussed.
Using two different approaches, we show that the field determination is
independent of the assumptions about the low energy electron spectrum within an
order of magnitude. As an example, we use our result to obtain a small
correction to the limiting brightness temperature.


title:
The jet-disk symbiosis model for Gamma Ray Bursts: cosmic ray and
  neutrino background contribution

abstract:
  The relation between the cosmological evolution of the jet-disk symbiosis
model for GRBs and the cosmic rays energy distribution is presented. We used
two different Star Formation Rates (SFR) as a function of redshift and a
Luminosity Function (LF) distribution to obtain the distribution in fluence of
GRBs in our model and compare it with the data. We show a good agreement
between the fluence distribution we obtain and the corrected data for the 4B
BATSE catalogue. The results we obtain are generally valid for models that use
jet physics to explain GRB properties. The fluence in the gamma ray band has
been used to calculate the energy in cosmic rays both in our Galaxy and at
extragalactic distances as a function of the redshift. This energy input has
been compared with the Galactic and extragalactic spectrum of cosmic rays and
neutrinos. Using our jet disk symbiosis model, we found that in both cases GRBs
cannot give any significant contribution to cosmic rays. We also estimate the
neutrino background, obtaining a very low predicted flux. We also show that the
fit of our model with the corrected fluence distribution of GRBs gives strong
constraints of the star formation rate as a function of the redshift.


title: Bimodal production of Be and B in the early Galaxy

abstract:
  Recently models based on the acceleration of metal-rich material inside
superbubbles have been proposed to account for the observed abundances of Be
and B in metal-poor halo stars. We analyse some of the implications of these
models for the distribution of the Be/O and B/O abundance ratios. In
particular, we discuss the possible scatter in the data and argue that LiBeB
production in the very early Galaxy was probably bimodal, with isolated
supernovae giving rise to a low-efficiency mechanism, and collective supernovae
exploding in an OB association inducing a high-efficiency mechanism. This
should produce two populations of halo stars, one with high L/M ratios (light
elements/metals), and the other with L/M ratios about ten times lower. The
relative weight of these two populations depends on the fraction of supernovae
exploding inside superbubbles. In this context, we discuss the recent
observation of the B-depleted, Li-normal star HD 160617 (Primas, et al., 1998),
as well as the reported spread in the Be data at [Fe/H] ~ -2.2 (Boesgaard, et
al., 1999). Finally, we predict that Be will be found to be even more deficient
than B in HD 160617.


title: Primordial black holes and gravitational memory

abstract:
  We discuss the various ways in which primordial black holes may have formed
in the early Universe and how the effects of such black holes can be used to
place constraints on cosmological models. We show that such constraints may be
severely modified if the value of the gravitational "constant" G varies with
cosmological epoch, a possibility which arises in many scenarios for the early
Universe. The nature of the modification depends upon whether the value of G
near a black hole maintains the value it had at its formation epoch
(corresponding to gravitational memory) or whether it tracks the background
cosmological value. This is still uncertain but we discuss various approaches
which might help to resolve the issue.


title: On the tilting of protostellar disks by resonant tidal effects

abstract:
  We consider the dynamics of a protostellar disk surrounding a star in a
circular-orbit binary system. Our aim is to determine whether, if the disk is
initially tilted with respect to the plane of the binary orbit, the inclination
of the system will increase or decrease with time. The problem is formulated in
the binary frame in which the tidal potential of the companion star is static.
We consider a steady, flat disk that is aligned with the binary plane and
investigate its linear stability with respect to tilting or warping
perturbations. The dynamics is controlled by the competing effects of the m=0
and m=2 azimuthal Fourier components of the tidal potential. In the presence of
dissipation, the m=0 component causes alignment of the system, while the m=2
component has the opposite tendency. We find that disks that are sufficiently
large, in particular those that extend to their tidal truncation radii, are
generally stable and will therefore tend to alignment with the binary plane on
a time-scale comparable to that found in previous studies. However, the effect
of the m=2 component is enhanced in the vicinity of resonances where the outer
radius of the disk is such that the natural frequency of a global bending mode
of the disk is equal to twice the binary orbital frequency. Under such
circumstances, the disk can be unstable to tilting and acquire a warped shape,
even in the absence of dissipation. The outer radius corresponding to the
primary resonance is always smaller than the tidal truncation radius. For disks
smaller than the primary resonance, the m=2 component may be able to cause a
very slow growth of inclination through the effect of a near resonance that
occurs close to the disk center. We discuss these results in the light of
recent observations of protostellar disks in binary systems.


title: A correction to the pp reaction

abstract:
  These descriptive comments are made to encourage detailed three-body,
relativistic, quantum collision calculations for the pp reaction. In stars,
coulomb barrier tunneling, as in the pp reaction, is not a two-body process.
Tunneling is mediated by an energetic electron that interacts with the
colliding particles. The presence of such an electron lowers the potential
barrier and increases the probability of tunneling by orders of magnitude. The
solar luminosity can be maintained with a central temperature near 10 million K
where the neutrino production rates correspond to the observed rates. Current
stellar interior and evolutionary models need substantial revision.


title: RXTE observation of NGC 6240: a search for the obscured active nucleus

abstract:
  The wide-band energy spectrum of NGC6240 over the range 0.5-200keV is
investigated using the RXTE and ASCA data. The RXTE data provide the spectrum
beyond the ASCA range (0.5-10 keV) with significant detection of signals up to
20keV and the upper limits above 20keV. The spectrum above 10keV is found to be
very flat. A strong iron-K emission line discovered in the previous ASCA
observation is also confirmed with the RXTE PCA. These results provide further
evidence for the dominance of a reflection component, i.e. emission from cool
material illuminated by an AGN. By fitting the spectra obtained with RXTE and
ASCA simultaneously, we satisfactorily modeled the AGN spectrum with a Compton
reflection component and probably a transmitted AGN component penetrating
through a thick absorber. The X-ray luminosity of the AGN is estimated to be in
the range 4x10^43 - 6x10^44 ergs/s in the range 2-10keV, which categorizes
NGC6240 among the most luminous Seyfert nuclei. The ratio of the 2-10keV X-ray
luminosity to the infrared luminosity, L_X(2-10keV)/L_IR, is 0.01 - 0.1, which
implies a quite substantial, if not dominant, contribution of AGN to the
infrared luminosity.


title: ISO Spectroscopy of the HH 7-11 Flow and its red-shifted Counterpart

abstract:
  We have used the two spectrometers on the Infrared Space Observatory to
observe the HH 7-11 flow, its red-shifted counterpart, and the candidate
exciting source SVS 13, in the star formation region NGC 1333. We detect atomic
([OI]63um and 145um, [SiII]34.8um, [CII]158um) and molecular (H_2, CO, H_2O)
lines at various positions along the bipolar flow.
  Most of the observed lines can be explained in terms of shock-excited
emission. In particular, our analysis shows that dissociative (J-type) and
non-dissociative (C-type) shocks are simultaneously present everywhere along
both lobes of the flow. We confirm the low-excitation nature of the Herbig-Haro
nebulosities, with shock velocities <40-50 km/s. Toward both lobes of the
outflow we find pre-shock densities of n_0~10^4 cm-3 for both the J and C
components, implying B_0~100uG. In the central region of the flow, close to the
exciting source, the pre-shock density deduced for the C-shock component is
n_0~10^5 cm-3, suggesting a magnetic field ~3 times stronger. We propose that
the deficiency of gas-phase water in the post C-shock regions is due to
freezing onto warm grains processed through the J-shock front and traveling
along the magnetic field lines. The total observed cooling from the
dissociative shock components is consistent with the power lost by a slow
molecular outflow accelerated by a fast neutral HI wind.
  Finally, the skin of the cloud seen in projection toward the flow appears to
be weakly photo-ionised by BD +30 549, the dominant illuminating source of the
NGC 1333 reflection nebula.


title:
Non-detection of a pulsar-powered nebula in Puppis A, and implications
  for the nature of the radio-quiet neutron star RX J0822-4300

abstract:
  We report on a deep radio search for a pulsar wind nebula associated with the
radio-quiet neutron star RX J0822-4300 in the supernova remnant Puppis A. The
well-determined properties of Puppis A allow us to constrain the size of any
nebula to less than 30 arcsec; however we find no evidence for such a source on
any spatial scale up to 30 arcmin. These non-detections result in an upper
limit on the radio luminosity of any pulsar-powered nebula which is three
orders of magnitude below what would be expected if RX J0822-4300 was an
energetic young radio pulsar beaming away from us, and cast doubt on a recent
claim of X-ray pulsations from this source. The lack of a radio nebula leads us
to conclude that RX J0822-4300 has properties very different from most young
radio pulsars, and that it represents a distinct population which may be as
numerous, or even more so, than radio pulsars.


title:
An Effective Temperature Scale for Late M and L Dwarfs, from Resonance
  Absorption Lines of CsI and RbI

abstract:
  We present Keck HIRES spectra of 6 late-M dwarfs and 11 L dwarfs. Our goal is
to assign effective temperatures to the objects using detailed atmospheric
models and fine analysis of the alkali resonance absorption lines of CsI and
RbI. These yield mutually consistent results (+-150 K) when we use
``cleared-dust'' models, which account for the removal of refractory species
from the molecular states but do not include dust opacities. We find a tendency
for the RbI line to imply a slightly higher temperature, which we ascribe to an
incomplete treatment of the overlying molecular opacities. The final effective
temperatures we adopt are based on the CsI fits alone, though the RbI fits
support the CsI temperature sequence. This work, in combination with results
from the infrared, hints that dust in these atmospheres has settled out of the
high atmosphere but is present in the deep photosphere. We also derive radial
and rotational velocities for all the objects, finding that the previously
discovered trend of rapid rotation for very low mass objects is quite
pervasive. To improve on our analysis, there is a clear need for better
molecular line lists and a more detailed understanding of dust formation and
dynamics.


title: Non-Linear Effects on the Angular Correlation Function

abstract:
  Extracting the three dimensional power spectrum from the 2D distribution of
galaxies has become a standard tool of cosmology. This extraction requires some
assumptions about the scaling of the power spectrum with redshift; all
treatments to date assume a simple power law scaling. In reality, different
scales grow at different rates, due to non-linearities. We show that angular
surveys are sensitive to a weighted average of the power spectrum over a
distribution of redshifts, where the weight function varies with wavenumber. We
compute this weight function and show that it is fairly sharply peaked at
\bar{z}, which is a function of k. As long as the extracted power spectrum is
understood to be $P(k,\bar{z})$, the error introduced by non-linear scaling is
quantifiable and small. We study these effects in the context of the APM and
SDSS photometric surveys. In general the weight matrix is peaked at larger z
and is broader for deeper surveys, leading to larger (but still quantifiable)
errors due to non-linear scaling. The tools introduced here -- in particular
the weight function and effective redshift $\bar{z}$ -- can also be profitably
applied to plan surveys to study the evolution of the power spectrum.


title:
J-Band Infrared Spectroscopy of a Sample of Brown Dwarfs Using Nirspec
  on Keck II

abstract:
  Near-infrared spectroscopic observations of a sample of very cool, low-mass
objects are presented with higher spectral resolution than in any previous
studies. Six of the objects are L-dwarfs, ranging in spectral class from L2 to
L8/9, and the seventh is a methane or T-dwarf. These new observations were
obtained during commissioning of NIRSPEC, the first high-resolution
near-infrared cryogenic spectrograph for the Keck II 10-meter telescope on
Mauna Kea, Hawaii. Spectra with a resolving power of R=2500 from 1.135 to 1.360
microns (approximately J-band) are presented for each source. At this
resolution, a rich spectral structure is revealed, much of which is due to
blending of unresolved molecular transitions. Strong lines due to neutral
potassium (K I), and bands due to iron hydride (FeH) and steam (H2O) change
significantly throughout the L sequence. Iron hydride disappears between L5 and
L8, the steam bands deepen and the K I lines gradually become weaker but wider
due to pressure broadening. An unidentified feature occurs at 1.22 microns
which has a temperature dependence like FeH but has no counterpart in the
available FeH opacity data. Because these objects are 3-6 magnitudes brighter
in the near-infrared compared to the I-band, spectral classification is
efficient. One of the objects studied (2MASSW J1523+3014) is the coolest
L-dwarf discovered so far by the 2-Micron All-Sky Survey (2MASS), but its
spectrum is still significantly different from the methane-dominated objects
such as Gl229B or SDSS 1624+0029.


title: A simple, stringent test on the nature of GRBs' progenitors

abstract:
  I show here that the time-delay between the gamma ray burst proper and the
onset of the afterglow is a sensitive function of the surrounding density,
going from ~10s for typical disk densities to several hours for IGM densities.
Since bulk Lorenz factor, explosion energy and environment density can be
simultaneously determined from observations of the time-delay and of the
afterglow, afterglow observations alone may establish which fraction, if any,
of all bursts resides inside galaxies, in their outer haloes, or outright in
the IGM medium, subjecting the NS/NS merger hypothesis to a definitive test. If
bursts are due to collapse of massive stars, no matter how short the burst, the
onset of the afterglow should be immediate.


title: Detection of CO in the inner part of M31's bulge

abstract:
  We report the first detection of CO in M31's bulge. The 12CO (1-0) and (2-1)
lines are both detected in the dust complex D395A/393/384, at 1.3" (~0.35 kpc)
from the centre. From these data and from visual extinction data, we derive a
CO-luminosity to reddening ratio (and a CO-luminosity to H_2 column density
ratio) quite similar to that observed in the local Galactic clouds. The (2-1)
to (1-0) line intensity ratio points to a CO rotational temperature and a gas
kinetic temperature > 10 K. The molecular mass of the complex, inside a 25'
(100 pc) region, is 1.5 10^4 Mo.


title: CMB B-polarization to map the Large-scale Structures of the Universe

abstract:
  We explore the possibility of using the B-type polarization of the CMB to map
the large-scale structures of the Universe taking advantage of the lens effects
on the CMB polarization. The functional relation between the B component with
the primordial CMB polarization and the line-of-sight mass distribution is
explicited. Noting that a sizeable fraction (at least 40%) of the dark halo
population which is responsible of this effect can also be detected in galaxy
weak lensing survey, we present statistical quantities that should exhibit a
strong sensitivity to this overlapping. We stress that it would be a sound test
of the gravitational instability picture, independent on many systematic
effects that may hamper lensing detection in CMB or galaxy survey alone.
Moreover we estimate the intrinsic cosmic variance of the amplitude of this
effect to be less than 8% for a 100, deg^2 survey with a 10' CMB beam. Its
measurement would then provide us with an original mean for constraining the
cosmological parameters, more particularly, as it turns out, the cosmological
constant Lambda.


title:
The ROSAT Bright Survey: II. Catalogue of all high-galactic latitude
  RASS sources with PSPC countrate CR > 0.2 s-1

abstract:
  We present a summary of an identification program of the more than 2000 X-ray
sources detected during the ROSAT All-Sky Survey (Voges et al. 1999) at high
galactic latitude, |b| > 30 degr, with countrate above 0.2 s-1. This program,
termed the ROSAT Bright Survey RBS, is to more than 99.5% complete. A
sub-sample of 931 sources with countrate above 0.2 s-1 in the hard spectral
band between 0.5 and 2.0 keV is to 100% identified. The total survey area
comprises 20391 sq.deg at a flux limit of 2.4 \times 10^{-12} erg/cmsq/s in the
0.5 -- 2.0 keV band. About 1500 sources of the complete sample could be
identified by correlating the RBS with SIMBAD and the NED. The remaining 500
sources were identified by low-resolution optical spectroscopy and CCD imaging
utilizing telescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea.
Apart from completely untouched sources, catalogued clusters and galaxies
without published redshift as well as catalogued galaxies with unusual high
X-ray luminosity were included in the spectroscopic identification program.
Details of the observations with an on-line presentation of the finding charts
and the optical spectra will be published separately. Here we summarize our
identifications in a table which contains optical and X-ray information for
each source. As a result we present the most massive complete sample of X-ray
selected AGNs with a total of 669 members and a well populated X-ray selected
sample of 302 clusters of galaxies with redshifts up to 0.70. Three fields
studied by us remain without optical counterpart (RBS0378, RBS1223, RBS1556).
While the first is a possible X-ray transient, the two latter are isolated
neutron star candidates (Motch et al. 1999, Schwope et al. 1999).


title: How flat is the Universe?

abstract:
  In order to answer this question, we combine ten independent astrophysical
constraints in the space of the density parameters $\Omega_m$ of gravitating
matter and $\Omega_{\Lambda}$ of vacuum energy. We find that $\Omega_m=0.31\pm
0.07$, $\Omega_{\Lambda}=0.63\pm 0.21$, and thus $\Omega_m +
\Omega_{\Lambda}=0.94\pm 0.22$. The total $\chi^2$ is 4.1 for 8 degrees of
freedom, testifying that the various systematic errors included are generous.
We also determine $\Omega_m$ in the exactly flat case. Five supplementary
flat-case constraints can then be included in our fit, with the result
$\Omega_m=1-\Omega_{\Lambda}= 0.337\pm0.031$. It follows that the age of the
Universe is $t_0 = 13.5\pm 1.3$ (0.68/h) Gyr.


title: Pixel and Micro-lensing with NGST

abstract:
  Within 8 years, the current microlensing surveys of M31 will provide several
hundred events affecting unresolved stars. They will thus allow a statistical
study of the dark matter in M31's halo. The NGST will resolve these stars and
constrain the mass of the corresponding lenses. In case of on-line alerts from
ground-based observations, real-time NGST follow-up with high signal-to-noise
ratio will provide further constraints on the lenses. In addition, high
resolution observations with NGST will complement XMM and the previous optical
data and thus enable a closer insight of X-ray binaries within M31 to be
obtained. The optimal instrumentation to achieve these scientific goals will be
discussed. Last, the study of the dark matter encompassed in the galaxy
clusters would be possible with high angular resolution observations on a large
field camera and would open a new field of research.


title:
Low Frequency Radio Emission of Pulsar PSR J1907+0919 Associated with
  the Magnetar SGR 1900+14

abstract:
  The soft gamma repeater SGR 1900+14 was observed in Pushchino observatory
since 1988 December using BSA radio telescope operating at 111 MHz. We have
detected the pulsed radio emission (Shitov 1999) with the same 5.16 s period
that was reported earlier for this object. The timing analysis has shown that
this new radio pulsar PSR J1907+0919 associated with SGR 1900+14 has a
superstrong magnetic field, which is 8.1 * 10^14 G, thereby confirming that it
is a "magnetar". The dispersion measure of PSR J1907+0919 is 281.4(9) pc *
cm^(-3) which gives an estimate of the pulsar's distance as about 5.8 kpc.


title: Jets from compact objects

abstract:
  Some topics in the theory of jets are reviewed. These include jet precession,
unconfined jets, the origin of knots, the internal shock model as a unifying
theme from protostellar jets to Gamma-ray bursts, relations between the
Blandford-Znajek and MHD disk-wind models, and jet collimation in magnetic
acceleration models.


title: Theory of solar luminosity variations

abstract:
  The theory of stellar structure can be used to identify the most plausible
mechanisms for the irradiance variations associated with the solar cycle.
Changes in surface emissivity, i.e. the reduced cooling in spots and enhanced
emission by small scale magnetic fields, are the most effective mechanisms and
account for most of the observed variation. Helioseismology will soon be able
to test the consequences of changes in surface emissivity, and distinguish them
from other scenarios for irradiance variability.


title: A High-Eccentricity Low-Mass Companion to HD 89744

abstract:
  HD 89744 is an F7 V star with mass 1.4 M, effective temperature 6166 K, age
2.0 Gy and metallicity [Fe/H]= 0.18. The radial velocity of the star has been
monitored with the AFOE spectrograph at the Whipple Observatory since 1996, and
evidence has been found for a low mass companion. The data were complemented by
additional data from the Hamilton spectrograph at Lick Observatory during the
companion's periastron passage in fall 1999. As a result, we have determined
the star's orbital wobble to have period P = 256 d, orbital amplitude K = 257
m/s, and eccentricity e = 0.7. From the stellar mass we infer that the
companion has minimum mass m2 sin i = 7.2 MJup in an orbit with semi-major axis
a2 = 0.88 AU. The eccentricity of the orbit, among the highest known for
extra-solar planets, continues the trend that extra-solar planets with
semi-major axes greater than about 0.15 AU tend to have much higher
eccentricities than are found in our solar system. The high metallicity of the
parent star reinforces the trend that parent stars of extra-solar planets tend
to have high metallicity


title: Self-interacting Warm Dark Matter

abstract:
  It has been shown by many independent studies that the cold dark matter
scenario produces singular galactic dark halos, in strong contrast with
observations. Possible remedies are that either the dark matter is warm so that
it has significant thermal motion or that the dark matter has strong self
interactions. We have combined these ideas to calculate the linear mass power
spectrum and the spectrum of cosmic microwave background (CMB) fluctuations for
self-interacting warm dark matter. Our results indicate that such models have
more power on small scales than is the case for the standard warm dark matter
model, with a CMB fluctuation spectrum which is nearly indistinguishable from
standard cold dark matter. This enhanced small-scale power may provide better
agreement with the observations than does standard warm dark matter.


title: Star formation properties of UCM galaxies

abstract:
  We present new near-infrared J and K imaging data for 67 galaxies from the
Universidad Complutense de Madrid survey used to determine the SFR density of
the local universe by Gallego et al. (1995). This is a sample of local
star-forming galaxies with redshift lower than 0.045, and they constitute a
representative subsample of the galaxies in the complete UCM survey. From the
new data, complemented with our own Gunn-r images and long-slit optical
spectroscopy, we have measured integrated K-band luminosities, r-J and J-K
colours, and Halpha luminosities and equivalent widths. Using a maximum
likelihood estimator and a complete set of evolutionary synthesis models, these
observations have allowed us to estimate the strength of the current (or most
recent) burst of star formation, its age, the star-formation rate and the total
stellar mass of the galaxies. An average galaxy in the sample has a stellar
mass of 5E10 Msun and is undergoing (or recently completed) a burst of star
formation involving about 2 per cent of its total stellar mass. We have
identified two separate classes of star-forming galaxies in the UCM sample: low
luminosity, high excitation galaxies (HII-like) and relatively luminous spirals
galaxies (starburst disk-like). The former show higher specific star formation
rates (SFR per unit mass) and burst strengths, and lower stellar masses than
the latter. With regard to their specific star formation rates, the UCM
galaxies are intermediate objects between normal quiescent spirals and the most
extreme HII galaxies.


title: WR146 - observing the OB-type companion

abstract:
  We present new radio and optical observations of the colliding-wind system
WR146 aimed at understanding the nature of the companion to the Wolf-Rayet star
and the collision of their winds. The radio observations reveal emission from
three components: the WR stellar wind, the non-thermal wind-wind interaction
region and, for the first time, the stellar wind of the OB companion. This
provides the unique possibility of determining the mass-loss rate and terminal
wind velocity ratios of the two winds, independent of distance. Respectively,
these ratios are determined to be 0.20+/-0.06 and 0.56+/-0.17 for the
OB-companion star relative to the WR star. A new optical spectrum indicates
that the system is more luminous than had been believed previously. We deduce
that the ``companion'' cannot be a single, low luminosity O8 star as previously
suggested, but is either a high luminosity O8 star, or possibly an O8+WC binary
system.


title: A limit for the mass transfer rate in soft X-ray transients

abstract:
  Many black hole X-ray transients are in a low state for several decades until
an outburst occurs. We interpret this outburst behaviour as a marginal
occurrence of a dwarf nova type disk instability in the cool outer accretion
disk. We compute the disk evolution including evaporation of matter from the
cool thin disk. This evaporation process causes the transition to a hot coronal
flow. The efficiency depends on the black hole mass.
  The new results are the dependence of the outburst recurrence time on the
mass transfer rate from the companion star and the fraction of the matter which
is accumulated in the disk for the outburst. We determine a lower limit of the
mass transfer rate, below which no disk instability can be triggered. We find
that for rates slightly lower than those in the known black hole X-ray
transients the disk would be stationary. We argue that many such optically
faint black hole X-ray binaries with stationary cool accretion disks exist.


title:
Systematic uncertainties in gravitational lensing models: a
  semi-analytical study of PG1115+080

abstract:
  While the Hubble constant can be derived from observable time delays between
images of lensed quasars, the result is often highly sensitive to assumptions
and systematic uncertainties in the lensing model. Unlike most previous authors
we explore a broad class of models with non-parametric lens radial profile and
allow for non-elliptical lens potentials. The axis ratio and position angle of
the lens can be determined from the image positions of quadruple gravitational
lensed systems by solving the lens equation semi-analytically, independent of
the radial profile. We give simple equations for estimating the power-law slope
of the lens density directly from the image positions and for estimating the
time delay ratios. Our method greatly simplifies the numerics for fitting
observations and is fast in exploring the model parameter space. As an
illustration we apply the model to PG1115+080. We show that the measured image
positions and time delays do not uniquely determine the Hubble constant. The
mass-to-light ratio of the lens galaxy increases with radius, higher at image C
than at image B.


title:
Bit-String Physics Prediction of eta, The Dark Matter/Baryon Ratio and
  Omega_M

abstract:
  Using a simple combinatorial algorithm for generating finite and discrete
events as our numerical cosmology, we predict that the baryon/photon ratio at
the time of nucleogenesis is $\eta= 1/256^4$, $\Omega_{DM}/\Omega_B= 12.7$ and
(for a cosmological constant of $\Omega_{\Lambda}=0.6\pm 0.1$ predicted on
general grounds by E.D.Jones) that $0.325 > \Omega_M > 0.183 $. The limits are
set not by our theory but by the empirical bounds on the renormalized Hubble
constant of $0.6 < h_0 < 0.8$. If we impose the additional empirical bound of
$t_0 < 14 Gyr$, the predicted upper bound on $\Omega_M$ falls to 0.26. The
predictions of $\Omega_M$ and $\Omega_{\Lambda}$were in excellent agreement
with Glanz' analysis in 1998, and are still in excellent agreement with
Lineweaver's recent analysis despite the reduction of observational uncertainty
by close to an order of magnitude.


title: The Propagation of Magneto-Centrifugally Launched Jets: I

abstract:
  We present simulations of the propagation of magnetized jets. This work
differs from previous studies in that the cross-sectional distributions of the
jets's state variables are derived from analytical models for
magneto-centrifugal launching. The source is a magnetized rotator whose
properties are specfied as boundary conditions. The jets in these simulations
are considerably more complex than the ``top-hat''constant density etc.
profiles used in previous work. We find that density and magnetic field
stratification (with radius) in the jet leads to new behavior including the
separation of an inner jet core from a low density collar. We find this {\it
jet within a jet} structure, along with the magnetic stresses, leads to
propagation behaviors not observed in previous simulation studies. Our
methodology allows us to compare MHD jets from different types of sources whose
properties could ultimately be derived from the behavior of the propagating
jets.


title: HST Observations of the Wolf-Rayet Nebula NGC 6888

abstract:
  We present Hubble Space Telescope WFPC2 images of a portion of the bright
northeast rim of NGC 6888, the nebular shell physically associated with the
Wolf-Rayet star HD 192163. The exposures are taken in the light of Halpha 6563,
[O III] 5007, and [S II] 6717,6731. The images are used to constrain models of
the ionization structure of nebular features. From these models we infer
physical conditions within features, and estimate elemental abundances within
the nebula. The results of our analysis, together with the degree of small
scale inhomogeneity apparent in the images, call into question the assumptions
underlying traditional methodologies for interpretation of nebular
spectroscopy.
  The thermal pressure of photoionized clumps is higher than the inferred
internal pressure of the shocked stellar wind, implying that the current
physical conditions have changed significantly over less than a few thousand
years. These results are discussed within the context of published three-wind
evolutionary scenarios for the formation of the nebula. We also discuss the
nature of a back-illuminated radiative shock driven into the cavity surrounding
NGC 6888.