title: The response of a dwarf nova disc to real mass transfer variations

abstract:
  We present simulations of dwarf nova outbursts taking into account realistic
variations of the mass loss rate from the secondary. The mass transfer
variation has been derived from 20 years of visual monitoring and from X-ray
observations covering various accretion states of the discless cataclysmic
variable AM Herculis. We find that the outburst behaviour of a fictitious dwarf
nova with the same system parameters as AM Her is strongly influenced by these
variations of the mass loss rate. Depending on the mass loss rate, the disc
produces either long outbursts, a cycle of one long outburst followed by two
short outbursts, or only short outbursts. The course of the transfer rate
dominates the shape of the outbursts because the mass accreted during an
outburst cycle roughly equals the mass transferred from the secondary over the
outburst interval. Only for less than 10% of the simulated time, when the mass
transfer rate is nearly constant, the disc is in a quasi-stationary state
during which it periodically repeats the same cycle of outbursts. Consequently,
assuming that the secondary stars in non-magnetic CV's do not differ from those
in magnetic ones, our simulation indicates that probably all dwarf novae are
rarely in a stationary state and are constantly adjusting to the prevailing
value of the mass transfer rate from the secondary.


title: Observations of the supernova remnant W28 at TeV energies

abstract:
  The atmospheric Cerenkov imaging technique has been used to search for
point-like and diffuse TeV gamma-ray emission from the southern supernova
remnant, W28, and surrounding region. The search, made with the CANGAROO 3.8m
telescope, encompasses a number of interesting features, the supernova remnant
itself, the EGRET source 3EG J1800-2338, the pulsar PSR J1801-23, strong 1720
MHz OH masers and molecular clouds on the north and east boundaries of the
remnant. An analysis tailored to extended and off-axis point sources was used,
and no evidence for TeV gamma-ray emission from any of the features described
above was found in data taken over the 1994 and 1995 seasons. Our upper limit
(E>1.5 TeV) for a diffuse source of radius 0.25deg encompassing both molecular
clouds was calculated at 6.64e-12 photons cm^-2 s^-1 (from 1994 data), and
interpreted within the framework of a model predicting TeV gamma-rays from
shocked-accelerated hadrons. Our upper limit suggests the need for some cutoff
in the parent spectrum of accelerated hadrons and/or slightly steeper parent
spectra than that used here (-2.1). As to the nature of 3EG J1800-2338, it
possibly does not result entirely from pi-zero decay, a conclusion also
consistent with its location in relation to W28.


title: Oscillations and convective motion in stars with URCA shells

abstract:
  It is shown that in presence of URCA shell pulsational energy losses due to
neutrino emission and nonequilibrium beta heating are much less than energy
losses by excitation of short-wavelength acoustic waves. Convective motion in
presence of URCA shell is considered, and equations generalizing the mean free
path model of convection are derived.


title: Cosmological Constraints from the Cosmic Microwave Background

abstract:
  Using an approximate likelihood method adapted to band--power estimates, we
analyze the ensemble of first generation cosmic microwave background anisotropy
experiments to deduce constraints over a six--dimensional parameter space
describing Inflation--generated adiabatic, scalar fluctuations. The basic
preferences of simple Inflation scenarios are consistent with the data set:
flat geometries $(\OmT \equiv 1-\Omk \sim 1)$ and a scale--invariant primeval
spectrum ($n\sim 1$) are favored. Models with significant negative curvature
($\OmT < 0.7$) are eliminated, while constraints on postive curvature are less
stringent. Degeneracies among the parameters prevent independent determinations
of the matter density $\OmM$ and the cosmological constant $\Lambda$, and the
Hubble constant $\Ho$ remains relatively unconstrained. We also find that the
height of the first Doppler peak relative to the amplitude suggested by data at
larger $l$ indicates a high baryon content ($\Omb h^2$), almost independently
of the other parameters. Besides the overall qualitative advance expected of
the next generation experiments, their improved dipole calibrations will be
particularly useful for constraining the peak height. Our analysis includes a
{\em Goodness--of--Fit} statistic applicable to power estimates and which
indicates that the maximum likelihood model provides an acceptable fit to the
data set.


title: CMB Cosmological Parameter Estimation: Methods and Current Results

abstract:
  The majority of present efforts to constrain cosmological parameters with
cosmic microwave background (CMB) anisotropy data employ approximate likelihood
functions, the time consuming nature of a complete analysis being a major
obstacle. We have performed a full (unapproximated) likelihood analysis on
several experiments that allows us to examine the various assumptions made in
these approximate methods and to evaluate their performance. Our results
indicate that care should be taken when using such approaches. With an improved
approximate method, we present some constraints on cosmological parameters
using the entire present CMB data set.


title: Direct Observation of the Fourth Star in the Zeta Cancri System

abstract:
  Direct imaging of the zeta Cnc system has resolved the fourth star in the
system, which is in orbit around zeta Cnc C. The presence of the fourth star
has been inferred for many years from irregularities in the motion of star C,
and recently from C's spectroscopic orbit. However, its mass is close to that
of C, making its non-detection puzzling. Observing at wavelengths of 1.2, 1.7,
and 2.2 microns with the adaptive-optics system of the CFHT, we have obtained
images which very clearly reveal star D and show it to have the color of an M2
star. Its brightness is consonant with its being two M stars, which are not
resolved in our observations but are likely to be in a short-period orbit,
thereby accounting for the large mass and the difficulty of detection at
optical wavelengths, where the magnitude difference is much larger. The
positions and colors of all four stars in the system are reported and are
consistent with the most recent astrometric observations.


title: A radio-microlensing caustic crossing in B1600+434?

abstract:
  First, we review the current status of the detection of strong `external'
variability in the CLASS gravitational B1600+434, focusing on the 1998 VLA
8.5-GHz and 1998/9 WSRT multi-frequency observations. We show that this data
can best be explained in terms of radio-microlensing. We then proceed to show
some preliminary results from our new multi-frequency VLA monitoring program,
in particular the detection of a strong feature (~30%) in the light curve of
the lensed image which passes predominantly through the dark-matter halo of the
lens galaxy. We tentatively interpret this event, which lasted for several
weeks, as a radio-microlensing caustic crossing, i.e. the superluminal motion
of a micro-arcsec-scale jet-component in the lensed source over a single
caustic in the magnification pattern, that has been created by massive compact
objects along the line-of-sight to the lensed image.


title:
Constraining the Accretion Rate Onto Sagittarius A* Using Linear
  Polarization

abstract:
  Two possible explanations for the low luminosity of the supermassive black
hole at the center of our galaxy are (1) an accretion rate of order the
canonical Bondi value (roughly 10^{-5} solar masses per year), but a very low
radiative efficiency for the accreting gas or (2) an accretion rate much less
than the Bondi rate. Both models can explain the broad-band spectrum of the
Galactic Center. We show that they can be distinguished using the linear
polarization of synchrotron radiation. Accretion at the Bondi rate predicts no
linear polarization at any frequency due to Faraday depolarization. Low
accretion rate models, on the other hand, have much lower gas densities and
magnetic field strengths close to the black hole; polarization may therefore be
observable at high frequencies. If confirmed, a recent detection of linear
polarization from Sgr A$^*$ above 150 GHz argues for an accretion rate of order
10^{-8} solar masses per year, much less than the Bondi rate. This test can be
applied to other low-luminosity galactic nuclei.


title:
The Optical Gravitational Lensing Experiment. A Hunt for Caustic
  Crossings in QSO 2237+0305

abstract:
  In 1998 The Optical Gravitational Lensing Experiment (OGLE) successfully
implemented automated data reductions for QSO 2237+0305. Using a new image
subtraction method we achieved a differential photometry scatter of 1-5 % for
images A-D respectively. Combined with a time sampling of 1-2 times a week this
is sufficient for early detection of caustic crossings. Nearly real time
photometry of QSO 2237+0305 is available from the OGLE web site
http://www.astro.princeton.edu/~ogle/ogle2/huchra.html . During the 1999
observing season, the apparent V magnitude of the A, B, C and D images changed
by 0.50, 0.15, 0.65 and 0.35 mag, respectively. Most likely however, none of
the microlensing events involved a caustic crossing. The most rapid variation
was 0.25 mag in 30 days, observed for image C. The alert system will continue
to be active in the next observing season from late April until September 2000,
when OGLE suspends operation for an upgrade. Observations will resume for
season of 2001.


title:
Nonlinear stochastic biasing from the formation epoch distribution of
  dark halos

abstract:
  We propose a physical model for nonlinear stochastic biasing of one-point
statistics resulting from the formation epoch distribution of dark halos. In
contrast to previous works on the basis of extensive numerical simulations, our
model provides for the first time an analytic expression for the joint
probability function. Specifically we derive the joint probability function of
halo and mass density contrasts from the extended Press-Schechter theory. Since
this function is derived in the framework of the standard gravitational
instability theory assuming the random-Gaussianity of the primordial density
field alone, we expect that the basic features of the nonlinear and stochastic
biasing predicted from our model are fairly generic. As representative
examples, we compute the various biasing parameters in cold dark matter models
as a function of a redshift and a smoothing length. Our major findings are (1)
the biasing of the variance evolves strongly as redshift while its
scale-dependence is generally weak and a simple linear biasing model provides a
reasonable approximation roughly at $R\simgt 2(1+z)\himpc$, and (2) the
stochasticity exhibits moderate scale-dependence especially on $R\simlt
20\himpc$, but is almost independent of $z$. Comparison with the previous
numerical simulations shows good agreement with the above behavior, indicating
that the nonlinear and stochastic nature of the halo biasing is essentially
understood by taking account of the distribution of the halo mass and the
formation epoch.


title:
The Influence of Galactic Outflows on the Formation of Nearby Dwarf
  Galaxies

abstract:
  We show that the gas in growing density perturbations is vulnerable to the
influence of winds outflowing from nearby collapsed galaxies that have already
formed stars. This suggests that the formation of nearby galaxies with masses
less than 10^9 solar masses is likely to be suppressed, irrespective of the
details of galaxy formation. An impinging wind may shock heat the gas of a
nearby perturbation to above the virial temperature, thereby mechanically
evaporating the gas, or the baryons may be stripped from the perturbation
entirely if they are accelerated to above the escape velocity. We show that
baryonic stripping is the most effective of these two processes, because
shock-heated clouds that are too large to be stripped are able to radiatively
cool within a sound-crossing time, limiting evaporation. The intergalactic
medium temperatures and star-formation rates required for outflows to have a
significant influence on the formation of low-mass galaxies are consistent with
current observations, but may soon be examined directly via associated
distortions in the cosmic microwave background, and with near-infrared
observations from the Next Generation Space Telescope, which may detect the
supernovae from early-forming stars.


title:
VLA 8.4-GHz monitoring observations of the CLASS gravitational lens
  B1933+503

abstract:
  The complex ten-component gravitational lens system B1933+503 has been
monitored with the VLA during the period February to June 1998 with a view to
measuring the time delay between the four compact components and hence to
determine the Hubble parameter. Here we present the results of an `A'
configuration 8.4-GHz monitoring campaign which consists of 37 epochs with an
average spacing of 2.8 days. The data have yielded light curves for the four
flat-spectrum radio components (components 1, 3, 4 and 6). We observe only
small flux density changes in the four flat-spectrum components which we do not
believe are predominantly intrinsic to the source. Therefore the variations do
not allow us to determine the independent time delays in this system. However,
the data do allow us to accurately determine the flux density ratios between
the four flat-spectrum components. These will prove important as modelling
constraints and could prove crucial in future monitoring observations should
these data show only a monotonic increase or decrease in the flux densities of
the flat-spectrum components.


title: X-rays from Radio-Galaxies: BeppoSAX Observations

abstract:
  We briefly review BeppoSAX observations of X-ray bright radio-galaxies. Their
X-ray spectra are quite varied, and perhaps surprisingly, any similarity
between radio-loud AGN and Seyfert galaxies is the exception rather than the
rule. When detected, reprocessing features (iron line and reflection) are
generally weak, suggesting two possible scenarios: either: (1) non-thermal
(jet?) radiation dilutes the X-ray emission from the disk in radio-loud
objects, or (2) the solid angle subtended by the X-ray reprocessing material is
smaller in radio-loud than in radio-quiet AGN due to different characteristics
of the accretion disk itself.


title: First results of the ROSEBUD Dark Matter experiment

abstract:
  Rare Objects SEarch with Bolometers UndergrounD) is an experiment which
attempts to detect low mass Weak Interacting Massive Particles (WIMPs) through
their elastic scattering off Al and O nuclei. It consists of three small
sapphire bolometers (of a total mass of 100 g) with NTD-Ge sensors in a
dilution refrigerator operating at 20 mK in the Canfranc Underground
Laboratory. We report in this paper the results of several runs (of about 10
days each) with successively improved energy thresholds, and the progressive
background reduction obtained by improvement of the radiopurity of the
components and subsequent modifications in the experimental assembly, including
the addition of old lead shields. Mid-term plans and perspectives of the
experiment are also presented.


title:
Starburst in the Ultra-luminous Galaxy Arp 220 - Constraints from
  Observations of Radio Recombination Lines and Continuum

abstract:
  We present observations of radio recombination lines from the starburst
galaxy Arp 220 at 1.4, 8.1, 84, 96 and 207 GHz (sensitive upper limit for the
1.4 GHz line and firm detections at the other frequencies), and the radio
continuum spectrum between 330 MHz and 207 GHz.
  We show that a model with three components of ionized gas with different
densities and area covering factors can consistently explain both RRL and
continuum data. The total mass of ionized gas in the three components is 3 x
10^7 M_sun requiring 3 x 10^5 O5 stars with a total Lyman continuum production
rate (NLyc) of 1.3 x 10^{55} photons /s. These values imply a dust extinction
A_V ~ 45 magnitudes and an SFR of ~240 M_sun/yr. The NLyc of ~3% associated
with the high density HII regions implies similar SFR at recent epochs. The
data is also consistent with multiple starbursts of very high SFR and short
durations. The derived value of 24 for the IR-excess favours a starburst rather
than an AGN as the origin of the observed FIR luminosity.
  (the abstract has been abridged)


title:
Radio recombination lines from starburst galaxies : high and low density
  ionized gas

abstract:
  Radio recombination lines (RRL) at 8 GHz and 15 GHz detected from four
starburst galaxies are shown to arise in compact high density HII regions,
which are undetectable below ~4 GHz. Detection of an RRL at 1.4 GHz towards one
galaxy and upper limits in the other three are consistent with the presence of
an equal amount of low density diffuse gas. Continuum flux density measurements
using the GMRT will be important in constraining the properties of the diffuse
gas.


title:
Vertical motions in the disk of NGC 5668 as seen with optical
  Fabry-Perot spectroscopy

abstract:
  We have observed the nearly face-on spiral galaxy NGC 5668 with the TAURUS II
Fabry-Perot interferometer at the William Herschel Telescope using the
$H\alpha$ line to study the kinematics of the ionized gas. From the extracted
data cube we construct intensity, velocity and velocity dispersion maps. We
calculate the rotation curve in the innermost 2 arcmin and we use the residual
velocity field to look for regions with important vertical motions. By
comparing the geometry of these regions in the residual velocity field with the
geometry in the intensity and velocity dispersion maps we are able to select
some regions which are very likely to be shells or chimneys in the disk. The
geometry and size of these regions are very similar to the shells or chimneys
detected in other galaxies by different means. Moreover, it is worth noting
than this galaxy has been reported to have a population of neutral hydrogen
high velocity clouds (Schulman et al. 1996) which, according to these
observations, could have been originated by chimneys similar to those reported
in this paper.


title: Black hole constraints on the running-mass inflation model

abstract:
  The running-mass inflation model, which has strong motivation from particle
physics, predicts density perturbations whose spectral index is strongly
scale-dependent. For a large part of parameter space the spectrum rises sharply
to short scales. In this paper we compute the production of primordial black
holes, using both analytic and numerical calculation of the density
perturbation spectra. Observational constraints from black hole production are
shown to exclude a large region of otherwise permissible parameter space.


title: Multiple Cyclotron Lines in the Spectrum of 4U 0115+63

abstract:
  We report phase resolved spectroscopy of the transient accreting pulsar,
4U0115+63. For the first time, more than two cylotron resonance scattering
features are detected in the spectrum of an X-ray pulsar. The shape of the
fundamental line appears to be complex, and this is in agreement with
predictions of Monte-Carlo models. As in other pulsars, the line energies and
optical depths are strong functions of pulse phase. One possible model for this
is an offset of the dipole of the neutron star magnetic field.


title:
Optical and Near Infrared Study of the Cepheus E outflow, a very low
  excitation object

abstract:
  We present images and spectra of the Cepheus E (Cep E) region at both optical
and infrared wavelengths. Only the brightest region of the southern lobe of the
Cep E outflow reveals optical emission, suggesting that the extinction close to
the outflow source plays an important r\^ole in the observed difference between
the optical and IR morphologies. Cep E is a unique object since it provides a
link between the spectroscopic properties of the optical Herbig-Haro (HH)
objects and those of deeply embedded outflows.


title: On The Cosmic Origins Of Carbon And Nitrogen

abstract:
  We analyze the behavior of N/O and C/O abundance ratios as a function of
metallicity as gauged by O/H in large, extant Galactic and extragalactic H II
region abundance samples. Numerical chemical evolution models are computed
using published stellar yields implied by comparing analytical models to the
observations. Our results suggest that carbon and nitrogen originate from
separate production sites and are decoupled from one another. Massive stars
(M>8M_sun) dominate the production of carbon, while intermediate-mass stars
between 4 and 8 solar masses, with a characteristic ejection delay time of 250
Myr after their formation, dominate nitrogen production. Carbon production is
positively sensitive to metallicity through mass loss processes in massive
stars and has a pseudo-secondary character. Nitrogen production in intermediate
mass stars is primary at low metallicity, but clearly secondary (and perhaps
tertiary) when 12+log(O/H)>8.3. The observed flat behavior of N/O versus O/H in
metal-poor galaxies is explained by invoking low star formation rates which
flatten the age-metallicity relation and thereby allow N/O to rise to observed
levels at low metallicities. The observed scatter and distribution of data
points for N/O challenge the popular idea that intermittent polluting by oxygen
from massive stars is occurring following star bursts. The effect of inflow of
gas into galactic systems on secondary production of nitrogen from carbon may
introduce some scatter into N/O ratios at high metallicities.


title: RXTE Studies of Cyclotron Lines in Accreting Pulsars

abstract:
  In this paper, we summarize the RXTE measurements of cyclotron resonance
scattering features in 8 accreting pulsars. The wide bandpass and modest
resolution of the RXTE instruments make them ideal for measuring these
generally broad features. In particular, the high energy response of HEXTE
provides a window for discovery of new lines not detectable with proportional
counters such as Ginga or the PCA alone.


title: Deficiency of `Thin' Stellar Bars in Seyfert Host Galaxies

abstract:
  Using all available major samples of Seyfert galaxies and their corresponding
control samples of closely matched non-active galaxies, we find that the bar
ellipticities (or axial ratios) in Seyfert galaxies are systematically
different from those in non-active galaxies. Overall, there is a deficiency of
bars with large ellipticities (i.e., `fat' or `weak' bars) in Seyferts,
compared to non-active galaxies. Accompanied with a large dispersion due to
small number statistics, this effect is strictly speaking at the `2sigma'
level. To obtain this result, the active galaxy samples of near-infrared
surface photometry were matched to those of normal galaxies in type, host
galaxy ellipticity, absolute magnitude, and, to some extent, in redshift. We
discuss possible theoretical explanations of this phenomenon within the
framework of galactic evolution, and, in particular, of radial gas
redistribution in barred galaxies. Our conclusions provide further evidence
that Seyfert hosts differ systematically from their non-active counterparts on
scales of a few kpc.


title:
A Possible Cepheid-Like Luminosity Estimator for the Long Gamma-Ray
  Bursts

abstract:
  We present a possible Cepheid-like luminosity estimator for the long
gamma-ray bursts based on the variability of their light curves. To construct
the luminosity estimator, we use CGRO/BATSE data for 13 bursts, Wind/KONUS data
for 5 bursts, Ulysses/GRB data for 1 burst, and NEAR/XGRS data for 1 burst.
Spectroscopic redshifts, peak fluxes, and high resolution light curves are
available for 11 of these bursts; partial information is available for the
remaining 9 bursts. We find that the isotropic-equivalent luminosities L of
these bursts positively correlate with a rigorously-constructed measure V of
the variability of their light curves. We fit a model to these data that
accommodates both intrinsic scatter (statistical variance) and extrinsic
scatter (sample variance). If one excludes GRB 980425 from the fit on the
grounds that its association with SN 1998bw at a redshift of z = 0.0085 is not
secure, the luminosity estimator spans approx. 2.5 orders of magnitude in L,
and the slope of the correlation between L and V is positive with a probability
of 1 - 1.4 x 10^-4 (3.8 sigma). Although GRB 980425 is excluded from this fit,
its L and V values are consistent with the fitted model, which suggests that
GRB 980425 may well be associated with SN 1998bw, and that GRB 980425 and the
cosmological bursts may share a common physical origin. If one includes GRB
980425 in the fit, the luminosity estimator spans approx. 6.3 orders of
magnitude in L, and the slope of the correlation is positive with a probability
of 1 - 9.3 x 10^-7 (4.9 sigma). Independently of whether or not GRB 980425
should be included in the fit, its light curve is unique in that it is much
less variable than the other approx. 17 light curves in our sample for which
the signal-to-noise is reasonably good.


title: Collapse of Low Mass Clouds in the Presence of UV Radiation Field

abstract:
  The collapse of marginally Jeans unstable primordial gas clouds in the
presence of UV radiation field is discussed. Assuming that the dynamical
collapse proceeds approximately in an isothermal self-similar fashion, we
investigate the thermal evolution of collapsing central core until H$_2$
cooling dominates photoheating and the temperature drops to below $10^4$K.
Consequently, the mass of the cooled core is evaluated as $M_{cool}=3.6\times
10^6 M_\odot(I_{21}/1)^{-0.32}$. This scale depends only on the incident UV
intensity, and provides a lower limit to the mass of collapsed objects in the
UV radiation field.


title:
Cluster Detection in Astronomical Databases: the Adaptive Matched Filter
  Algorithm and Implementation

abstract:
  Clusters of galaxies are the most massive objects in the Universe and mapping
their location is an important astronomical problem. This paper describes an
algorithm (based on statistical signal processing methods), a software
architecture (based on a hybrid layered approach) and a parallelization scheme
(based on a client/server model) for finding clusters of galaxies in large
astronomical databases. The Adaptive Matched Filter (AMF) algorithm presented
here identifies clusters by finding the peaks in a cluster likelihood map
generated by convolving a galaxy survey with a filter based on a cluster model
and a background model. The method has proved successful in identifying
clusters in real and simulated data. The implementation is flexible and readily
executed in parallel on a network of workstations.


title: Probing the Properties of Neutron Stars with Type-I X-Ray Bursts

abstract:
  The increase in spin frequency as the burning atmospheres of TypeI X-ray
bursts cool provides a strong constraint on the radius of the underlying
neutron star. If the change in spin frequency is due to a change in the
thickness of the atmosphere, the radius of the star must exceed $3 G M/c^2$ for
any equation of state and approximately $3.5 G M/c^2$ for most physically
reasonable equations of state. This constraint arises because the direction of
the Coriolis force for radial motion reverses for $R < G / c^2 (M + I / R^2)$.
Furthermore, the marked change in the magnitude of the Coriolis force near
compact stars provides a straightforward explanation for why the frequency of
the quickly rotating bursters shifts by the same amount as the slow rotators;
they are slightly more massive, 1.6M$_\odot$ versus 1.4M$_\odot$. \


title: Resolving the radio nebula around beta Lyrae

abstract:
  In this paper we present high spatial resolution radio images of the puzzling
binary system beta Lyrae obtained with MERLIN at 5 GHz. We find a nebula
surrounding the binary with a brightness temperature of 11000+-700K
approximately 40AU across. This definitively confirms the thermal origin of the
radio emission, which is consistent with emission from the wind of the B6-8II
component (mass loss of order of 10^-7 Msun per year), ionized by the radiation
field of the hotter companion. This nebula, surrounding the binary, is the
proof that beta Layrae evolved in a non-conservative way, i. e. not all the
mass lost by the primary is accretted by the secondary, and present
measurements indicate that almost 0.015Msun had been lost from the system since
the onset of the Roche lobe overflow phase. Moreover, the nebula is aligned
with the jet-like structures inferred from recent optical measurements,
indicating a possible connection among them.


title:
Search for radio recombination lines towards the gravitational lens PKS
  1830-211

abstract:
  A search for radio recombination lines near 20 cm at z=0.193 and z=0.886
towards the gravitational lens system PKS 1830-211 has yielded upper limits on
the line optical depth of 5 x 10^{-5} and 5 x10^{-4} at the two redshifts
respectively. Based on the non-detections, we derive upper limits to the
emission measure of the ionized gas in the absorbing systems. We also present
continuum flux density measurements over the frequency range 0.3-45 GHz made at
a single epoch.


title:
Background discrimination capabilities of a heat and ionization
  germanium cryogenic detector

abstract:
  The discrimination capabilities of a 70 g heat and ionization Ge bolometer
are studied. This first prototype has been used by the EDELWEISS Dark Matter
experiment, installed in the Laboratoire Souterrain de Modane, for direct
detection of WIMPs. Gamma and neutron calibrations demonstrate that this type
of detector is able to reject more than 99.6% of the background while retaining
95% of the signal, provided that the background events distribution is not
biased towards the surface of the Ge crystal. However, the 1.17 kg.day of data
taken in a relatively important radioactive environment show an extra
population slightly overlapping the signal. This background is likely due to
interactions of low energy photons or electrons near the surface of the
crystal, and is somewhat reduced by applying a higher charge-collecting inverse
bias voltage (-6 V instead of -2 V) to the Ge diode. Despite this
contamination, more than 98% of the background can be rejected while retaining
50% of the signal. This yields a conservative upper limit of 0.7
event.day^{-1}.kg^{-1}.keV^{-1}_{recoil} at 90% confidence level in the 15-45
keV recoil energy interval; the present sensitivity appears to be limited by
the fast ambient neutrons. Upgrades in progress on the installation are
summarized.


title:
Four Measures of the Intracluster Medium Temperature and Their Relation
  to a Cluster's Dynamical State

abstract:
  We employ an ensemble of hydrodynamic cluster simulations to create spatially
and spectrally resolved images of quality comparable to Chandra's expected
performance. Emission from simulation mass elements is represented using the
XSPEC mekal program assuming 0.3 solar metallicity, and the resulting spectra
are fit with a single-temperature model. Despite significant departures from
isothermality in the cluster gas, single-temperature models produce acceptable
fits to 20,000 source photon spectra. The spectral fit temperature T_s is
generally lower than the mass weighted average temperature T_m due to the
influence of soft line emission from cooler gas being accreted as part of the
hierarchical clustering process. In a Chandra-like bandpass of 0.5 to 9.5 keV
we find a nearly uniform fractional bias of (T_m-T_s)/T_s = 20% with occasional
large deviations in smaller clusters. In the more traditional 2.0 to 9.5 keV
bandpass, the fractional deviation is scale-dependent and on average follows
the relation (T_m-T_s)/T_s = 0.2 log(T_m). This bias results in a spectral
mass-temperature relationship with slope about 1.6, intermediate between the
virial relation M ~ T_m^{3/2} and the observed relation M_{ICM} ~ T^2. Imaging
each cluster in the ensemble at 16 epochs in its evolutionary history, we
catalogue merger events with mass ratios exceeding 10% in order to investigate
the relationship between spectral temperature and proximity to a major merger
event. Clusters that are very cool relative to the mean mass-temperature
relationship lie preferentially close to a merger, suggesting a viable
observational method to cull a subset of dynamically young clusters from the
general population.


title: On the late spectral types of cataclysmic variable secondaries

abstract:
  We investigate why the spectral type of most cataclysmic variable (CV)
secondaries is significantly later than that of a ZAMS star with the same mean
density. Using improved stellar input physics, tested against observations of
low-mass stars at the bottom of the main sequence, we calculate the secular
evolution of CVs with low-mass donors. We consider sequences with different
mass transfer rates and with a different degree of nuclear evolution of the
donor prior to mass transfer. Systems near the upper edge of the gap ($P \sim 3
- 6$ h) can be reproduced by models with a wide range of mass transfer rates
from $1.5 \times 10^{-9} \msolyr$ to $10^{-8} \msolyr$. Evolutionary sequences
with a small transfer rate and donors that are substantially evolved off the
ZAMS (central hydrogen content $0.05-0.5$) reproduce CVs with late spectral
types above $P \simgr$ 6 h. Systems with the most discrepant (late) spectral
type should have the smallest donor mass at any given $P$. Consistency with the
period gap suggests that the mass transfer rate increases with decreasing donor
mass for evolved sequences above the period gap. In this case, a
single-parameter family of sequences with varying $\xc$ and increasing mass
transfer rate reproduces the full range of observed spectral types. This would
imply that CVs with such evolved secondaries dominate the CV population.


title: The Instability Strip of M3

abstract:
  We present new multicolour CCD photometry of the central part of the globular
cluster M3, mapping the precise position of ~120 RR Lyrae stars (RRab, RRd,
RRc) on the horizontal branch (HB). The location of the double-mode variables
(RRd) is in perfect agreement with recent theoretical results. We find a
significant internal spread of metallicity amongst the RRab variables.


title:
Circumbinary disks around T Tauri stars: HST/NICMOS near-infrared images
  and polarimetric maps

abstract:
  We have obtained new near-infrared images of both GG Tau and UY Aur
circumbinary disks with the polarimetric modes of NICMOS aboard the Hubble
Space Telescope. The 1 micron intensity map of GG Tau reveals a more complete
elliptical shape than in previous ground-based images. Its eastern and western
sides are definitely asymmetric. Our image strongly supports the ring geometry
proposed by Guilloteau, Dutrey & Simon (1999) on the basis of their millimetre
interferometry images: a geometrically thick and sharply edged ring surrounding
an empty gap around the binary. Around UY Aur, we identify structures that are
in excellent agreement with the optical images of M\'enard et al. (1999), which
confirms that the inclination of the system to the line-of-sight is about 60
degrees. We also find tentative new structures closer to the stars.


title:
Can a Large Neutron Excess Help Solve the Baryon Loading Problem in
  Gamma-Ray Burst Fireballs?

abstract:
  We point out that the baryon-loading problem in Gamma-Ray Burst (GRB) models
can be amelioriated if a significant fraction of the baryons which inertially
confine the fireball are converted to neutrons. A high neutron fraction in some
circumstances can result in a reduced transfer of energy from relativistic
light particles in the fireball to baryons. The energy needed to produce the
required relativistic flow in the GRB is consequently reduced, in some cases by
orders of magnitude. This could be relevant to GRB models because a high
neutron-to-proton ratio has been calculated in neutron star-merger fireball
environments. Significant neutron excess also could occur near compact objects
with high neutrino fluxes.


title: Spectral Templates from Multicolor Redshift Surveys

abstract:
  Understanding how the physical properties of galaxies (e.g. their spectral
type or age) evolve as a function of redshift relies on having an accurate
representation of galaxy spectral energy distributions. While it has been known
for some time that galaxy spectra can be reconstructed from a handful of
orthogonal basis templates, the underlying basis is poorly constrained. The
limiting factor has been the lack of large samples of galaxies (covering a wide
range in spectral type) with high signal-to-noise spectrophotometric
observations. To alleviate this problem we introduce here a new technique for
reconstructing galaxy spectral energy distributions directly from samples of
galaxies with broadband photometric data and spectroscopic redshifts.
Exploiting the statistical approach of the Karhunen-Loeve expansion, our
iterative training procedure increasingly improves the eigenbasis, so that it
provides better agreement with the photometry. We demonstrate the utility of
this approach by applying these improved spectral energy distributions to the
estimation of photometric redshifts for the HDF sample of galaxies. We find
that in a small number of iterations the dispersion in the photometric
redshifts estimator (a comparison between predicted and measured redshifts) can
decrease by up to a factor of 2.


title: Evidence for Newly Synthesized Lithium in Interstellar Space

abstract:
  As a part of a study on the variation in the 7Li/6Li ratio, high signal to
noise, high resolution spectroscopy of Li I (6708 Angstroms) and K I (4044
Angstroms) were obtained toward zeta and o Per. The lithium isotope shift is
approximately the same as its fine structure splitting (0.158 Angstroms),
resulting in a blend of the 7Li and 6Li lines. Use of the K I line as a
velocity template allowed a straightforward determination of the lithium
isotopic ratio.
  Our high resolution observations show remarkably disparate isotope ratios:
7Li/6Li = 1 0.6 +/- 2.9 toward zeta Per and 3.6 +/- 0.6 and 1.7 +/- 0.3 toward
o Per. While the results toward zeta Per are consistent with the meteoritic
value, the ratio toward o Per's clouds suggest recent Galactic cosmic ray
spallation. This is the first clear and unambiguous evidence for an
interstellar 7Li/6Li ratio that differs dramatically from the Solar System
value. The low ratio indicates a higher cosmic ray flux toward o Per than
toward zeta Per. A higher flux of cosmic rays toward o Per was suggested
earlier in a study of OH and HD chemistry (Federman, Weber, & Lambert 1996, ApJ
463, 181).
  The total lithium abundances and the K/Li ratio are useful in placing the
7Li/6Li ratio into context. We obtained total lithium abundances of (12.2 +/-
2.2) x 10^(-10) toward zeta Per and (9.8 +/- 3.5) x 10^(-10) toward o Per. The
columns of K I and Li I yielded K/Li ratios that revealed no discernable trend.
Since a Li isotope ratio ~2 toward o Per suggests newly processed lithium, the
comparable total Li abundances and K/Li ratios are unexpected.


title:
Short term pulse frequency fluctuations of OAO 1657-415 from RXTE
  observations

abstract:
  We present new X-ray observations of the high mass X-ray binary (HMXRB)
pulsar OAO 1657-415, obtained during one orbital period (10.44 days) with the
Rossi X-Ray Timing Explorer (RXTE). Using the binary orbital parameters,
obtained from Burst and Transient Source Experiment (BATSE) observations, we
resolve the fluctuations in the pulse frequency at time scales on the order of
one day for the first time. Recent BATSE results showed that OAO 1657-415 has
spin-up/down trends in its pulse frequency time series, without any correlation
with the X-luminosity at energies $>$20 keV (Baykal 1997). In the present RXTE
observations the source is found to be in an extended phase of spin-down. We
also find a gradual increase in the X-ray luminosity which is correlated with a
marginal spin-up episode. The marginal correlation between the gradual spin-up
(or decrease in spin-down rate) and increase in X-ray luminosity suggests that
the OAO 1657-415 is observed during a stable accretion episode where the
prograde accretion disk is formed.


title: The Properties of Matter in White Dwarfs and Neutron Stars

abstract:
  White dwarfs and neutron stars are stellar objects with masses comparable to
that of our sun. However, as the endpoint stages of stellar evolution, these
objects do not sustain any thermonuclear burning and therefore can no longer
support the gravitational load of their own mass by generating thermal
pressure. Rather, matter in their interiors is compressed to much higher
densities than commonly found in normal stars, and pressure is created by
degenerate fermion kinetic energy and particle interactions. As a result, white
dwarfs and neutron stars offer unique cosmic laboratories for studying matter
at very high densities. In this review we discuss the basic properties of
condensed matter at extreme densities and summarize the extent to which these
properties can be examined by observations of compact objects.


title: 2MASS Extended Source Catalog: Overview and Algorithms

abstract:
  The 2 Micron All-Sky Survey (2MASS) will observe over one-million galaxies
and extended Galactic sources covering the entire sky at wavelengths between 1
and 2 microns. The survey catalog will have both high completeness and
reliability down to J = 15.0 and Ks = 13.5 mag, equivalent to 1.6 mJy and 2.9
mJy, respectively. This paper describes the basic algorithms used to detect and
characterize extended sources in the 2MASS database and catalog. Critical
procedures include tracking the point spread function, image background
removal, artifact removal, photometry and basic parameterization, star-galaxy
discrimination and object classification using a decision tree technique.
Examples of extended sources encountered in 2MASS are given.


title: The Hubble Deep Fields

abstract:
  The Hubble space telescope observations of the northern Hubble deep field,
and more recently its counterpart in the south, provide detections and
photometry for stars and field galaxies to the faintest levels currently
achievable, reaching magnitudes V ~ 30. Since 1995, the northern Hubble deep
field has been the focus of deep surveys at nearly all wavelengths. These
observations have revealed many properties of high redshift galaxies, and have
contributed important data on the stellar mass function in the Galactic halo.


title:
Morphology-Density Relation for Simulated Clusters of Galaxies in Cold
  Dark Matter Dominated Universe

abstract:
  We present a model to investigate the formation and evolution of cluster
galaxies using cosmological N-body simulations. The N-body simulations are used
to make merging history trees of dark halos. Gas cooling, star formation,
supernova feedback and mergers of galaxies within dark halos are included by
using simple prescriptions taken from semi-analytic models of galaxy formation.
In this {\it Letter} we exam the merger-driven bulge formation model and
represent the morphology-density relation of cluster galaxies at z = 0. We find
that this morphological evolution model can explain the distribution of
elliptical galaxies in the clusters well and cannot reproduce the distribution
of S0 galaxies. This result suggests that the elliptical galaxies are mainly
formed by the major mergers, while, in the S0 formation, the processes other
than major mergers play important role.


title: Can the Universe escape eternal acceleration?

abstract:
  Recent astronomical observations of distant supernovae light-curves suggest
that the expansion of the universe has recently begun to accelerate.
Acceleration is created by an anti-gravitational repulsive stress, like that
produced by a positive cosmological constant, or universal vacuum energy. It
creates a rather bleak eschatological picture. An ever-expanding universe's
future appears to be increasingly dominated by its constant vacuum energy. A
universe doomed to accelerate forever will produce a state of growing
uniformity and cosmic loneliness. Structures participating in the cosmological
expansion will ultimately leave each others' horizons and
information-processing must eventually die out. Here, we examine whether this
picture is the only interpretation of the observations. We find that in many
well-motivated scenarios the observed spell of vacuum domination is only a
transient phenomenon. Soon after acceleration starts, the vacuum energy's
anti-gravitational properties are reversed, and a matter-dominated decelerating
cosmic expansion resumes. Thus, contrary to general expectations, once an
accelerating universe does not mean always an accelerating universe.


title: Looking for non-Gaussianity in the COBE-DMR data with spherical wavelets

abstract:
  We present an analysis of the Gaussianity of the 4-year COBE-DMR data (in
HEALPix pixelisation) based on spherical wavelets. The skewness, kurtosis and
scale-scale correlation spectra are computed from the detail wavelet
coefficients at each scale. The sensitivity of the method to the orientation of
the data is also taken into account. We find a single detection of
non-Gaussianity at the $>99%$ confidence level in one of our statistics. We use
Monte-Carlo simulations to assess the statistical significance of this
detection and find that the probability of obtaining such a detection by chance
for an underlying Gaussian field is as high as 0.69. Therefore, our analysis
does not show evidence of non-Gaussianity in the COBE-DMR data.


title: NGC1333/IRAS4: A multiple star formation laboratory

abstract:
  We present SCUBA observations of the protomultiple system NGC1333/IRAS4 at
450um and 850um. The 850um map shows significant extended emission which is
most probably a remnant of the initial cloud core. At 450um, the component 4A
is seen to have an elongated shape suggestive of a disk. Also we confirm that
in addition to the 4A and 4B system, there exists another component 4C, which
appears to lie out of the plane of the system and of the extended emission.
Deconvolution of the beam reveals a binary companion to IRAS4B. Simple
considerations of binary dynamics suggest that this triple 4A-4BI-4BII system
is unstable and will probably not survive in its current form. Thus IRAS4
provides evidence that systems can evolve from higher to lower multiplicity as
they move towards the main sequence. We construct a map of spectral index from
the two wavelengths, and comment on the implications of this for dust evolution
and temperature differences across the map. There is evidence that in the
region of component 4A the dust has evolved, probably by coagulating into
larger or more complex grains. Furthermore, there is evidence from the spectral
index maps that dust from this object is being entrained in its associated
outflow.


title: Planetary Transits Toward the Galactic Bulge

abstract:
  The primary difficulty with using transits to discover extrasolar planets is
the low probability a planet has of transiting its parent star. One way of
overcoming this difficulty is to search for transits in dense stellar fields,
such as the Galactic bulge. Here I estimate the number of planets that might be
detected from a monitoring campaign toward the bulge. A campaign lasting 10
nights on a 10 meter telescope (assuming 8 hours of observations per night and
a 5'x5' field of view) would detect about 100 planets with radius $\rp=1.5
\rjup$, or about 30 planets with $\rp=1.0 \rjup$, if the frequency and
distribution of planets in the bulge is similar to that in the solar
neighborhood. Most of these planets will be discovered around stars just below
the turn-off, i.e. slightly evolved G-dwarfs. Campaigns involving 1- or 4-m
class telescopes are unlikely to discover any planets, unless there exists a
substantial population of companions with $\rp > 1.5 \rjup$.


title:
The Sun, stellar-population models, and the age estimation of
  high-redshift galaxies

abstract:
  Given sufficiently deep optical spectroscopy, the age estimation of
high-redshif t ($z > 1$) galaxies has been claimed to be a relatively robust
process (e.g. Dunlop et al. 1996) due to the fact that, for ages $< 5$Gyr, the
near-ultraviolet light of a stellar population is expected to be dominated by
`well-understood' main-sequence (MS) stars. Recently, however, the reliability
of this process has been called into question by Yi et al (2000), who claim to
have developed models in which the spectrum produced by the main sequence
reddens much more rapidly than in the models of Jimenez et al (2000a), leading
to much younger age estimates for the reddest known high-redshift ellipticals.
In support of their revised age estimates, Yi et al cite the fact that their
models can reproduce the spectrum of the Sun at an age of 5 Gyr, whereas the
solar spectrum is not reproduced by the Jimenez et al models until $\simeq 10$
Gyr. Here we confirm this discrepancy, but point out that this is in fact a
{\it strength} of the Jimenez et al models and indicative of some flaw in the
models of Yi et al (which, in effect, imply that the Sun will turn into a red
giant any minute now). We have also explored the models of Worthey (1994)
(which are known to differ greatly from those of Jimenez et al in the treatment
of post-MS evolution) and find that the main-sequence component of Worthey's
models also cannot reproduce the solar spectrum until an age of 9-10 Gyr. We
conclude that either the models of Yi et al are not as main-sequence dominated
at 4-5 Gyr as claimed, or that the stellar evolutionary timescale in these
models is in error by a factor possibly as high as two. (abridged)


title: Sub-Saturn Planet Candidates to HD 16141 and HD 46375

abstract:
  Precision Doppler measurements from the Keck/HIRES spectrometer reveal
periodic Keplerian velocity variations in the stars HD 16141 and HD 46375. HD
16141 (G5 IV) has a period of 75.8 d and a velocity amplitude of 11 m/s,
yielding a companion having Msini = 0.22 Mjup and a semimajor axis, a = 0.35
AU. HD 46375 (K1 IV/V) has a period of 3.024 d and a velocity amplitude of 35
m/s, yielding a companion with Msini=0.25 Mjup, a semimajor axis of a = 0.041
AU, and an eccentricity of 0.04 (consistent with zero). These companions
contribute to the rising planet mass function toward lower masses.


title:
Density dependent strong coupling constant of QCD derived from compact
  star data

abstract:
  The present work is an endeavour to connect the properties of tiny nearly
massless objects with those of some of the most massive ones, the compact
stars.
  Since 1996 there is major influx of X-ray and $\gamma$ ray data from binary
stars, one or both of which are compact objects that are difficult to explain
as neutron stars since they contain a mass M in too small a radius R . The
suggestion has been put forward that these are strange quark stars (SS)
explainable in a simple model with chiral symmetry restoration (CSR) for the
quarks and the M, R and other properties like QPOs (quasi periodic
oscillations) in their X-ray power spectrum.
  It would be nice if this astrophysical data could shed some light on
fundamental properties of quarks obeying QCD. One can relate the strong
coupling constant of QCD, $\alpha_s$ to the quark mass through the
Dyson-Schwinger gap equation using the real time formalism of Dolan and Jackiw.
This enables us to obtain the density dependence of $\alpha_s$ from the simple
CSR referred to above. This way fundamental physics, difficult to extract from
other models like for example lattice QCD, can be constrained from present-day
compact star data and may be put back to modelling the dense quark phase of
early universe.


title: Vacuum Energy: Cosmological Constant or Quintessence?

abstract:
  For a flat universe presently dominated by smooth energy, either cosmological
constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed
mass fraction as function of the present ratio of smooth energy to matter
energy $\mathcal R_0$. Identifying the normalized collapsed fraction as a
conditional probability for habitable galaxies, we observe that the observed
present ratio $\mathcal R_0 \sim 2$ is likely in LCDM, but more likely in QCDM.
Inverse application of Bayes' Theorem makes the Anthropic Principle a
predictive scientific principle: the data implies that the prior probability
for $\mathcal R_0$ must be essentially flat over the anthropically allowed
range. Interpreting this prior as a distribution over {\em theories} lets us
predict that any future theory of initial conditions must be indifferent to
$\mathcal R_0$. This application of the Anthropic Principle does not demand the
existence of other universes.


title: A Self-Occulting Accretion Disk in the SW Sex Star DW UMa

abstract:
  We present the ultraviolet spectrum of the SW Sex star and nova-like variable
DW UMa in an optical low state, as observed with the Space Telescope Imaging
Spectrograph on board the Hubble Space Telescope (HST). The data are well
described by a synthetic white dwarf (WD) spectrum with T_eff = 46,000 +/- 1000
K, log g = 7.60 +/- 0.15, v*sin(i) = 370 +/- 100 km/s and Z/Z_solar = 0.47 +/-
0.15. For this combination of T_eff and log g, WD models predict M_WD = 0.48
+/- 0.06 M_solar and R_WD = (1.27 +/- 0.18) * 10^9 cm. Combining the radius
estimate with the normalization of the spectral fit, we obtain a distance
estimate of d = 830 +/-150 pc.
  During our observations, DW UMa was approximately 3 magnitudes fainter in V
than in the high state. A comparison of our low-state HST spectrum to a
high-state spectrum obtained with the International Ultraviolet Explorer shows
that the former is much bluer and has a higher continuum level shortward of
1450 A. Since DW UMa is an eclipsing system, this suggests that an optically
thick accretion disk rim blocks our view of the WD primary in the high state.
If self-occulting accretion disks are common among the SW Sex stars, we can
account for (i) the preference for high-inclination systems within the class
and (ii) their V-shaped continuum eclipses. Moreover, even though the emission
lines produced by a self-obscured disk are generally still double-peaked, they
are weaker and narrower than those produced by an unobscured disk. This may
allow a secondary line emission mechanism to dominate and produce the
single-peaked, optical lines that are a distinguishing characteristic of the SW
Sex stars.


title: Discovery of Two High-Magnetic-Field Radio Pulsars

abstract:
  We report the discovery of two young isolated radio pulsars with very high
inferred magnetic fields. PSR J1119-6127 has period P = 0.407 s, and the
largest period derivative known among radio pulsars, Pdot = 4.0e-12. Under
standard assumptions these parameters imply a characteristic spin-down age of
only tau = 1.6 kyr and a surface dipole magnetic field strength of B = 4.1e13
G. We have measured a stationary period-second-derivative for this pulsar,
resulting in a braking index of n = 2.91+-0.05. We have also observed a glitch
in the rotation of the pulsar, with fractional period change Delta_P/P =
-4.4e-9. Archival radio imaging data suggest the presence of a previously
uncataloged supernova remnant centered on the pulsar. The second pulsar, PSR
J1814-1744, has P = 3.975 s and Pdot = 7.4e-13. These parameters imply tau = 85
kyr, and B = 5.5e13 G, the largest of any known radio pulsar.
  Both PSR J1119-6127 and PSR J1814-1744 show apparently normal radio emission
in a regime of magnetic field strength where some models predict that no
emission should occur. Also, PSR J1814-1744 has spin parameters similar to the
anomalous X-ray pulsar (AXP) 1E 2259+586, but shows no discernible X-ray
emission. If AXPs are isolated, high magnetic field neutron stars
(``magnetars''), these results suggest that their unusual attributes are
unlikely to be merely a consequence of their very high inferred magnetic
fields.


title:
Perturbations of self-gravitating, ellipsoidal superfluid-normal fluid
  mixtures

abstract:
  We study the perturbation modes of rotating superfluid ellipsoidal figures of
equilibrium in the framework of the two-fluid superfluid hydrodynamics and
Newtonian gravity. Our calculations focus on linear perturbations of background
equilibria in which the two fluids move together, the total density is uniform,
and the densities of the two components are proportional to one another, with
ratios that are independent of position. The motions of the two fluids are
coupled by mutual friction, as formulated by Khalatnikov. We show that there
are two general classes of modes for small perturbations: one class in which
the two fluids move together and the other in which there is relative motion
between them. The former are identical to the modes found for a single fluid,
except that the rate of viscous dissipation, when computed in the secular (or
``low Reynolds number'') approximation under the assumption of a constant
kinematic viscosity, is diminished by a factor $f_N$, the fraction of the total
mass in the normal fluid. The relative modes are completely new, and are
studied in detail for a range of values for the phenomenological mutual
friction coefficients, relative densities of the superfluid and normal
components, and, for Roche ellipsoids, binary mass ratios. We find that there
are no new secular instabilities connected with the relative motions of the two
fluid components. Moreover, although the new modes are subject to viscous
dissipation (a consequence of viscosity of the normal matter), they do not emit
gravitational radiation at all.


title: Quintessence and Scalar Dark Matter in the Universe

abstract:
  Continuing with previous works, we present a cosmological model in which dark
matter and dark energy are modeled by scalar fields $\Phi $ and $\Psi$,
respectively, endowed with the scalar potentials $V(\Phi)=V_{o}[ \cosh
{(\lambda \sqrt{\kappa_{o}}\Phi)}-1] $ and $\tilde{V}(\Psi)=\tilde{V_{o}}[
\sinh {(\alpha \sqrt{\kappa_{o}}\Psi)}] ^{\beta}$. This model contains 95% of
scalar field. We obtain that the scalar dark matter mass is $m_{\Phi}\sim
10^{-26}eV.$ The solution obtained allows us to recover the success of the
standard CDM. The implications on the formation of structure are reviewed. We
obtain that the minimal cutoff radio for this model is $r_{c}\sim 1.2 kpc.$


title: On the supernovae heating of intergalactic medium

abstract:
  We present estimates of the energy input from supernovae (SNe) into the
intergalactic medium using (i) recent measurements of Si and Fe abundances in
the intracluster medium (ICM) and (ii) self-consistent gasdynamical galaxy
formation simulations that include processes of cooling, star formation, SNe
feedback, and a multi-phase model of the interstellar medium. We estimate the
energy input from observed abundances using two different assumptions: (i)
spatial uniformity of metal abundances in the ICM and (ii) radial abundance
gradients. We show that these two cases lead to energy input estimates which
are different by an order of magnitude, highlighting a need for observational
data on large-scale abundance gradients in clusters. Analysis of galaxy
formation results and estimates from observed Fe and Si abundances indicates
that the SNe energy input can be important for heating of the entire ICM
(providing energy of ~1 keV per particle) only if the ICM abundances are
uniform and the efficiency of gas heating by SN explosions is close to 100%
(implying that all of the initial kinetic energy of the explosion goes into
heating of the ICM). We conclude that unless these most favorable conditions
are met, SNe alone are unlikely to provide sufficient energy input to heat all
of the cluster ICM and may need to be supplemented or even substituted by some
other heating process(es). (Abridged)


title: Linear Redshift Distortions and Power in the PSCz Survey

abstract:
  We present a state-of-the-art linear redshift distortion analysis of the
recently published IRAS Point Source Catalog Redshift Survey (PSCz). The
procedure involves linear compression into 4096 Karhunen-Loeve modes culled
from a potential pool of about 3 x 10^5 modes, followed by quadratic
compression into three separate power spectra, the galaxy-galaxy,
galaxy-velocity, and velocity-velocity power spectra. Least squares fitting to
the decorrelated power spectra yields a linear redshift distortion parameter
beta = Omega_m^0.6/b = 0.41(+0.13,-0.12).


title: Temporal variations of the rotation rate in the solar interior

abstract:
  The temporal variations of the rotation rate in the solar interior are
studied using frequency splittings from Global Oscillations Network Group
(GONG) data obtained during the period 1995-99. We find alternating latitudinal
bands of faster and slower rotation which appear to move towards the equator
with time - similar to the torsional oscillations seen at the solar surface.
This flow pattern appears to persist to a depth of about 0.1R_sun and in this
region its magnitude is well correlated with solar activity indices. We do not
find any periodic or systematic changes in the rotation rate near the base of
the convection zone.


title:
Photon acceleration in variable ultra-relativistic outflows and
  high-energy spectra of Gamma-Ray Bursts

abstract:
  MeV seed photons produced in shocks in a variable ultra-relativistic outflow
gain energy by the Fermi mechanism, because the photons Compton scatter off
relativistically colliding shells. The Fermi-modified high-energy photon
spectrum has a non-universal slope and a universal cutoff. A significant
increase in the total radiative efficiency is possible. In some gamma ray
bursts, most of the power might be emitted at the high-energy cutoff for this
mechanism, which would be close to 100 MeV for outflows with a mean bulk
Lorentz factor of 100.


title:
Non-LTE Abundances and Consequences for the Evolution of the alpha
  elements in the Galaxy

abstract:
  Abundances of alpha-elements such as Ca and Mg in disk and halo stars are
usually derived from equivalent widths lines measured on high resolution
spectra, and assuming Local Thermodynamic Equilibrium (LTE) . In this paper, we
present non-LTE differential abundances derived by computing the statistical
equilibrium of CaI and MgI atoms, using high resolution equivalent widths
available in the literature for 252 dwarf to subgiant stars. These non-LTE
abundances combined with recent determination of non-LTE abundances of iron,
seem to remove the dispersion of the [Ca/Fe] and [Mg/Fe] ratios in the galactic
halo and disk phases, revealing new and surprising structures. These results
have important consequences for chemical evolution models of the Galaxy. In
addition, non-LTE abundance ratios for stars belonging to the M92 cluster
apparently have the same behavior. More high resolution observations, mainly of
globular clusters, are urgently needed to confirm our results.


title:
HST/NICMOS Observations of IZw18 - revealing a population of old AGB
  stars

abstract:
  I present the first results from a HST/NICMOS imaging study of the most
metal-poor blue compact dwarf galaxy IZw18. The near infrared color-magnitude
diagram (CMD) is dominated by two populations, one 10-20 Myr population of red
super giants and one 0.5-5 Gyr population of asymptotic giant branch stars.
Stars older than 1 Gyr are required to explain the observed CMD at the adopted
distance of 12.6 Mpc, showing that IZw18 is not a young galaxy. The results
hold also if the distance to IZw18 i s significantly larger. This rules out the
possibility that IZw18 is a truly young galaxy formed recently in the local
universe.


title: Rotation and activity in the solar-type stars of NGC 2547

abstract:
  We present high resolution spectroscopy of a sample of 24 solar-type stars in
the young (15-40 Myr), open cluster, NGC 2547. We use our spectra to confirm
cluster membership in 23 of these stars, determine projected equatorial
velocities and chromospheric activity, and to search for the presence of
accretion discs. We have found examples of both fast (vsini>50kms) and slow
(vsini<10kms) rotators, but find no evidence for active accretion in any of the
sample. The distribution of projected rotation velocities is indistinguishable
from the slightly older IC 2391 and IC 2602 clusters, implying similar initial
angular momentum distributions and circumstellar disc lifetimes. The presence
of very slow rotators indicates that either long (10-40 Myr) disc lifetimes or
internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602)
were born with more slowly rotating stars than are presently seen in even
younger clusters and associations. The solar-type stars in NGC 2547 follow a
similar rotation-activity relationship to that seen in older clusters. X-ray
activity increases until a saturation level is reached for vsini>15-20kms. We
are unable to explain why this saturation level, of log (L_x/L_bol)~-3.3, is a
factor of two lower than in other clusters, but rule out anomalously slow
rotation rates or uncertainties in X-ray flux calculations.


title:
Kinematic Evidence of Minor Mergers in Normal Sa Galaxies: NGC3626,
  NGC3900, NGC4772 and NGC5854

abstract:
  BVRI and H-alpha imaging and long-slit optical spectroscopic data are
presented for four morphologically normal and relatively isolated Sa galaxies,
NGC3626, NGC3900, NGC4772 and NGC5854. VLA HI synthesis imaging is presented
for the first 3 objects. In all 4 galaxies, evidence of kinematic decoupling of
ionized gas components is found; the degree and circumstances of the distinct
kinematics vary from complete counterrotation of all of the gas from all of the
stars (NGC3626) to nuclear gas disks decoupled from the stars (NGC5854) to
anomalous velocity central gas components (NGC3900 and NGC4772). In the 3
objects mapped in HI, the neutral gas extends far beyond the optical radius,
R_HI/R_25 > 2. In general, the HI surface density is very low and the outer HI
is patchy and asymmetric or found in a distinct ring, exterior to the optical
edge. While the overall HI velocity fields are dominated by circular motions,
strong warps are suggested in the outer regions. Optical imaging is also
presented for NGC 4138 previously reported by Jore et al. (1996) to show
counterrotating stellar components. The multiwavelength evidence is interpreted
in terms of the kinematic "memory" of past minor mergers in objects that
otherwise exhibit no morphological signs of interaction.


title: Non-Gaussianity vs. non-linearity of cosmological perturbations

abstract:
  Following the discovery of the CMB, the hot big-bang model has become the
standard cosmological model. In this theory, small primordial fluctuations are
subsequently amplified by gravity to form the large-scale structure seen today.
Different theories for unified models of particle physics, lead to different
predictions for the statistical properties of the primordial fluctuations, that
can be divided in two classes: gaussian and non-gaussian. Convincing evidence
against or for gaussian initial conditions would rule out many scenarios and
point us towards a physical theory for the origin of structures. The
statistical distribution of cosmological perturbations, as we observe them, can
deviate from the gaussian distribution in several different ways. Even if
perturbations start off gaussian, non-linear gravitational evolution can
introduce non-gaussian features. Additionally, our knowledge of the Universe
comes principally from the study of luminous material such as galaxies, but
these might not be faithful tracers of the underlying mass distribution. The
relationship between fluctuations in the mass and in the galaxies distribution
(bias), is often assumed to be local, but could well be non-linear. Moreover,
galaxy catalogues use the redshift as third spatial coordinate: the resulting
redshift-space map is non-linearly distorted by peculiar velocities. Non-linear
gravitational evolution, biasing, and redshift-space distortion introduce
non-gaussianity, even in an initially gaussian fluctuation field. I will
investigate the statistical tools that allow us, in principle, to disentangle
the above different effects, and the observational datasets we require to do so
in practice.


title: Microlensing searches of dark matter

abstract:
  The evolution of the observational results of microlensing towards the
  LMC and some of the suggested interpretations to account for them are
discussed. It is emphasized that the results at present are indicative of a
lensing population of white dwarfs, possibly in the spheroid (not dark halo) of
the Galaxy, together with the more standard backgrounds of stellar populations
in the Magellanic Clouds and in the
  Galaxy. This is also hinted by dynamical estimates of the spheroid mass and
by recent direct searches of old white dwarfs.


title: Variability Model for the Microquasar GRS 1915+105

abstract:
  We analyse the radiation pressure instability of the standard, optically
thisc disc and propose it as an explanation of the obseved variability of the
superluminal microquasar GRS 1915+105. The quantitave agreement with the
observed outburst amplitudes is obtained under the assumption that substantial
part of the energy is carried away by the jet.


title: The Orbital Light Curve of Aquila X-1

abstract:
  We obtained R- and I-band CCD photometry of the soft X-ray transient/neutron-
star binary Aql X-1 in 1998 June while it was at quiescence. We find that its
light curve is dominated by ellipsoidal variations, although the ellipsoidal
variations are severely distorted and have unequal maxima. After we correct for
the contaminating flux from a field star located only 0.46" away, the
peak-to-peak amplitude of the modulation is ~0.25 mag in the R band, which
requires the orbital inclination to be greater than 36 degrees. The orbital
period we measure is consistent with the 18.95 h period measured by Chevalier &
Ilovaisky (1998). During its outbursts the light curve of Aql X-1 becomes
single humped. The outburst light curve observed by Garcia et al. (1999) agrees
in phase with our quiescent light curve. We show that the single humped
variation is caused by a ``reflection effect,'' that is, by heating of the side
of the secondary star facing towards the neutron star.


title: On 7Li Enrichment by Low Mass Metal Poor Red Giant Branch Stars

abstract:
  First-ascent red giants with strong and very strong Li lines have just been
discovered in globular clusters. Using the stellar internal prompt 7Li
enrichment - mass loss scenario, we explore the possibility of 7Li enrichment
in the interstellar matter of the globular cluster M3 produced by these Li-rich
giants. We found that enrichment as large as 70% or more compared to the
initial 7Li content of M3 can be obtained during the entire life of this
cluster. However, due to the several crossings of M3 into the Galactic plane,
the new 7Li will be very probably removed by ram pressure into the disk.
Globular clusters appear then as possible new sources of 7Li in the galactic
disk. It is also suggested that the known Na/Al variations in stars of globular
clusters could be somehow related to the 7Li variations and that the cool
bottom mixing mechanism acting in the case of 7Li could also play a role in the
case of Na and Al surface enrichments.


title: Cluster Populations in A115 and A2283

abstract:
  This paper presents four color narrow-band photometry of clusters A115
($z=0.191$) and A2283 ($z=0.182$) in order to follow the star formation history
of various galaxy types. Although located at similar redshifts, the two
clusters display very different fractions of blue galaxies (i.e. the
Butcher-Oemler effect, $f_B = 0.13$ for A115, $f_B = 0.30$ for A2283). A system
of photometric classification is applied to the cluster members that divides
the cluster population into four classes based on their recent levels of star
formation. It is shown that the blue population of each cluster is primarily
composed of normal starforming (SFR $< 1 M_{\sun}$ yrs$^{-1}$) galaxies at the
high luminosity end, but with an increasing contribution from a dwarf starburst
population below $M_{5500}= -20$. This dwarf starburst population appears to be
the same population of low mass galaxies identified in recent HST imaging (Koo
et al 1997), possible progenitors to present-day cluster dwarf ellipticals,
irregulars and BCD's. Deviations in the color-magnitude relationship for the
red galaxies in each cluster suggest that a population of blue S0's is evolving
into present-day S0 colors at this epoch. The radial distribution of the blue
population supports the prediction of galaxy harassment mechanisms for tidally
induced star formation operating on an infalling set of gas-rich galaxies.


title: Rotational Evolution During Type I X-Ray Bursts

abstract:
  The rotation rates of six weakly-magnetic neutron stars accreting in low-mass
X-ray binaries have most likely been measured by Type I X-ray burst
observations with RXTE. The nearly coherent oscillations detected during the
few seconds of thermonuclear burning are most simply understood as rotational
modulation of brightness asymmetries on the neutron star surface. We show that,
as suggested by Strohmayer and colleagues, the frequency changes of 1-2 Hz
observed during bursts are consistent with angular momentum conservation as the
burning shell hydrostatically expands and contracts. We calculate how vertical
heat propagation through the radiative outer layers of the atmosphere and
convection affect the coherence of the oscillation. We show that the evolution
of the rotational profile depends strongly on whether the burning layers are
composed of pure helium or mixed hydrogen/helium. Our results help explain the
absence (presence) of oscillations from hydrogen-burning (helium-rich) bursts
that was found by Muno and collaborators.
  We investigate angular momentum transport within the burning layers and the
recoupling of the burning layers with the star. We show that the
Kelvin-Helmholtz instability is quenched by the strong stratification, and that
mixing between the burning fuel and underlying ashes by the baroclinic
instability does not occur. However, the baroclinic instability may have time
to operate within the differentially rotating burning layer, potentially
bringing it into rigid rotation.


title: Stellar Populations and Galaxy Morphology at High Redshift

abstract:
  In this article we investigate the morphology and stellar populations of
high-redshift galaxies through multi-waveband HST imaging and ground-based
spatially-resolved spectroscopy. We study the redshift evolution of galaxy
morphology in the Hubble Deep Field, using the deep IDT-NICMOS near-infrared
HST imaging coupled with spectroscopic and photometric redshifts. Using the
multi-waveband data to compare the appearance of galaxies at the same
rest-frame wavelengths reveals that "morphological k-corrections" (the change
in appearance when viewing high-z objects at shorter rest-frame wavelengths)
are only important in a minority of cases, and that galaxies were intrinsically
more peculiar at high redshift. One example of significant morphological
k-corrections is spiral galaxies, which often show more pronounced barred
structure in the near-infrared than in the optical. Therefore, the apparent
decline in the fraction of barred spirals at faint magnitudes in the optical
HDF may be due to band-shifting effects at the higher redshifts, rather than
intrinsic evolution. Using such features as the age-sensitive Balmer+4000Ang
break, the spatially- resolved colours of distant galaxies in
optical/near-infrared imaging can also be used to study their component stellar
populations. We supplement this with deep Keck/LRIS spectroscopy of two
extended sources: a chain galaxy at z=2.8 (HDF4-555.1, the "Hot Dog" - the
brightest U-drop Lyman-break galaxy in the HDF) and a pair of z=4.04
gravitationally lensed arcs behind the cluster Abell 2390. The absence of
measurable rotation across the z=2.8 chain galaxy implies that it is unikely to
be a disk viewed edge on. With the resolution enhancement from lensing, we
detect stellar populations of different ages in the z=4 arcs.


title: Mixing Metals in the Early Universe

abstract:
  We investigate the evolution of the metallicity of the intergalactic medium
(IGM) with particular emphasis on its spatial distribution. We propose that
metal enrichment occurs as a two step process. First, supernova (SN) explosions
eject metals into relatively small regions confined to the surroundings of
star-forming galaxies. From a comprehensive treatment of blowout we show that
SNae by themselves fail by more than one order of magnitude to distribute the
products of stellar nucleosynthesis over volumes large enough to pollute the
whole IGM to the metallicity levels observed. Thus, a additional (but as yet
unknown) physical mechanism must be invoked to mix the metals on scales
comparable to the mean distance between the galaxies which are most efficient
pollutants. From this simple hypothesis we derive a number of testable
predictions for the evolution of the IGM metallicity. Specifically, we find
that: (i) the fraction of metals ejected over the star formation history of the
universe is about 50% at z=0; that is, approximately half of the metals today
are found in the IGM; (ii) if the ejected metals were homogeneously mixed with
the baryons in the universe, the average IGM metallicity would be <Z> ~ 1/25
Z(solar) at z=3. However, due to spatial inhomogeneities, the mean of the
distribution of metallicities in the diffusive zones has a wide (more than 2
orders of magnitude) spread around this value; (iii) if metals become more
uniformly distributed at z < 1, as assumed, at z = 0 the metallicity of the IGM
is narrowly confined within the range Z ~ 0.1 +/- 0.03 Z(solar). Finally, we
point out that our results can account for the observed metal content of the
intracluster medium.


title: Extremely Metal-Poor Stars. VII. The Most Metal-Poor Dwarf, CS 22876-032

abstract:
  We report high-resolution, high-signal-to-noise, observations of the
extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system
has a long period : P = 424.7 $\pm$ 0.6 days. It comprises two main sequence
stars having effective temperatures 6300 K and 5600 K, with a ratio of
secondary to primary mass of 0.89 $\pm$ 0.04. The metallicity of the system is
[Fe/H] = -3.71 $\pm$ 0.11 $\pm$ 0.12 (random and systematic errors) -- somewhat
higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of
less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have
significantly lower values, ~ 0.0-0.1, suggesting that the heavier elements
might have been underproduced relative to Mg in the material from which this
object formed. In the context of the hypothesis that the abundance patterns of
extremely metal-poor stars are driven by individual enrichment events and the
models of Woosley and Weaver (1995), the data for CS 22876-032 are consistent
with its having been enriched by a zero-metallicity supernova of mass 30
M$_{\odot}$. As the most metal-poor near-main-sequence-turnoff star currently
known, the primary of the system has the potential to strongly constrain the
primordial lithium abundance. We find A(Li) (= log(N(Li)/N(H)) + 12.00) = 2.03
$\pm$ 0.07, which is consistent with the finding of Ryan et al. (1999) that for
stars of extremely low metallicity A(Li) is a function of [Fe/H].


title: Photon frequency conversion induced by gravitational radiation

abstract:
  We consider propagation of gravitational radiation in a magnetized
multicomponent plasma. It is shown that large density perturbations can be
generated, even for small deviations from flat space, provided the cyclotron
frequency is much larger than the plasma frequency. Furthermore, the induced
density gradients can generate frequency conversion of electromagnetic
radiation, which may give rise to indirect observational effect of the
gravitational waves.


title: Is a Simple Collisionless Relic Dark Matter Particle Ruled Out?

abstract:
  The central densities of dark matter (DM) halos are much lower than predicted
in cold DM models of structure formation. Confirmation that they have cores
with a finite central density would allow us to rule out many popular types of
collisionless particle as candidates for DM. Any model that leads to cusped
halos (such as cold DM) is already facing serious difficulties on small scales
and hot DM models have been excluded. Here I show that fermionic warm DM is
inconsistent with the wide range of phase space densities in the DM halos of
well-observed nearby galaxies.


title: Galactic Gamma-Ray Background Radiation from Supernova Remnants

abstract:
  The contribution of the Source Cosmic Rays (SCRs), confined in Supernova
Remnants, to the diffuse high energy \gr emission above 1 GeV from the Galactic
disk is studied. \grs produced by the SCRs have a much harder spectrum compared
with those generated by the Galactic Cosmic Rays which occupy a much larger
residence volume uniformly. SCRs contribute less than 10% at GeV energies and
become dominant at \gr energies above 100 GeV. The contributions from
$\pi^0$-decay and Inverse Compton \grs have comparable magnitude and spectral
shape, whereas the Bremsstrahlung component is negligible. At TeV energies the
contribution from SCRs increases the expected diffuse \gr flux almost by an
order of magnitude. It is shown that for the inner Galaxy the discrepancy
between the observed diffuse intensity and previous model predictions at
energies above a few GeV can be attributed to the SCR contribution.


title:
TeV Gamma Rays Expected from Supernova Remnants in Different Uniform
  Interstellar Media

abstract:
  Calculations of the expected TeV $\gamma$-ray emission, produced by
accelerated cosmic rays (CRs) in nuclear collisions, from supernova remnants
evolving in a uniform interstellar medium (ISM) are presented. The aim is to
study the sensitivity of $\gamma$-ray production to a physical parameter set.
Apart from its general proportionality to N_H, it is shown that the
$\gamma$-ray production essentially depends upon the ratio of the CR diffusion
coefficient $\kappa$ to a critical value $\kappa_{crit}=10(B_0/5
\mu{G})(N_H/0.3 {cm}^{-3})^{-1/3}\kappa_B$, where B_0 and N_H are the magnetic
field and hydrogen number density of the ISM, and $\kappa_B$ denotes the Bohm
diffusion coefficient. If $\kappa$ is of the same order or lower than
$\kappa_{crit}$, then the peak TeV $\gamma$-ray flux in the Sedov evolutionary
phase, normalized to a distance of 1 kpc, is about 10^{-10}(N_H/0.3 {cm}^{-3})
photons cm^{-2} s^{-1}. For a CR diffusion coefficient that is significantly
larger than $\kappa_{crit}$, the CR cutoff energy is less than 10 TeV and the
expected $\gamma$-ray flux at 1 TeV is considerably below the presently
detectable level of 10^{-12} photons cm^{-2} s^{-1}. The same is of course true
for a SNR in the rarified, so-called hot ISM.


title:
Constraints on the Cosmic Infra-Red Background based on BeppoSAX and CAT
  spectra of Mkn 501

abstract:
  The TeV and X-ray data obtained by the imaging Cherenkov telescope CAT and
X-ray satellite BeppoSAX during the remarkable flare of Mkn 501 in April 16,
1997 are used to constrain the flux of the Cosmic Infrared Background (CIB)
using different CIB models. We show that a non-negligible absorption of
gamma-rays due to the CIB could take place already in the low-energy (sub-TeV)
domain of the spectrum of Mkn 501. This implies that the data of the low-energy
threshold CAT telescope contain very important information about the CIB at
short wavelengths, 0.4 mum <= lambda <= 3. mum. The analysis of almost
simultaneous spectroscopic measurements of Mkn 501 in a high state by CAT and
BeppoSAX in the framework of the standard homogeneous Synchrotron-Self-Compton
(SSC) framework model leads to the conclusion that the density of the
near-infrared background with typical ``starlight spectrum'' around 1 mum
should be between 5 and 35 nW m^-2 sr^-1 (99 % CL), with most likely value
around 20 nW m^-2 sr^-1. Also we argue that the CAT gamma-ray data alone allow
rather robust upper limits on the CIB, lambda F_lambda <= 60 nW m^-2 sr^-1 at 1
mum, taking into account that for any reasonable scenario of gamma-ray
production the differential intrinsic spectrum of gamma-ray hardly could be
flatter than dN/dE == E^-1. This estimate agrees, within statistical and
systematic uncertainties, with recent reports about tentative detections of the
CIB at 2.2 and 3.5 mum by the Diffuse Infrared Background Experiment (DIRBE),
as well as with the measurements of the background radiation at optical
wavelengths from absolute photometry. We also discuss the impact of the
intergalactic absorption effect in derivation of the SSC parameters for the jet
in Mkn 501.


title: Dark matter caustics

abstract:
  The late infall of cold dark matter onto an isolated galaxy such as our own
produces flows with definite velocity vectors at any physical point in the
galactic halo. It also produces caustics which are places where the dark matter
density is very large. The outer caustics are topological spheres whereas the
inner caustics are rings. The self-similar model of galactic halo formation
predicts that the caustic ring radii $a_n$ follow the approximate law $a_n \sim
1/n$. In a recent study of 32 extended and well-measured galactic rotation
curves, we found evidence for this law.


title:
A measurement at the first acoustic peak of the CMB with the 33 GHz
  interferometer

abstract:
  This paper presents the results from the Jodrell Bank-IAC two-element 33 GHz
interferometer operated with an element separation of 32.9 wavelengths and
hence sensitive to 1 deg scale structure on the sky. The level of CMB
fluctuations, assuming a flat CMB spatial power spectrum over the range of
multipoles l = 208 +- 18, was found using a likelihood analysis to be \Delta
T_l = 63^{+7}_{-6} mu K at the 68% confidence limit, after the subtraction of
the contribution of monitored point sources. Other possible foreground
contributions have been assessed and are expected to have negligible impact on
this result.


title: Kinematics and morphology of ionized gas in Hickson Compact Group 18

abstract:
  We present new observations of $H\alpha $ emission in the Hickson Compact
Group 18 (HCG 18) obtained with a scanning Fabry-Perot interferometer. The
velocity field does not show motions of individual group members but, instead,
a complex common velocity field for the whole group. The gas distribution is
very asymmetric with clumps of maximum intensity coinciding with the optically
brightest knots. Comparing $H\alpha $ and HI data we conclude that HCG 18 is
not a compact group but instead a large irregular galaxy with several clumps of
star formation.


title:
Windows through the Dusty Disks Surrounding the Youngest Low Mass
  Protostellar Objects

abstract:
  The formation and evolution of young low mass stars are characterized by
important processes of mass loss and accretion ocurring in the innermost
regions of their placentary circumstellar disks. Because of the large
obscuration of these disks at optical and infrared wavelengths in the early
protostellar stages (Class 0 Sources), they were previously detected only at
radio wavelengths using interferometric techniques. We have detected with the
Infrared Space Observatory (ISO) the mid-infrared emission associated with the
Class 0 protostar VLA1 in the HH1-2 region located in the Orion nebula. The
emission arises in the three wavelength windows at 5.3, 6.6 and 7.5 micras
where the absorption due to ices and silicates has a local minimum that exposes
the central parts of the youngest protostellar systems to mid-infrared
investigations. The mid-infrared emission arises from a central source with 4
AU diameter at an averaged temperature of 700 K, deeply embedded in a dense
region with a visual extinction of Av=80-100mag.


title: On the Efficiency of Internal Shocks in Gamma-Ray Bursts

abstract:
  The fraction of a fireball kinetic kinetic energy that can be radiated away
by internal shocks is sensitive to the amplitude of initial fluctuations in the
fireball. We give a simple analytical description for dissipation of
modest-amplitude fluctuations and confirm it with direct numerical simulations.
At high amplitudes, the dissipation occurs in a non-linear regime with
efficiency approaching 100 %. Most of the explosion energy is then radiated
away by the prompt GRB and only a fraction remains to be radiated by the
afterglow.


title:
New Neighbors from 2MASS: Activity and Kinematics at the Bottom of the
  Main Sequence

abstract:
  We have combined 2MASS and POSS II data in a search for nearby ultracool
(later than M6.5) dwarfs with K_s<12. Spectroscopic follow-up observations
identify 53 M7 to M9.5 dwarfs and seven L dwarfs. The observed space density is
0.0045 +- 0.0008 M8-M9.5 dwarfs per cubic parsec, without accounting for
biases, consistent with a mass function that is smooth across the
stellar/substellar limit. We show the observed frequency of H alpha emission
peaks at ~100% for M7 dwarfs and then decreases for cooler dwarfs. In absolute
terms, however, as measured by the ratio of H alpha to bolometric luminosity,
none of the ultracool M dwarfs can be considered very active compared to
earlier M dwarfs, and we show that the decrease that begins at spectral type M6
continues to the latest L dwarfs. We find that flaring is common among the
coolest M dwarfs and estimate the frequency of flares at 7% or higher. We show
that the kinematics of relatively active (EW_{H alpha}>6 Angstroms) ultracool M
dwarfs are consistent with an ordinary old disk stellar population, while the
kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old
population. The early L dwarfs in the sample have kinematics consistent with
old ages, suggesting that the hydrogen burning limit is near spectral types
L2-L4. We use the available data on M and L dwarfs to show that chromospheric
activity drops with decreasing mass and temperature, and that at a given (M8 or
later) spectral type, the younger field (brown) dwarfs are less active than
many of the older, more massive field stellar dwarfs. Thus, contrary to the
well-known stellar age-activity relationship, low activity in field ultracool
dwarfs can be an indication of comparative youth and substellar mass.


title: Microlensing and Galactic Structure

abstract:
  Because we know little about the Galactic force-field away from the plane,
the Galactic mass distribution is very ill-determined. I show that a
microlensing survey of galaxies closer than 50 Mpc would enable us to map in
three dimensions the Galactic density of stellar mass, which should be strictly
less than the total mass density. A lower limit can be placed on the stellar
mass needed at R<R_0 to generate the measured optical depth towards sources in
the bulge. If the Galaxy is barred, this limit is lower by a factor of up to
two than in the axisymmetric case. Even our limited knowledge of the Galactic
force field suffices to rule out the presence of the amount of mass an
axisymmetric Galaxy needs to generate the measured optical depth. Several lines
of argument imply that the Galaxy is strongly barred only at R < 4 kpc, and if
this is the case, even barred Galaxy models cannot generate the measured
optical depth without violating some constraint on the Galactic force-field.
Galactic mass models that are based on the assumption that light traces mass,
for which there is significant support in the inner Galaxy, yield microlensing
optical depths that are smaller than the measured value by a factor of more
than 2.5.


title: ISO Spectroscopy of the Young Bipolar Nebulae S106 IR and Cep A East

abstract:
  We present the results of ISO SWS and LWS grating scans towards the embedded
Young Stellar Objects (YSOs) S106 IR and Cep A East. Emission from the pure
rotational lines of H2 and the infrared fine structure lines of [C II], [O I],
[S I], [Si II] and [Fe II], as well as absorption bands due to H2O, CO and CO2
ice were detected toward Cep A. In S106 we detected emission lines of H2, CO, H
I, and a large number of ionized species including Fe, O, N, C, Si, S, Ne and
Ar. S106 also shows many of the infrared PAH bands in emission. Excitation
temperatures and molecular hydrogen masses were derived from the low-lying pure
rotational levels of H2 and are 500 and 730 K and 8 and 3 x 10^{-3} solar
masses for S106 and Cep A, respectively. Since both objects are expected to
have several solar masses of H2 in their environment, we conclude that in both
cases the bulk of the H2 is cooler than a few hundred Kelvins. Excitation
temperatures and line ratios were compared with those predicted by theoretical
models for PDRs and dissociative and non-dissociative shocks. The [S I] 25.2
micron/[Si II] 34.8 micron ratio is a particularly useful shock versus PDR
discriminant and we conclude that S106 IR is dominated by PDR emission while
Cep A East has a large shock component. From an analysis of the ionic lines in
S106 we conclude that the central star must have a temperature around 37,000 K,
corresponding to a spectral type of O8. From its luminosity it is concluded
that the driving source of Cep A must also be a massive early-type star. The
absence of strong high-ionization ionic lines in its ISO spectrum shows that
Cep A has not yet created a significant H II region and must be younger than
S106, illustrating the process of the clearing of the surroundings of a massive
young star.


title: Spectral Variations in Early-Type Galaxies as a Function of Mass

abstract:
  We report on the strengths of three spectral indicators - Mg_2, Hbeta, and
Hn/Fe - in the integrated light of a sample of 100 field and cluster E/S0
galaxies. The measured indices are sensitive to age and/or and metallicity
variations within the galaxy sample. Using linear regression analysis for data
with non-uniform errors, we determine the intrinsic scatter present among the
spectral indices of our galaxy sample as a function of internal velocity
dispersion. Our analysis indicates that there is significantly more intrinsic
scatter in the two Balmer line indices than in the Mg_2 index, indicating that
the Balmer indices provide more dynamic range in determining the age of a
stellar population than does the Mg_2 index. Furthermore, the scatter is much
larger for the low velocity dispersion galaxies, indicating that star formation
has occurred more recently in the lower mass galaxies.


title:
ROSAT HRI and ASCA Observations of the Spiral Galaxy NGC 6946 and Its
  Northeast Complex of Luminous SNRs

abstract:
  Analysis of 80 ksec ASCA and 60 ksec ROSAT HRI observations of the face-on
spiral galaxy NGC 6946 are presented. The ASCA image is the first observation
of this galaxy above ~2 keV. Diffuse emission may be present in the inner ~4'
extending to energies above ~2-3 keV. In the HRI data, fourteen point-like
sources are detected, the brightest two being a source very close to the
nucleus and a source to the northeast that corresponds to a luminous complex of
interacting supernova remnants (SNRs). We detect a point source that lies
\~30$''$ west of the SNR complex but with a luminosity ~1/15 of it. None of the
point sources shows evidence of strong variability; weak variability would
escape our detection.
  The ASCA spectrum of the SNR complex shows evidence for an emission line at
\~0.9 keV which could be either Ne IX at ~0.915 keV or a blend of ion stages of
Fe L-shell emission if the continuum is fit with a power law. However, a two
component, Raymond-Smith thermal spectrum with no lines gives an equally valid
continuum fit and may be more physically plausible given the observed spectrum
below 3 keV. Adopting this latter model, we derive a density for the SNR
complex of 10-35 cm^{-3}, consistent with estimates inferred from optical
emission line ratios. The complex's extraordinary X-ray luminosity may be
related more to the high density of the surrounding medium than to a small but
intense interaction region where two of the complex's SNRs are apparently
colliding.


title: Broad iron lines in Active Galactic Nuclei

abstract:
  An intrinsically narrow line emitted by an accretion disk around a black hole
appears broadened and skewed as a result of the Doppler effect and
gravitational redshift. The fluorescent iron line in the X-ray band at
6.4-6.9keV is the strongest such line and is seen in the X-ray spectrum of many
active galactic nuclei and, in particular, Seyfert galaxies. It is an important
diagnostic with which to study the geometry and other properties of the
accretion flow very close to the central black hole. The broad iron line
indicates the presence of a standard thin accretion disk in those objects,
often seen at low inclination. The broad iron line has opened up strong
gravitational effects around black holes to observational study with
wide-reaching consequences for both astrophysics and physics.


title:
Theoretical Predictions for Surface Brightness Fluctuations and
  Implications for Stellar Populations of Elliptical Galaxies

abstract:
  (Abridged) We present new theoretical predictions for surface brightness
fluctuations (SBFs) using models optimized for this purpose. Our predictions
agree well with SBF data for globular clusters and elliptical galaxies. We
provide refined theoretical calibrations and k-corrections needed to use SBFs
as standard candles. We suggest that SBF distance measurements can be improved
by using a filter around 1 micron and calibrating I-band SBFs with the
integrated V-K galaxy color. We also show that current SBF data provide useful
constraints on population synthesis models, and we suggest SBF-based tests for
future models. The data favor specific choices of evolutionary tracks and
spectra in the models among the several choices allowed by comparisons based on
only integrated light. In addition, the tightness of the empirical I-band SBF
calibration suggests that model uncertainties in post-main sequence lifetimes
are less than +/-50% and that the IMF in ellipticals is not much steeper than
that in the solar neighborhood. Finally, we analyze the potential of SBFs for
probing unresolved stellar populations. We find that optical/near-IR SBFs are
much more sensitive to metallicity than to age. Therefore, SBF magnitudes and
colors are a valuable tool to break the age/metallicity degeneracy. Our initial
results suggest that the most luminous stellar populations of bright cluster
galaxies have roughly solar metallicities and about a factor of three spread in
age.