title: The modified dynamics is conducive to galactic warp formation

abstract:
  There is an effect in the modified dynamics (MOND) that is conducive to
formation of warps. Because of the nonlinearity of the theory the internal
dynamics of a galaxy is affected by a perturber over and above possible tidal
effects. For example, a relatively distant and light companion or the mean
influence of a parent cluster, with negligible tidal effects, could still
produce a significant warp in the outer part of a galactic disk. We present
results of numerical calculations for simplified models that show, for
instance, that a satellite with the (baryonic) mass and distance of the
Magellanic clouds can distort the axisymmetric field of the Milky Way enough to
produce a warp of the magnitude (and position) observed. Details of the warp
geometry remain to be explained: we use a static configuration that can produce
only warps with a straight line of nodes. In more realistic simulations one
must reckon with the motion of the perturbing body, which sometimes occurs on
time scales not much longer than the response time of the disk.


title:
Comparing the Evolution of the Galaxy Disk Sizes with CDM Models: The
  Hubble Deep Field

abstract:
  The intrinsic sizes of the field galaxies with I<26 in the Hubble and ESO-NTT
Deep Fields are shown as a function of their redshifts and absolute magnitudes
using photometric redshifts derived from the multicolor catalogs and are
compared with the CDM predictions. Extending to lower luminosities and to
higher z our previous analysis performed on the NTT field alone, we find that
the distribution of the galaxy disk sizes at different cosmic epochs is within
the range predicted by typical CDM models. However, the observed size
distribution of faint (M_B>-19) galaxies is skewed with respect to the CDM
predictions and an excess of small-size disks (R_d<2 kpc) is already present at
z~ 0.5. The excess persists up to z~3 and involves brighter galaxies . Such an
excess may be reduced if luminosity-dependent effects, like starburst activity
in interacting galaxies, are included in the physical mechanisms governing the
star formation history in CDM models.


title:
chi sup 2_ testing of optimal filters for gravitational wave signals: An
  experimental implementation

abstract:
  We have implemented likelihood testing of the performance of an optimal
filter within the online analysis of AURIGA, a sub-Kelvin resonant-bar
gravitational wave detector. We demonstrate the effectiveness of this technique
in discriminating between impulsive mechanical excitations of the resonant-bar
and other spurious excitations. This technique also ensures the accuracy of the
estimated parameters such as the signal-to-noise ratio. The efficiency of the
technique to deal with non-stationary noise and its application to data from a
network of detectors are also discussed.


title: Looking for GRB progenitors

abstract:
  Using stellar binary population synthesis code we calculate the production
rates and lifetimes of several types of possible GRB progenitors. We consider
mergers of double neutron stars, black hole neutron stars, black hole white
dwarfs and helium star mergers. We calibrate the results with the measured star
formation rate history. We discuss the viability of each GRB model, and
alternatively assuming that all bursts are connected with one model we
constrain the required collimation of GRBs. We also show the importance of
widely used evolutionary parameters on the merger rates of calculated binary
populations.


title: Distribution of binary mergers around galaxies

abstract:
  We use a stellar binary population synthesis code to find the lifetimes and
velocities of several types of possible GRB progenitors: double neutron stars,
black hole neutron stars, black hole white dwarfs, helium star mergers.
Assuming that they are born in different types of galaxies we compute their
spatial distribution and compare it with the observed locations of GRB
afterglows within their hosts. We discuss constraints on the compact object
merger model of GRBs imposed by this comparison and find that the observations
of afterglows and their host galaxies appear inconsistent with the GRB compact
object merger model.


title: The Lake Baikal neutrino experiment: selected results

abstract:
  We review the present status of the lake Baikal Neutrino Experiment and
present selected physical results gained with the consequetive stages of the
stepwise increasing detector: from NT-36 to NT-96. Results cover atmospheric
muons, neutrino events, very high energy neutrinos, search for neutrino events
from WIMP annihilation, search for magnetic monopoles and environmental
studies. We also describe an air Cherenkov array developed for the study of
angular resolution of NT-200.


title: W UMa-type Binary Stars in Globular Clusters

abstract:
  A sample of 86 contact binary systems in 14 globular clusters with available
color index data in (B-V) or in (V-I) has been analyzed. At least one third of
all systems are numerous foreground Galactic Disk projections over long lines
of sight to the clusters. Since the selection of the cluster members has been
based on the MV(logP, color) calibrations, the matter of a
metallicity-correction required particular attention with the result that such
a correction is apparently not needed at the present level of accuracy.
Analysis of the color-magnitude and period-color relations shows that globular
cluster members are under-luminous relative to the Galactic Disk contact
systems mainly because of the smaller sizes and, consequently, shorter orbital
periods; the color-index effect of the diminished blanketing is less important,
especially for (V-I). The variability amplitudes for the Blue Straggler systems
show a significantly different distribution from that for systems below the
Turn Off Point (TOP): The BS systems show only small amplitudes while the
distribution for the systems below the TOP is peculiar in containing only large
amplitude systems. This difference is linked to an observational selection
effect so that efforts at determining the frequency of occurrence of the
contact systems below the TOP have been judged to be premature; the frequency
among the BS stars could be at about 45+-10 BS stars per one contact BS binary.


title:
Resolving the jeans mass in hydrodynamic simulations of hierarchical
  clustering

abstract:
  We examine the necessity of resolving the Jeans mass in hydrodynamical
simulations of the hierarchical formation of cosmological structures. We
consider a standard two-component fluid and use smoothed particle hydrodynamics
to model the baryonic component. It is found that resolution of the Jeans mass
is not necessary to extract bulk properties of bound objects that are
independent of resolution, provided that the objects have undergone a number of
merger events. The degree of merging must be sufficient to populate the objects
with substructure of similar total mass. After this criterion is met, neither
the density profiles of structures nor their density-temperature distributions
depend appreciably on the resolution of the simulation. Resolution of the Jeans
mass by the imposition of a minimum temperature is not found to adversely
affect the state of the matter in the final clusters. The baryon concentration
profiles still indicate a deficit of gas with respect to the dark matter in the
interiors of structures.


title: The Stellar Content of the Halo of NGC 5907 from Deep HST NICMOS Imaging

abstract:
  We present H-band images obtained with NICMOS of a field 75'' (5kpc) above
the plane of the disk of the edge-on spiral galaxy NGC 5907. Ground-based
observations have shown that NGC 5907 has a luminous halo with a shallow radial
profile between 4 and 8 kpc that roughly traces the dark matter distribution of
the galaxy deduced from its rotation curve. Our NICMOS observations were
designed to resolve bright giants in the halo of NGC 5907 to constrain its
stellar composition with the goal of understanding its nature and origin. More
than 100 stars are expected in the NICMOS images if the dwarf-to-giant ratio in
the halo of NGC 5907 is consistent with that expected from standard stellar
initial mass functions, and if ground-based estimates of the distance to NGC
5907 and the integrated colors of its halo are correct. Instead we observe only
one candidate giant star. This apparent discrepancy can be resolved by assuming
either a significantly larger distance than suggested by several studies, or a
halo metallicity much lower than suggested by ground-based colors and as low as
metal-poor Galactic globular clusters. If previous distance and halo color
estimates for NGC 5907 are correct, our NICMOS results suggest that its
extended light is composed of stars that formed with an initial mass function
different than that observed locally, leading to a much higher ratio of dwarfs
to giants. We describe how these three possible explanations for the absence of
bright giants in our NICMOS images of the halo of NGC 5907 might be constrained
by future observations.


title: ISO SWS-LWS observations of the prototypical reflection nebula NGC 7023

abstract:
  We present SWS and LWS ISO observations towards a strip across the
photodissociation region (PDR) in the reflection nebula NGC 7023. SWS02 and
LWS01 spectra have been taken towards the star and the brightest infrared
filaments located NW and SW from the star (hereafter NW and SW PDRs). In
addition, SWS02 spectra have been taken towards two intermediate positions.
  While all the PDR tracers (the [CII] 157.7$\mic$, [OI] 63.2 and 145.6$\mic$,
[HI] 21cm and the H$_2$ rotational lines) present a ring-like morphology with
the peaks toward the NW and SW PDRs and a minimum around the star, the SiII
emission is filling the hole of this ring with the peak towards the star. This
morphology can only be explained if the SiII emission arise in the lowest
extinction layers of the PDR (A$_v$ $<$ 2 mag) and the HII region. At least 20%
-- 30% of the Si must be in gas phase in these layers. For A$_v$ $\geq$ 2 mag,
the Si is mainly in solid form ($\delta$ Si = -1.3).
  In both, the NW and SW PDRs, the intensities of the H$_2$ rotational lines
can only be fitted by assuming an ortho-to-para-H$_2$ ratio lower than 3 in gas
with rotation temperatures from 400 to 700 K. Therefore, there is a
non-equilibrium OTP ratio in the region. The comparison between the OTP ratio
derived from H$_2$ vibrational lines and the pure H$_2$ rotational lines, shows
that the OTP ratio increases from $\sim$ 1.5 to 3 across the photodissociation
region with larger values in the less shielded gas (A$_v$$<$ 0.7 mag). This
behavior is interpreted as a consequence of an advancing photodissociation
front.


title:
Discovery of 9 Ly alpha emitters at redshift z~3.1 using narrow-band
  imaging and VLT spectroscopy

abstract:
  Narrow-band imaging surveys aimed at detecting the faint emission from the
5007 [O III] line of intracluster planetary nebulae in Virgo also probe high
redshift z=3.1 Ly alpha emitters. Here we report on the spectroscopic
identification of 9 Ly alpha emitters at z=3.13, obtained with the FORS
spectrograph at Unit 1 of the ESO Very Large Telescope (VLT UT1). The spectra
of these high redshift objects show a narrow, isolated Ly alpha emission with
very faint (frequently undetected) continuum, indicating a large equivalent
width. No other features are visible in our spectra. Our Ly alpha emitters are
quite similar to those found by Hu (1998), Cowie & Hu (1998) and Hu et al.
(1998). Using simple population synthesis models, on the assumption that these
sources are regions of star formation, we conclude that the nebulae are nearly
optically thick and must have a very low dust content, in order to explain the
high observed Ly alpha equivalent widths. For the cosmological and star
formation parameters we adopted, the total stellar mass produced would seem to
correspond to the formation of rather small galaxies, some of which are perhaps
destined to merge. The implied star formation density in our sampled comoving
volume is probably somewhat smaller than, but of the same order of magnitude as
the star formation density at z=3 derived by other authors from Lyman-break
galaxy surveys. This result agrees with the expectation that the Ly alpha
emitters are a low-metallicity (or low-dust) tail in a distribution of star
forming regions at high redshifts. Finally, the Ly alpha emitters may
contribute as many H-ionizing photons as QSOs at z=3.


title: The Stellar Content of Obscured Galactic Giant H II Regions: II. W42

abstract:
  We present near infrared J, H, and K images and K-band spectroscopy in the
giant HII region W42. A massive star cluster is revealed; the color-color plot
and K-band spectroscopic morphology of two of the brighter objects suggest the
presence of young stellar objects. The spectrum of the bright central star is
similar to unobscured stars with MK spectral types of O5-O6.5. If this star is
on the zero age main sequence, then the derived spectrophotometric distance is
considerably smaller than previous estimates. The Lyman continuum luminosity of
the cluster is a few times that of the Trapezium. The slope of the K-band
luminosity function is similar to that for the Trapezium cluster and
significantly steeper than that for the massive star cluster in M17 or the
Arches cluster near the Galactic center.


title: The Off State of GX 339-4

abstract:
  We report BeppoSAX and optical observations of the black hole candidate GX
339-4 during its X-ray `off' state in 1999. The broad-band (0.8-50 keV) X-ray
emission can be fitted by a single power law with spectral index, \alpha ~1.6.
The observed luminosity is 6.6e33 erg s^{-1} in the 0.5-10 keV band, which is
at the higher end of the flux distribution of black hole soft X-ray transients
in quiescence, comparable to that seen in GS 2023+338 and 4U 1630-47. An
optical observation just before the BeppoSAX observation shows the source to be
very faint at these wavelengths as well (B=20.1, V=19.2). By comparing with
previously reported `off' and low states (LS), we conclude that the `off' state
is actually an extension of the LS, i.e. a LS at lower intensities. We propose
that accretion models such as the advection-dominated accretion flows are able
to explain the observed properties in such a state.


title: Probing the width of the MACHO mass function

abstract:
  The simplest interpretation of the microlensing events observed towards the
Large Magellanic Cloud is that approximately half of the mass of the Milky Way
halo is in the form of MAssive Compact Halo Objects with $M \sim 0.5
M_{\odot}$. This poses severe problems for stellar MACHO candidates, and leads
to the consideration of more exotic objects such as primordial black holes
(PBHs). Constraining the MACHO mass function will shed light on their nature.
Using the current data we find, for four halo models, the best fit
delta-function, power law and PBH mass functions. The best fit PBH mass
functions, despite having significant finite width, have likelihoods which are
similar to, and for one particular halo model greater than, those of the best
fit delta functions. We also find that if the correct halo model is known then
$\sim$ 500 events will be sufficient to determine whether the MACHO mass
function has significant width, and will also allow determination of the mass
function parameters to $\sim 5%$.


title: The Synchrotron Spectrum of Fast Cooling Electrons Revisited

abstract:
  We discuss the spectrum arising from synchrotron emission by fast cooling
(FC) electrons, when fresh electrons are continually accelerated by a strong
blast wave, into a power law distribution of energies. The FC spectrum was so
far described by four power law segments divided by three break frequencies
$\nu_{sa} < \nu_{c} < \nu_{m}$. This is valid for a homogeneous electron
distribution. However, hot electrons are located right after the shock, while
most electrons are farther down stream and have cooled. This spatial
distribution changes the optically thick part of the spectrum, introducing a
new break frequency, $\nu_{ac} < \nu_{sa}$, and a new spectral slope, $F_{\nu
}\propto \nu^{11/8}$ for $\nu_{ac} < \nu < \nu_{sa}$. The familiar $F_{\nu
}\propto \nu ^{2}$ holds only for $\nu < \nu_{ac}$. This ordering of the break
frequencies is relevant for typical gamma-ray burst (GRB) afterglows in an ISM
environment. Other possibilities arise for internal shocks or afterglows in
dense circumstellar winds. We discuss possible implications of this spectrum
for GRBs and their afterglows, in the context of the internal-external shock
model. Observations of $F_{\nu} \propto \nu^{11/8}$ would enable us to probe
scales much smaller than the typical size of the system, and constrain the
amount of turbulent mixing behind the shock.


title: Hubble Space Telescope Imaging of the Young Planetary Nebula GL 618

abstract:
  We present narrow-band Hubble Space Telescope images of the young planetary
nebula GL 618. These images have allowed us to study the detailed morphology of
shock-excited emission present in the bipolar lobes of this object. These
images reveal the presence of three highly collimated outflows emanating from
the central regions of GL 618. We discuss the significance of the detection of
these outflows and the possible origins of these features.


title: X-ray total mass estimate for the nearby relaxed cluster A3571

abstract:
  We constrain the total mass distribution in the cluster A3571, combining
spatially resolved ASCA temperature data with ROSAT imaging data with the
assumption that the cluster is in hydrostatic equilibrium. The total mass
within r_500 (1.7/h_50 Mpc) is M_500 = 7.8[+1.4,-2.2] 10^14/ h_50 Msun at 90%
confidence, 1.1 times smaller than the isothermal estimate. The Navarro, Frenk
& White ``universal profile'' is a good description of the dark matter density
distribution in A3571. The gas density profile is shallower than the dark
matter profile, scaling as r^{-2.1} at large radii, leading to a monotonically
increasing gas mass fraction with radius. Within r_500 the gas mass fraction
reaches a value of f_gas = 0.19[+0.06,-0.03] h_50^{-3/2} (90% confidence
errors). Assuming that this value of f_gas is a lower limit for the the
universal value of the baryon fraction, we estimate the 90% confidence upper
limit of the cosmological matter density to be Omega_m < 0.4.


title:
Correlated X-ray Spectral and Timing Behavior of the Black Hole
  Candidate XTE J1550-564: A New Interpretation of Black Hole States

abstract:
  We present an analysis of RXTE data of the X-ray transient XTE J1550-564. The
source went through several states, which were divided into spectrally soft and
hard states. These states showed up as distinct branches in the color-color
diagram, forming a structure with a comb-like topology; the soft state branch
forming the spine and the hard state branches forming the teeth. Variability
was strongly correlated with the position on the branches. The broad band noise
became stronger, and changed from power law like to band limited, as the
spectrum became harder. Three types of QPOs were found: 1-18 Hz and 102-284 Hz
QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The
frequencies of the high and low frequency QPOs on the hard branches were
correlated with each other, and anti-correlated with spectral hardness. The
changes in QPO frequency suggest that the inner disc radius only increases by a
factor of 3-4 as the source changes from a soft to a hard state. Our results on
XTE J1550-564 strongly favor a 2-dimensional description of black hole
behavior, where the regions near the spine of the comb in the color-color
diagram can be identified with the high state, and the teeth with transitions
from the high state, via the intermediate state (which includes the very high
state) to the low state, and back. The two physical parameters underlying this
behavior vary to a large extent independently and could for example be the mass
accretion rate through the disk and the size of a Comptonizing region.


title:
The Effect of Resistivity on the Nonlinear Stage of the
  Magnetorotational Instability in Accretion Disks

abstract:
  We present three-dimensional magnetohydrodynamic simulations of the nonlinear
evolution of the magnetorotational instability (MRI) with a non-zero Ohmic
resistivity. The properties of the saturated state depend on the initial
magnetic field configuration. In simulations with an initial uniform vertical
field, the MRI is able to support angular momentum transport even for large
resistivities through the quasi-periodic generation of axisymmetric radial
channel solutions rather than through the maintenance of anisotropic
turbulence. Simulations with zero net flux show that the angular momentum
transport and the amplitude of magnetic energy after saturation are
significantly reduced by finite resistivity, even at levels where the linear
modes are only slightly affected. This occurs at magnetic Reynolds numbers
expected in low, cool states of dwarf novae, these results suggest that finite
resistivity may account for the low and high angular momentum transport rates
inferred for these systems.


title:
Three-Layered Atmospheric Structure in Accretion Disks Around
  Stellar-Mass Black Holes

abstract:
  Modeling of the x-ray spectra of the Galactic superluminal jet sources GRS
1915+105 and GRO J1655-40 reveal a three-layered atmospheric structure in the
inner region of their accretion disks. Above the cold and optically thick disk
of a temperature 0.2-0.5 keV, there is a warm layer with a temperature of
1.0-1.5 keV and an optical depth around 10. Sometimes there is also a much
hotter, optically thin corona above the warm layer, with a temperature of 100
keV or higher and an optical depth around unity. The structural similarity
between the accretion disks and the solar atmosphere suggest that similar
physical processes may be operating in these different systems.


title: A Definitive Optical Detection of a Supercluster at z = 0.91

abstract:
  We present the results from a multi-band optical imaging program which has
definitively confirmed the existence of a supercluster at z = 0.91. Two massive
clusters of galaxies, CL1604+4304 at z = 0.897 and CL1604+4321 at z = 0.924,
were originally observed in the high-redshift cluster survey of Oke, Postman &
Lubin (1998). They are separated by 4300 km/s in radial velocity and 17
arcminutes on the plane of the sky. Their physical and redshift proximity
suggested a promising supercluster candidate. Deep BRi imaging of the region
between the two clusters indicates a large population of red galaxies. This
population forms a tight, red sequence in the color--magnitude diagram at (R-i)
= 1.4. The characteristic color is identical to that of the
spectroscopically-confirmed early-type galaxies in the two member clusters. The
red galaxies are spread throughout the 5 Mpc region between CL1604+4304 and
CL1604+4321. Their spatial distribution delineates the entire large scale
structure with high concentrations at the cluster centers. In addition, we
detect a significant overdensity of red galaxies directly between CL1604+4304
and CL1604+4321 which is the signature of a third, rich cluster associated with
this system. The strong sequence of red galaxies and their spatial distribution
clearly indicate that we have discovered a supercluster at z = 0.91.


title: Millisecond Oscillations in X-Ray Binaries

abstract:
  The first millisecond X-ray variability phenomena from accreting compact
objects have recently been discovered with the Rossi X-ray Timing Explorer.
Three new phenomena are observed from low-mass X-ray binaries containing
low-magnetic-field neutron stars: millisecond pulsations, burst oscillations
and kiloHertz quasi-periodic oscillations. Models for these new phenomena
involve the neutron star spin, and orbital motion closely around the neutron
star and rely explicitly on our understanding of strong gravity and dense
matter. I review the observations of these new neutron-star phenomena and
possibly related ones in black-hole candidates, and describe the attempts to
use them to perform measurements of fundamental physical interest in these
systems.


title:
Clues to the origin of parsec to kilo-parsec jet misalignments in EGRET
  sources

abstract:
  The apparent position angle difference $\Delta$PA between parsec and
kilo-parsec jets in blazars can be related to the bending properties of the
jets. We present correlations between the misalignment $\Delta$PA and the ratio
of radio to broad-line emission for a sample of $\gamma$-ray blazars. The
present study is limited to EGRET sources due to uniform data for the radio and
optical properties of these sources being easily available. The broad-line
emission is known to be a good indicator of the accretion power for both steep
and flat-spectrum quasars. We find significant correlations between $\Delta$PA
and the ratio of radio flux (total and radio core flux respectively) to
broad-line flux, which is consistent with the beaming scenario. A weak
correlation between $\Delta$PA and the ratio of extended radio to broad-line
flux might imply that the intrinsic bend of the jet is related with the ratio
of jet mechanical power to accretion power.


title: Formation of Primordial Galaxies under UV background Radiation

abstract:
  The pancake collapse of pregalactic clouds under UV background radiation is
explored with a one-dimensional sheet model. Here, attention is concentrated on
elucidating the basic physics on the thermal evolution of pregalactic clouds
exposed to diffuse UV radiation. So, we treat accurately the radiation transfer
for the ionizing photons, with solving chemical reactions regarding hydrogen
molecules as well as atoms. The self-shielding against UV radiation by H$_2$
Lyman-Werner bands, which regulates the photo-dissociation of hydrogen
molecules, is also taken into account. As a result, it is found that when the
UV background radiation is at a level of $10^{-22} (\nu/\nu_L)^{-1} {\rm erg~
s^{-1} cm^{-2} Hz^{-1} str^{-1}}$, the cloud evolution bifurcates with a
critical mass as $M_{\rm SB} = 2.2\times 10^{11} M_\odot [(1+z_c)/5]^{-4.2}$,
where $z_c$ is the final collapse epoch. A cloud more massive than $M_{\rm SB}$
cools below $5\times 10^3$K due to H$_2$ line emission at the pancake collapse
and would undergo the initial starburst. The pancake possibly evolves into a
virialized system in a dissipationless fashion. Consequently, this leads to the
dissipationless galaxy formation at $3\lsim z_c \lsim 10$. A cloud less massive
than $M_{\rm SB}$ cannot cool by H$_2$ emission shortly after the pancake
collapse, but could cool in the course of shrinking to the rotation barrier.
This is likely to lead to the dissipational galaxy formation at relatively low
redshifts as $0\lsim z_c \lsim 4$. The present results provide a solid physical
mechanism which controls the star formation efficiency in the pregalactic
clouds. In the context of a standard CDM cosmology, $M_{\rm SB}$ lies between
1$\sigma$ and 2$\sigma$ density fluctuations.


title: Synthesis of 19F in Wolf-Rayet stars

abstract:
  Meynet and Arnould (1993) have suggested that Wolf-Rayet (WR) stars could
significantly contaminate the Galaxy with 19F. In their scenario, 19F is
synthesized at the beginning of the He-burning phase from the 14N left over by
the previous CNO-burning core, and is ejected in the interstellar medium when
the star enters its WC phase. Recourse to CNO seeds makes the 19F yields
metallicity-dependent. These yields are calculated on grounds of detailed
stellar evolutionary sequences for an extended range of initial masses (from 25
to 120 Msol) and metallicities (Z = 0.008, 0.02 and 0.04). The adopted mass
loss rate prescription enables to account for the observed variations of WR
populations in different environments. The 19F abundance in the WR winds of 60
Msol model stars is found to be about 10 to 70 times higher than its initial
value, depending on the metallicity. This prediction is used in conjunction
with a very simple model for the chemical evolution of the Galaxy to predict
that WR stars could be significant (dominant?) contributors to the solar system
fluorine content. We also briefly discuss the implications of our model on the
possible detection of fluorine at high redshift.


title: 2 micron Spectroscopy within 0.3 arcseconds of SgrA*

abstract:
  We present moderate (R~$\approx$~2,700) and high resolution
(R~$\approx$~22,000) 2.0$-$2.4 \micron\ spectroscopy of the central 0.1 square
arcseconds of the Galaxy obtained with NIRSPEC, the facility near-infrared
spectrometer for the Keck II telescope. The composite spectra do not have any
features attributable to the brightest stars in the central cluster, i.e.\
after background subtraction, W$_{\rm ^{12}CO(2-0)}$~$<$~2~\AA. This stringent
limit leads us to conclude that the majority, if not all, of the stars are
hotter than typical red giants. Coupled with previously reported photometry, we
conclude that the sources are likely OB main sequence stars. In addition, the
continuum slope in the composite spectrum is bluer than that of a red giant and
is similar to that of the nearby hot star, IRS16NW. It is unlikely that they
are late-type giants stripped of their outer envelopes because such sources
would be much fainter than those observed. Given their inferred youth
($\tau_{\rm age}$~$<$~20~\Myr), we suggest the possibility that the stars have
formed within 0.1 pc of the supermassive black hole. We find a newly-identified
broad-line component (V$_{\rm FWHM}$ $\approx$ 1,000 \kms) to the 2.2178
\micron\ [\ion{Fe}{3}] line located within a few arcseconds of Sgr~A$^*$. A
similar component is not seen in the Br-$\gamma$ emission.


title:
The discovery of photospheric nickel in the hot DO white dwarf
  REJ0503-289

abstract:
  We present the first evidence for the direct detection of nickel in the
photosphere of the hot DO white dwarf REJ0503$-$289. While this element has
been seen previously in the atmospheres of hot H-rich white dwarfs, this is one
of the first similar discoveries in a He-rich object. Intriguingly, iron, which
is observed to be more abundant than Ni in the hot DA stars, is not detected,
the upper limit to its abundance (Fe/He$=10^{-6}$) implying a Fe/Ni ratio a
factor 10 lower than seen in the H-rich objects (Ni/He$=10^{-5}$ for
REJ0503$-$289). The abundance of nickel and various other elements heavier than
He were determined from GHRS spectra. We used two completely independent sets
of NLTE model atmospheres which both provide the same results. This not only
reduces the possibility of systematic errors in our analysis but is also an
important consistency check for both model atmosphere codes.
  We have also developed a more objective method of determining $T_{\rm eff}$
and log g, from the He lines in the optical spectrum, in the form of a formal
fitting of the line profiles to a grid of model spectra, an analogue of the
standard procedure utilising the Balmer lines in DA white dwarfs. This gives
the assigned uncertainties in $T_{\rm eff}$ and log g a firm statistical basis
and allows us to demonstrate that inclusion of elements heavier than H, He and
C in the spectral calculations, exclusively considered in most published
optical analyses, yields a systematic downward shift in the measured value of
$T_{\rm eff}$.


title:
Large-scale shocks in the starburst galaxy NGC253. Interferometer
  mapping of a 600 pc SiO/H13CO+ circumnuclear disk

abstract:
  This paper presents the first high-resolution SiO map made in an external
galaxy. The nucleus of the nearby barred spiral NGC253 has been observed
simultaneously in the v=0, J=2-1 line of SiO and in the J=1-0 line of H13CO+
with the IRAM interferometer, with a resolution of 7.5"x2.6". Emission from SiO
and H13CO+ is extended in the nucleus of NGC253. The bulk of the SiO/H13CO+
emission arises from a 600pcx250pc circumnuclear disk (CND) with a double
ringed structure. The inner ring, of radius r=60pc, viewed edge-on along
PA=51deg, hosts the nuclear starburst; the outer pseudo-ring opens out as a
spiral-like arc up to r=300pc. The kinematics of the gaseous disk,
characterized by strong non-circular motions, is interpreted in terms of the
resonant response of the gas to the barred potential. The inner ring would
correspond to the inner Inner Lindblad Resonance(iILR), whereas the outer
region is linked to the onset of a trailing spiral wave across the outer ILR
(oILR). Most notably, we report the detection of a molecular gas counterpart of
the giant outflow of hot gas, previously seen in X-ray and optical lines, and
tentatively identified as a dust chimney in the the 450mu continuum band.The
SiO shows a high average fractional abundance in the CND of X(SiO)=1.5x10E-10.
This is more than an order of magnitude above the predicted value of a PDR.
Moreover, X(SiO) varies at least by an order of magnitude between the inner
starburst region, which dominates the global emission, where we derive
X(SiO)=1--2x10E-10, and the outer region, where X(SiO) reaches a few 10E-9. SiO
abundance is also significantly enhanced in the outflow (X(SiO)=3--5x10E-10).
Different mechanisms are explored to explain the unlike chemical processing of
molecular gas within the nucleus.


title: Origin of Color Gradients in Elliptical Galaxies

abstract:
  The origin of color gradients in elliptical galaxies is examined by comparing
model gradients with those observed in the Hubble Deep Field. The models are
constructed so as to reproduce color gradients in local elliptical galaxies
either by a metallicity gradient or by an age gradient. By looking back a
sequence of color gradient as a function of redshift, the age - metallicity
degeneracy is solved. The observed color gradients in elliptical galaxies at
$z=0.1-1.0$ agree excellently with those predicted by the metallicity gradient,
while they deviates significantly from those predicted by the age gradient even
at $z \sim 0.3$ and the deviation is getting larger with increasing redshift.
This result does not depend on cosmological parameters and parameters for an
evolutionary model of galaxy within a reasonable range. Thus our results
clearly indicate that the origin of color gradients is not the age but the
stellar metallicity.


title: Ammonia observations of the nearby molecular cloud MBM 12

abstract:
  We present NH3(1,1) and NH3(2,2) observations of MBM 12, the closest known
molecular cloud (65 pc distance), aimed to find evidence for on-going star
formation processes. No local temperature (with a T_rot upper limit of 12 K)
nor linewidth enhancement is found, which suggests that the area of the cloud
we mapped (~ 15' size) is not currently forming stars. Therefore, this close
``starless'' molecular gas region is an ideal laboratory to study the physical
conditions preceding new star formation.
  A radio continuum source was found in Very Large Array archive data, close
but outside the NH3 emission. This source is likely to be a background object.


title: X-rays and Cosmology

abstract:
  The role of X-ray observations for cosmological studies and conclusions is
briefly explored. X-rays currently yield cosmologically interesting results on
the abundances, evolution and gas content of clusters of galaxies, on the
clustering and evolution of active galaxies and on the X-ray Background. They
are unlikely in the long term future to give the most precise values of the
cosmological parameters, although in the short term the baryon fraction of
clusters and the Sunyaev-Zeldovich effect will remain important determinants
and checks for some parameters. X-rays will however continue to play an
important role in studying the astrophysics of the formation and growth of
black holes, galaxies, groups and clusters. It is possible that this role will
be crucial, if winds from active galaxies are responsible for breaking the
simple gravitational scaling laws for clusters.


title:
Observational Limits on Terrestrial-Sized Inner Planets Around the CM
  Draconis System Using the Photometric Transit Method with a Matched-Filter
  Algorithm

abstract:
  A lightcurve of the eclipsing binary CM Draconis has been analyzed for the
presence of transits of planets of size >= 2.5 Earth-radii (Re), with periods
of 60 days or less, and in co-planar orbits around the binary system. About 400
million model lightcurves, representing transits from planets with periods
ranging from 7 to 60 days, have been matched/correlated against these data.
This process we call the "transit detection algorithm" or TDA. The resulting
`transit-statistics' for each planet candidate allow the quantification of
detection probabilities, and of false alarm rates.
  Our current lightcurve of CM Dra has a coverage of 1014 hours with 26,043
individual points, at a photometric precision between 0.2% and 0.7%. Planets
significantly larger then 3Re would constitute a `supra-noise' detection, and
for periods of 60 days or less, they would have been detected with a
probability of 90%. `Subnoise' detections of smaller planets are more
constrained. For example, 2.5 Re planets with 10-day periods or less would have
been detected with an 80% probability. The necessity for predicted observations
is illustrated with the nine top planet candidates that emerged from our TDA
analysis. They are the planet candidates with the highest transit-statistics
from the 1994-1998 observing seasons and, for them, transits for the 1999
observing season were predicted. Of the seven candidates that were then
observationally tested in 1999, all were ruled out except one, which needs
further observational confirmation. We conclude that the photometric transit
method is a viable way to search for relatively small, inner extrasolar planets
with moderate-sized telescopes using CCD photometry with a matching-filter
analysis.


title: The Energy Output of the Universe

abstract:
  The total energy emitted by the growth of massive black holes is large and
can be 10-50 per cent of that emitted by stars in the universe. I show how the
X-ray Background provides a good measure of this energy and also why most
accretion power is absorbed and re-emitted in the far infrared band. A model
for the obscured growth of massive black holes is presented which accounts for
both the high absorption and the observed black hole to galaxy spheroid mass
correlation. Future missions should detect the obscured X-ray sources
associated with the growth of massive black holes.


title: A deep ROSAT survey XV. the average QSO spectrum and its evolution

abstract:
  Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields we
investigate the X-ray spectral properties of an ``average''radio-quiet broad
line QSO as a function of redshift. The QSO stacked spectra, in the observers
0.1-2 keV band, in five redshift bins over the range 0.1<z<3.2 apparently
harden from an equivalent photon index of 2.6 at z=0.4 to ~2.1 at z=2.4 as seen
in other QSO samples. In contrast, the spectra in the 0.5-2 keV band show no
significant variation in spectral index with redshift. This suggests the
presence of a spectral upturn at low energies (<0.5 keV). Indeed, while at high
redshifts (z>1.0) the single power-law model gives an acceptable fit to the
data over the full energy band, at lower redshifts the spectra need a second
component at low energies, a `soft excess'. Inclusion of a simple model for the
soft excess, i.e. a black-body component (kT~100 eV), results in a significant
improvement to the model fit, and yields power-law slopes of 1.8-1.9, for all
redshift bins. This power-law is not inconsistent, within the error bars, with
those of nearby AGN in the 2-10 keV band, suggesting that the same intrinsic
power-law slope may continue from 10 keV down to below 0.5 keV. We caution that
there is a possibility that the spectral upturn observed may not represent a
real physical component but could be due to co-adding spectra with a large
dispersion in spectral indices. Regardless of the origin of the soft excess,
the average QSO spectrum has important consequences for the origin of the X-ray
background: the average spectra of the typical, faint, high redshift QSO are
significantly steeper than the spectrum of the X-ray background extending the
spectral paradox into the soft 0.1-2 keV X-ray band.


title: Simulating Supernovae Remnants in Gas Clouds

abstract:
  The Hydra $N$-body hydrodynamics code has been modified to model, from the
end of the Sedov phase, the effects of supernovae on the surrounding medium.
The motivation is to investigate the feedback of energy into the
interstellar/intergalactic medium. We compare our results for supernova
remnants (SNRs) in a uniform medium to previous detailed work on the late
evolution of SNRs. The code is found to reproduce the bulk characteristics of
SNRs well. Results on the effects of a single central SNR on Plummer clouds are
presented. The feedback of kinetic energy and the percentage mass loss can be
parameterised in terms of the cloud mass and characteristic radius in a simple
way. The kinetic energy fraction returned to the ISM from a SNR is $<3$ per
cent. The removal of gas from the cold, dense phase and the addition of energy
due to the lowering of the potential energy of a cloud is at least as
significant, if not much more so, than the kinetic energy leaving a cloud.


title: On the optical pulsations from the Geminga pulsar

abstract:
  We present a model for generation mechanisms of the optical pulsations
recently detected from the Geminga pulsar. We argue that this is just a
synchrotron radiation emitted along open magnetic field lines at altitudes of a
few light cylinder radii (which requires that Geminga is an almost aligned
rotator), where charged particles acquire non-zero pitch-angles as a result of
the cyclotron absorption of radio waves in the magnetized pair plasma. This
explains self-consistently both the lack of apparent radio emission, at least
at frequencies higher than about 100 MHz, and the optical pulsations from the
Geminga pulsar. From our model it follows that the synchrotron radiation is a
maximum in the infrared band, which suggests that Geminga should also be a
source of a pulsed infrared emission.


title: X-ray sources as tracers of the large-scale structure in the Universe

abstract:
  We review the current status of studies of large-scale structure in the X-ray
Universe. After motivating the use X-rays for cosmological purposes, we discuss
the various approaches used on different angular scales including X-ray
background multipoles, cross-correlations of the X-ray background with galaxy
catalogues, clustering of X-ray selected sources and small-scale fluctuations
and anisotropies in the X-ray background. We discuss the implications of the
above studies for the bias parameter of X-ray sources, which is likely to be
moderate for X-ray selected AGN and the X-ray background (~1-2). We finally
outline how all-sky X-ray maps at hard X-rays and medium surveys with large sky
coverage could provide important tests for the cosmological models.


title:
Evidence for a substellar secondary in the magnetic cataclysmic binary
  EF Eridani

abstract:
  Low-state spectrophotometry of the short-period polar EF Eridani (Porb=81
min) found the system at V=18.0 with no trace of the companion (Wheatley and
Ramsay 1998). We show that the lack of such spectral features implies that the
companion to the white dwarf in EF Eri has a spectral type later than M9 and is
either a transition object at the brink of hydrogen burning or a brown dwarf.
The optical low state spectrum indicates a temperature of the white dwarf of
Teff=9500+-500K. This is one of the coldest white dwarfs in cataclysmic
variables, implying a cooling age t_cool>=10e9 yrs or accretional heating at a
rate as given by gravitational radiation. The large age of the system excludes
a warm brown dwarf as companion. EF Eri has either just passed through the
period minimum of cataclysmic variable stars or has started mass transfer from
an old brown dwarf secondary.


title: Testing for substructure in optical and X-ray clusters

abstract:
  We present a detailed study of the morphological features of 22 rich galaxy
clusters. We systematically compare cluster images and morphological parameters
in an attempt to reliably identify possible substructure in both optical and
X-ray images. To this end, we compute moments of the surface-brightness
distribution to estimate ellipticities, center-of-mass shifts and orientations.
We find important correlations between the optical and X-ray morphological
shape parameters. Most of our clusters (17) have a good 1-to-1 correspondence
between the optical and the X-ray images and at least 9 appear to have strong
indications of substructure. This corresponds to a percentage of about 40% in
good accordance with other similar analyses. Finally, 4 out of 22 systems seem
to have distinct subclumps in the optical which are not verified in the X-ray
images, and thus are suspect of being due to optical projection effects. We
assess the significance of results using Monte Carlo simulations.


title: The Galactic Center: An Interacting System of Unusual Sources

abstract:
  The region bounded by the inner tens of light years at the center of the
Milky Way contains five principal components that coexist within the central
deep gravitational potential well. These constituents are a black hole
candidate (Sgr~A*) with a mass equivalent to $2.6\pm0.2\times 10^6$ suns, a
surrounding cluster of evolved stars, a complex of young stars, molecular and
ionized gas clouds, and a powerful supernova-like remnant. The interaction of
these components is responsible for many of the phenomena occurring in this
complex and unique portion of the Galaxy. Developing a consistent picture of
the primary interactions between the components at the Galactic Center will
improve our understanding of the nature of galactic nuclei in general, and will
provide with a better defined set of characteristics of black holes. For
example, the accretion of stellar winds by Sgr A* appears to produce far less
radiation than indicated by estimates based on models of galactic nuclei.


title: On the gamma-ray fluxes expected from Cassiopeia A

abstract:
  Based on the results of our previous study of the broad band synchrotron
emission of Cas A in the framework of a spatial inhomogeneous model, we
calculate the fluxes of gamma-radiation that can be expected from this
supernova remnant in different energy bands. We show that at energies below 10
GeV gamma-ray fluxes detectable by forthcoming space-borne detectors should be
produced due to bremsstrahlung of radio emitting electrons. We predict that the
power-law index of the photon flux in the GeV region should be hard, close to
the index ~2.2 expected for the acceleration spectrum of the electrons. Photon
fluxes accessible to future ground-based gamma-ray detectors could also be
expected at very high energies. The fluxes to be expected due to bremsstrahlung
and inverse Compton radiation of relativistic electrons at TeV energies should
be very steep, and strongly dependent on the characteristic magnetic fields in
Cas A. We could expect also significant fluxes of pi^0 -decay gamma-rays
produced by relativistic protons. These fluxes may extend with a hard spectrum
beyond TeV energies as far as the protons would be accelerated to energies
beyond 100 TeV. The hardness of the gamma-ray spectrum at TeV energies could in
principle allow one to distinguish between electronic and hadronic origins of
those gamma-rays. We discuss also other implications, such as relativistic
particle content, or physical parameters in the source, that could be derived
from future gamma-ray flux measurements in different energy bands.


title:
On energy-dependent propagation effects and acceleration sites of
  relativistic electrons in Cassiopeia A

abstract:
  We consider the effect of energy dependent propagation of relativistic
electrons in a spatially inhomogeneous medium in order to interpret the
broad-band nonthermal radiation of the young supernova remnant (SNR) Cassiopeia
A. A two-zone model is proposed that distinguishes between compact, bright
steep-spectrum radio knots and the bright fragmented radio ring on the one
hand, and the remainder of the shell - the diffuse `plateau' - on the other
hand. In the framework of this model it is possible to explain the basic
features of the spectral and temporal evolution of the synchrotron radiation of
Cas A if one assumes that these compact structures correspond to sites of
efficient electron acceleration producing hard spectra of accelerated particles
with power-law indices ~2.2. The resulting energy distribution of radio
electrons in these compact structures becomes significantly steeper than the
electron production spectrum on timescales of the energy dependent escape of
these electrons into the surrounding diffuse plateau region. We argue that the
steepness, rather than the hardness, of the radio spectra of compact bright
structures in clumpy sources can in general be considered as a typical
signature of sites where strong electron acceleration has built up high
gradients in the spatial distribution of radio electrons. Subsequent diffusive
escape then modifies their energy distribution, leading to spatial variations
of spectral indices within the radio source. Qualitative and quantitative
interpretations of a number of observational data of Cas A are given.
Predictions following from the model are discussed.


title: N-body simulations of interactions and mergings in small galaxy groups

abstract:
  In this paper I focus on three topics related to the dynamical evolution of
small galaxy groups, for which the input of N-body simulations has been
decisive. These are the merging rates in compact groups, the properties of
remnants of multiple mergers, and the evolution of disc galaxies surrounded by
one or more satellites. The short dynamical times of compact groups make it
difficult to understand why such groups are observed at all. N-body simulations
have pointed out two possible classes of solutions to this problem. The first
one proposes that there is on-going formation of compact groups, or that the
longevity of the group is due to secondary infall. For the second class of
solutions the longevity of compact groups is due either to their specific
initial conditions, or to a massive common halo, encompassing the whole group.
I discuss here these alternatives, together with their respective advantages
and disadvantages. I then turn to the structure of remnants of multiple mergers
and compare the results of N-body simulations with the properties of observed
elliptical galaxies. Finally I discuss the dynamical evolution of a disc galaxy
surrounded by one or more spherical satellites.


title:
Thermonuclear Runaways on Accreting White Dwarfs: Models of Classical
  Novae Explosions

abstract:
  The mechanism of classical novae explosions is explained, together with some
of their observational properties. The scarce but not null impact of novae in
the chemical evolution of the Milky Way is analyzed, as well as their relevance
for the radioactivity in the Galaxy. A special emphasis is given to the
predicted gamma-ray emission from novae and its relationship with the
thermonuclear model itself and its related nucleosynthesis.


title:
The modulation effect for supersymmetric dark matter detection with
  asymmetric velocity dispersion

abstract:
  The detection of the theoretically expected dark matter is central to
particle physics cosmology. Current fashionable supersymmetric models provide a
natural dark matter candidate which is the lightest supersymmetric particle
(LSP). Such models combined with fairly well understood physics like the quark
substructure of the nucleon and the nuclear form factor and the spin response
function of the nucleus, permit the evaluation of the event rate for
LSP-nucleus elastic scattering. The thus obtained event rates are, however,
very low or even undetectable. So it is imperative to exploit the modulation
effect, i.e. the dependence of the event rate on the earth's annual motion. In
this review we study such a modulation effect in directional and undirectional
experiments. We calculate both the differential and the total rates using
symmetric as well as asymmetric velocity distributions. We find that in the
symmetric case the modulation amplitude is small, less than 0.07. There exist,
however, regions of the phase space and experimental conditions such that the
effect can become larger. The inclusion of asymmetry, with a realistic enhanced
velocity dispersion in the galactocentric direction, yields the bonus of an
enhanced modulation effect, with an amplitude which for certain parameters can
become as large as 0.46.


title: Clustering in the X-ray Universe

abstract:
  In this paper we discuss various possibilities of using X-ray observations to
gain information about the large-scale structure of the Universe. After
reviewing briefly the current status of these investigations we explore
different ways of making progress in this field, using deep surveys, large area
surveys and X-ray background observations.


title: An Instability-driven Dynamo for Gamma Ray Bursts

abstract:
  We show that an MHD-instability driven dynamo (IDD) operating in a hot
accretion disk is capable of generating energetically adequate magnetic flux
deposition rates above and below a mildly advective accretion disk structure.
  The dynamo is driven by the magnetorotational instability (MRI) of a toroidal
field in a shear flow and is limited by the buoyancy of `horizontal' flux and
by reconnection in the turbulent medium. The efficiency of magnetic energy
deposition is estimated to be comparable to the neutrino losses although an MHD
collimation mechanism may deem this process a more viable alternative to
neutrino-burst-driven models of gamma ray bursts.


title: Astrometric Resolution of Severely Degenerate Binary Microlensing Events

abstract:
  We investigate whether the "close/wide" class of degeneracies in
caustic-crossing binary microlensing events can be broken astrometrically.
Dominik showed that these degeneracies are particularly severe because they
arise from a degeneracy in the lens equation itself rather than a mere
"accidental" mimicking of one light curve by another. A massive observing
campaign of five microlensing collaborations was unable to break this
degeneracy photometrically in the case of the binary lensing event MACHO
98-SMC-1. We show that this degeneracy indeed causes the image centroids of the
wide and close solutions to follow an extremely similar pattern of motion
during the time when the source is in or near the caustic. Nevertheless, the
two image centroids are displaced from one another and this displacement is
detectable by observing the event at late times. Photometric degeneracies
therefore can be resolved astrometrically, even for these most severe cases.


title:
Discovery of a Brown Dwarf Companion to Gliese 570ABC: A 2MASS T Dwarf
  Significantly Cooler than Gliese 229B

abstract:
  We report the discovery of a widely separated (258$\farcs3\pm0\farcs$4) T
dwarf companion to the Gl 570ABC system. This new component, Gl 570D, was
initially identified from the Two Micron All Sky Survey (2MASS). Its
near-infrared spectrum shows the 1.6 and 2.2 $\micron$ CH$_4$ absorption bands
characteristic of T dwarfs, while its common proper motion with the Gl 570ABC
system confirms companionship. Gl 570D (M$_J$ = 16.47$\pm$0.07) is nearly a
full magnitude dimmer than the only other known T dwarf companion, Gl 229B, and
estimates of L = (2.8$\pm$0.3)x10$^{-6}$ L$_{\sun}$ and T$_{eff}$ = 750$\pm$50
K make it significantly cooler and less luminous than any other known brown
dwarf companion. Using evolutionary models by Burrows et al. and an adopted age
of 2-10 Gyr, we derive a mass estimate of 50$\pm$20 M$_{Jup}$ for this object.


title: Physical morphology and triggers of starburst galaxies

abstract:
  We present a method of determining the likely triggering mechanisms for
luminous, nearby starbursts based on their optical asymmetry and (B-V) color --
what we term as indices of `physical morphology.' By comparing the starbursts'
locations in a color-asymmetry diagram to an independent, nearby galaxy sample,
we identify which starbursts are likely triggered by an interaction/merger. We
confirm our morphological interpretation based on a comparison to 20% and 50%
HI velocity-width ratios. We further explain how this use of physical
morphology can put constraints on the dynamical history of other galaxies,
including star-forming galaxies at high redshifts.


title: Current Development of the Photo-Ionization Code Cloudy

abstract:
  This paper describes the most important differences between Cloudy 90 and
Cloudy 94, the most recent upgrade of the code which has now been posted on the
WWW. It also describes the upgrade of the grain model in Cloudy which is
currently underway.


title: The Beginning of the End of the Anthropic Principle

abstract:
  We argue that if string theory as an approach to the fundamental laws of
physics is correct, then there is almost no room for anthropic arguments in
cosmology. The quark and lepton masses and interaction strengths are
determined.


title:
Young Massive Star Clusters in Spiral Galaxies. III. Correlations
  between cluster populations and host galaxy properties

abstract:
  We present an analysis of correlations between integrated properties of
galaxies and their populations of young massive star clusters. Data for 21
nearby galaxies presented by Larsen & Richtler (1999) are used together with
literature data for 10 additional galaxies, spanning a range in specific U-band
luminosity T_L(U) from 0 to 15. We find that T_L(U) correlates with several
observable host galaxy parameters, in particular the ratio of Far-Infrared
(FIR) to B-band flux and the optical surface brightness. Taking the FIR
luminosity as an indicator of the star formation rate (SFR), it is found that
T_L(U) correlates very well with the SFR per unit area. A similar correlation
is seen between T_L(U) and the atomic hydrogen surface density. The cluster
formation efficiency seems to depend on the SFR in a continuous way, rather
than being related to any particularly violent mode of star formation. We
discuss fundamental features of possible scenarios for cluster formation. One
possibility is that the correlation between T_L(U) and SFR is due to a common
controlling parameter, most probably the density of the ISM. Another scenario
conceives a high T_L(U) as resulting from the energy input from many massive
stars in case of a high SFR.


title: Superluminal Caustics of Close, Rapidly-Rotating Binary Microlenses

abstract:
  The two outer triangular caustics (regions of infinite magnification) of a
close binary microlens move much faster than the components of the binary
themselves, and can even exceed the speed of light. When $\epsilon > 1$, where
$\epsilon c$ is the caustic speed, the usual formalism for calculating the lens
magnification breaks down. We develop a new formalism that makes use of the
gravitational analog of the Li\'enard-Wiechert potential. We find that as the
binary speeds up, the caustics undergo several related changes: First, their
position in space drifts. Second, they rotate about their own axes so that they
no longer have a cusp facing the binary center of mass. Third, they grow larger
and dramatically so for $\epsilon >> 1$. Fourth, they grow weaker roughly in
proportion to their increasing size. Superluminal caustic-crossing events are
probably not uncommon, but they are difficult to observe.


title: APM 08279+5255: Keck Near- and Mid-IR High-Resolution Imaging

abstract:
  We present Keck high-resolution near-IR (2.2 microns; FWHM~0.15") and mid-IR
(12.5 microns; FWHM~0.4") images of APM08279+5255, a z=3.91 IR-luminous BALQSO
with a prodigious apparent bolometric luminosity of 5x10^{15} Lsun, the largest
known in the universe. The K-band image shows that this system consists of
three components, all of which are likely to be the gravitationally lensed
images of the same background object, and the 12.5 micron image shows a
morphology consistent with such an image configuration. Our lens model suggests
that the magnification factor is ~100 from the restframe UV to mid-IR, where
most of the luminosity is released. The intrinsic bolometric luminosity and IR
luminosity of APM08279+5255 are estimated to be 5x10^{13} Lsun and 1x10^{13}
Lsun, respectively. This indicates that APM 08279+5255 is intriniscally
luminous, but it is not the most luminous object known. As for its dust
contents, little can be determined with the currently available data due to the
uncertainties associated with the dust emissivity and the possible effects of
differential magnification. We also suggest that the lensing galaxy is likely
to be a massive galaxy at z~3.


title:
A Long Observation of NGC 5548 by BeppoSAX: the High Energy Cut-off,
  Intrinsic Spectral Variability and a Truly Warm Absorber

abstract:
  NGC 5548 was observed by BeppoSAX in a single long (8 day) observation from
0.2 to 200 keV. We find (1) the spectral variation of the source is produced by
a change of the intrinsic power law slope; (2) a high energy cut-off at $E_c=
115^{+39}_{-27}$ keV with a hint of change of $E_c$ with flux; (3) OVII and
OVIII absorption K edges, and a possible blended OVII-OVIII K$\alpha,\beta$
emission feature at $0.54^{+0.07}_{-0.06}$ keV, inconsistent with a purely
photoionized gas in equilibrium. We propose that the temperature of the
absorbing and emitting gas is $\sim 10^6$ K so that both collisional ionization
and photoionization contribute.


title: Stellar-Dynamics of Young Star Clusters

abstract:
  The stellar-dynamical evolution of bound star clusters during the first few
Myr is dominated by binary-binary and binary-star interactions, the rapid
sinking of the most massive stars to the centre of the clusters and mass loss
from evolving stars. The consequences of these processes for the binary and
stellar population in clusters, and for the star clusters as a whole, are
studied by following the evolution over 150 Myr of a library of compact cluster
models containing up to 10^4 stars.


title:
Disc instability models for X-ray transients: evidence for evaporation
  and low alpha-viscosity ?

abstract:
  We construct time-dependent models of accretion discs around black holes and
neutron stars. We investigate the effect of evaporating the disc inner regions
during quiescence on the predictions of the Disc Instability Model (DIM) for
these systems. We do not include irradiation of the disc in the models.
  Removing the inner, most unstable parts of the accretion disc increases the
predicted recurrence times. However, DIMs with values of the viscosity
parameter alpha_hot ~ 0.1 and alpha_cold ~ 0.02 (typical of applications of the
DIM to standard dwarf nova outbursts) fail to reproduce the long recurrence
times of SXTs (unless we resort to fine-tuning of the parameters) independent
of the evaporation strength. We show that models with evaporation and a smaller
value of alpha_cold (~ 0.005) do reproduce the long recurrence times and the
accretion rates at the level of the Eddington rate observed in outburst. The
large difference between the values of alpha_hot and alpha_cold, if confirmed
when disc irradiation is included, suggests that several viscosity mechanisms
operate in these accretion discs.
  For some choices of parameters our models predict reflares during the decline
from outburst. They are a physical property of the model and result from a
heating front forming in the wake of an initial cooling front and subsequent,
multiple front reflections. The reflares disappear in low-alpha models where
front reflection can not occur.


title: A large high-energy gamma-ray flare from the blazar 3C 273

abstract:
  The Compton Gamma-Ray Observatory (CGRO) experiments EGRET and COMPTEL
observed the Virgo sky region continuously for 7 weeks between December 10,
1996 and January 28, 1997. The prominent quasar 3C~273 was found to be the
brightest source in gamma-rays and was significantly detected by EGRET and
COMPTEL. The EGRET experiment observed a time-variable flux at energies above
100 MeV, which reached in a 2-week flaring period (December 30, 1996 to January
14, 1997) its highest flux level observed during the CGRO-era. COMPTEL,
however, does not observe obvious time variability at energies below ~30 MeV
contemporaneous to EGRET. In particular, no flare was observed, indicating that
this outburst is solely a high-energy (>100 MeV) phenomenon. The energy
spectrum between 3 MeV and 10 GeV is well represented by a simple power-law
model. Below 3 MeV a spectral turnover is indicated. Performing spectral
analysis for different time periods, we found evidence for a spectral hardening
during the flaring period, which is consistent with the flare occurring mainly
at the higher energies and with its absence at COMPTEL energies of a few MeV.
This may be interpreted as an indication that the emission in the EGRET energy
range is dominated by a different radiation mechanism than the MeV emission. We
argue that the most likely mechanism for the high-energy flare is
inverse-Compton scattering of reprocessed accretion-disk radiation.


title: Regular magnetic fields in the dwarf irregular galaxy NGC 4449

abstract:
  We present a high-resolution VLA study of the total power and polarized radio
continuum emission at 8.46 and 4.86 GHz of the irregular galaxy NGC 4449, known
for its weak rotation and non-systematic gas motions. We found strong
galaxy-scale regular magnetic fields, which is surprising because of a lack of
ordered rotation required for the dynamo action. The strength of the regular
field reaches 8 $\mu$G and that of the total field 14 $\mu$G, comparable to
that of the total magnetic field strength in radio-bright spirals. The magnetic
vectors in NGC 4449 form radial ``fans'' in the central region and fragments of
a spiral pattern in the galaxy's outskirts. These structures are associated
with large regions of systematic Faraday rotation, implying genuine
galaxy-scale magnetic fields rather than random ones compressed and stretched
by gas flows. The observed pattern of polarization B-vectors is similar to
dynamo-type fields in normal spirals. Nonstandard, fast dynamo concepts are
required to explain the observed field strengths, though it is unknown what
kind of magnetic field geometry can be produced in slowly and chaotically
rotating objects. The so far neglected role of magnetic fields for the dynamics
and star formation in dwarf irregulars also needs to be revised.


title:
Extended Gas in Seyfert Galaxies: Near-Infrared Observations of 15
  Active Nuclei

abstract:
  Results from an analysis of low resolution (R~250) near-IR long-slit spectra
covering simultaneously the I, J, H, and K bands, for a sample of 15 Seyfert
galaxies and the N5253 starburst nucleus, are presented. The Seyfert galaxies
were selected as presenting `linear' or cone-like high excitation emission line
in the optical, most probably due to the collimation of the central source's
radiation by a dusty molecular torus. Our goal was to look for signatures of
this torus, and to investigate the gaseous distribution, excitation and
reddening. The IR emission lines are spatially extended in most cases, and we
have used the [FeII]/Pa(beta) ratio as a measure of the gaseous excitation in
Mrk573, N1386, and N7582. Values for this ratio between 1.5 and 6 are found,
suggesting excitation of [FeII] by X-rays or shock waves in some regions.
Nuclear Pa(beta) in N1365, and possibly nuclear Br(gama) in Mrk573, are broad.
  From analysis of the spatial distribution of the continuum (J-H) and (H-K)
colours derived from our spectra, we find redder colours for the nucleus than
the nearby bulge in most of the Seyfert 2s observed. Comparison with models
including emission from dust and stars shows that hot (T~1000 K) dust emission
dominates the nuclear continuum in N1365, N2110, N3281, N7582, and ESO362-G18.
In N1386, N5643, and N5728 the main contributor is the underlying stellar
population, combined with some foreground reddening and/or cool dust emission.
In a few cases, the (J-H) colours on opposite sides of the nucleus differ by
0.3-0.8 mag, an effect that we interpret as partly due to differences in the
local stellar population, and possibly extinction gradients.


title: The redshift evolution of bias and baryonic matter distribution

abstract:
  We study the distribution of baryonic and luminous matter within the
framework of a hierarchical scenario. Using an analytical model for structure
formation which has already been checked against observations for galaxies,
Lyman-$\alpha$ clouds, clusters and reionization processes, we present its
predictions for the bias of these objects. We describe its dependence on the
luminosity (for galaxies or quasars) or the column density (for Lyman-$\alpha$
absorbers) of the considered objects. We also study its redshift evolution,
which can exhibit an intricate behaviour. These astrophysical objects do not
trace the dark matter density field, the Lyman-$\alpha$ forest clouds being
undercorrelated and the bright galaxies overcorrelated, while the intermediate
class of Lyman-limit systems is seen to sample the matter field quite well.
  We also present the distribution of baryonic matter over these various
objects. We show that light does not trace baryonic mass, since bright galaxies
which contain most of the stars only form a small fraction of the mass
associated with virialized and cooled halos. We consider two cosmologies: a
critical density universe and an open universe. In both cases, our results
agree with observations and show that hierarchical scenarios provide a good
model for structure formation and can describe a wide range of objects which
spans at least the seven orders of magnitude in mass for which data exist. More
detailed observations, in particular of the clustering evolution of galaxies,
will constrain the astrophysical models involved.


title: Structure of the solar core: Effect of asymmetry of peak profiles

abstract:
  Recent studies have established that peaks in solar oscillation power spectra
are not Lorentzian in shape, but have a distinct asymmetry. Fitting a symmetric
Lorentzian profile to the peaks therefore produces a shift in frequency of the
modes. Accurate determination of low-frequency modes is essential to infer the
structure of the solar core by inversion of the mode frequencies. In this paper
we investigate how the changes in frequencies of low-degree modes obtained by
fitting symmetric and asymmetric peak profiles change the inferred properties
of the solar core. We use data obtained by the Global Oscillations at Low
Frequencies (GOLF) project on board the SoHO spacecraft. Two different solar
models and inversion procedures are used to invert the data to determine the
sound speed in the solar core. We find that for a given set of modes no
significant difference in the inferred sound-speed results from taking
asymmetry into account when fitting the low-degree modes.


title: A ROSAT HRI survey of bright nearby galaxies

abstract:
  We use the extensive public archive of ROSAT High Resolution Imager (HRI)
observations to carry out a statistical investigation of the X-ray properties
of nearby galaxies. Specifically we focus on the sample of 486 bright (B_T <
12.5) northern galaxies studied by Ho, Filippenko and Sargent (HFS) in the
context of their exploration of the optical spectroscopic properties of nearby
galactic nuclei. Over 20% of HFS galaxies are encompassed in ROSAT HRI fields
of reasonable (> 10ks) exposure. The X-ray sources detected within the optical
extent of each galaxy are categorised as either nuclear or non-nuclear
depending on whether the source is positioned within or outside of a 25
arcsecond radius circle centred on the optical nucleus. A nuclear X-ray source
is detected in over 70% of the galaxies harbouring either a Seyfert or LINER
nucleus compared to a detection rate of only ~40% in less active systems. The
correlation of the H alpha luminosity with nuclear X-ray luminosity previously
observed in QSOs and bright Seyfert 1 galaxies appears to extend down into the
regime of ultra-low luminosity (L(x)~10^38 - 10^40 erg/s) active galactic
nuclei (AGN). The inferred accretion rates for this sample of low-luminosity
AGN are significantly sub-Eddington. In total 142 non-nuclear sources were
detected. In combination with published data for M31 this leads to a luminosity
distribution (normalised to an optical blue luminosity of L(B) = 10^10
L(solar)) for the discrete X-ray source population in spiral galaxies of the
form dN/dL38 = 1.0 +/- 0.2 L38^-1.8, where L38 is the X-ray luminosity in units
of 10^38 erg/s. The implied L(x)/L(B) ratio is ~1.1 x 10^39 erg/s/(10^10
L(solar)). The nature of the substantial number of ``super-luminous''
non-nuclear objects detected in the survey is discussed.


title: The stellar populations of early-type galaxies in the Fornax cluster

abstract:
  We have measured central line strengths for a magnitude-limited sample of
early-type galaxies in the Fornax cluster, comprising 11 elliptical (E) and 11
lenticular (S0) galaxies, more luminous than MB=-17. When compared with
single-burst stellar population models we find that the centres of Fornax
ellipticals follow a locus of fixed age and have metallicities varying roughly
from half solar to two times solar. The centres of (lower luminosity)
lenticular galaxies, however, exhibit a substantial spread to younger
luminosity-weighted ages indicating a more extended star formation history.
Galaxies with old stellar populations show tight scaling relations between
metal-line indices and the central velocity dispersion. Remarkably also the
Fe-lines are well correlated with sigma. Our detailed analysis of the stellar
populations suggests that these scaling relations are driven mostly by
metallicity. Galaxies with a young stellar component do generally deviate from
the main relation. In particular the lower luminosity S0s show a large spread.
Furthermore we find that elliptical galaxies and the bulge of one bright S0 are
overabundant in magnesium, where the most luminous galaxies show the strongest
overabundances. The stellar populations of young and faint S0s are consistent
with solar abundance ratios or a weak Mg underabundance. (abriged)


title:
The 1996 outburst of GRO J1655-40: disc irradiation and enhanced mass
  transfer

abstract:
  We show that the 1996 outburst of the X-ray binary transient system GRO
J1655-40 can be explained by the standard dwarf-nova type disc instability,
followed by an episode of enhanced mass transfer from the secondary if the mass
transfer rate in GRO J1655-40 is within a factor < 10 of the stability limit.
We argue that irradiation of the secondary during the onset of the outburst
driven by the thermal instability in the outer disc can increase the mass
transfer rate above the minimum value required for stable accretion. This will
then produce the period of near-constant X-ray emission seen in this system.
This scenario can also explain the observed anti-correlation between the
optical and X-ray fluxes. It is generally accepted that optical emission in
low-mass X-ray binaries is produced by irradiation of the outer disc by X-rays.
There is also strong circumstantial evidence that in order for the outer disc
to see the irradiating flux, it must be warped. Depending on the warp
propagation mechanism, either a burst of mass from the secondary or viscous
decay are likely to decrease the degree of warping, thereby causing the
decrease in the observed optical flux while the X-ray flux remains constant or
even increases, exactly as observed in GRO J1655-40. Finally, the decrease of
the disc warping and, therefore, irradiation will cause the disc to become
unstable once again, terminating the outburst.


title: The triple degenerate star WD1704+481

abstract:
  WD1704+481 is a visual binary in which both components are white dwarfs. We
present spectra of the H-alpha line of both stars which show that one component
(WD1704+481.2 = Sanduleak B = GR 577) is a close binary with two white dwarf
components. Thus, WD1704+481 is the first known triple degenerate star. From
radial velocity measurements of the close binary we find an orbital period of
0.1448d, a mass ratio, q=Mbright/Mfaint of q=0.70+-0.03 and a difference in the
gravitational redshifts of 11.5+-2.3km/s. The masses of the close pair of white
dwarfs predicted by the mass ratio and gravitational redshift difference
combined with theoretical cooling curves are 0.39+-0.05 solar mass and
0.56+-0.07 solar masses. WD1704+481 is therefore also likely to be the first
example of a double degenerate in which the less massive white dwarf is
composed of helium and the other white dwarf is composed of carbon and oxygen.


title: Superconducting cosmic strings as gamma ray burst engines

abstract:
  Cusps of superconducting strings can serve as GRB engines. A powerful beamed
pulse of electromagnetic radiation from a cusp produces a jet of accelerated
particles, whose propagation is terminated by the shock responsible for GRB. A
single free parameter, the string scale of symmetry breaking $\eta \sim 10^{14}
GeV$, together with reasonable assumptions about the magnitude of cosmic
magnetic fields and the fraction of volume that they occupy, explains the GRB
rate, duration and fluence, as well as the observed ranges of these quantities.
The wiggles on the string can drive the short-time structures of GRB. This
model predicts that GRBs are accompanied by strong bursts of gravitational
radiation which should be detectable by LIGO, VIRGO and LISA detectors.


title:
ISO Spectroscopy of Young Intermediate-Mass Stars in the BD+40 4124
  Group

abstract:
  We present the results of ISO SWS and LWS grating scans towards the three
brightest members of the BD+40 4124 group in the infrared: BD+40 4124 (B2Ve),
LkHalpha 224 (A7e) and the embedded source LkHalpha 225. Emission from the pure
rotational lines of H_2, from ro-vibrational transitions of CO, from PAHs, from
H I recombination lines and from the infrared fine structure lines of [Fe II],
[Si II], [S I], [O I], [O III] and [C II] was detected. These emission lines
arise in the combination of a low-density (approx 10^2 cm^-3) H II region with
a clumpy PDR in the case of BD+40 4124. The lower transitions of the infrared H
I lines observed in BD+40 4124 are optically thick; most likely they arise in
either a dense wind or a circumstellar disk. This same region is also
responsible for the optical H I lines and the radio continuum emission. In the
lines of sight towards LkHalpha 224 and LkHalpha 225, the observed emission
lines arise in a non-dissociative shock produced by a slow (approx 20 km/s)
outflow arising from LkHalpha 225. Toward LkHalpha 225 we also observe a
dissociative shock, presumably located closer to the outflow source than the
non-dissociative shock. In the line of sight towards LkHalpha 225 we observed
absorption features due to solid water ice and amorphous silicates, and due to
gas-phase H_2O, CO and CO_2. No solid CO_2 was detected towards LkHalpha 225,
making this the first line of sight where the bulk of the CO_2 is in the
gas-phase.


title:
H_3^(2+) molecular ion in a strong magnetic field: a triangular
  configuration

abstract:
  The bound state in the system of three protons and an electron (pppe) under a
homogeneous strong magnetic field where the protons are situated in the
vertices of an equilateral triangle perpendicular to the magnetic field lines
is found. It is shown that for magnetic fields B = 10^11 - 4.414x10^13 G the
potential energy curves as a function of the internuclear distance R have an
explicit minimum. For all magnetic fields studied, the binding energy of the
triangular configuration is less than the binding energy of the linear parallel
configuration (A. Turbiner et al. JETP Lett. 69, p. 844). In the contrary to
the linear case, the binding energy decreases with a magnetic field growth,
while the equilibrium internuclear distance slowly increases.


title:
A New Model for the Spiral Structure of the Galaxy. Superposition of
  2+4-armed patterns

abstract:
  We investigate the possibility of describing the spiral pattern of the Milky
Way in terms of a model of superposition 2- and 4-armed wave harmonics (the
simplest description, besides pure modes). Two complementary methods are used:
a study of stellar kinematics, and direct tracing of positions of spiral arms.
In the first method, the parameters of the galactic rotation curve and the free
parameters of the spiral density waves were obtained from Cepheid kinematics,
under different assumptions. To turn visible the structure corresponding to
these models, we computed the evolution of an ensemble of N-particles,
simulating the ISM clouds, in the perturbed galactic gravitational field. In
the second method, we present a new analysis of the longitude-velocity (l-v)
diagram of the sample of galactic HII regions, converting positions of spiral
arms in the galactic plane into locii of these arms in the l-v diagram. Both
methods indicate that the ``self-sustained'' model, in which the 2-armed and
4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively)
is a good description of the disk structure. An important conclusion is that
the Sun happens to be practically at the corotation circle. As an additional
result of our study, we propose an independent test for localization of the
corotation circle in a spiral galaxy: a gap in the radial distribution of
interstellar gas has to be observed in the corotation region.


title:
Comparison of Star Formation Rate estimations from $H_{\alpha}$, FIR and
  radio data

abstract:
  We used three indicators of massive star formation: $H_{\alpha}$, FIR and
non-thermal radio luminosities, to compare estimations of Star Formation Rate
(SFR) for the sample of 34 spiral galaxies. To adjust SFR values, obtained from
different indicators, we considered the slope $\alpha$ and/or upper mass limit
$M_{up}$ of Initial Mass Function (IMF) as free parameters. The best agreement
between these indicators reaches for $M_{up} \approx 60 - 100 ~M_{\odot}$ and
$\alpha \approx -3.1 $ in the high mass end of IMF ($M > 10 ~M_{\odot}$).
Parallel with SFR we also estimated FIR excess $X_{FIR}$ defined as a fraction
of the observed FIR which is not related to young massive stars directly.
$X_{FIR}$ is found to be well correlated with types of spiral galaxies and
their colors (B-V): the redder a galaxy the higher is its FIR excess. We
conclude that for any parameters of IMF the observed FIR flux of early type
spiral galaxies needs the additional source of energy but the massive star
radiation.


title:
Gravitational waves from inspiralling compact binaries with magnetic
  dipole moments

abstract:
  We investigate the effects of the magnetic dipole-dipole coupling and the
electromagnetic radiation on the frequency evolution of gravitational waves
from inspiralling binary neutron stars with magnetic dipole moments. This study
is motivated by the discovery of the superstrongly magnetized neutron stars,
i.e., magnetar. We derive the contributions of the magnetic fields to the
accumulated cycles in gravitational waves as $N_{mag} \sim 6 \times 10^{-3}
(H/10^{16}{\rm G})^{2}$, where $H$ denotes the strength of the polar magnetic
fields of each neutron star in the binary system. It is found that the effects
of the magnetic fields will be negligible for the detection and the parameter
estimation of gravitational waves, if the upper limit for magnetic fields of
neutron stars are less than $\sim 10^{16}$G, which is the maximum magnetic
field observed in the soft gamma repeaters and the anomalous X-ray pulsars up
to date. We also discuss the implications of electromagnetic radiation from the
inspiralling binary neutron stars for the precursory X-ray emission prior to
the gamma ray burst observed by the Ginga satellite.


title:
Disc galaxy evolution models in a hierarchical formation scenario:
  structure and dynamics

abstract:
  We predict the structure and dynamics of disc galaxies using galaxy evolution
models within a hierarchical formation scenario The halo mass aggregation
histories, for a Lambda CDM model, were generated and used to calculate the
virialization of dark matter (DM) haloes. A diversity of halo density profiles
were obtained, the most typical one being close to the NFW profile. We modeled
the formation of discs in centrifugal equilibrium within the evolving DM haloes
using gas accretion rates proportional to the halo mass aggregation rates, and
assuming detailed angular momentum conservation. We calculated the
gravitational interactions between halo and disc, and the hydrodynamics, star
formation, and evolution of the galaxy discs. We found that the slope and
zero-point of the infrared Tully-Fisher relations (TFR) may be explained as a
direct consequence of the cosmological initial conditions. This relation is
almost independent of the assumed disc mass fraction. The rms scatter of the
TFR originates mainly from the scatter in the DM halo structure and, to a minor
extension, from the dispersion of the primordial spin parameter. The scatter
obtained does not disagree with the observational estimates. Our models allow
us to understand why the residuals of the TFR do not correlate significantly
with disc size or surface brightness (SB), and why low and high SB galaxies
have the same TFR. The correlations between gas fraction and SB, and between
scale length and V_max agree with those observed. The discs present nearly
exponential SB distributions. The shape of the rotation curves changes with the
SB and is nearly flat for most cases. The rotation curve decompositions show a
dominance of DM down to very small radii. The introduction of shallow cores in
the DM halo attenuates this difficulty.


title:
Probing the Site for r-Process Nucleosyntheis with Abundances of Barium
  and Magnesium in Extremely Metal-Poor Stars

abstract:
  We suggest that if the astrophysical site for r-process nucleosynthesis in
the early Galaxy is confined to a narrow mass range of Type II supernova (SN
II) progenitors, with a lower mass limit of Mms = 20 Msun, a unique feature in
the observed distribution of [Ba/Mg] vs.[Mg/H] for extremely metal-poor stars
can be adequately reproduced. We associate this feature, a bifurcation of the
observed elemental ratios into two branches in the Mg abundance interval -2.7 <
[Mg/H] < -2.3, with two distinct processes. The first branch, which we call the
``y''-branch, is associated with the production of Ba and Mg from individual
massive supernovae. We conclude that SNe II with Mms = 20 Msun are the dominant
source of r-process nucleosynthesis in the early Galaxy. An SN-induced chemical
evolution model with this Mms-dependent Ba yield creates the y-branch,
reflecting the different nucleosynthesis yields of [Ba/Mg] for each SN II with
Mms > 20 Msun. The second branch, which we call the ``i''-branch, is associated
with the elemental abundance ratios of stars which were formed in the dense
shells of the interstellar medium swept up by SNe II with Mms < 20 Msun that do
not synthesize r-process elements, and applies to stars with observed Mg
abundances in the range [Mg/H] < -2.7. The Ba abundances in these stars reflect
those of the interstellar gas at the (later) time of their formation. The
existence of a [Ba/Mg] i-branch strongly suggests that SNe II which are
associated with stars of progenitor mass Mms < 20 Msun are infertile sources
for the production of r-process elements. We predict the existence of this
i-branch for other r-process elements, such as europium (Eu), to the extent
that their production site is in common with Ba.


title: Electromagnetic showers in a strong magnetic field

abstract:
  We present the results concerning the main shower characteristics in a strong
magnetic field obtained through shower simulation. The processes of magnetic
bremsstrahlung and pair production were taken into account for values of the
parameter $\chi \gg 1$. We compare our simulation results with a recently
developed cascade theory in a strong magnetic field.


title: Rings in the Planetesimal Disk of Beta Pic

abstract:
  The nearby main sequence star Beta Pictoris is surrounded by an edge-on disk
of dust produced by the collisional erosion of larger planetesimals. Here we
report the discovery of substructure within the northeast extension of the disk
midplane that may represent an asymmetric ring system around Beta Pic. We
present a dynamical model showing that a close stellar flyby with a quiescient
disk of planetesimals can create such rings, along with previously unexplained
disk asymmetries. Thus we infer that Beta Pic's planetesimal disk was highly
disrupted by a stellar encounter in the last hundred thousand years.


title: Deconvolving the Beam in Small Angular Scale CMB Experiments

abstract:
  This paper is concerned with experiments which measure CMB anisotropies on
small angular scales. A certain coverage, a beam structure and a level of
uncorrelated noise define each experiment. We focus our atention on the
reversion of the beam average. In each experiment, we look for the best
pixelization for reversion, namely, for the pixelization that -after reversion-
leads to good maps containing right spectra for the most wide range of angular
scales. Squared pixels having different sizes "smaller" than the beam radius
are considered. For a given size, the following question arises: How well can
we assign a temperature to each pixel? Various mathematical methods are used to
show that, in practice, this assignation -beam reversion or deconvolution- only
leads to right spectra for pixel sizes greater than a certain lower limit close
to the half of the beam radius. This limit is estimated for negligible and
relevant levels of noise and also for spherically symmetric and asymmetric
beams. After this general study, we focus our attention on two feasible
detectors (which have been proposed to be on board of PLANCK satellite). For
each of them, we estimate the size of the most appropriate pixelization
compatible with beam reversion, difraction, observational strategy et cetera
and, then, we answer the following question: Which is the part of the angular
power spectrum which can be extracted from appropriately pixelized maps after
deconvolution?


title: Error analysis for stellar population synthesis as an inverse problem

abstract:
  Stellar population synthesis can be approached as an inverse problem.
  The physical information is extracted from the observations through an
inverse model.
  The process requires the transformation of the observational errors into
model errors.
  A description is given for the error analysis to obtain objectively the
errors in the model. Finding a solution for overdetermined and under-determined
case was the purpose of two preceding papers. This new one completes the
problem of stellar populations synthesis by means of a data base, by providing
practical formul\ae defining the set of acceptable solutions. All solutions
within this set are compatible, at a given confidence level, with the
observations.


title:
Spectra of High Redshift Galaxies using a Cluster as a Gravitational
  Telescope

abstract:
  Using the Focal Reducer Spectrograph (FORS) at the Very Large Telescope (VLT)
during the FORS commissioning time in December 1998 we took long slit spectra
of the gravitational arc visible on images of the galaxy cluster 1E 0657 (z =
0.296). This cluster is one of the hottest (massive) cluster known so far and
hence perfectly acts as a gravitational telescope magnifying the flux of
background sources up to a factor of 17. Here we present the spectra of the
gravitational arc (z = 3.23) and 4 additional high redshift objects (z=2.35 to
3.09), that also fall on the slit by chance coincidence. We briefly discuss the
stellar contents of these galaxies and show first models of the observed
spectra. Furthermore we point out the effectivity of using FORS in combination
with available gravitational telescopes.


title: Wind properties of Wolf-Rayet stars at low metallicity: Sk41 (SMC)

abstract:
  The stellar properties of Sk41 (AB4, WN5h), the only known single Wolf-Rayet
star in the SMC, are derived from ultraviolet (IUE), optical (AAT) and near-IR
(NTT) spectroscopy. Contrary to expectations, the stellar properties of Sk41
are typical of equivalent WN stars in the Galaxy and LMC, with T=42kK,
log(L/Lo)=5.7, v_inf=1300 km/s, Mdot/sqrt(f)=3x10^-5 Mo/yr and H/He=2 by
number, where f is the volume filling factor. The stellar luminosity of Sk41 is
50% below the minimum value predicted by single star evolutionary models at the
metallicity of the SMC.
  Emission line luminosities of HeII 4686A and CIV 5801-12A in SMC WR stars are
not systematically lower than their Galactic and LMC counterparts. From 43
late-type and 59 early-type WN stars, log L(HeII)=36.0 erg/s and 35.8 erg/s
respectively, while log L(CIV)=36.5 erg/s from 25 early-type WC stars. This new
calibration has application in deriving WR populations in young starburst
galaxies.
  Synthetic WN models are calculated with identical parameters except that
metal abundances are varied. Following the Smith et al. WN classification
scheme, CNO equilibrium models reveal that earlier spectral types are predicted
at lower metallicity, i.e. WN3--4 at 0.04 Z(solar) versus WN6 at 1.0 Z(solar).
This provides an explanation for the trend towards earlier WN spectral types at
low metallicity.


title:
Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO
  J1655--40)

abstract:
  We have obtained intermediate resolution optical spectra of the black-hole
candidate Nova Sco 1994 in June 1996, when the source was in an X-ray/optical
active state (R~15.05). We measure the radial velocity curve of the secondary
star and obtain a semi-amplitude of 279+/-10 km/s; a value which is 30 per cent
larger than the value obtained when the source is in quiescence. Our large
value for K_2 is consistent with 60 +9,-7 per cent of the secondary star's
surface being heated; compared to 35 per cent, which is what one would expect
if only the inner face of the secondary star were irradiated. Effects such as
irradiation-induced flows on the secondary star may be important in explaining
the observed large value for K_2.


title:
Quantitative analysis of WC stars: Constraints on neon abundances from
  ISO/SWS spectroscopy

abstract:
  Neon abundances are derived in four Galactic WC stars -- gamma Vel (WR11,
WC8+O7.5III), HD156385 (WR90, WC7), HD192103 (WR135, WC8), and WR146 (WC5+O8) -
using mid-infrared fine structure lines obtained with ISO/SWS. Stellar
parameters for each star are derived using a non-LTE model atmospheric code
(Hillier & Miller 1998) together with ultraviolet (IUE), optical (INT, AAT) and
infrared (UKIRT, ISO) spectroscopy. In the case of gamma Vel, we adopt results
from De Marco et al. (2000), who followed an identical approach.
  ISO/SWS datasets reveal the [NeIII] 15.5um line in each of our targets, while
[NeII] 12.8um, [SIV] 10.5um and [SIII] 18.7um are observed solely in gamma Vel.
Using a method updated from Barlow et al. (1988) to account for clumped winds,
we derive Ne/He=3-4x10^-3 by number, plus S/He=6x10^-5 for gamma Vel. Neon is
highly enriched, such that Ne/S in gamma Vel is eight times higher than cosmic
values. However, observed Ne/He ratios are a factor of two times lower than
predictions of current evolutionary models of massive stars. An imprecise
mass-loss and distance were responsible for the much greater discrepancy in
neon content identified by Barlow et al.
  Our sample of WC5--8 stars span a narrow range in T* (=55--71kK), with no
trend towards higher temperature at earlier spectral type, supporting earlier
results for a larger sample by Koesterke & Hamann (1995). Stellar luminosities
range from 100,000 to 500,000 Lo, while 10^-5.1 < Mdot/(Mo/yr) < 10^-4.5,
adopting clumped winds, in which volume filling factors are 10%. In all cases,
wind performance numbers are less than 10, significantly lower than recent
estimates. Carbon abundances span 0.08 < C/He < 0.25 by number, while oxygen
abundances remain poorly constrained.


title: Ages, Distances, and the Initial Mass Functions of Stellar Clusters

abstract:
  We provide a review of the current status of several topics on the ages,
distances, and mass functions of open clusters, with a particular emphasis on
illuminating the areas of uncertainty. Hipparcos has obtained parallaxes for
nearby open clusters that have expected accuracies much better than has been
previously achievable. By using the lithium depletion boundary method and
isochrone fitting based on much improved new theoretical evolutionary models
for low mass stars, it is arguable that we will soon have have much better age
scales for clusters and star-forming regions. With improved optical and near-IR
cameras, we are just now beginning to extend the mass function of open clusters
like the Pleiades into the regime below the hydrogen burning mass limit.
Meanwhile, observations in star-forming regions are in principle capable of
identifying objects down to of order 10 Jupiter masses.


title: 7Li in Metal-Poor Stars: The Spread of the Li Plateau

abstract:
  A highly homogeneous study of 23 halo field dwarf stars has achieved a Li
abundance accuracy of 0.033 dex per star. The work shows that the intrinsic
spread of the Li abundances of these stars at a given metallicity is <0.02 dex,
and consistent with zero. That is, the Spite Li plateau for halo field dwarfs
is incredibly thin. The thinness rules out depletion by more than 0.1 dex by a
rotational-induced extra-mixing mechanism. Despite the thinness of the plateau,
an increase of Li with [Fe/H] is seen, interpreted as evidence of Galactic
chemical evolution (GCE) of Li, primarily due to Galactic cosmic ray (GCR)
spallation reactions in the era of halo formation. The rate of Li evolution is
concordant with: (1) observations of spallative 6Li in halo dwarfs; (2) GCE
models; and (3) data on Li in higher metallicity halo stars. New data have also
revealed four new ultra-Li-deficient halo dwarfs, doubling the number known.
Based on their propensity to cluster at the halo main sequence turnoff and also
to exist redward of the turnoff, we hypothesise that they are the products of
binary mergers that ultimately will become blue stragglers. We explain their
low Li abundances by normal pre-main-sequence (and possibly main-sequence)
destruction in the low mass stars prior to their merging. If this explanation
is correct, then such stars need no longer be considered an embarrassment to
the existence of negligible Li destruction in the majority of field halo
dwarfs.