title: Are There Three Peaks in the Power Spectra of GX 339-4 and Cyg X-1?

abstract:
  Among the variability behaviour exhibited by neutron star systems are the
so-called ``horizontal branch oscillations'' (HBO, with frequencies ~50 Hz),
the ``lower-frequency kHz quasi-periodic oscillation'' (QPO) and the
``upper-frequency kHz QPO'', with the latter two features being separated in
frequency by an amount comparable to, but varying slightly from, the suspected
spin-frequency of the neutron star. Recently, Psaltis, Belloni, & van der Klis
(1999) have suggested that there exists a correlation between these three
frequencies that, when certain identifications of variability features are
made, even encompasses black hole sources. We consider this hypothesis by
reanalyzing a set of GX 339-4 observations. The power spectral density (PSD)
constructed from a composite of 7 separate, but very similar, observations
shows evidence for three broad peaks in the PSD. If the peak frequencies of
these features are identified with ``QPO'', then their frequencies
approximately fit the correlations suggested by Psaltis, Belloni, & van der
Klis (1999). We also reanalyze a Cyg X-1 observation and show that the
suggested QPO correlation may also hold, but that complications arise when the
``QPOs'' (which, in reality, are fairly broad features) are considered as a
function of energy band. These fits suggest the existence of at least three
separate, independent physical processes in the accretion flow, a hypothesis
that is also supported by consideration of the Fourier frequency-dependent time
lags and coherence function between variability in different energy bands.


title: What Does It Take to Stabilize Gravitational Clustering?

abstract:
  An analytical understanding of the strongly nonlinear regime of gravitational
collapse has been difficult to achieve. The only insight has been the stable
clustering hypothesis, which assumes that the number of neighbors for objects
averaged over small length scales is constant in time. Our recently proposed
analytic halo model for N-point correlation functions now provides a tool for
calculating gravitational clustering properties in the strongly nonlinear
regime. This model also provides a new physical framework for an independent
evaluation of the validity of the stable clustering hypothesis. We derive the
asymptotic nonlinear behavior of the N-point correlation functions and pairwise
peculiar velocities in terms of dark matter halo properties. We show that these
statistics exhibit stable clustering only when the halo mass function and halo
density profile obey specific relations. The long-cherished stable clustering
hypothesis therefore is not necessarily realized in practice.


title:
An excess of submm sources near 4C41.17: a candidate proto-cluster at
  z=3.8?

abstract:
  Biased galaxy-formation theories predict that massive galaxies at high
redshifts should act as signposts to high-density environments in the early
Universe, which subsequently evolve into the cores of the richest clusters seen
at the present day. These regions are characterised by over-densities of young
galaxies, perhaps including a population of dusty, interaction-driven
starbursts - the progenitors of massive cluster ellipticals. By searching for
this population at submm wavelengths we can therefore test both galaxy- and
structure-formation models. We have undertaken such a search in the field of a
z=3.8 radio galaxy, 4C41.17, with the SCUBA submm camera. Our extremely deep
450- and 850-micron maps reveal an order-of-magnitude over-density of luminous
submm galaxies compared to typical fields (the likelihood of finding such an
over-density in a random field is <2E-3). The SCUBA galaxies have bolometric
luminosities, >10^13 L(solar), which imply star-formation rates consistent with
those required to form a massive galaxy in only a few 10^8 years. We also note
that this field exhibits an over-density of extremely red objects (EROs), some
of which may be associated with the submm sources, and Lyman-break galaxies. We
propose that the over-densities of both submm and ERO sources in this field
represent young dusty, starburst galaxies forming within a proto-cluster
centered on the radio galaxy at z=3.8, which is also traced by a less-obscured
population of Lyman-break galaxies.


title:
E&F White conference on Radio Frequency Interference Mitigation
  Strategies

abstract:
  The conference brought together expertise on a range of interference
mitigation techniques from CSIRO, Australian and international industry and
universities. Key goals were to: enhance the understanding of techniques and
their inter-relationship, increase awareness of advanced technologies such as
software radios and photonics, and foster a cooperative approach to the
development of interference mitigation techniques. The foremost application in
mind was the square kilometre array (SKA) and the need to find ways to develop
an hierarchical scheme for removing unwanted signals from astronomical data.
This paper gives an overview of the topics discussed at the conference and
summarises some of the key ideas and results that were presented.
  Many of the interference challenges we face can be overcome with technical
solutions. There will always be a place for regulatory and legal approaches to
spectrum management, but too much reliance has been placed on these in the
past. The strategy of avoidance has worked well in the past and will continue
to be useful in the future, but is now limited by the growth of space based
communications systems, which are visible everywhere on Earth. There is no
technical solution which is likely to provide a silver bullet. Rather a
hierarchical approach will be required, combining a range of techniques. We
have just begun exploring technical solutions and their toxicity to
astronomical data. We need to continue on this path, determining which
techniques are best and how they interact in order to develop a flexible and
powerful system for interference suppression.


title: The All Sky Automated Survey. A Catalog of almost 3900 variable stars

abstract:
  Results of the first two years of observations using the All Sky Automated
Survey prototype camera are presented. More than 140 000 stars in 50 Selected
Fields covering 300 sq. degrees were monitored each clear night in the I-band
resulting in the ASAS Photometric I-band Catalog containing over 50 x 10^6
individual measurements. Nightly monitoring over 100 standard stars confirms
that most of our data remains within sigma_I=0.03 of the standard I system.
Search for the stars varying on the time scales longer than a few days revealed
almost 4000 variables (mostly irregular, pulsating and binaries) brighter than
13 mag. Only 155 of them are known variables included in GCVS, 56 were observed
by Hipparcos satellite (46 were marked as variable). Among the stars brighter
than I ~ 7.5 (which are saturated on our frames) we have found about 50
variables (12 are in GCVS, 6 other in Hipparcos (Perryman etal 1997) catalog).
Because of the large volume of the data we present here only selected tables
and light-curves, but the complete ASAS Catalog of Variable Stars (currently
divided into Periodic and Miscellaneous sections) and all photometric data are
available on the Internet http://www.astrouw.edu.pl/~gp/asas/asas.html or
http://archive.princeton.edu/~asas/


title: Embedded Stellar Clusters in the W3/W4/W5 Molecular Cloud Complex

abstract:
  We analyze the embedded stellar content in the vicinity of the W3/W4/W5 HII
regions using the FCRAO Outer Galaxy 12CO(J=1-0) Survey, the IRAS Point Source
Catalog, published radio continuum surveys, and new near-infrared and molecular
line observations. Thirty-four IRAS Point Sources are identified that have
far-infrared colors characteristic of embedded star forming regions, and we
have obtained K' mosaics and 13CO(J=1-0) maps for 32 of them. Ten of the IRAS
sources are associated with an OB star and 19 with a stellar cluster, although
three OB stars are not identified with a cluster. Half of the embedded stellar
population identified in the K' images is found in just the 5 richest clusters,
and 61% is contained in IRAS sources associated with an embedded OB star. Thus
rich clusters around OB stars contribute substantially to the stellar
population currently forming in the W3/W4/W5 region. Approximately 39% of the
cluster population is embedded in small clouds with an average mass of ~130 Mo
that are located as far as 100 pc from the W3/W4/W5 cloud complex. We speculate
that these small clouds are fragments of a cloud complex dispersed by previous
episodes of massive star formation. Finally, we find that 4 of the 5 known
embedded massive star forming sites in the W3 molecular cloud are found along
the interface with the W4 HII region despite the fact that most of the
molecular mass is contained in the interior regions of the cloud. These
observations are consistent with the classical notion that the W4 HII region
has triggered massive star formation along the eastern edge of the W3 molecular
cloud.


title: Catalog of supersoft X-ray sources

abstract:
  This catalog comprises an up-to-date (December 1999) list of luminous (>10^36
erg/s), binary supersoft X-ray sources. This electronic version (including the
accompannying Web-pages) supersedes the printed version of Greiner (1996).


title: The Identification of the Submillimeter Galaxy SMM J00266+1708

abstract:
  We report the detection of 1.3mm continuum and near-infrared K-band
(2.2-micron) emission from the submillimeter galaxy SMM J00266+1708. Although
this galaxy is among the brightest sub-mm sources detected in the blank-sky
surveys (L~10^{13} L[sun]), SMM J00266+1708 had no reliable
optical/near-infrared counter-part. We used sensitive interferometric 1.3mm
observations with the Owens Valley Millimeter Array to accurately determine the
position of the sub-mm galaxy. Follow-up near-infrared imaging with the Keck I
telescope uncovered a new faint red galaxy at K=22.5 mag which is spatially
coincident with the 1.3mm emission. This is currently the faintest confirmed
counter-part of a sub-mm galaxy. Although the redshift of SMM J00266+1708 is
still unknown, its high sub-mm/radio spectral index suggests that the system is
at high redshift (z>2). Approximately 50% or more of the sub-mm galaxies are
faint/red galaxies similar to that of SMM J00266+1708. These ultraluminous
obscured galaxies account for a significant fraction of the total amount of
star-formation at high redshift despite being missed by optical/ultraviolet
surveys.


title:
RATAN-600 - VLA - BTA-6m ("BIG TRIO") Project: multicolour studying of
  distant radio galaxies

abstract:
  Powerful radio galaxies belong to the population of massive stellar systems
that can be picked at high redshifts. With measured redshift, morphology and
age determination these objects can help greatly in understanding of the way of
formation and evolution of giant elliptical galaxies with massive Black Holes
in the Early Universe. We presents some recent results of study of the sample
of 105 faint steep spectrum radio sources from RATAN-600 RC catalogue.
  All the objects has been mapped with the VLA and optical identification have
been carried out with the 6m BTA telescope. Using multicolour CCD photometry in
B,V,R_c,I_c bands photometric redshift and age of stellar population are
estimated for a subsample of 50 FRII and CSS radio galaxies. RC J0105+0501 is a
very probable candidate for radio galaxy at z ~ 3.5.
  Distribution of objects on colour-colour and magnitude-redshift diagrams
shows a rough agreement with expectations of theoretical models of SED
evolution for giant elliptical galaxies. On the [m_R - z_{phot}] diagram two
populations of faint (m_R = 22^m-24^m) radio galaxies are possibly revealed.
The first population at z~ 1 consists of mainly old (a half with age >= 5 Gyr)
and large (LAS~20") objects, the second one at z = 1.5 - 3.5 includes both
younger (<= 3 Gyr) and smaller (LAS ~ 5") objects.
  There are 12 more faint or still undetected (m_R >= 24.5^m) sources in the
sample which may presumably be a very distant radio galaxies (z>3.5) or
intermediate redshift old or dusty ones.


title: Evidence for a two-armed spiral in the Milky Way

abstract:
  Emission profiles of the Galactic plane in K and at $240\micron$ are
presented, and features associated with the tangents of the spiral arms are
identified. In the K band, which traces stellar emission and suffers little
from absorbtion, features associated with the arm tangents indicate that a
two-armed logarithmic spiral dominates the nonaxisymmetric structure of the
Milky Way. In contrast, the $240\micron$ emission from dust entrained in the
interstellar gas is consistent with a four-armed model, in concordance with
radio data and optical spiral tracers. This suggests that the non-axisymmetric
mass perturbation responsible for the four-armed spiral structure in the gas
has a two rather than four-armed structure.


title: Infrared observations of serendipitous hard Chandra X-ray sources

abstract:
  We present observations of a sample of optically-faint, hard X-ray sources of
the kind likely to be responsible for much of the hard X-ray background. We
confirm that such sources are easily detected in the near-infrared, and find
that they have a featureless continuum suggesting that the active nucleus is
heavily obscured. The infrared colours of the majority of the targets observed
are consistent with absorbed elliptical host galaxies at z=1-2. It is likely
that we are observing some of the brighter members of the important new class
of X-ray Type II quasars.


title:
Dust Streamers in the Virgo Galaxy M86 from Ram Pressure Stripping of
  its Companion VCC 882

abstract:
  The giant elliptical galaxy M86 in Virgo has a ~28 kpc long dust trail inside
its optical halo that points toward the nucleated dwarf elliptical galaxy, VCC
882. The trail seems to be stripped material from the dwarf. Extinction
measurements suggest that the ratio of the total gas mass in the trail to the
blue luminosity of the dwarf is about unity, which is comparable to such ratios
in dwarf irregular galaxies. The ram pressure experienced by the dwarf galaxy
in the hot gaseous halo of M86 was comparable to the internal gravitational
binding energy density of the presumed former gas disk in VCC 882. Published
numerical models of this case are consistent with the overall trail-like
morphology observed here. Three concentrations in the trail may be evidence for
the predicted periodicity of the mass loss. The evaporation time of the trail
is comparable to the trail age obtained from the relative speed of the galaxies
and the trail length. Thus the trail could be continuously formed from stripped
replenished gas if the VCC 882 orbit is bound. However, the high gas mass and
the low expected replenishment rate suggest that this is only the first
stripping event. Implications for the origin of nucleated dwarf ellipticals are
briefly discussed.


title:
Synchrotron Emission from Hot Accretion Flows and the Cosmic Microwave
  Background Anisotropy

abstract:
  Current estimates of number counts of radio sources in the frequency range
where the most sensitive Cosmic Microwave Background (CMB) experiments are
carried out significantly under-represent sources with strongly inverted
spectra. Hot accretion flows around supermassive black holes in the nuclei of
nearby galaxies are expected to produce inverted radio spectra by thermal
synchrotron emission. We calculate the temperature fluctuations and power
spectra of these sources in the Planck Surveyor 30 GHz energy channel, where
their emission is expected to peak. We find that their potential contribution
is generally comparable to the instrumental noise, and approaches the CMB
anisotropy level at small angular scales. Forthcoming CMB missions, which will
provide a large statistical sample of inverted-spectra sources, will be crucial
for determining the distribution of hot accretion flows in nearby quiescent
galactic nuclei. Detection of these sources in different frequency channels
will help constrain their spectral characteristics, hence their physical
properties.


title:
On the origin of the difference between the runaway velocities of the
  OB-supergiant X-ray Binaries and the Be/X-ray Binaries

abstract:
  The recent finding by Chevalier & Ilovaisky (1998) that OB-supergiant X-ray
binaries have relatively large runaway velocities whereas Be/X-ray binaries
have low runaway velocities, provides confirmation of the current models for
the formation of these two types of systems. These predict a difference in
runaway velocity of an order of magnitude. This difference basically results
from the variation of the fractional helium core mass as a function of stellar
mass, in combination with the conservation of orbital angular momentum during
the mass transfer phase that preceded the formation of the compact object in
the system. This combination results into: (i) Systematically narrower
pre-supernova orbits in the OB-supergiant systems than in the Be-systems, and
(ii) A larger fractional amount of mass ejected in the supernovae in high-mass
systems relative to systems of lower mass. Regardless of possible kick
velocities imparted to neutron stars at birth, this combination leads to a
considerable difference in average runaway velocity between these two groups.
The observed low runaway velocities of the Be/X-ray binaries confirm that in
most cases not more than 1 to 2Msun was ejected in the supernovae that produced
their neutron stars. This, in combination with the --on average-- large orbital
eccentricities of these systems, indicates that their neutron stars must have
received a velocity kick in the range 60 - 250 km/s at birth.


title: First Structure Formation and the First Stars

abstract:
  We discuss the results of recent 3D simulations of first structure formation
in relationship to the formation of the first stars. On the basis of a new,
high-resolution AMR simulation (spatial dynamic range = 30,000,000), we
conclude that the first stars are likely to be massive.


title:
Five High-Redshift Quasars Discovered in Commissioning Imaging Data of
  the Sloan Digital Sky Survey

abstract:
  We report the discovery of five quasars with redshifts of 4.67 - 5.27 and
z'-band magnitudes of 19.5-20.7 M_B ~ -27. All were originally selected as
distant quasar candidates in optical/near-infrared photometry from the Sloan
Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift
quasars by supplementing the SDSS data with J and K measurements. The quasars
possess strong, broad Lyman-alpha emission lines, with the characteristic sharp
cutoff on the blue side produced by Lyman-alpha forest absorption. Three
quasars contain strong, broad absorption features, and one of them exhibits
very strong N V emission. The amount of absorption produced by the Lyman-alpha
forest increases toward higher redshift, and that in the z=5.27 object (D_A ~
0.7) is consistent with a smooth extrapolation of the absorption seen in lower
redshift quasars. The high luminosity of these objects relative to most other
known objects at z >~ 5 makes them potentially valuable as probes of early
quasar properties and of the intervening intergalactic medium.


title:
Star cluster ecology IVa: Dissection of an open star
  cluster---photometry

abstract:
  The evolution of star clusters is studied using N-body simulations in which
the evolution of single stars and binaries are taken self-consistently into
account. Initial conditions are chosen to represent relatively young Galactic
open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations
include a realistic mass function, primordial binaries and the external
potential of the parent Galaxy. Our model clusters are generally significantly
flattened in the Galactic tidal field, and dissolve before deep core collapse
occurs. The binary fraction decreases initially due to the destruction of soft
binaries, but increases later because lower mass single stars escape more
easily than the more massive binaries. At late times, the cluster core is quite
rich in giants and white dwarfs. There is no evidence for preferential
evaporation of old white dwarfs, on the contrary the formed white dwarfs are
likely to remain in the cluster. Stars tend to escape from the cluster through
the first and second Lagrange points, in the direction of and away from the
Galactic center. Mass segregation manifests itself in our models well within an
initial relaxation time. As expected, giants and white dwarfs are much more
strongly affected by mass segregation than main-sequence stars. Open clusters
are dynamically rather inactive. However, the combined effect of stellar mass
loss and evaporation of stars from the cluster potential drives its dissolution
on a much shorter timescale than if these effects are neglected. The often-used
argument that a star cluster is barely older than its relaxation time and
therefore cannot be dynamically evolved is clearly in error for the majority of
star clusters.


title: The chemical evolution of gas-rich dwarf galaxies

abstract:
  A numerical double burst model of the chemical evolution of gas-rich dwarf
galaxies has been developed. The model is fitted to a sample of N/O, O/H, Y and
gas fraction observations, where N/O and O/H are the relative abundances by
number of nitrogen to oxygen and oxygen to hydrogen, respectively. Y is the
abundance by mass of helium. Closed models as well as models including enriched
outflow, ordinary outflow and ordinary outflow combined with inflow are
considered.The bursts are assumed to be instantaneous but ordered in pairs to
explain the scatter in N/O-O/H. The method of gas fraction fitting is revised,
and it is found that it is very important to specify whether dwarf irregulars
(dIrrs) or blue compact galaxies (BCGs) are considered. Effective enriched
winds fail when fitting N/O, whereas closed models, models with ordinary winds
or a combination of ordinary winds and inflow are all viable.


title: Event reconstruction in high resolution Compton telescopes

abstract:
  The development of germanium Compton telescopes for nuclear gamma-ray
astrophysics (~0.2-20 MeV) requires new event reconstruction techniques to
accurately determine the initial direction and energy of photon events, as well
as to consistently reject background events. This paper describes techniques
for event reconstruction, accounting for realistic instrument/detector
performance and uncertainties. An especially important technique is Compton
Kinematic Discrimination, which allows proper interaction ordering and
background rejection with high probabilities. The use of these techniques are
crucial for the realistic evaluation of the performance and sensitivity of any
germanium Compton telescope configuration.


title: Our Second Look at the Immature Universe: The Infrared View

abstract:
  The long-awaited promise of studying high-redshift galaxies at long
wavelengths has been partially eclipsed by progress at optical wavelengths,
mostly because of the number of available pixels. It is nonetheless essential
to study optically selected high-redshift galaxies at wavelengths longward of
1\mic, for several reasons. One of the indications from such studies to date is
that interstellar dust (as an absorber of UV photons and re-emitter of IR
photons) plays an energetically significant role at high redshifts, just as it
does in present-day star-forming galaxies. This also provides a strong
motivation for new searches which detect high-redshift galaxies based on IR
observations. Some of these are already succeeding in clarifying our view of
galaxy evolution in the Immature Universe.


title:
Semi analytic approach to understanding the distribution of neutral
  hydrogen in the universe

abstract:
  Analytic derivations of the correlation function and the column density
distribution for neutral hydrogen in the IGM are presented, assuming that the
non-linear baryonic mass density distribution in the IGM is lognormal. This
ansatz was used earlier by Bi & Davidsen (1997) to perform 1D simulations of
lines-of-sight and analyse the properties of absorption systems. Our approach
is completely analytic, which allows us to explore a wide region of the
parameter space for our model. The analytic results have been compared with
observations, whenever possible. Two kinds of correlation functions are
defined: along the line-of-sight (LOS) and across the transverse direction. We
find that the effects on the LOS correlation due to change in cosmology and the
slope of the equation of state of the IGM, \gamma are of the same order, which
means that we cannot constrain both the parameters simultaneously. However, it
is possible to constrain \gamma and its evolution using the observed LOS
correlation function at different epochs, provided one knows the background
cosmology. We suggest that the constraints on the evolution of \gamma obtained
using the LOS correlation can be used as an independent tool to probe the
reionisation history of the universe. From the transverse correlation function,
we find that the excess probability, over random, of finding two neutral
hydrogen overdense regions separated by an angle \theta, is always less than 1
per cent for redshifts greater than 2. Our models also reproduce the observed
column density distribution for neutral hydrogen and the shape of the
distribution depends on \gamma. Our calculations suggest that one can rule out
\gamma > 1.6 for z \simeq 2.31 using the column density distribution. However,
one cannot rule higher values of \gamma at higher redshifts.


title:
On The Synchrotron Self-Compton Emission from Relativistic Shocks and
  Its Implications for Gamma-Ray Burst Afterglows

abstract:
  We consider the effects of inverse Compton scattering of synchrotron photons
from relativistic electrons in GRB afterglows. We compute the spectrum of the
inverse Compton emission and find that it can dominate the total cooling rate
of the afterglow for several months or even years after the initial explosion.
We demonstrate that the presence of strong inverse Compton cooling can be
deduced from the effect it has on the time-evolution of the cooling break in
the synchrotron spectral component, and therefore on the optical and X-ray
afterglow lightcurves. We then show how the physical interpretation of the
observed characteristics of the synchrotron spectrum must be modified to take
into consideration this extra source of cooling, and give a revised
prescription for computing physical parameters characterizing the expanding
shock wave from the observed quantities. We find that for a given set of
observables (synchrotron break frequencies and fluxes) there is either no
consistent physical interpretation or two of them. Finally we discuss the
prospects of directly detecting the inverse Compton emission with Chandra. We
argue that such a detection is possible for GRBs exploding in a reasonably
dense (n>1 cm^-3) medium.


title: Measurement of [OIII] Emission in Lyman Break Galaxies

abstract:
  Measurements of [OIII] emission in Lyman Break galaxies (LBGs) at z>3 are
presented. Four galaxies were observed with narrow-band filters using the
Near-IR Camera on the Keck I 10-m telescope. A fifth galaxy was observed
spectroscopically during the commissioning of NIRSPEC, the new infrared
spectrometer on Keck II. The emission-line spectrum is used to place limits on
the metallicity. Comparing these new measurements with others available from
the literature, we find that strong oxygen emission in LBGs may suggest
sub-solar metallicity for these objects. The [OIII]5007 line is also used to
estimate the star formation rate (SFR) of the LBGs. The inferred SFRs are
higher than those estimated from the UV continuum, and may be evidence for dust
extinction.


title: Chandra Discovery of a 100 kpc X-ray Jet in PKS 0637--752

abstract:
  The quasar PKS 0637-753, the first celestial X-ray target of the Chandra
X-ray Observatory, has revealed asymmetric X-ray structure extending from 3 to
12 arcsec west of the quasar, coincident with the inner portion of the jet
previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a
redshift of z=0.651, the jet is the largest (~100 kpc) and most luminous
(~10^{44.6} ergs/s) of the few so far detected in X-rays. This letter presents
a high resolution X-ray image of the jet, from 42 ks of data when PKS 0637-753
was on-axis and ACIS-S was near the optimum focus. For the inner portion of the
radio jet, the X-ray morphology closely matches that of new ATCA radio images
at 4.8 and 8.6 GHz. Observations of the parsec scale core using the VSOP space
VLBI mission show structure aligned with the X-ray jet, placing important
constraints on the X-ray source models. HST images show that there are three
small knots coincident with the peak radio and X-ray emission. Two of these are
resolved, which we use to estimate the sizes of the X-ray and radio knots. The
outer portion of the radio jet, and a radio component to the east, show no
X-ray emission to a limit of about 100 times lower flux.
  The X-ray emission is difficult to explain with models that successfully
account for extra-nuclear X-ray/radio structures in other active galaxies. We
think the most plausible is a synchrotron self-Compton (SSC) model, but this
would imply extreme departures from the conventional minimum-energy and/or
homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung
models for the jet X-rays, unless multicomponent or ad hoc geometries are
invoked.


title:
Search for the Identification of 3EG J1835+5918: Evidence for a New Type
  of High-Energy Gamma-ray Source

abstract:
  The EGRET source 3EG J1835+5918 is the brightest and most accurately
positioned of the as-yet unidentified high-energy gamma-ray sources at high
Galactic latitude (l,b=89,25). We present a multiwavelength study of the region
around it, including X-ray, radio, and optical imaging surveys, as well as
optical spectroscopic classification of most of the active objects in this
area. The identified X-ray sources in or near the EGRET error ellipse are
radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radio
sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. In
addition there are no flat-spectrum radio sources in the vicinity. Since no
blazar-like or pulsar-like candidate has been found as a result of these
searches, 3EG J1835+5918 must be lacking one or more of the physically
essential attributes of these known classes of gamma-ray emitters. If it is an
AGN it lacks the beamed radio emission of blazars by at least a factor of 100
relative to identified EGRET blazars. If it is an isolated neutron star, it
lacks the steady thermal X-rays from a cooling surface and the magnetospheric
non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a
pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and
probably an even more efficient or beamed gamma-ray engine.


title: Identifying Old Tidal Dwarf Irregulars

abstract:
  We examine the observational consequences of the two possible origins for
irregular galaxies: formation from collapse of a primordial cloud of gas early
in the age of the Universe, and formation from tidal tails in an interaction
that could have occured any time in the history of the Universe. Because the
formation from tidal tails could have occurred a long time ago, proximity to
larger galaxies is not sufficient to distinguish tidal dwarfs from traditional
dwarfs. We consider the effects of little or no dark matter on rotation speeds
and the Tully-Fisher relationship, the metallicity-luminosity relationship,
structure, and stellar populations. From these selection criteria, we identify
a small list of dwarf irregular galaxies that are candidates for having formed
as tidal dwarfs.


title: Time-Dependence of the Mass Accretion Rate in Cluster Cooling Flows

abstract:
  We analyze two time-dependent cluster cooling flow models in spherical
symmetry. The first assumes that the intracluster gas resides in a static
external potential, and includes the effects of optically thin radiative
cooling and mass deposition. This corresponds to previous steady-state cooling
flow models calculated by White & Sarazin (1987). Detailed agreement is found
between steady-state models and time-dependent models at fixed times in the
simulations. The mass accretion rate is found either to increase or remain
nearly constant once flows reach a steady state. The time rate of change of the
accretion rate is strongly sensitive to the value of the mass deposition
parameter q, but only mildly sensitive to the ratio beta of gravitational
binding energy to gas temperature. We show that previous scaling arguments
presented by Bertschinger (1988) and White (1988) are valid only for mature
cooling flows with weak mass deposition (q ~< 1). The second set of models
includes the effects of a secularly deepening cluster potential and secondary
infall of gas from the Hubble flow. We find that such heating effects do not
prevent the flows from reaching a steady state within an initial central
cooling time.


title: Clustering of Galaxy Clusters in CDM Universes

abstract:
  We use very large cosmological N--body simulations to obtain accurate
predictions for the two-point correlations and power spectra of mass-limited
samples of galaxy clusters. We consider two currently popular cold dark matter
(CDM) cosmogonies, a critical density model ($\tau$CDM) and a flat low density
model with a cosmological constant ($\Lambda$CDM). Our simulations each use
$10^9$ particles to follow the mass distribution within cubes of side
$2h^{-1}$Gpc ($\tau$CDM) and $3h^{-1}$Gpc ($\Lambda$CDM) with a force
resolution better than $10^{-4}$ of the cube side. We investigate how the
predicted cluster correlations increase for samples of increasing mass and
decreasing abundance. Very similar behaviour is found in the two cases. The
correlation length increases from $r_0=12$ -- 13$h^{-1}$Mpc for samples with
mean separation $d_{\rm c}=30h^{-1}$Mpc to $r_0=22$-- 27$h^{-1}$Mpc for samples
with $d_{\rm c}=100h^{-1}$Mpc. The lower value here corresponds to $\tau$CDM
and the upper to $\Lambda$CDM. The power spectra of these cluster samples are
accurately parallel to those of the mass over more than a decade in scale. Both
correlation lengths and power spectrum biases can be predicted to better than
10% using the simple model of Sheth, Mo & Tormen (2000). This prediction
requires only the linear mass power spectrum and has no adjustable parameters.
We compare our predictions with published results for the APM cluster sample.
The observed variation of correlation length with richness agrees well with the
models, particularly for $\Lambda$CDM. The observed power spectrum (for a
cluster sample of mean separation $d_{\rm c}=31h^{-1}$Mpc) lies significantly
above the predictions of both models.


title: Mass function of dark matter halos

abstract:
  We combine data from a number of N-body simulations to predict the abundance
of dark halos in Cold Dark Matter universes over more than 4 orders of
magnitude in mass. A comparison of different simulations suggests that the
dominant uncertainty in our results is systematic and is smaller than 10--30%
at all masses, depending on the halo definition used. In particular, our
``Hubble Volume'' simulations of \tcdm and \lcdm cosmologies allow the
abundance of massive clusters to be predicted with uncertainties well below
those expected in all currently planned observational surveys. We show that for
a range of CDM cosmologies and for a suitable halo definition, the simulated
mass function is almost independent of epoch, of cosmological parameters, and
of initial power spectrum when expressed in appropriate variables. This
universality is of exactly the kind predicted by the familiar Press-Schechter
model, although this model predicts a mass function shape which differs from
our numerical results, overestimating the abundance of ``typical'' halos and
underestimating that of massive systems.


title: Using the acoustic peak to measure cosmological parameters

abstract:
  Recent measurements of the cosmic microwave background radiation by the
Boomerang experiment indicate that the universe is spatially flat. Here some
simple back-of-the-envelope calculations are used to explain their result. The
main result is a simple formula for the angular scale of the acoustic peak in
terms of the standard cosmological parameters:
l=193*[1+3(1-Omega_0)/5+(1-h)/5+Omega_Lambda/35].


title: Reionization by Hard Photons: I. X-rays from the First Star Clusters

abstract:
  Observations of the Ly-alpha forest at z~3 reveal an average metallicity
Z~0.01 Z_solar. The high-redshift supernovae that polluted the IGM also
accelerated relativistic electrons. Since the energy density of the CMB scales
as (1+z)^4, at high redshift these electrons cool via inverse Compton
scattering. Thus, the first star clusters emit X-rays. Unlike stellar UV
ionizing photons, these X-rays can escape easily from their host galaxies. This
has a number of important physical consequences: (i) Due to their large mean
free path, these X-rays can quickly establish a universal ionizing background
and partially reionize the universe in a gradual, homogeneous fashion. If
X-rays formed the dominant ionizing background, the universe would have more
closely resembled a single-phase medium, rather than a two-phase medium. (ii)
X-rays can reheat the universe to higher temperatures than possible with UV
radiation. (iii) X-rays counter the tendency of UV radiation to
photo-dissociate H2, an important coolant in the early universe, by promoting
gas phase H2 formation. The X-ray production efficiency is calibrated to local
observations of starburst galaxies, which imply that ~10% of the supernova
energy is converted to X-rays. While direct detection of sources in X-ray
emission is difficult, the presence of relativistic electrons at high redshift
and thus a minimal level of X-ray emission may be inferred by synchrotron
emission observations with the Square Kilometer Array. These sources may
constitute a significant fraction of the unresolved hard X-ray background, and
can account for both the shape and amplitude of the gamma-ray background. This
paper discusses the existence and observability of high-redshift X-ray sources,
while a companion paper models the detailed reionization physics and chemistry.


title: Probing High Redshift Radiation Fields with Gamma-Ray Absorption

abstract:
  The next generation of gamma-ray telescopes may be able to observe gamma-ray
blazars at high redshift, possibly out to the epoch of reionization. The
spectrum of such sources should exhibit an absorption edge due to
pair-production against UV photons along the line of sight. One expects a sharp
drop in the number density of UV photons at the Lyman edge E_{L}. This implies
that the universe becomes transparent after gamma-ray photons redshift below E
(m_{e}c^2)^{2}/E_{L} 18 GeV. Thus, there is only a limited redshift interval
over which GeV photons can pair produce. This implies that any observed
absorption will probe radiation fields in the very early universe, regardless
of the subsequent star formation history of the universe. Furthermore,
measurements of differential absorption between blazars at different redshifts
can cleanly isolate the opacity due to UV emissivity at high redshift. An
observable absorption edge should be present for most reasonable radiation
fields with sufficient energy to reionize the universe. Ly-alpha photons may
provide an important component of the pair-production opacity. Observations of
a number of blazars at different redshifts will thus allow us to probe the rise
in comoving UV emissivity with time.


title:
On the Kinematic Origin of the Luminosity-Pulse Lag Relationship in
  Gamma-ray Bursts

abstract:
  This paper presents an interpretation based on gamma-ray burst source
kinematics for the relationship found by Norris et al. between peak luminosity
and energy-dependent pulse lag. I argue that the correlation should instead be
between "number" luminosity and pulse lag. This interpretation improves the
least-squares fit of this correlation for the known bursts by 25 percent or
more. It also suggests a distance estimation scheme. I propose that this
relationship is due to the variation in line-of-sight velocity among bursts.
This interpretation allows one to speculate on the range of gamma-ray burst
expansion velocities or the size of their jet opening angles.


title:
The Cosmological Constant Problems (Talk given at Dark Matter 2000,
  February, 2000)

abstract:
  The old cosmological constant problem is to understand why the vacuum energy
is so small; the new problem is to understand why it is comparable to the
present mass density. Several approaches to these problems are reviewed.
Quintessence does not help with either; anthropic considerations offer a
possibility of solving both. In theories with a scalar field that takes random
initial values, the anthropic principle may apply to the cosmological constant,
but probably to nothing else.


title: ORFEUS II echelle spectra : H_2 measurements in the Magellanic Clouds

abstract:
  More than 20 years after the Copernicus satellite, ORFEUS allows the
investigation of molecular hydrogen (H_2) in the diffuse interstellar medium by
way of FUV absorption spectroscopy once again. This time, targets in the
Magellanic Clouds were also observed, allowing the first investigation of cold
H_2 in a metal-poor environment outside the Milky Way disk. This paper presents
new H_2 measurements in LMC gas along the lines of sight toward HD 269698 (Sk
67 166), HD 269546 (Sk -68 82) and HD 36402 (Sk 67 104) and summarizes the
ORFEUS H_2 measurements in the the Magellanic Clouds. For seven lines of sight
we investigate correlations between N(H I), N(H_2), E(B-V) and compare the
results with H_2 observations in the Milky Way disk. We find that the abundance
of H_2 is low in comparison to the total gas quantity and speculate that the
fraction of hydrogen in molecular form is limited by the lower dust content of
the Magellanic Clouds.


title: An Extragalactic HI Cloud with No Optical Counterpart?

abstract:
  We report the discovery, from the HI Parkes All-Sky Survey (HIPASS), of an
isolated cloud of neutral hydrogen which we believe to be extragalactic. The HI
mass of the cloud (HIPASS J1712-64) is very low, 1.7 x 10^7 Msun, using an
estimated distance of ~3.2 Mpc. Most significantly, we have found no optical
companion to this object to very faint limits (mu(B)~ 27 mag arcsec^-2). HIPASS
J1712-64 appears to be a binary system similar to, but much less massive than,
HI 1225+01 (the Virgo HI Cloud) and has a size of at least 15 kpc. The mean
velocity dispersion, measured with the Australia Telescope Compact Array
(ATCA), is only 4 km/s for the main component and because of the weak or
non-existent star-formation, possibly reflects the thermal linewidth (T<2000 K)
rather than bulk motion or turbulence. The peak column density for HIPASS
J1712-64, from the combined Parkes and ATCA data, is only 3.5 x 10^19 cm^-2,
which is estimated to be a factor of two below the critical threshold for star
formation. Apart from its significantly higher velocity, the properties of
HIPASS J1712-64 are similar to the recently recognised class of Compact High
Velocity Clouds. We therefore consider the evidence for a Local Group or
Galactic origin, although a more plausible alternative is that HIPASS J1712-64
was ejected from the interacting Magellanic Cloud/Galaxy system at
perigalacticon ~ 2 x 10^8 yr ago.


title: First results of UVES at VLT: abundancesin the Sgr dSph

abstract:
  Two giants of the Sagittarius dwarf spheroidal have been observed with the
UVES spectrograph on the ESO 8.2m Kueyen telescope, during the commissioning of
the instrument. Sgr 139 has [Fe/H]=-0.28 and Sgr 143 [Fe/H]=-0.21, these values
are considerably higher than photometric estimates of the metallicity of the
main population of Sgr. We derived abundances for O, Na, Mg, Al, Si, Ca, Sc,
Ti, V, Cr, Mn, Co, Ni, Cu, Y, Ba, La, Ce, Nd and Eu; the abundance ratios found
are essentially solar with a few exceptions: Na shows a strong overdeficiency,
the heavy elements Ba to Eu, are overabundant, while Y is underabundat. The
high metallicity derived implies that the Sgr galaxy has experienced a high
level of chemical processing. The stars had been selected to be representative
of the two main stellar populations of Sagittarius, however, contrary to what
expected from the photometry, the two stars show a very similar chemical
composition. We argue that the most likely explanation for the difference in
the photometry of the two stars is a different distance, Sgr 143 being about
2Kpc nearer than Sgr 139. This result suggests that the interpretation of
colour -- magnitude diagrams of Sgr is more complex than previously thought and
the effect of the line of sight depth should not be neglected. It also shows
that spectroscopic abundances are required for a correct interpretation of Sgr
populations.


title:
Intrinsic Correlation of Galaxy Shapes: Implications for Weak Lensing
  Measurements

abstract:
  Weak gravitational lensing is now established as a powerful method to measure
mass fluctuations in the universe. It relies on the measurement of small
coherent distortions of the images of background galaxies. Even low-level
correlations in the intrinsic shapes of galaxies could however produce a
significant spurious lensing signal. These correlations are also interesting in
their own right, since their detection would constrain models of galaxy
formation. Using $3\times 10^{4} - 10^5$ halos found in N-body simulations, we
compute the correlation functions of the intrinsic ellipticity of spiral
galaxies assuming that the disk is perpendicular to the angular momentum of the
dark matter halo. We also consider a simple model for elliptical galaxies, in
which the shape of the dark matter halo is assumed to be the same as that of
the light. For deep lensing surveys with median redshifts $\sim 1$, we find
that intrinsic correlations of $\sim 10^{-4}$ on angular scales $\theta \sim
0.1-10'$ are generally below the expected lensing signal, and contribute only a
small fraction of the excess signals reported on these scales. On larger scales
we find limits to the intrinsic correlation function at a level $\sim 10^{-5}$,
which gives a (model-dependent) range of separations for which the intrinsic
signal is about an order of magnitude below the ellipticity correlation
function expected from weak lensing. Intrinsic correlations are thus negligible
on these scales for dedicated weak lensing surveys. For wider but shallower
surveys such as SuperCOSMOS, APM and SDSS, we cannot exclude the possibility
that intrinsic correlations could dominate the lensing signal. We discuss how
such surveys could be used to calibrate the importance of this effect, as well
as study spin-spin correlations of spiral galaxies.


title: BeppoSAX discovery of a new Seyfert 2 galaxy: 1SAXJ2234.8-2541

abstract:
  In the present work we report the BeppoSAX serendipitous discovery of the
type 2 AGN 1SAXJ2234.8-2541 in the MECS field of view when pointing at the
Seyfert 2 galaxy NGC7314. The source is optical identified with the bright
(m_B=14.40) galaxy ESO533-G50 at redshift z=0.026. The source is clearly
detected at energies above 4 keV but barely visible below this energy implying
heavy obscuration intrinsic to the source. Spectral analysis indicates a column
density of the order of 2-3 x 10^(23) cm^-2$ and a power law photon index
compatible with values often seen in active galaxies. These X-ray
characteristics suggest a Seyfert 2 classification for 1SAXJ2234.8-2541.
Subsequent spectroscopy of the optical conterpart, ESO533-G50, performed with
the ESO 1.52m telescope at La Silla Observatory, confirms the type 2 nature of
this source and therefore its identification with the X-ray source. We also
report the marginal detection of an iron line centered at 7.29 +/- 0.47 keV and
having an equivalent width of 464 (+636 -398) eV; although marginal, this
result is indicative of the presence of warm material in the source. If instead
the line is associated to cold material, we estimate an upper limit to its
equivalent width of 290 eV. The overall characteristics of 1SAXJ2234.8-2541
strongly suggest that the source is Compton thin.


title: Stellar indices and kinematics in Seyfert 1 nuclei

abstract:
  We present spectra of 6 type 1 Seyfert galaxies, 2 Seyfert 2, a starburst
galaxy and a compact narrow line radiogalaxy, taken in two spectral ranges
centered around the near--IR CaII triplet (CaT) (at ~8600 Angstroms), and the
Mgb stellar feature at 5180 Angstroms. We measured the equivalent width (EWs)
of these features and the Fe52 and Fe53 spectral indices. We found that the
strength of the CaT in type 1 Seyfert galaxies with prominent central point
sources, is larger than what would be expected from the observed strength of
the blue indices. This could be explained by the presence of red supergiants in
the nuclei of Seyfert 1 galaxies. On the other hand, the blue indices of these
galaxies could also be diluted by the strong FeII multiplets that can be seen
in their spectra.
  We have also measured the stellar and gas velocity dispersions of the
galaxies in the sample. The stellar velocity dispersions were measured using
both, the Mgb and CaT stellar features. The velocity dispersion of the gas in
the narrow line region (NLR) was measured using the strong emission lines
[OIII] 5007, 4959 and [SIII] 9069. We compare the gas and star velocity
dispersions and find that both magnitudes are correlated in Seyfert galaxies.
Most of the Seyfert 1 we observe have stellar velocity dispersion somehow
greater than that of the gas in the NLR.


title: The Infrared Jet In 3C66B

abstract:
  We present images of infrared emission from the radio jet in 3C66B. Data at
three wavelengths (4.5, 6.75 and 14.5 microns) were obtained using the Infrared
Space Observatory. The 6.75 micron image clearly shows an extension aligned
with the radio structure. The jet was also detected in the 14.5 micron image,
but not at 4.5 micron. The radio-infrared-optical spectrum of the jet can be
interpreted as synchrotron emission from a population of electrons with a
high-energy break of 4e11 eV. We place upper limits on the IR flux from the
radio counter-jet. A symmetrical, relativistically beamed twin-jet structure is
consistent with our results if the jets consist of multiple components.


title: Detection of interstellar CH_3

abstract:
  Observations with the Short Wavelength Spectrometer (SWS) onboard the {\it
Infrared Space Observatory} (ISO) have led to the first detection of the methyl
radical ${\rm CH_3}$ in the interstellar medium. The $\nu_2$ $Q-$branch at 16.5
$\mu$m and the $R$(0) line at 16.0 $\mu$m have been unambiguously detected
toward the Galactic center SgrA$^*$. The analysis of the measured bands gives a
column density of (8.0$\pm$2.4)$\times10^{14}$ cm$^{-2}$ and an excitation
temperature of $(17\pm 2)$ K. Gaseous ${\rm CO}$ at a similarly low excitation
temperature and ${\rm C_2H_2}$ are detected for the same line of sight. Using
constraints on the ${\rm H_2}$ column density obtained from ${\rm C^{18}O}$ and
visual extinction, the inferred ${\rm CH_3}$ abundance is
$(1.3{{+2.2}\atop{-0.7}}) \times 10^{-8}$. The chemically related ${\rm CH_4}$
molecule is not detected, but the pure rotational lines of ${\rm CH}$ are seen
with the Long Wavelength Spectrometer (LWS). The absolute abundances and the
${\rm CH_3/CH_4}$ and ${\rm CH_3/CH}$ ratios are inconsistent with published
pure gas-phase models of dense clouds. The data require a mix of diffuse and
translucent clouds with different densities and extinctions, and/or the
development of translucent models in which gas-grain chemistry, freeze-out and
reactions of ${\rm H}$ with polycyclic aromatic hydrocarbons and solid
aliphatic material are included.


title:
Wide-field imaging at mid infrared wavelengths: reconstruction of
  chopped and nodded data

abstract:
  Ground-based astronomical observations at thermal infrared wavelengths face
the problem of extracting the weak astronomical signal from the large and
rapidly variable background flux. The observing strategy most commonly used,
the so-called ``chopping and nodding'' differential technique, provides
reliable representations of the target uniquely in the case of compact sources
while extended and complex sources can be easily distorted by their negative
counterparts. A restoration method, designed to remove the negative values and
to provide reliable representations of extended sources has been proposed. We
show that the algorithm successfully removes the distortions due to the
negative counterparts and, in addition, provides noise reduction.


title: High redshift radio galaxies

abstract:
  There is considerable evidence that powerful radio quasars and radio galaxies
are orientation-dependent manifestations of the same parent population: massive
spheroids containing correspondingly massive black holes. Following the
recognition of this unification, research is directed to the task of
elucidating the structure and composition of the active nuclei and their hosts
to understand the formation and evolution of what we expect to become the most
massive of galaxies. In contrast to the quasars, where the nucleus can outshine
the galaxy at optical/near infrared wavelengths by a large factor, the radio
galaxies contain a 'built-in coronograph' that obscures our direct view to the
nucleus. These objects present our best opportunity to study the host galaxy in
detail. Of particular interest are those sources with redshifts greater than
about 2 that represent an epoch when nuclear activity was much more common that
it is now and when we believe these objects were in the process of assembly. In
combination with high resolution imaging from space (HST), optical
spectropolarimetry with Keck II allows us to clearly separate the scattered
nuclear radiation from the stellar and gaseous emission from the host galaxy.
The rest-frame ultraviolet emission line spectra suggest that rapid chemical
evolution is occurring at this epoch. Near infrared spectroscopy with the VLT
is giving us access to both the lines and continuum in the rest-frame optical
spectrum, allowing investigations of the evolved stellar population and
extending the composition analysis with measurements of the familiar
forbidden-line spectrum.


title: Pulsational instability domain of Delta Scuti variables

abstract:
  An updated theoretical instability domain of the Delta Scuti star models in
the Hertzsprung-Russell diagram, in the log g - log Teff diagram and in
diagrams for dereddened uvby-beta photometric indices is presented. The
sensitivity of both the position of the evolutionary tracks and the Blue Edge
of the instability domain to changes in the chemical composition parameters (X,
Z) and to changes in the convection theory parameters (mixing-length in the
stellar envelope, the extent of the overshooting from the convective core) is
discussed.


title: Astronomers and the Science Citation Index, 1981--1997

abstract:
  The Institute for Science Information (ISI) has generated two lists of
citation information for astronomers that are uniquely restricted both as to
the years surveyed for the cited papers, and the years surveyed for the citing
papers. The main list gives citation data for 62,813 physicists and astronomers
whose journal papers were cited 100 times or more from 1981.0 to 1997.5 by
papers published during the same time interval. The second list gives the 200
most-cited papers/year published in refereed astronomical journals from
1981-1996, as cited in papers in those same journals from 1981.0-1998.0.
Astronomer names were selected from various sources. From this work an
Astronomy Citation Database (ACD) has been constructed, containing citation
data for 6458+ astronomers, plus additional data for 173 astronomers and the
top-10 cited papers published 1981-1996. Various problems, both substantial and
subtle, of producing a reasonably fair citation database are detailed. Chief
among these are whether to assign either parital or full credit for each author
on a given paper. Whether one is honored with one of the top astronomy
lifetime-awards is a strong function of how well your work stands out as your
own, and in what kind of publications you publish (e.g., the ISI does not keep
track of papers in meeting proceedings/books/catalogs). Name confusion affects
this kind of analysis to the extent that that it would take an enormous effort
to disentangle its effects and, even then, not all name confusion would be
settled. A ``modest proposal'' is made that our professional field (and others)
go to a system of uniquely associating an identification number to each author
on each paper to solve the name confusion problem.


title: An alpha theory of time-dependent warped accretion discs

abstract:
  The non-linear fluid dynamics of a warped accretion disc was investigated in
an earlier paper by developing a theory of fully non-linear bending waves in a
thin, viscous disc. That analysis is here extended to take proper account of
thermal and radiative effects by solving an energy equation that includes
viscous dissipation and radiative transport. The problem is reduced to simple
one-dimensional evolutionary equations for mass and angular momentum, expressed
in physical units and suitable for direct application. This result constitutes
a logical generalization of the alpha theory of Shakura & Sunyaev to the case
of a time-dependent warped accretion disc. The local thermal-viscous stability
of such a disc is also investigated.


title: The European Large Area ISO Survey: Latest Result

abstract:
  We present some recent results from the European Large Area ISO Survey
(ELAIS). This survey was the largest non-serendipitous ISO field survey. A
preliminary reduction has recently been completed and catalogues of sources
released to the community. Early results show strongly evolving source counts.
A comprehensive identification programme is underway and a number of extremely
luminous objects have already been discovered. This survey provides an exciting
legacy from the ISO mission and (amongst many goals) will allow us to provide
important constraints on the obscured star-formation history of the Universe.


title:
The Nature of the Lithium Rich Giants. Mixing episodes on the RGB and
  early-AGB

abstract:
  We present a critical analysis of the nature of the so-called Li-rich RGB
stars. For a majority of the stars, we have used Hipparcos parallaxes to
determine masses and evolutionary states by comparing their position on the
Hertzsprung-Russell diagram with theoretical evolutionary tracks. Among the
twenty Li-rich giants whose location on the HR diagram we were able to
determine precisely, eight should be re-classified as Li-normal. For the
remaining stars, the high Li abundance must be a result of fresh synthesis of
this fragile element. We identify two distinct episodes of Li production which
occur in advanced evolutionary phases depending upon the mass of the star. We
suggest that the Li-rich stars are formed mainly as a precursor to the deeper
mixing process which produces anomalously low carbon isotopic ratios. Low-mass
RGB stars, which later undergo the helium flash, produce Li at the phase
referred to as the bump in the luminosity function. In intermediate-mass stars,
the Li-rich phase occurs on the early-AGB. The compiled data provide
constraints on the time scales for extra mixing and some insight on processes
suggested in the literature. However, our results do not suggest any specific
trigger mechanism. Since the Li-rich phases are extremely short, enrichment of
the Li content of the ISM as a result of these episodes is negligible.


title: A Dynamical Study of Galaxies in the Hickson Compact Groups

abstract:
  In order to investigate dynamical properties of spiral galaxies in the
Hickson compact groups (HCGs), we present rotation curves of 30 galaxies in 20
HCGs. We found as follows. 1) There is not significant relation between
dynamical peculiarity and morphological peculiarity in HCG spirals. 2) There is
no significant relation between the dynamical properties and the frequency
distribution of nuclear activities in HCG spirals. 3) There are no significant
correlations between the dynamical properties of HCG spirals and any group
properties (i.e., the size, the velocity dispersion, the galaxy number density,
and the crossing time). 4) Asymmetric and peculiar rotation curves are more
frequently seen in the HCG spirals than in field spirals and in cluster ones.
However, this tendency is more obviously seen in late-type HCG spirals. These
results suggest that the dynamical properties of HCG spirals do not strongly
correlate with the morphology, the nuclear activity, and the group properties.
Our results also suggest that more frequent galaxy collisions occur in the HCGs
than in the field and in the clusters.


title: New Detremination Method of Primordial Li Abundance

abstract:
  We discuss the primordial nucleosynthesis in lepton asymmetric Universe
models. In order to better estimate the universal baryon-mass density parameter
$\Omega_b$, we try to remove the uncertainty from the theoretical prediction of
primordial $^7$Li abundance. We propose a new method to determine the
primordial $^7$Li by the use of isotopic abundance ratio $^7$Li/$^6$Li in the
interstellar medium which exhibits the minimum effects of the stellar
processes.


title: Monitoring All Sky for Variability

abstract:
  A few percent of all stars are variable, yet over 90% of variables brighter
than 12 magnitude have not been discovered yet. There is a need for an all sky
search and for the early detection of any unexpected events: optical flashes
from gamma-ray bursts, novae, dwarf novae, supernovae, `killer asteroids'. The
ongoing projects like ROTSE, ASAS, TASS, and others, using instruments with
just 4 inch aperture, have already discovered thousands of new variable stars,
a flash from an explosion at a cosmological distance, and the first partial
eclipse of a nearby star by its Jupiter like planet. About one million
variables may be discovered with such small instruments, and many more with
larger telescopes. The critical elements are software and full automation of
the hardware. A complete census of the brightest eclipsing binaries is needed
to select objects for a robust empirical calibration of the the accurate
distance determination to the Magellanic Clouds, the first step towards the
Hubble constant. An archive to be generated by a large number of small
instruments will be very valuable for data mining projects. The real time
alerts will provide great targets of opportunity for the follow-up observations
with the largest telescopes.


title:
Multiwavelength study of the nuclei of a volume-limited sample of
  galaxies I: X-ray observations

abstract:
  We discuss ROSAT HRI X-ray observations of 33 very nearby galaxies, sensitive
to X-ray sources down to a luminosity of approximately 10^38 erg/s. The
galaxies are selected from a complete, volume limited sample of 46 galaxies
with d<7 Mpc for which we have extensive multi-wavelength data. For an almost
complete sub-sample with M_B<-14 (29/31 objects) we have HRI images. Contour
maps and source lists are presented within the central region of each galaxy,
together with nuclear upper limits where no nuclear source was detected.
Nuclear X-ray sources are found to be very common, occurring in ~35% of the
sample. Nuclear X-ray luminosity is statistically connected to host galaxy
luminosity - there is not a tight correlation, but the probability of a nuclear
source being detected increases strongly with galaxy luminosity and the
distribution of nuclear luminosities seems to show an upper envelope that is
roughly proportional to galaxy luminosity. While these sources do seem to be a
genuinely nuclear phenomenon rather than nuclear examples of the general X-ray
source population, it is far from obvious that they are miniature Seyfert
nuclei. The more luminous nuclei are very often spatially extended, and HII
region nuclei are detected just as often as LINERs. Finally, we also note the
presence of fairly common super-luminous X-ray sources in the off-nuclear
population - out of 29 galaxies we find 9 sources with a luminosity larger than
10^39 erg/s. These show no particular preference for more luminous galaxies.
One is already known to be a multiple SNR system, but most have no obvious
optical counterpart and their nature remains a mystery.


title:
The Size-Frequency Distribution of the Zodiacal Cloud: Evidence from the
  Solar System Dust Bands

abstract:
  Recent observations of the size-frequency distribution of zodiacal cloud
particles obtained from the cratering record on the LDEF satellite (Love and
Brownlee 1993) reveal a significant large particle population (100 micron
diameter or greater) near 1 AU. Our previous modeling of the Solar System dust
bands (Grogan et al 1997), features of the zodiacal cloud associated with the
comminution of Hirayama family asteroids, has been limited by the fact that
only small particles (25 micron diameter or smaller) have been considered. This
was due to the prohibitively large amount of computing power required to
numerically analyze the dynamics of larger particles. The recent availability
of cheap, fast processors has finally made this work possible. Models of the
dust bands are created, built from individual dust particle orbits, taking into
account a size-frequency distribution of the material and the dynamical history
of the constituent particles. These models are able to match both the shapes
and amplitudes of the dust band structures observed by IRAS in multiple
wavebands. The size-frequency index, q, that best matches the observations is
approximately 1.4, consistent with the LDEF results in that large particles are
shown to dominate. However, in order to successfully model the `ten degree'
band, which is usually associated with collisional activity within the Eos
family, we find that the mean proper inclination of the dust particle orbits
has to be approximately 9.35 degrees, significantly different to the mean
proper inclination of the Eos family (10.08 degrees).


title: Large Scale Pressure Fluctuations and Sunyaev-Zel'dovich Effect

abstract:
  The Sunyaev-Zel'dovich (SZ) effect associated with pressure fluctuations of
the large scale structure gas distribution will be probed with current and
upcoming wide-field small angular scale cosmic microwave background
experiments. We study the generation of pressure fluctuations by baryons which
are present in virialized dark matter halos and by baryons present in small
overdensities. For collapsed halos, assuming the gas distribution is in
hydrostatic equilibrium with matter density distribution, we predict the
pressure power spectrum and bispectrum associated with the large scale
structure gas distribution by extending the dark matter halo approach which
describes the density field in terms of correlations between and within halos.
The projected pressure power spectrum allows a determination of the resulting
SZ power spectrum due to virialized structures. The unshocked photoionized
baryons present in smaller overdensities trace the Jeans-scale smoothed dark
matter distribution. They provide a lower limit to the SZ effect due to large
scale structure in the absence of massive collapsed halos. We extend our
calculations to discuss higher order statistics, such as bispectrum and
skewness in SZ data. The SZ-weak lensing cross-correlation is suggested as a
probe of correlations between dark matter and baryon density fields, while the
probability distribution functions of peak statistics of SZ halos in wide field
CMB data can be used as a probe of cosmology and non-Gaussian evolution of
large scale structure pressure fluctuations.


title: MHD Stellar and Disk Winds: Application to Planetary Nebulae

abstract:
  MHD winds can emanate from both stars and surrounding accretion disks. It is
of interest to know how much wind power is available and which (if either) of
the two rotators dominates that power. We investigate this in the context of
multi-polar planetary nebulae (PNe) and proto-planetary nebulae (PPNe), for
which recent observations have revealed the need for a wind power source in
excess of that available from radiation driving, and a possible need for
magnetic shaping. We calculate the MHD wind power from a coupled disk and star,
where the former results from binary disruption. The resulting wind powers
depend only on the accretion rate and stellar properties. We find that if the
stellar envelope were initially slowly rotating, the disk wind would dominate
throughout the evolution. If the envelope of the star were rapidly rotating,
the stellar wind could initially be of comparable power to the disk wind until
the stellar wind carries away the star's angular momentum. Since an initially
rapidly rotating star can have its spin and magnetic axes misaligned to the
disk, multi-polar outflows can result from this disk wind system. For times
greater than a spin-down time, the post-AGB stellar wind is slaved to the disk
for both slow and rapid initial spin cases and the disk wind luminosity
dominates. We find a reasonably large parameter space where a hybrid star+disk
MHD driven wind is plausible and where both or either can account for PPNe and
PNe powers. We also speculate on the morphologies which may emerge from the
coupled system. The coupled winds might help explain the shapes of a number of
remarkable multi-shell or multi-polar nebulae. Magnetic activity such as X-ray
flares may be associated with the both central star and the disk and would be a
valuable diagnostic for the dynamical role of MHD processes in PNe.


title: The infrared counterpart of the Z source GX5-1

abstract:
  We have obtained UKIRT infrared observations of the field of the bright
Galactic Z source GX5-1. From an astrometric plate solution tied to Tycho-ACT
standards we have obtained accurate positions for the stars in our field which,
combined with an accurate radio position, have allowed us to identify the
probable infrared counterpart of GX5-1. Narrow-band photometry marginally
suggests excess Br-gamma emission in the counterpart, supporting its
association with an accretion-disc source. No significant variability is
observed in a limited number of observations. We compare the H and K magnitudes
with those of other Z sources, and briefly discuss possible sources of infrared
emission in these systems.


title: From Stars to Super-planets: the Low-Mass IMF in the Young Cluster IC348

abstract:
  We investigate the low-mass population of the young cluster IC348 down to the
deuterium-burning limit, a fiducial boundary between brown dwarf and planetary
mass objects, using a new and innovative method for the spectral classification
of late-type objects. Using photometric indices, constructed from HST/NICMOS
narrow-band imaging, that measure the strength of the 1.9 micron water band, we
determine the spectral type and reddening for every M-type star in the field,
thereby separating cluster members from the interloper population. Due to the
efficiency of our spectral classification technique, our study is complete from
approx 0.7 Msun to 0.015 Msun. The mass function derived for the cluster in
this interval, dN/dlogM \propto M^{0.5}, is similar to that obtained for the
Pleiades, but appears significantly more abundant in brown dwarfs than the mass
function for companions to nearby sun-like stars. This provides compelling
observational evidence for different formation and evolutionary histories for
substellar objects formed in isolation vs. as companions. Because our
determination of the IMF is complete to very low masses, we can place
interesting constraints on the role of physical processes such as fragmentation
in the star and planet formation process and the fraction of dark matter in the
Galactic halo that resides in substellar objects.


title: Statistical characteristics of simulated walls

abstract:
  The large scale matter distribution in three different simulations of CDM
models is investigated and compared with corresponding results of the
Zel'dovich theory of nonlinear gravitational instability. We show that the
basic characteristics of wall-like structure elements are well described by
this theory, and that they can be expressed by the cosmological parameters and
a few spectral moments of the perturbation spectrum. Therefore the
characteristics of such elements provide reasonable estimates of these
parameters. We show that the compressed matter is relaxed and gravitationally
confined, what manifests itself in the existence of walls as (quasi)stationary
structure elements with life time restricted by their disruption into high
density clouds.
  The matter distribution is investigated both in the real and redshift spaces.
In both cases almost the same particles form the walls, and we estimate
differences in corresponding wall characteristics. The same methods are applied
to several mock catalogues of 'galaxies' what allows us to characterize a large
scale bias between the spatial distribution of dark matter and of simulated
`galaxies'.


title:
Embedded, Self-Gravitating Equilibria in Sheetlike and Filamentary
  Molecular Clouds

abstract:
  Numerical solutions of the isothermal Lane-Emden equation are presented,
corresponding to self-gravitating gaseous cores embedded within a finite
density envelope of overall cylindrical symmetry. These structures may be
members of a fragmentation hierarchy proceeding from sheets, to filaments, to
elongated, prolate clumps. The embedded solutions are the first of their kind,
and as such represent a significant improvement upon the isolated cloud
paradigm used almost exclusively by previous authors. The properties of the
equilibria are in reasonable agreement with observations of dense molecular
cores in star-forming clouds, despite the fact that there is only one free
parameter in the models. We show that this parameter may be identified with the
critical wavelength for instability in the parent filament. The implications of
further fragmentation and the possible influence of magnetic fields are briefly
discussed.


title:
Observations of the Soft Gamma-Ray early Afterglow emission from Two
  Bright Gamma-ray Bursts

abstract:
  We present the results of observations of the soft gamma-ray early afterglows
with the energy above 100keV from two bright Gamma-ray bursts detected by the
PHEBUS instrument of the GRANAT orbital observatory. We show that the light
curves of GRB910402 and GRB920723 events present the afterglow emission with
fading fluxes. During our observations ~700s for these gamma-ray bursts the
afterglow emission was fading as the power law of time with indices equal to
-0.70+/-0.04 and -0.60+/-0.05 (at 1sigma confidence level). In both cases just
after the end of the GRB event we observed the energy spectrum of the afterglow
emissions which was softer than the energy spectrum of the main GRB events. The
average photon index of the main GRB event (in 100-800keV range) is equal to
\~1.5. In the beginning of the afterglow emission the energy spectra of both
events correspond to the much softer spectra with the photon index of ~2.5. We
found that the times of abrupt softening of the burst spectra correspond within
statistical errors to the moment when the afterglow emission begins to dominate
over GRB emission. We found that during the afterglow emission of GRB910402 the
statistically significant hardening of its spectra was observed. This is the
first observation of hardening of GRB afterglow emission. Analysis of GRB910402
and its afterglow showed that this GRB source emits during ~700s of our
observations in soft gamma-rays (100-500keV) only ~1.6% of its total energy
released during the main event. For the GRB920723 we found that in afterglow
during ~700s of our observations ~6% of the GRB total energy was released.


title: Exploring the evolution of spiral galaxies

abstract:
  We have constructed a family of simple models for spiral galaxy evolution to
allow us to investigate observational trends in star formation history with
galaxy parameters. The models are used to generate broad band colours from
which ages and metallicities are derived in the same way as the data. We
generate a grid of model galaxies and select only those which lie in regions of
parameter space covered by the sample. The data are consistent with the
proposition that the star formation history of a region within a galaxy depends
primarily on the local surface density of the gas but that one or two
additional ingredients are required to fully explain the observational data.
The observed age gradients appear steeper than those produced by the density
dependent star formation law, indicating that the star formation law or infall
history must vary with galactocentric radius. Furthermore, the
metallicity-magnitude and age-magnitude correlations are not reproduced by a
local density dependence alone. These correlations require one or both of the
following: (i) a combination of mass dependent infall and metal enriched
outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these
possibilities on the basis of current data is extremely difficult.


title: Constraints on diffuse neutrino background from primordial black holes

abstract:
  We calculated the energy spectra and the fluxes of electron neutrino emitted
in the process of evaporation of primordial black holes (PBHs) in the early
universe. It was assumed that PBHs are formed by a blue power-law spectrum of
primordial density fluctuations. We obtained the bounds on the spectral index
of density fluctuations assuming validity of the standard picture of
gravitational collapse and using the available data of several experiments with
atmospheric and solar neutrinos. The comparison of our results with the
previous constraints (which had been obtained using diffuse photon background
data) shows that such bounds are quite sensitive to an assumed form of the
initial PBH mass function.


title: The Star Formation History of the Local Group

abstract:
  The star formation histories of Local Group galaxies are summarized. The
three large spirals are discussed individually, whereas the discussion of the
Local Group dwarfs concentrates on differences and commonalities. While Local
Group galaxies exhibit an amazing diversity in their star formation and
enrichment histories even within the same morphological type, they share a
number of common global properties. Major determining factors for their
evolution are galaxy mass and environmental effects such as ram pressure and
tidal stripping. It seems likely that low-mass dwarf irregular galaxies evolve
into dwarf spheroidals.


title: Cosmic Microwave Background Temperature at 1280 MHz

abstract:
  The absolute temperature of the cosmic microwave background (CMB) has been
measured at a frequency of 1280 MHz. The observation was made with a modified
version of the L-band receiver used in the Giant Metre wavelength Radio
Telescope (GMRT): the feed horn was replaced by a corrugated plate and the
receiver was placed on the ground, directed at zenith, and shielded from ground
radiation by an aluminium screen with corrugated edges. Novel techniques have
been adopted for (1) reducing and cancelling unwanted contributions to the
system temperature of the receiver and (2) calibrating the contributions from
the feed assembly and receiver. The thermodynamic temperature of the CMB is
estimated to be 3.45 +/- 0.78 K.


title:
Spectrum Analysis of the Type Ib Supernova 1999dn: Probable
  Identifications of C II and H-alpha

abstract:
  Low resolution spectra of SN 1999dn at early times are presented and compared
with synthetic spectra generated with the parameterized supernova
synthetic-spectrum code SYNOW. We find that the spectra of SN 1999dn strongly
resemble those of SN 1997X and SN 1984L, and hence we classify it as a Type Ib
event. Line-identifications are established through spectrum synthesis. Strong
evidence of both H-alpha and C II 6580 is found. We infer that H-alpha appears
first, before the time of maximum brightness, and then is blended with and
finally overwhelmed by the C II line after maximum; this favors a thin
high-velocity hydrogen skin in this Type Ib supernova.


title:
A Decaying Ultra Heavy Dark Matter (WIMPZILLA): Review of Recent
  Progress

abstract:
  Recent theoretical and observational motivations for existence of a decaying
Ultra Heavy Dark Matter (UHDM) are reviewed. We show that present data from
Ultra High Energy Cosmic Rays (UHECRs) and SN - Ia are compatible with a
relatively short lifetime of UHDM.


title: The radiation driven winds of rotating B[e] supergiants

abstract:
  We have formulated the momentum equation for sectorial line driven winds from
rotating stars including: (a) the oblateness of the star, (b) gravity darkening
(von Zeipel effect), (c) conservation of angular momentum, (d) line driving
specified by the force multiplier parameters (k, alpha, delta), (e) finite disk
correction factors for an oblate star with gravity darkening for both the
continuum and the line driving. The equations are solved numerically. We
calculated the distribution of the mass flux and the wind velocity from the
pole to the equator for the winds of B[e]-supergiants. Rotation decreases the
terminal velocity in the equatorial region but hardly affects the wind velocity
from the poles; it enhances the mass flux from the poles while the mass flux
from the equator remains nearly the same. These effects increase with
increasing rotation rates.
  We also calculated models with a bi-stability jump around 25 000 K, using
force multipliers recently calculated with a Monte Carlo technique. In this
case the mass flux increases drastically from the pole to the equator and the
terminal velocity decreases drastically from pole to equator. This produces a
density contrast in the wind rho(equator/pole) of about a factor 10 independent
of the rotation rate of the star. We suggest that the observed density contrast
of a factor 100 of the disks of B[e] stars may be reached by taking into
account the wind compression due to the trajectories of the gas above the
critical point towards the equatorial plane.


title:
General relativistic treatment of the thermal, magnetic and rotational
  evolution of neutron stars with crustal magnetic fields

abstract:
  We investigate the thermal, magnetic and rotational evolution of isolated
neutron stars assuming that the dipolar magnetic field is confined to the
crust. Our treatment, for the first time, uses a fully general relativistic
formalism not only for the thermal but also for the magnetic part, and includes
partial general relativistic effects in the rotational part. Due to the fact
that the combined evolution depends crucially upon the compactness of the star,
three different equations of state have been employed in the calculations. In
the absence of general relativistic effects, while upon increasing compactness
a decrease of the crust thickness takes place leading into an accelerating
field decay, the inclusion of general relativistic effects intend to
``decelerate this acceleration''. As a consequence we find that within the
crustal field hypothesis, a given equation of state is compatible with the
observed periods $P$ and period derivative $\dot P$ provided the initial field
strength and current location as well as the magnitude of the impurity content
are constrained appropriately. Finally, we access the flexibility of the soft,
medium and stiff classes of equations of state as candidates in describing the
state of the matter in the neutron star interiors. The comparison of our model
calculations with observations, together with the consideration of independent
information about neutron star evolution, suggests that a not too soft equation
of state describes neutron star interiors and its cooling proceeds along the
`standard' scenario.


title: Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts

abstract:
  We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts
are cosmological and they are beamed in the form of a conical jet with a large
bulk Lorentz factor $\sim 100$. In such a conic beam, the relativistic ejecta
may have a spatial variation in the bulk Lorentz factor and the density
distribution of gamma-ray emitting jet material. An apparent luminosity
function arises because the axis of the cone is randomly oriented with respect
to the observer's line of sight. The width and the shape of the luminosity
function are determined by the ratio of the beam opening angle of the conical
jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and
the jet material density is uniform on the photon emitting jet surface. We
calculate effects of spatial variation of the Lorentz factor and the spatial
density fluctuations within the cone on the luminosity function and the
statistics of gamma-ray bursts. In particular, we focus on the redshift
distribution of the observed gamma-ray bursts. The maximum distance to and the
average redshift of the gamma-ray bursts are strongly affected by the
beaming-induced luminosity function. The bursts with the angle-dependent
Lorentz factor which peaks at the center of the cone have substantially higher
average gamma-ray burst redshifts. When both the jet material density and the
Lorentz factor are inhomogeneous in the conical beam, the average redshift of
the bursts could be 5 times higher than that of the case in which relativistic
jet is completely homogeneous and structureless. Even the simplest models for
the gamma-ray burst jets and their apparent luminosity distributions have a
significant effect on the redshift distribution of the gamma-ray bursts.


title: Near-infrared imaging polarimetry of the GG Tau circumbinary ring

abstract:
  We present 1 micron Hubble Space Telescope/NICMOS resolved imaging
polarimetry of the GG Tau circumbinary ring. We find that the ring displays
east-west asymmetries in surface brightness as well as several pronounced
irregularities, but is smoother than suggested by ground-based adaptive optics
observations. The data are consistent with a 37 deg system inclination and a
projected rotational axis at a position angle of 7 deg east of north,
determined from millimeter imaging. The ring is strongly polarized, up to 50%,
which is indicative of Rayleigh-like scattering from sub-micron dust grains.
Although the polarization pattern is broadly centrosymmetric and clearly
results from illumination of the ring by the central stars, departures from
true centrosymmetry and the irregular flux suggest that binary illumination,
scattering through unresolved circumstellar disks, and shading by these disks,
may all be factors influencing the observed morphology. We confirm a 0.25
arcsec shift between the inner edges of the NIR and millimeter images and find
that the global morphology of the ring and the polarimetry provide strong
evidence for a geometrically thick ring. A simple Monte Carlo scattering
simulation is presented which reproduces these features and supports the thick
ring hypothesis. We cannot confirm filamentary streaming from the binary to the
ring, also observed in the ground-based images, although it is possible that
there is material inside the dynamically cleared region which might ontribute
to filamentary deconvolution artifacts. Finally, we find a faint 5th point
source in the GG Tau field which, if it is associated with the system, is
almost certainly a brown dwarf.


title:
The Detection Rate of Molecular Gas in Elliptical Galaxies: Constraints
  on Galaxy Formation Theories

abstract:
  In order to constrain parameters in galaxy formation theories, especially
those for a star formation process, we investigate cold gas in elliptical
galaxies. We calculate the detection rate of cold gas in them using a
semi-analytic model of galaxy formation and compare it with observations. We
show that the model with a long star formation time-scale (~20 Gyr) is
inconsistent with observations. Thus, some mechanisms of reducing the mass of
interstellar medium, such as the consumption of molecular gas by star formation
and/or reheating from supernovae, are certainly effective in galaxies. Our
model predicts that star formation induced when galaxies in a halo collide each
other reduces the cold gas left until the present. However, we find that the
reduction through random collisions of satellite (non-central) galaxies in mean
free time-scale in a halo is not required to explain the observations. This may
imply that the collisions and mergers between satellite galaxies do not occur
so often in clusters or that they do not stimulate the star formation activity
as much as the simple collision model we adopted. For cD galaxies, the
predicted detection rate of cold gas is consistent with observations as long as
the transformation of hot gas into cold gas is prevented in halos whose
circular velocities are larger than 500 km s^-1. Moreover, we find that the
cold gas brought into cDs through captures of gas-rich galaxies is little. We
also show that the fraction of galaxies with observable cold gas should be
small for cluster ellipticals in comparison with that for field ellipticals.


title:
The Nature of the Halo Population of NGC 5128 Resolved with NICMOS on
  the Hubble Space Telescope

abstract:
  We present the first infrared color-magnitude diagram (CMD) for the halo of a
giant elliptical galaxy. The CMD for the stars in the halo of NGC 5128
(Centaurus A) was constructed from HST NICMOS observations of the WFPC2 CHIP-3
field of Soria et al. (1996) to a 50% completeness magnitude limit of
[F160W]=23.8. This field is located at a distance of 08'50" (~9 kpc) south of
the center of the galaxy. The luminosity function (LF) shows a marked
discontinuity at [F160W]=20.0. This is 1-2 mag above the tip of the red giant
branch (TRGB) expected for an old population (~12 Gyr) at the distance modulus
of NGC 5128. We propose that the majority of stars above the TRGB have
intermediate ages (~2 Gyr), in agreement with the WFPC2 observations of Soria
et al. (1996). Five stars with magnitudes brighter than the LF discontinuity
are most probably due to Galactic contamination. The weighted average of the
mean giant branch color above our 50% completeness limit is
[F110W]-[F160W]=1.22+-0.08 with a dispersion of 0.19 mag. From our
artificial-star experiments we determine that the observed spread in color is
real, suggesting a real spread in metallicity. We estimate the lower and upper
bounds of the stellar metallicity range by comparisons with observations of
Galactic star clusters and theoretical isochrones. Assuming an old population,
we find that, in the halo field of NGC 5128 we surveyed, stars have
metallicities ranging from roughly 1% of solar at the blue end of the color
spread to roughly solar at the red end, with a mean of [Fe/H]=-0.76 and a
dispersion of 0.44 dex.


title:
Objective Classification of Galaxy Spectra using the Information
  Bottleneck Method

abstract:
  A new method for classification of galaxy spectra is presented, based on a
recently introduced information theoretical principle, the `Information
Bottleneck'. For any desired number of classes, galaxies are classified such
that the information content about the spectra is maximally preserved. The
result is classes of galaxies with similar spectra, where the similarity is
determined via a measure of information. We apply our method to approximately
6000 galaxy spectra from the ongoing 2dF redshift survey, and a mock-2dF
catalogue produced by a Cold Dark Matter-based semi-analytic model of galaxy
formation. We find a good match between the mean spectra of the classes found
in the data and in the models. For the mock catalogue, we find that the classes
produced by our algorithm form an intuitively sensible sequence in terms of
physical properties such as colour, star formation activity, morphology, and
internal velocity dispersion. We also show the correlation of the classes with
the projections resulting from a Principal Component Analysis.


title: Disk mass accretion rate and infrared flares in GRS 1915+105

abstract:
  We have analyzed in detail a set of Rossi X-ray Timing Explorer (RXTE)
observations of the galactic microquasar GRS 1915+105 corresponding to times
when quasi-periodic oscillations in the infrared have been reported. From
time-resolved spectral analysis, we have estimated the mass accretion rate
through the (variable) inner edge of the accretion disk. We compare this
accretion rate to an estimate of the mass/energy outflow rate in the jet. We
discuss the possible implications of these results in terms of disk-instability
and jet ejection, and in particular note an apparent anti-correlation between
the accretion and ejection rates, implying that the gas expelled in the jet
must leave the accretion disk before reaching its innermost radius.


title: Kinematics of young stars (I): Local irregularities

abstract:
  The local velocity field of young stars is dominated by the galactic
rotation, the kinematics of the Gould Belt and the nearest OB associations and
open clusters, and the kinematics of the spiral structure. We re-examined here
this local velocity field by using a large sample of nearby O and B stars from
the Hipparcos Catalogue. The high quality astrometric data are complemented
with a careful compilation of radial velocities and Str\"omgren photometry,
which allows individual photometric distances and ages to be derived. The Gould
Belt extends up to 600 pc from the Sun with an inclination with respect to the
galactic plane of $i_{\mathrm{G}} = 16$-$22\degr$ and the ascending node placed
at $\Omega_{\mathrm{G}} = 275$-$295\degr$. Approximately 60% of the stars
younger than 60 Myr belong to this structure. The values found for the Oort
constants when different samples selected by age or distance were used allowed
us to interpret the systematic trends observed as signatures induced by the
kinematic behaviour of the Gould Belt. The contribution of Sco-Cen and Ori OB1
complexes in the characterization of the expansion of the Gould Belt system is
also discussed. We found that a positive $K$-term remains when these aggregates
are excluded. From the kinematic behaviour of the stars and their spatial
distribution we derive an age for the Gould Belt system in the interval 30-60
Myr.


title: Ultrahigh energy neutrino interactions and weak-scale string theories

abstract:
  It has been suggested that ultrahigh energy neutrinos can acquire
cross-sections approaching hadronic size if the string scale is as low as 1-10
TeV. In this case, the vertical air showers observed with energies above the
Greisen-Zatsepin-Kuzmin cutoff at E approximately 6x10^{19} eV could be
initiated by neutrinos which are the only known primaries able to travel long
distances unimpeded. We have calculated the neutrino-nucleon cross-section due
to the exchange of Kaluza-Klein excitations of the graviton in a field
theoretical framework. We have found that the neutrino-nucleon cross section
and the transferred energy per interaction are too small to explain vertical
showers even in the most optimistic scenario.


title:
Hyperbolic character of the angular moment equations of radiative
  transfer and numerical methods

abstract:
  We study the mathematical character of the angular moment equations of
radiative transfer in spherical symmetry and conclude that the system is
hyperbolic for general forms of the closure relation found in the literature.
Hyperbolicity and causality preservation lead to mathematical conditions
allowing to establish a useful characterization of the closure relations. We
apply numerical methods specifically designed to solve hyperbolic systems of
conservation laws (the so-called Godunov-type methods), to calculate numerical
solutions of the radiation transport equations in a static background. The
feasibility of the method in any kind of regime, from diffusion to
free-streaming, is demonstrated by a number of numerical tests and the effect
of the choice of the closure relation on the results is discussed.


title: Molecular gas in blue compact dwarf galaxies

abstract:
  Blue compact dwarf galaxies (BCDGs) are currently undergoing strong bursts of
star formation. Nevertheless, only a few of them have been clearly detected in
CO, which is thought to trace the "fuel" of star formation: H_2. In this paper,
we present a deep search for CO J=1-->0 and J=2-->1 emission lines in a sample
of 8 BCDGs and two companions. Only 2 of them (Haro 2 and UM 465) are detected.
For the other galaxies we have obtained more stringent upper limits on the CO
luminosity than published values. We could not confirm the previously reported
``detection'' of CO for the galaxies UM 456 and UM 462. We analyze a possible
relation between metallicity, CO luminosity, and absolute blue magnitude of the
galaxies. We use previously determined relations between X = N(H_2)/I_CO and
the metallicity to derive molecular cloud masses or upper limits for them. With
these ``global'' X_CO values we find that for those galaxies which we detect in
CO, the molecular gas mass is similar to the HI mass, whereas for the
non-detections, the upper limits on the molecular gas masses are significantly
lower than the HI mass. Using an LVG (Large Velocity Gradient) model we show
that X_CO depends not only on metallicity, but also on other physical
parameters such as volume density and kinetic temperature, which rises the
question on the validity of ``global'' X_CO factors.


title: Evolution in the Clustering of Galaxies for z < 1.0

abstract:
  Measuring the evolution in the clustering of galaxies over a large redshift
range is a challenging problem. We have developed a new technique which uses
photometric redshifts to measure the angular correlation function in redshift
shells. This novel approach minimizes the galaxy projection effect inherent in
standard angular correlation measurements, and allows for a measurement of the
evolution in the galaxy correlation strength with redshift. In this paper, we
present new results which utilize more accurate photometric redshifts, which
are derived from a multi-band dataset (U,B,R, and I) covering almost two
hundred square arcminutes to B_{AB} ~ 26.5, to quantify the evolution in the
clustering of galaxies for z < 1. We also extend our technique to incorporate
absolute magnitudes, which provides a simultaneous measurement of the evolution
of clustering with both redshift and intrinsic luminosity. Specifically, we
find a gradual decline in the strength of clustering with redshift out to z ~
1, as predicted by semi-analytic models of structure formation. Furthermore, we
find that r_0(z=0) ~ 4.0 h^{-1} Mpc for the predictions of linear theory in an
Omega_0 = 0.1 universe.