title:
Possible Method to Reconstruct the Cosmic Equation of State from Strong
  Gravitational Lensing Systems

abstract:
  A possible method to reconstruct the cosmic equation of state using strong
gravitational lensing systems is proposed. The feasibility of the method is
investigated by carrying out the reconstruction on the basis of a simple
Monte-Carlo simulation. We show that the method can work and that the cosmic
equation of state $w(z)$ can be determined within errors of $\Delta w\sim
\pm0.1$ -- $\pm0.2$ when a sufficiently large number of lensing systems ($N\sim
20$) for $z\simlt 1$ are precisely measured. Statistics of lensed sources in a
wide and deep survey like the SDSS are also briefly discussed.


title: Was SN1997ff at z~1.7 magnified by gravitational lensing?

abstract:
  The quest for the cosmological parameters has come to fruition with the
identification of a number of supernovae at a redshift of $z\sim1$. Analyses of
the brightness of these standard candles reveal that the Universe is dominated
by a large cosmological constant. The recent identification of the $z\sim1.7$
SN1997ff in the northern Hubble Deep Field has provided further evidence for
this cosmology. Here we examine the case for gravitational lensing of SN1997ff
due to the presence of galaxies lying along our line of sight. We find that,
while the alignment of SN1997ff with foreground masses was not favorable for it
to be multiply imaged and strongly magnified, two galaxies did lie close enough
to result in significant magnification: $\mu\sim1.4$ for the case where these
elliptical galaxies have velocity dispersion $200 {\rm km/s}$. Given the small
difference between supernova brightnesses in different cosmologies, detailed
modeling of the gravitational lensing properties of the intervening matter is
therefore required before the true cosmological significance of SN1997ff can be
deduced.


title: White Dwarfs and Dark Matter

abstract:
  Oppenheimer et al. (2001) have argued recently that at least 2% of the
Galactic halo is comprised of white dwarfs If true, this finding has crucial
implications for understanding the formation and evolution of the Milky Way. We
draw attention to three potential shortcomings in the Oppenheimer et al.
analysis which lead us to conclude that the density of white dwarfs with halo
kinematics may have been significantly overestimated.


title: The Limiting Effects of Dust in Brown Dwarf Model Atmospheres

abstract:
  We present opacity sampling model atmospheres, synthetic spectra and colors
for brown dwarfs and very low mass stars in two limiting case of dust grain
formation: 1) inefficient gravitational settling i.e. the dust is distributed
according to the chemical equilibrium predictions, 2) efficient gravitational
settling i.e. the dust forms and depletes refractory elements from the gas, but
their opacity does not affect the thermal structure. The models include the
formation of over 600 gas phase species, and 1000 liquids and crystals, and the
opacities of 30 different types of grains including corundum (Al$_2$O$_3$), the
magnesium aluminum spinel MgAl$_2$O$_4$, iron, enstatite (MgSiO$_3$),
forsterite (Mg$_2$SiO$_4$), amorphous carbon, SiC, and a number of calcium
silicates. The models extend from the beginning of the grain formation regime
well into the condensation regime of water ice ($\teff= 3000 - 100$ K) and
encompasses the range of $\log g= 2.5 - 6.0$ at solar metallicity.
  We find that silicate dust grains can form abundantly in the outer
atmospheric layers of red and brown dwarfs with spectral type later than M8.
The greenhouse effects of dust opacities provide a natural explanation for the
peculiarly red spectroscopic distribution of the latest M dwarfs and young
brown dwarfs. The grainless (Cond) models on the other hand, correspond closely
to methane brown dwarfs such as Gliese 229B. We also recover that the
$\lambda$5891,5897\AA Na I D and $\lambda$7687,7701\AA K I resonance doublets
plays a critical role in T dwarfs where their red wing define the
pseudo-continuum from the $I$ to the $Z$ bandpass.


title:
Constraints on the Progenitors of SNeIa & Implications for the
  Cosmological Equation of State

abstract:
  Detailed calculations for the stellar evolution, explosion and light curves
have been performed to quantify the influence of the main sequence mass M(MS)
and the metallicity of the progenitor on the structure of the exploding WD
which are thought to be the progenitors of SNe Ia. In particular,we study the
effects of progenitors on the brightness decline relation M(dM_15) which is a
corner stone for the use of SNe Ia as cosmological yard-stick.M(MS) has been
identified as the decisive factor to change the energetics of the explosion and
dominates the variations in the rise-time to decline relation of light curves.
M(MS) has little effect on the color index B-V. For similar decline rates
dM_15, the flux at maximum brightness relative to the flux on the radioactive
tail decreases systematically with M(MS) by about 0.2mag. This change goes
along with a reduc- tion of the photospheric expansion velocity $v_{ph}$ by
about 2000 km/sec. A change in the central density has the opposite dependency.
The metallicity Z affects mainly the intrinsic color index B-V by up to
-0.06mag, and it alters the fluxes in the U band and the UV. B-V is critical if
extinction corrections are applied. The spread in the fiducial rise-time to
decline relation in local SNe Ia restricts the range of main sequence masses to
a factor of 2. The upper limit of 1 day for the difference between the local
and distance sample support the need for a positive cosmological constant. The
size of evolutionary effects are small (dM abou 0.2mag) but are absolutely
critical for the reconstruction of the cosmological equation of state.


title:
Parallel Implementation of the PHOENIX Generalized Stellar Atmosphere
  Program. III: A parallel algorithm for direct opacity sampling

abstract:
  We describe two parallel algorithms for line opacity calculations based on a
local file and on a global file approach. The performance and scalability of
both approaches is discussed for different test cases and very different
parallel computing systems. The results show that a global file approach is
more efficient on high-performance parallel supercomputers with dedicated
parallel I/O subsystem whereas the local file approach is very useful on farms
of workstations, e.g., cheap PC clusters.


title:
Three-dimensional Simulations of the Parker Instability in a
  Uniformly-rotating Disk

abstract:
  We investigate the effects of rotation on the evolution of the Parker
instability in an exponentially-stratified and uniformly-rotating magnetized
gas disk by carrying out three-dimensional numerical simulations with an
isothermal magnetohydrodynamic code. The instability has been initialized by
random velocity perturbations. In the linear stage, the evolution is not much
different from that without rotation and the mixed (undular + interchange) mode
regulates the system. The interchange mode induces alternating dense and
rarefied regions with small radial wavelengths, while the undular mode bends
the magnetic field lines in the plane of azimuthal and vertical directions. In
the nonlinear stage, flow motion overall becomes chaotic as in the case without
rotation. However, as the gas in higher positions slides down along field lines
forming supersonic flows, the Coriolis force becomes important. As oppositely
directed flows fall into valleys along both sides of magnetic field lines, they
experience the Coriolis force toward opposite directions, which twists magnetic
field lines there. Hence, we suggest that the Coriolis force plays a role in
randomizing magnetic field. The three-dimensional density structure formed by
the instability is still sheet-like with the short dimension along the radial
direction, as in the case without rotation. However, the long dimension is now
slightly tilted with respect to the mean field direction. The shape of high
density regions is a bit rounder. The maximum enhancement factor of the
vertical column density relative to its initial value is about 1.5, which is
smaller than that in the case without rotation. We conclude that uniform
rotation doesn't change our point of view that the Parker instability alone is
not a viable mechanism for the formation of giant molecular clouds.


title: Radiation Pressure Acceleration by X-rays in Active Galactic Nuclei

abstract:
  We present calculations of the dynamics of highly ionized gas clouds that are
confined by external pressure, and are ionized by AGN continuum. We focus on
the gas that is seen in absorption in the X-ray spectrum of many AGN and show
that such gas can reach hydrostatic equilibrium under various conditions. The
principal conclusion is that the clouds can be accelerated to high velocities
by the central X-ray source. The dynamical problem can be reduced to the
calculation of a single parameter, the average force multiplier, <M>. The
typical value of <M> is ~10 suggesting that radiation pressure acceleration by
X-rays is efficient for L/L_Edd>0.1. The terminal velocity scales with the
escape velocity at the base of the flow and can exceed it by a large factor.
The typical velocity for a HIG flow that originates at R=1e17 cm in a source
with L_x=1e44 erg/s is ~1000 km/s, i.e. similar to the velocities observed in
several X-ray and UV absorption systems.
  Highly ionized AGN clouds are driven mainly by bound-free absorption and
bound-bound processes are less important unless the lines are significantly
broadened or the column density is very small. Pressure laws that result in
constant or outward decreasing ionization parameters are most effective in
accelerating the flow.


title: The Galaxy Population of Cluster RXJ0848+4453 at z=1.27

abstract:
  We present a study of the galaxy population in the cluster RXJ0848+4453 at
z=1.27, using deep HST NICMOS and WFPC2 images. We morphologically classify all
galaxies to K_s=20.6 that are covered by the HST imaging, and determine
photometric redshifts using deep ground based BRIzJK_s photometry. Of 22 likely
cluster members with morphological classifications, eleven (50%) are classified
as early-type galaxies, nine (41%) as spiral galaxies, and two (9%) as
``merger/peculiar''. At HST resolution the second brightest cluster galaxy is
resolved into a spectacular merger between three red galaxies of similar
luminosity, separated from each other by ~6 kpc, with an integrated magnitude
K=17.6 (~3 L* at z=1.27). The two most luminous early-type galaxies also show
evidence for recent or ongoing interactions. Mergers and interactions between
galaxies are possible because RXJ0848+4453 is not yet relaxed. The fraction of
early-type galaxies in our sample is similar to that in clusters at 0.5<z<1,
and consistent with a gradual decrease of the number of early-type galaxies in
clusters from z=0 to z=1.3. We find evidence that the color-magnitude relation
of the early-type galaxies is less steep than in the nearby Coma cluster. This
may indicate that the brightest early-type galaxies have young stellar
populations at z=1.27, but is also consistent with predictions of single age
``monolithic'' models with a galactic wind. The scatter in the color-magnitude
relation is ~0.04 in rest frame U-V, similar to that in clusters at 0<z<1.
Taken together, these results show that luminous early-type galaxies exist in
clusters at z~1.3, but that their number density may be smaller than in the
local Universe. Additional observations are needed to determine whether the
brightest early-type galaxies harbor young stellar populations.


title: Irradiated Planets

abstract:
  We have modeled irradiated planets located near a dM5 and a G2 primary star.
The impinging radiation field was explicitly included in the solution of the
radiative transfer equation and in the computation of the atmospheric
structure. We find that large errors in both the thermal and reflected flux
will result from models which do not include the impinging radiation in a
self-consistent manner. A cool (teff = 500K) and a hot (teff = 1000K) planet
were modeled at various orbital separations from both the dM5 and the G2
primary. In all scenarios, we compared the effects of the irradiation in two
extreme cases: one where dust clouds form and remain suspended in the
atmosphere, and another where dust clouds form but completely settle out of the
atmosphere. The atmospheric structure and emergent spectrum strongly depend on
the presence or absence of dust clouds. We find that, in the absence of dust
opacity, the impinging radiation significantly alters the innermost layers of
an EGP atmosphere and that they are actually brighter in the optical than dusty
planets. Our models also indicate that the planet-to-star brightness ratio in
the optical will be less that 1 x 10^-5 for objects like tau bootis which is
consistent with recently reported upper limit values.


title:
Evolution of Active Galaxies: Black-hole Mass - Bulge relations for
  Narrow Line Objects

abstract:
  Mathur (2000) has proposed that the narrow line Seyfert 1 galaxies (NLS1s)
are likely to be the active galaxies in early stage of evolution. To test this
hypothesis, we have calculated the black hole (BH) masses and the host galaxy
bulge masses for a sample of NLS1s. We find that the mean BH to bulge mass
ratio of NLS1s is significantly smaller than that for normal Seyfert galaxies.
We also find that the ratio of BH mass to bulge velocity dispersion is also
significantly smaller for NLS1s. A scenario of BH growth is our preferred
interpretation, though alternative explanations are discussed. Assuming that
the BHs grow with accretion with a radiative efficiency of 0.1, it will take
them about $3.3\times 10^8$ years to become as massive as in normal Seyfert 1s.
These timescales are consistent with the theoretical estimates of quasar
timescale $t_Q$ less than about $4.5\times 10^8$ years calculated by Haehnelt,
Natarajan & Rees (1998). Studies of low redshift NLS1s thus provide a powerful,
and due to their proximity relatively easy, way to understand the high redshift
quasars and their evolution.


title: Variability of the Vela Pulsar-wind Nebula Observed with Chandra

abstract:
  The observations of the pulsar-wind nebula (PWN) around the Vela pulsar with
the Advanced CCD Imaging Spectrometer aboard the Chandra X-ray Observatory,
taken on 2000 April 30 and November 30, reveal its complex morphology
reminiscent of that of the Crab PWN. Comparison of the two observations shows
changes up to 30% in the surface brightness of the PWN features. Some of the
PWN elements show appreciable shifts, up to a few arcseconds (about 10^{16}
cm), and/or spectral changes. To elucidate the nature of the observed
variations, further monitoring of the Vela PWN is needed.


title:
Cataclysmic Variables: An Empirical Angular Momentum Loss Prescription
  From Open Cluster Data

abstract:
  We apply the angular momentum loss rates inferred from open cluster stars to
the evolution of cataclysmic variables (CVs). We show that the angular momentum
prescriptions used in earlier CV studies are inconsistent with the measured
rotation data in open clusters. The timescale for angular momentum loss above
the fully convective boundary is ~ 2 orders of magnitude longer than inferred
from the older model, and the observed angular momentum loss properties show no
evidence for a change in a behavior at the fully convective boundary. This
provides evidence against the hypothesis that the period gap is caused by an
abrupt change in the angular momentum loss law when the secondary becomes fully
convective. It also implies that the timescale for CV evolution is much longer
than it was than previously thought, comparable to a Hubble time. For the same
reason, it will be more difficult to produce CVs from the products of common
envelope evolution and implies a lower space density of CVs. The empirical loss
law is consistent with the observed period minimum (1.3 hours) contrary to the
minimum predicted by angular momentum loss due to gravitational radiation alone
(1.1 hours).
  We introduce a method to infer the time-averaged mass accretion rate and
derive mass-period relation for different evolutionary states of the secondary.
The mass-period relationship is more consistent with evolved secondaries than
with unevolved secondaries above the period gap. Implications for the CV period
gap are discussed, including the possibility that two populations of
secondaries could produce the gap.


title:
Estimating Stochastic Gravitational Wave Backgrounds with Sagnac
  Calibration

abstract:
  Armstrong et al. have recently presented new ways of combining signals to
precisely cancel laser frequency noise in spaceborne interferometric
gravitational wave detectors such as LISA. One of these combinations, the
symmetrized Sagnac observable, is much less sensitive to external signals at
low frequencies than other combinations, and thus can be used to determine the
instrumental noise level. We note here that this calibration of the
instrumental noise permits smoothed versions of the power spectral density of
stochastic gravitational wave backgrounds to be determined with considerably
higher accuracy than earlier estimates, at frequencies where one type of noise
strongly dominates and is not substantially correlated between the six main
signals generated by the antenna. We illustrate this technique by analyzing
simple estimators of gravitational wave background power, and show that the
instrumental sensitivity to broad-band backgrounds at some frequencies can be
improved by more than an order of magnitude over the standard method,
comparable to that which would be achieved by cross-correlating two separate
antennas. The applications of this approach to studies of astrophysical
gravitational wave backgrounds generated after recombination and to searches
for a possible primordial background are discussed.


title:
A New Code for Nonradial Stellar Pulsations and its Application to Low -
  Mass, Helium White Dwarfs

abstract:
  We present a finite difference code intended for computing linear, adiabatic,
nonradial pulsations of spherical stars. This code is based on a general Newton
- Raphson technique in order to handle the relaxation of the eigenvalue (square
of the eigenfrequency) of the modes and their corresponding eigenfunctions.
This code has been tested computing the pulsation spectra of polytropic spheres
finding a good agreement with previous work. Then, we have coupled this code to
our evolutionary code and applied it to the computation of the pulsation
spectrum of a low mass, pure - helium white dwarf of 0.3 M_{sun} for a wide
range of effective temperatures. In making this calculation we have taken an
evolutionary time step short enough such that eigenmodes corresponding to a
given model are used as initial approximation to those of the next one.
Specifically, we have computed periods, period spacing, eigenfunctions, weight
functions, kinetic energies and variational periods for a wide range of modes.
To our notice this is the first effort in studying the pulsation properties of
helium white dwarfs. The solution we have found working with these realistic
white dwarf models are in good accord with the predictions of the asymptotic
theory of Tassoul (1980) for high order modes. This indicates that the code
presented here is able to work adequately also with realistic stellar models.


title: The Variability of T Tau, RY Tau and RW Aur from 1899 to 1952

abstract:
  We present the historical light curve of T Tau derived from photographic
plates in the Harvard College Observatory archives. We find that the optical
light of T Tau varied by 2-3 (or more) magnitudes on time scales as short as a
month prior to ~1917, consistent with the results of Lozinskii (1949). Extreme
light fluctuations of greater than 2 magnitudes abruptly ceased in the late
1910's and, to the best of our knowledge, have not repeated since this time. We
compare the observed light variations of T Tau to the T Tauri stars RY Tau and
RW Aur, whose light curves we also constructed from inspection of the archival
plates. We find that variable extinction along the line of sight to the star is
the most likely explanation for the observed light fluctuation of T Tau during
the early part of the 20th century.


title: Variable H-alpha Line Emission from the Central Star of the Helix Nebula

abstract:
  The central star of the Helix Nebula is a hot white dwarf with unexpected
hard X-ray emission. With an effective temperature of ~100,000 K, the star is a
powerful source of H-ionizing radiation; the atmosphere of a stellar or
planetary companion, if present, will be ionized and emit recombination lines.
To probe the origin of hard X-ray emission from the Helix Nebula's central
star, we have obtained multi-epoch high-dispersion spectra of the star, and
found temporal variation in the H-alpha line profile over a time span of one
week. The observed width and strength of the variable H-alpha emission
component are consistent with the hypothesized dMe companion proposed to
explain the hard X-ray emission. A dMe companion, however, cannot explain the
possible detection of variable HeII and [NII] emission. Follow-up spectroscopic
monitoring of the Helix Nebula central star is needed to better establish the
identification of the spectral lines and their temporal behavior in order to
determine the origin of the optical variability and hard X-ray emission.


title:
The Enigmatic X-ray Point Sources at the Central Stars of NGC 6543 and
  NGC 7293

abstract:
  Recent Chandra ACIS-S observations have detected a point source at the
central star of NGC 6543 and confirmed the point source nature of the hard
X-ray emission from NGC 7293. The X-ray spectra of both sources peak between
0.5 keV and 1.0 keV and show line features indicating a thin plasma at
temperatures of a few times 10^6 K. Their X-ray luminosities are 10^30 erg/s
and 3x10^29 erg/s, respectively. We have considered four different mechanisms
to explain the nature of these sources. The X-ray emission from the central
star of NGC 6543 may originate from the coronal activity of an undetected
companion star or from shocks in its fast stellar wind, while the hard X-ray
emission from NGC 7293 might be ascribed to an undetected dMe companion.
Follow-up observations are needed to determine the existence and natures of
these stellar companions.


title: The Fragmentation of Pre-enriched Primordial Objects

abstract:
  Recent theoretical investigations have suggested that the formation of the
very first stars, forming out of metal-free gas, was fundamentally different
from the present-day case. In this paper, we study the effect of metallicity on
the evolution of the gas in a collapsing dark matter mini-halo. We model such a
system as an isolated 3\sigma peak of mass 2x10^6 M_sun that collapses at
z_coll=30, using smoothed particle hydrodynamics. The gas has a supposed level
of pre-enrichment of either 10^-4 Z_sun or 10^-3 Z_sun. We find that the
evolution proceeds very differently for the two cases. The gas in the lower
metallicity simulation fails to undergo continued collapse and fragmentation,
whereas the gas in the higher metallicity case dissipatively settles into the
center of the dark matter halo. The central gas, characterized by densities n >
10^4 cm^-3, and a temperature, T \sim 90 K, which closely follows that of the
CMB, is gravitationally unstable and undergoes vigorous fragmentation. We
discuss the physical reason for the existence of a critical metallicity, Z_crit
\sim 5x10^-4 Z_sun, and its possible dependence on redshift. Compared to the
pure H/He case, the fragmentation of the 10^-3 Z_sun gas leads to a larger
relative number of low-mass clumps.


title:
Model-independent insights into the nature of the Lyman-alpha forest and
  the distribution of matter in the universe

abstract:
  Straightforward physical arguments are used to derive the properties of
Lyman-alpha forest absorbers. It is shown that many aspects of the current
physical picture of the forest, in particular the fact that the absorption
arises in extended structures of moderate overdensities which contain a large
fraction of the baryons in the universe, can be derived directly from the
observations without making any specific assumptions about the presence and
distribution of dark matter, the values of the cosmological parameters or the
mechanism for structure formation. The key argument is that along any line of
sight intersecting a gravitationally confined gas cloud, the size of the region
over which the density is of order the maximum density, is typically of order
the local Jeans length. This is true for overdense absorbers, regardless of the
shape of the cloud and regardless of whether the cloud as a whole is in
dynamical equilibrium. The simple analytic model is used to derive the mass
distribution of the photoionized gas directly from the observed column density
distribution. It is demonstrated that the shape of the column density
distribution, in particular the observed deviations from a single power-law,
and its evolution with redshift, reflect the shape of the matter distribution
and can be understood in terms of the growth of structure via gravitational
instability in an expanding universe.


title: A quasi-spherical inner accretion flow in Seyfert galaxies ?

abstract:
  We study a phenomenological model for the continuum emission of Seyfert
galaxies. In this quasi-spherical accretion scenario, the central X-ray source
is constituted by a hot spherical plasma region surrounded by spherically
distributed cold dense clouds. The cold material is radiatively coupled with
the hot thermal plasma. Assuming energy balance, we compute the hard X-ray
spectral slope Gamma and reflection amplitude R.
  This simple model enables to reproduce both the range of observed hard X-ray
spectral slopes, and reflection amplitude R. It also predicts a correlation
between R and Gamma which is very close to what is observed. Most of the
observed spectral variations from source to source, would be due to differences
in the cloud covering fraction. If some internal dissipation process is active
in the cold clouds, darkening effects may provide a simple explanation for the
observed distributions of reflection amplitudes, spectral slopes, and UV to
X-ray flux ratios.


title:
HST/NICMOS detection of a partially embedded, intermediate-mass
  pre-main-sequence population in the 30 Doradus Nebula

abstract:
  We present the detection of an intermediate-mass pre-main-sequence population
embedded in the nebular filaments surrounding the 30 Doradus region in the
Large Magellanic Cloud (LMC) using HST/NICMOS. In addition to four previously
known luminous Class I infrared ``protostars,'' the NICMOS data reveal 20 new
sources with intrinsic infrared excess similar to Galactic pre-main sequence
stars. Based on their infrared brightness, these objects can be identified as
the LMC equivalent of Galactic pre-main sequence stars. The faintest LMC Young
Stellar Objects in the sample have colors similar to T Tauri and have about the
same brightness as T Tauri if placed at the distance of the LMC. We find no
evidence for a lower-mass cut-off in the initial mass function. Instead, the
whole spectrum of stellar masses from pre-main sequence stars with ~1.5Mo to
massive O stars still embedded in dense knots appears to be present in the
nebular filaments. The majority of the young stellar objects can be found to
the north of the central starburst cluster R136. This region is very likely
evolving into an OB association.


title:
Galaxy Formation: One Star at a Time. New Information from the
  Kinematics of Field Stars in the Galaxy

abstract:
  We present a review of recent studies of the formation and evolution of the
Milky Way galaxy, in particular based on large samples of non-kinematically
selected stars with available proper motions. The Milky Way is argued to be a
reasonable template for the formation of large spiral galaxies, the only one in
which complete kinematical and abundance information can be readily obtained.
Ongoing and future projects to obtain proper motions and spectral information
for much larger samples of stars that will sharpen our perspective are
discussed.


title:
The Implications of the New Z=0 Stellar Models and Yields on the Early
  Metal Pollution of the Intergalactic Medium

abstract:
  Motivated by the recent detection of metals in different components of the
high redshift universe and by the abundance ratios measured in the extremely
metal-poor stars of our Galaxy, we study the nucleosynthesis constraints that
this imposes on an early generation of stars (Population III). To do so we take
into account the chemical yields obtained from homogeneous evolutionary
calculations of zero metal stars in the mass range $3\la m/M_\odot\la 40$
(Limongi et al. 2000, Chieffi et al. 2001). We also consider the role played by
metal-free very massive objects (m$>100$ M$_\odot$). Using both analytical and
numerical chemical evolution models, we confront model predictions from the
different choices of the mass function proposed for Population III with the
observational constraints. We show that low values of star formation efficiency
($<1%$) are required so as not to exceed the minimum metallicity ([C/H]$\approx
-2.4$) measured in the high redshift systems for any of the IMFs proposed. We
also show that the observational constraints require $\Omega_{sr}< 3\times
10^{-3}\Omega_b$, confirming previous claims that the possible contribution of
the stellar remnants from Population III to the baryonic dark matter is
insignificant. At present, however, the scarcity of abundance measurements for
high redshift systems does not permit us to put severe limitations on the
nature of the initial mass function for Population III. In fact, overabundances
of alpha-elements with respect to iron of the order of those measured in damped
Lyman-$\alpha$ systems are obtained for any of the IMFs tested. Nevertheless,
to account for the very large [C,N/Fe] ratios found in a considerable number of
extremely metal-poor stars of our Galaxy, an IMF peaking at the intermediate
stellar mass range (4-8 M$_\odot$) is needed.


title:
Space Debris Hazards from Explosions in the collinear Sun-Earth Lagrange
  points

abstract:
  The collinear Lagrange points of the Sun-Earth system provide an ideal
environment for highly sensitive space science missions. Consequently many new
missions are planed by ESA and NASA that require satellites close to these
points. For example, the SOHO spacecraft built by ESA is already installed in
the first collinear Lagrange point. Neither uncontrolled spacecraft nor escape
motors will stay close to the Lagrange points for a long time. In case an
operational satellite explodes, the fragmentation process will take place close
to the Lagrange point. Apparently a number of spacecraft will accumulate close
to the Lagrange points over the next decades. We investigate the space debris
hazard posed by these spacecraft if they explode and fall back to an Earth
orbit. From our simulation we find that, as expected, about half of the
fragments drift towards the Earth while the other half drifts away from it.
Around 2% of the simulated fragments even impact the Earth within one year
after the explosion.


title: Interstellar Dust Module for the ESA Meteoroid Model

abstract:
  The ESA meteoroid model predicts impacts of meteoroids in the mass range
between $10^{-18}$ to $10^0$ g on spacecraft surfaces. It covers heliocentric
distances from 0.3 to 20 AU. Measurements of the dust detector on board the
highly successful joint ESA/NASA mission Ulysses have shown, that the flux of
meteoroids with masses between $10^{-15}$ and $10^{-12}$ g is, at least in the
outer Solar System, dominated by interstellar dust grains that traverse the
Solar System as it travels through the local interstellar cloud. We present a
simple semi-analytic interstellar dust model that can easily be included in the
ESA meteoroid model, together with a more precise determination of the flux
direction of the interstellar dust stream. The model is based on the assumption
that interstellar dust dynamics have two effects: solar gravitation and
radiation pressure determines the spatial distribution, and Lorentz-interaction
of the charged particles creates a temporal variation.


title: The FIRST Bright Quasar Survey III. The South Galactic Cap

abstract:
  We present the results of an extension of the FIRST Bright Quasar Survey
(FBQS) to the South Galactic cap, and to a fainter optical magnitude limit.
Radio source counterparts with SERC R magnitudes brighter than 18.9 which meet
the other FBQS criteria are included. We supplement this list with a modest
number of additional objects to test our completeness for quasars with extended
radio morphologies. The survey covers 589 square degrees in two equatorial
strips in the southern cap. We have obtained spectra for 86% of the 522
candidates, and find 321 radio-selected quasars of which 264 are reported here
for the first time. A comparison of this fainter sample with the FBQS sample
shows the two to be generally similar.
  Fourteen new broad absorption line (BAL) quasars are included in this sample.
When combined with the previously identified BAL quasars in our earlier papers,
we can discern a break in the frequency of BAL quasars with radio loudness,
namely that the relative number of high-ionization BAL quasars drops by a
factor of four for quasars with a radio-loudness parameter R* > 100.


title:
Cooling Rates of Molecular Clouds Based on Numerical MHD Turbulence and
  non-LTE Radiative Transfer

abstract:
  We have computed line emission cooling rates for the main cooling species in
models of interstellar molecular clouds. The models are based on numerical
simulations of super-sonic magneto-hydrodynamic (MHD) turbulence. Non-LTE
radiative transfer calculations have been performed to properly account for the
complex density and velocity structures in the MHD simulations. Three models
are used. Two of the models are based on MHD simulations with different
magnetic field strength and the third includes the computation of self-gravity
(in the super-Alfvenic regime of turbulence). The density and velocity fields
in the simulations are determined self-consistently by the dynamics of
super-sonic turbulence. The models are intended to represent molecular clouds
with linear size L~6 pc and mean density <n>~300 cm^-3, with the density
exceeding 10^4 cm^-3 in the densest cores. We present 12CO, 13CO, C18O, O2, OI,
CI and H2O cooling rates in isothermal clouds with kinetic temperatures 10-80K.
Analytical approximations are derived for the cooling rates. The inhomogeneity
of the models reduces photon trapping and enhances the cooling in the densest
parts of the clouds. Compared with earlier models the cooling rates are less
affected by optical depth effects and are therefore higher. The main effects
comes, however, from the density variation since cooling efficiency increases
with density. This is very important for the cooling of the clouds as a whole
since most cooling is provided by gas with density above the average.


title: A two-phase model for the Narrow Line Region of NGC 4151

abstract:
  NGC 4151 is one of the brightest and best-studied Seyfert galaxies. Here, we
present a two-phase model of the narrow-line region (NLR) of NGC 4151. This
study is motivated by (i) the fact that the X-ray spectrum of NGC 4151 is among
the flattest known for Seyferts, and (ii) the recent Chandra detection of an
X-ray narrow-line region in this galaxy. X-ray spectra as flat as that of NGC
4151 (Gamma_x = -1.5) are expected to favor the presence of two gas phases in
pressure equilibrium (Krolik, McKee & Tarter 1981). In the present study, we
show that a pronounced two-phase equilibrium develops in the extended
emission-line region if we use the observed multi-wavelength spectrum of NGC
4151 to ionize the clouds. The material is stable to isobaric perturbations
over a wide range of temperatures. We therefore propose that such a condition
has arisen in the NLR of NGC 4151, and that it explains the detection of hot,
extended X-ray gas which we identify as the NLR-cloud confining medium.


title: A Search for Fine Structure of the Knee in EAS Size Spectra

abstract:
  28 size spectra of extensive air showers from 7 different experiments are
analysed consistently. They are fitted by adjusting either 4 or 5 parameters:
knee position, power law exponents above and below the knee, overall intensity
and, in addition, a parameter describing the smoothness of the bend. The
residuals are then normalized to the same knee position and averaged. When 5
parameters are employed no systematic deviation from a single smooth knee is
apparent at the 1% level up to a factor of 4 above the knee. At larger shower
sizes a moderately significant deviation can be seen whose shape and position
are compatible with a second knee caused by iron group nuclei.


title:
WeCAPP - The Wendelstein Calar Alto Pixellensing Project I Tracing Dark
  and Bright Matter in M31

abstract:
  We present WeCAPP, a long term monitoring project searching for microlensing
events towards M 31. Since 1997 the bulge of M 31 was monitored in two
different wavebands with the Wendelstein 0.8 m telescope. In 1999 we extended
our observations to the Calar Alto 1.23 m telescope. Observing simultaneously
at these two sites we obtained a time coverage of 53 % during the observability
of M 31. To check thousands of frames for variability of unresolved sources, we
used the optimal image subtraction method (OIS) by Alard & Lupton (1998) This
enabled us to minimize the residuals in the difference image analysis (DIA) and
to detect variable sources with amplitudes at the photon noise level. Thus we
can detect microlensing events with corresponding amplifications A > 10 of red
clump giants with M_I = 0.


title:
Deep near-IR observations of the Chandra Deep Field and of the HDF-South
  - Color and Number Counts

abstract:
  We present near-IR (J and Ks) number counts and colors of galaxies detected
in deep VLT-ISAAC images centered on the Chandra Deep Field and Hubble Deep
Field-South for a total area of 13.6 arcmin$^2$. The limiting surface
brightness obtained is Ks$\simeq$22.8 mag/arcsec$^2$ and J$\simeq$24.5
(1$\sigma$) on both fields. A d$log$N/dm relation with a slope of $\sim0.34$ in
J and $\sim0.28$ in Ks is found in both fields with no evidence of decline near
the magnitude limit. The median J-Ks color of galaxies becomes bluer at
magnitudes fainter than Ks$\sim18$, in agreement with the different number
counts slope observed in the two bands. We find a fraction ($\le5%$ of the
total sample) of sources with color redder than J-Ks=2.3 at magnitudes Ks$>20$.
Most of them appear as isolated sources, possibly elliptical or dusty starburst
galaxies at redshift $z>2$. The comparison of the observed number counts with
models shows that our J-band and Ks-band counts are consistent with the
prediction of a model based on a small amount of merging in a $\Omega=1$
cosmology. On the other hand, we fail to reproduce the observed counts if we do
not consider merging independently of the parameters defining the universe.


title: The first outburst of SAX J1808.4-3658 revisited

abstract:
  Data of the 1996 outburst of the single-known accreting millisecond pulsar
SAX J1808.4-3658, taken with the Wide Field Cameras (WFCs) on BeppoSAX, are
revisited with more complete data coverage and more comprehensive analysis
techniques than in a previous report. An additional type-I X-ray burst was
identified which occurred at a time when the persistent emission is below the
detection limit, roughly 30 days after outburst maximum. This burst is three
times longer than the first two bursts, and 50% brighter. It is the brightest
burst within the ~1700 type-I bursts detected so far with the WFCs. A spectral
analysis of the data reveals a distance to SAX J1808.4-3658 of ~2.5 kpc. This
is an update from a previously reported value of 4 kpc. We present the evidence
that we have for the presence of oscillations at the pulsar frequency during
part of the newly found burst. Such an oscillation would lend support to the
idea that the frequency of millisecond burst oscillations in other objects is
very close to the neutron star rotation frequency.


title:
The Nearby Field Galaxy Survey: a spectrophotometric and photometric
  study of nearby galaxies

abstract:
  We report on our observing program to obtain integrated spectrophotometry,
intermediate and high resolution major axis spectra, and U,B,R surface photo-
metry of a representative sample of ~200 galaxies in the nearby field. The main
goal of this program is to provide a comparison sample for high redshift
studies and to study the variation in star formation rates (SFR), star forma-
tion history (SFH), excitation, metallicity, and internal kinematics over a
large range in galaxy luminosity and morphological type. In particular, we
extend the work of Kennicutt (1992) to lower luminosity systems.
  We present the main results of our analysis sofar. In these proceedings, we
condense the presented two atlases of (1) images and radial surface brightness
profiles and color profiles, and (2) of images and integrated spectra into
several example images, profiles and spectra, showing the general trends
observed. For the original atlasses we refer to the electronic version,
available at http://www.astro.rug.nl/~nfgs/ .


title: Pulsar Spindown by a Fall-Back Disk and the P-P_dot Diagram

abstract:
  Neutron stars may be surrounded by fall-back disks formed from supernova
core-collapse. If the disk circumscribes the light-cylinder, the neutron star
will be an active radio pulsar spinning down under the propeller spin-down
torque applied by the disk as well as the usual magnetic dipole radiation
torque. Evolution across the P-P_dot diagram is very rapid when pulsar
spin-down is dominated by the propeller torque. This explains the distribution
of pulsars in the P-P_dot diagram.


title: SS-433/W50 at TeV Energies

abstract:
  The HEGRA CT-System was used to search for TeV gamma-rays from the
microquasar and supernova remnant combination SS-433/W50 over the 1998 and 1999
observing seasons, resulting in about 20 hours of useful data. An analysis of
various extended sources of location guided by X-Ray observations reveals no
evidence for TeV emission at photon energies above 1 TeV, with resulting 99%
upper limits in the range 8 to 10 of the Crab flux. For one of these locations,
the eastern jet termination, a model predicting a presently detectable TeV
gamma-ray flux via the inverse Compton upscattering of microwave background
photons is now somewhat constrained. The results and model contraints are
briefly discussed.


title: Centaurus A as the Source of ultra-high energy cosmic rays?

abstract:
  We present numerical simulations for energy spectra and angular distributions
of nucleons above 10^{19} eV injected by the radio-galaxy Centaurus A at a
distance 3.4 Mpc and propagating in extra-galactic magnetic fields in the
sub-micro Gauss range. We show that field strengths B~0.3 micro Gauss, as
proposed by Farrar and Piran, cannot provide sufficient angular deflection to
explain the observational data. A magnetic field of intensity ~1 micro Gauss
could reproduce the observed large-scale isotropy and could marginally explain
the observed energy spectrum. However, it would not readily account for the
E=320 plusminus 93 EeV Fly's Eye event that was detected at an angle 136
degrees away from Cen-A. Such a strong magnetic field also saturates
observational upper limits from Faraday rotation observations and X-ray
bremsstrahlung emission from the ambient gas (assuming equipartition of
energy). This scenario may already be tested by improving magnetic field limits
with existing instruments. We also show that high energy cosmic ray experiments
now under construction will be able to detect the level of anisotropy predicted
by this scenario. We conclude that for magnetic fields B~0.1-0.5 micro Gauss,
considered as more reasonable for the local Supercluster environment, in all
likelihood at least a few sources within ~10 Mpc from the Earth should
contribute to the observed ultra high energy cosmic ray flux.


title: On the variation of the gauge couplings during inflation

abstract:
  It is shown that the evolution of the (Abelian) gauge coupling during an
inflationary phase of de Sitter type drives the growth of the two-point
function of the magnetic inhomogeneities. After examining the constraints on
the variation of the gauge coupling arising in a standard model of inflationary
and post-inflationary evolution, magnetohydrodynamical equations are
generalized to the case of time evolving gauge coupling. It is argued that
large scale magnetic fields can be copiously generated. Other possible
implications of the model are outlined.


title:
Ultra-High Energy Cosmic Rays: A Probe of Physics and Astrophysics at
  Extreme Energies

abstract:
  The origin of cosmic rays is one of the major unresolved questions in
astrophysics. In particular, the highest energy cosmic rays observed possess
macroscopic energies and their origin is likely associated with the most
energetic processes in the Universe. They thus provide a probe of physics and
astrophysics at energies that are unreached in laboratory experiments.
Theoretical explanations range from acceleration of charged particles in
astrophysical environments to particle physics beyond the well established
Standard Model, and processes taking place at the earliest moments of our
Universe. Distinguishing between these scenarios requires detectors with
effective areas in the 1000 km^2 range which are now under construction or in
the planning stage. Close connections with gamma-ray and neutrino astrophysics
add to the interdisciplinary character of this field.


title:
Towards Better Age Estimates for Stellar Populations: The Y^2 Isochrones
  for Solar Mixture

abstract:
  We have constructed a new set of isochrones, called the Y^2 Isochrones, that
represent an update of the Revised Yale Isochrones (RYI), using improved
opacities and equations of state. Helium diffusion and convective core
overshoot have also been taken into consideration. This first set of isochrones
is for the scaled solar mixture. A subsequent paper will consider the effects
of alpha-element enhancement, believed to be relevant in many stellar systems.
Two additionally significant features of these isochrones are that (1) the
stellar models start their evolution from the pre-main sequence birthline
instead of from the zero-age main sequence, and (2) the color transformation
has been performed using both the latest table of Lejeune et al., and the
older, but now modified, Green et al. table. The isochrones have performed well
under the tests conducted thus far. The reduction in the age of the Galactic
globular clusters caused by this update in stellar models alone is
approximately 15% relative to RYI-based studies. When the suggested
modification for the alpha-element enhancement is made as well, the total age
reduction becomes approximately 20%. When post-RGB evolutionary stages are
included, we find that the ages of globular clusters derived from integrated
colors are consistent with the isochrone fitting ages. Isochrones are available
from http://achee.srl.caltech.edu.


title: Direct Detection of Galactic Halo Dark Matter

abstract:
  The Milky Way Galaxy contains a large, spherical component which is believed
to harbor a substantial amount of unseen matter. Recent observations indirectly
suggest that as much as half of this ``dark matter'' may be in the form of old,
very cool white dwarfs, the remnants of an ancient population of stars as old
as the Galaxy itself. We conducted a survey to find faint, cool white dwarfs
with large space velocities, indicative of their membership in the Galaxy's
spherical halo component. The survey reveals a substantial, directly observed
population of old white dwarfs, too faint to be seen in previous surveys. This
newly discovered population accounts for at least 2% of the halo dark matter.
It provides a natural explanation for the indirect observations, and represents
a direct detection of Galactic halo dark matter.


title: The Large Area Lyman Alpha Survey

abstract:
  The Lyman-$\alpha$ line is expected to be strong in the presence of active
star formation and the absence of dust, making it a good tool for finding
chemically primitive galaxies in the early universe. We report on a new survey
for high redshift Lyman-$\alpha$ sources, the Large Area Lyman Alpha (LALA)
survey. Our survey achieves an unprecedented combination of volume and
sensitivity by using narrow-band filters on the new $8192^2$ pixel CCD Mosaic
Camera at the 4 meter Mayall telescope of Kitt Peak National Observatory.
  Well-detected sources with flux and equivalent width matching known high
redshift Lyman-$\alpha$ galaxies have an observed surface density corresponding
to $11000 \pm 700$ per square degree per unit redshift at $z=4.5$. Early
spectroscopic followup from the Keck telescope suggests that $\sim 1/3$ of
these are actually at $z\approx 4.5$, and has confirmed five $z > 4$
Lyman-$\alpha$ emitters so far. Combining our photometric survey with
spectroscopic results, we estimate a net density of $\sim 4000 $ Lyman-$\alpha$
emitters per square degree per unit redshift at $z\approx 4.5$. The star
formation rate density (estimated both from UV continuum and from line
emission) is comparable to that of the Lyman break galaxy population within
present uncertainties. The most extreme Lyman-$\alpha$ emitters in our sample
have rest frame equivalent widths $> 100\AA$, consistent with the expectations
for the first burst of star formation in a primitive, dust-free galaxy.


title:
RR Lyrae pulsational temperature scales: On the consistency between
  different empirical relations

abstract:
  In this paper, by assuming as correct equilibrium temperatures of RRab Lyrae
variables those defined from Carney, Storm & Jones (1992, CSJ), we show that
temperatures derived from (B-V) colour (mean colour over the pulsational cycle
calculated on the magnitude scale) by Bessel, Castelli & Plez 1998 (BCP)
transformations, are consistent with CSJ equilibrium temperatures within a
probable error of $\delta \log T_e = \pm 0.003$. As a consequence it is shown
that the pulsational temperature scale, temperature-period-blue amplitude
($T_{eff}=f(P,A_B)$), provided by De Santis (1996, DS96) by (B-V) colour of
about 70 stars of Lub's sample, is a suitable relation, reddening and
metallicity free, to calculate equilibrium temperatures for RRab variables.
This relation is independent from variable mass and luminosity within a large
range of period-shift from the mean period-amplitude relation valid for the
Lub's sample of variables. On the contrary, it is also shown that a
temperature-amplitude-metallicity relation is strictly dependent from the
period-amplitude relation of the sample used for calibrating it: we prove this
means that it is dependent from both mass and luminosity variations of
variables.


title:
Statistical properties of ultraluminous IRAS galaxies from an HST
  imaging survey

abstract:
  We perform photometric measurements on a large HST snapshot imaging survey
sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative
nuclei from bright clumps in all the sample targets, mainly based on a
quantitative criterion of I-band luminosity as well as the global and local
morphological information. All the sources are then classified into three
categories with multiple, double and single nucleus/nuclei. The resultant
fractions of multiple, double and single nucleus/nuclei ULIRGs are 18%, 39% and
43%, respectively. This supports the multiple merger scenario as a possible
origin of ULIRGs, in addition to the commonly-accepted pair merger model.
Further statistical studies indicate that the AGN fraction increases from
multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs.
For the single nucleus category, there is a high luminosity tail in the
luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This
indicates that active galactic nuclei tend to appear at final merging stage.
For multiple/double nuclei galaxies, we also find a high fraction of very close
nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This
strengthens the conclusion that systems at late merging phase preferentially
host ULIRGs.


title:
The second ROSAT PSPC survey of M31 and the complete ROSAT PSPC source
  list

abstract:
  This paper reports the results of the analysis of the second ROSAT PSPC
survey of M31 performed in summer 1992. We compare our results with those of
the first survey. Within the ~10.7 deg^2 field of view, 396 individual X-ray
sources are detected in the second survey data, of which 164 are new
detections. When combined with the first survey, this result in a total of 560
X-ray sources in the field of M31. Their (0.1 keV - 2.0 keV) fluxes range from
7 x 10^-15 to 7.6 x 10^-12 erg cm^-2 s^-1, and of these 560 sources, 55 are
tentatively identified with foreground stars, 33 with globular clusters, 16
with supernova remnants, and 10 with radio sources and galaxies (including
M32). A comparison with the results of the Einstein M31 survey reveals 491
newly detected sources, 11 long term variable sources, and 7 possible transient
sources. Comparing the two ROSAT surveys, we come up with 34 long term variable
sources and 8 transient candidates. For the M31 sources, the observed
luminosities range from 4 x 10^35 to 4 x 10^38 erg s^-1. The total (0.1 keV -
2.0 keV) luminosity of M31 is (3.4+-0.3) x 10^39 erg s^-1, distributed
approximately equally between the bulge and disk. Within the bulge region, the
luminosity of a possible diffuse component combined with faint sources below
the detection threshold is (2.0+-0.5) x 10^38 erg s^-1. An explanation in terms
of hot gaseous emission leads to a maximum total gas mass of (1.0+-0.3) x 10^6
M_sun.


title:
Ionization Properties and Elemental Abundances in Damped Ly alpha
  Systems

abstract:
  We analyze extant data of Al++, Al+ and other low ions with the aim of
studying the ionization properties of Damped Ly alpha systems (DLAs) from the
analysis of the ratio R(Al++/Al+) = N(Al++)/N(Al+). We find good correlations
logN(Al+)-logN(Si+) and logN(Al+)-logN(Fe+) that we use to indirectly estimate
N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way we determine the
ratio R(Al++/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio
can attain relatively high values (up to 0.6), suggesting that gas of
intermediate ionization state plays an important role in DLAs. On the other
hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe,
and R(Al++/Al+) indicates that abundances are not severely influenced by
ionization effects. We find a logR(Al++/Al+) - logN(H I) anticorrelation that
we use, in conjunction with idealized photoionization equilibrium calculations,
to constrain the ionization properties and to predict ionization corrections in
DLAs. We consider two possible origins for the species of low and intermediate
ionization state: (1) neutral regions devoid of Al++ and/or (2) partially
ionized, (Al++)-bearing regions. The logR(Al++/Al+) - logN(H I) anticorrelation
can be naturally explained in terms of a two-region model with a soft,
stellar-type ionizing radiation field. We present abundance ionization
corrections for 14 elements of astrophysical interest derived with different
types of ionizing spectra. For most of these elements the corrections are
generally below measurements errors, contrary to the predictions of recent
models presented in the literature. We briefly discuss the potential effects of
inaccuracies of the Al recombination rates used in the photoionization
calculations.


title: X-ray transients in the galactic center region

abstract:
  In the last 15 years, 6 dedicated observation programs were carried out to
monitor ~20 deg around the galactic center for transient activity above a few
keV. Transient activity from low-mass X-ray binaries shows a strong preference
for this field. Two programs are currently active, with the Wide Field Cameras
on BeppoSAX and the Proportional Counter Array on RXTE. The coverage of these
programs is fairly extensive, with typical sensitivities of a few mCrab and an
angular resolution of a few arcminutes which is better than that of typical
all-sky monitor devices. Fifteen transients with peak fluxes above 10 mCrab
were discovered with these instruments so far (equivalent to 12 +/- 4 such
transients per year over the whole sky), and on top of that BeppoSAX-WFC
detected about 2000 X-ray bursts. We summarize some recent results.


title:
The discriminating power of wavelets to detect non-Gaussianity in the
  CMB

abstract:
  We investigate the power of wavelet techniques in detecting non-Gaussianity
in the cosmic microwave background (CMB). We use the method to discriminate
between an inflationary and a cosmic strings model using small simulated
patches of the sky. We show the importance of the choice of a good test
statistic in order to optimise the discriminating power of the wavelet
technique. In particular, we construct the Fisher discriminant function, which
combines all the information available in the different wavelet scales. We also
compare the performance of different decomposition schemes and wavelet bases.
For our case, we find that the Mallat and {\it \`a trous} algorithms are
superior to the 2D-tensor wavelets. Using this technique, the inflationary and
strings models are clearly distinguished even in the presence of a superposed
Gaussian component with twice the rms amplitude of the original cosmic string
map.


title: The First Detection of Cobalt in a Damped Lyman Alpha System

abstract:
  We present the first ever detection of Cobalt in a Damped Lyman Alpha system
(DLA) at z = 1.92. In addition to providing important clues to the star
formation history of these high redshift galaxies, we discuss how studying the
Co abundance in DLAs may also help to constrain models of stellar
nucleosynthesis in a regime not probed by Galactic stars.


title: MHD instability in differentially rotating cylindric flows

abstract:
  The possibility that the magnetic shear-flow instability (MRI, Balbus-Hawley
instability) might give rise to turbulence in a cylindric Couette flow is
investigated through numerical simulations. The study is linear and the fluid
flow is supposed to be incompressible and differentially rotating with a simple
velocity profile with $\Omega = a+b/R^2$. The simplicity of the model is
counterbalanced by the fact that the study is fully global in all three spatial
directions with boundaries on each side; finite diffusivities are also allowed.
The investigation is also carried out for several values of the azimuthal
wavenumber of the perturbations in order to analyse whether non-axisymmetric
modes might be preferred, which might produce, in a non-linear extension of the
study, a self-sustained magnetic field.
  We find that with magnetic field the instability is easier to be excited than
without magnetic field. The critical Reynolds number for Pm$=1$ is of order 50
independent of whether the nonmagnetic flow is stable or not. Also we find that
i) the magnetic field strongly reduces the number of Taylor vortices, ii) the
angular momentum is transported outwards and iii) for finite c ylinders a netto
dynamo-alpha effect results which is negative for the upper part of the
cylinder and which is positive for the lower part of the cylinder.
  For magnetic Prandtl number smaller than unity the critical Reynolds number
scales with Pm$^{-0.65}$. If this was true even for very small magnetic Prandtl
number (e.g. liquid sodium) the critical Reynolds number should reach the value
10$^5$ which is, however, also characteristic for nonlinear fi nite-amplitude
hydrodynamic Taylor-Couette turbulence -- so that we easily have to expect the
simultaneous existence of both sorts of instabi lities in cold accretion disks.


title:
STIS Echelle Observations of the Intrinsic UV Absorption in the Seyfert
  1 Galaxy NGC 3783

abstract:
  We present observations of the UV absorption lines in the Seyfert 1 galaxy
NGC 3783, obtained with the medium resolution echelle gratings of the Space
Telescope Imaging Spectrograph(STIS) on the Hubble Space Telescope (HST). The
spectra reveal the presence of three kinematic components of absorption in
Ly-alpha, C IV, and N V, at radial velocities of -1365, -548, and -724 km/s
with respect to the systemic velocity of the host galaxy (Components 1, 2 and
3, respectively); Component 1 also shows absorption by Si IV. Component 3 was
not detected in any of the earlier Goddard High Resolution Spectrograph (GHRS)
spectra, and the C IV absorption in the other components has changed since the
most recent GHRS observation obtained 5 yr earlier. Somewhat unexpectedly, each
component has a covering factor (of the continuum source + broad emission line
region) of about 0.6. We have calculated photoionization models to match the UV
column densities in each of the three components. Based on the model results,
there should be strong absorption in the bandpass of the Far Ultraviolet
Spectroscopic Explorer (FUSE), 912 - 1200 A, including saturated O VI lines at
each component velocity. The models also predict large O VII and O VIII column
densities but suggest that the UV absorbers cannot account for all of the X-ray
absorption detected in recent Chandra spectra. Finally, there is no evidence
for a correlation between the characteristics of the UV absorbers and the UV
continuum flux, which suggests that the variations observed in the GHRS and
STIS spectra are due in large part to changes in the column densities of the
absorbers as the result of transverse motion.


title:
Multi-method-modeling of interacting galaxies. I. A unique scenario for
  NGC 4449?

abstract:
  (abridged) We combined several N-body methods in order to investigate the
interaction scenario between NGC 4449 and DDO 125, a close companion in
projected space. In a first step fast restricted N-body models are used to
confine a region in parameter space reproducing the main observational
features. In a second step a genetic algorithm is applied for a uniqueness test
of our preferred parameter set. We show that our genetic algorithm reliably
recovers orbital parameters, provided that the data are sufficiently accurate,
i.e. all the key features are included.
  In the third step the results of the restricted N-body models are compared
with self-consistent N-body simulations. In the case of NGC 4449, the
applicability of the simple restricted N-body calculations is demonstrated.
Additionally, it is shown that the HI gas can be modeled here by a purely
stellar dynamical approach.
  In a series of simulations, we demonstrate that the observed features of the
extended HI disc can be explained by a gravitational interaction between NGC
4449 and DDO 125. According to these calculations the closest approach between
both galaxies happened $\sim 4-6 \cdot 10^8$ yr ago at a minimum distance of
$\sim 25$ kpc on a parabolic or slightly elliptic orbit. In the case of an
encounter scenario, the dynamical mass of DDO 125 should not be smaller than
10% of NGC 4449's mass. Before the encounter, the observed HI gas was arranged
in a disc with a radius of 35-40 kpc around the center of NGC 4449. It had the
same orientation as the central ellipsoidal HI structure. The origin of this
disc is still unclear, but it might have been caused by a previous interaction.


title:
Simulating Electron Transport and Synchrotron Emission in Radio
  Galaxies: Shock Acceleration and Synchrotron Aging in Three-Dimensional Flows

abstract:
  We present the first three-dimensional MHD radio galaxy simulations that
explicitly model transport of relativistic electrons, including diffusive
acceleration at shocks as well as radiative and adiabatic cooling in smooth
flows. We discuss three simulations of light Mach 8 jets, designed to explore
the effects of shock acceleration and radiative aging on the nonthermal
particle populations that give rise to synchrotron and inverse-Compton
radiations. We also conduct detailed synthetic radio observations of our
simulated objects. We have gained several key insights from this approach: 1.
The jet head in these multidimensional simulations is extremely complex. The
classical jet termination shock is often absent, but motions of the jet
terminus spin a ``shock-web complex'' within the backflowing jet material of
the head. 2. Understanding the spectral distribution of energetic electrons in
these simulations relies partly upon understanding the shock-web complex, for
it can give rise to distributions that confound interpretation in terms of the
standard model for radiative aging of radio galaxies. 3. The magnetic field
outside of the jet itself becomes very intermittent and filamentary in these
simulations, yet adiabatic expansion causes most of the cocoon volume to be
occupied by field strengths considerably diminished below the nominal jet
value. Thus population aging rates vary considerably from point to point.


title: Regularization and Inverse Problems

abstract:
  An overview is given of Bayesian inversion and regularization procedures. In
particular, the conceptual basis of the maximum entropy method (MEM) is
discussed, and extensions to positive/negative and complex data are
highlighted. Other deconvolution methods are also discussed within the Bayesian
context, focusing mainly on the comparison of Wiener filtering, Massive
Inference and the Pixon method, using examples from both astronomical and
non-astronomical applications.


title: Searching for the in-plane Galactic bar and ring in DENIS

abstract:
  New evidence for a long thin Galactic bar (in contradistinction to the
bulge), as well as for the existence of the ring and the truncation of the
inner disc, are sought in the DENIS survey. First, we examine DENIS and Two
Micron Galactic Survey star counts for the characteristic signatures of an
in-plane bar and ring. The star counts in the plane for 30 deg.>l>-30 deg. are
shown to be highly asymmetric with considerably more sources at positive than
at negative longitudes. At |b|\approx 1.5 deg., however, the counts are nearly
symmetric. Therefore, the asymmetry is not due to the disc, which is shown to
have an inner truncation, or to the bulge, so there has to be another major
component in the inner Galaxy that is causing the asymmetries. This component
provides up to 50% of the detected sources in the plane between the bulge and
l=27 deg. or l=-14 deg. This component is shown to be consistent with an
in-plane bar with a position angle of 40 deg. and half-length of 3.9 kpc.
However, there is also a major peak in the counts at l=-22 deg., which
coincides with the tangential point of the so-called 3 kpc arm. This is shown
to be most probably a ring or a pseudo-ring. The extinction in the plane is
also shown to be asymmetric with more extinction at negative than at positive
longitudes. For l<8 deg. the extinction is shown to be slightly tilted with
respect to b=0 deg. in the same manner as the HI disc. We conclude that the
Galaxy is a fairly typical ringed barred spiral galaxy.


title:
Chemical Enrichment at High Redshifts: Understanding the Nature of
  Damped Ly$\alpha$ Systems in Hierarchical Models

abstract:
  We use cosmological hydrodynamical simulations including star formation and
metal enrichment to study the evolution of the chemical properties of
galaxy-like objects at high redshift in the range $0.25<z< 2.35$ in a
hierarchical clustering scenario. As the galactic objects are assembled we find
that their gaseous components exhibit neutral Hydrogen column densities with
abundances and scatter comparable to those observed in damped Lyman-$\alpha$
systems (DLAs).The unweighted mean of abundance ratios and least square linear
regressions through the simulated DLAs yield intrinsic metallicity evolution
for the [Zn/H] and [Fe/H], consistent with results obtained from similar
analysis of available observations. Our model statistically reproduces the mild
evolution detected in the metallicity of the neutral hydrogen content of the
Universe, given by mass-weighted means,if observational constraints are
considered (as suggested by Boiss\'ee et al. 1998). For the $\alpha$-elements
in the simulated DLAs, we find neither enhancement nor dependence on
metallicity. Our results support the hypotheses that DLAs trace a variety of
galactic objects with different formation histories and that both SNI and SNII
are contributing to the chemical enrichment of the gas component at least since
$z \approx 2$. This study indicates that DLAs could be understood as the
building blocks that merged to form today normal galaxies within a hierarchical
clustering scenario.


title: The Inner Light Year of the Nearest Seyfert 1 Nucleus in NGC 4395

abstract:
  The NRAO VLBA was used at 1.4 GHz to image the inner 550 mas (23 lt-years) of
the nearest known Seyfert 1 nucleus, in the Sm galaxy NGC 4395. One continuum
source was detected, with flux density $530\pm130 \mu$Jy, diameter d<11 mas
(0.46 lt-year), and brightness temperature $T_{\rm b} > 2.0\times10^6$ K. The
spectral power $P$ of the VLBA source is intermediate between those of
Sagittarius A and Sagittarius A$^*$ in the Galactic Center. For the VLBA source
in NGC 4395, the constraints on $T_b$, $d$, and $P$ are consistent with an
origin from a black hole but exclude an origin from a compact starburst or a
supernova remnant like Cassiopeia A. Moreover, the spectral powers of NGC 4395
at 1.4 and 4.9 GHz appear to be too low and too constant to allow analogy with
SN 1988Z, a suggested prototype for models of compact supernova remnants. The
variable and warm X-ray absorber in NGC 4395 has a free-free optical depth much
larger than unity at 1.4 GHz and, therefore, cannot fully cover the VLBA
source.


title: The environments of radio-loud quasars

abstract:
  We have obtained multi-colour imaging of a representative, statistically
complete sample of low-frequency selected (S_408MHz > 0.95Jy) radio loud
quasars at intermediate (0.6 < z < 1.1) redshifts. These sources are found in a
variety of environments, from the field through to rich clusters. We show that
statistical measures of environmental richness, based upon single-band
observations are inadequate at these redshifts for a variety of reasons.
Environmental richness seems correlated with the size and morphology of the
radio source, as expected if the energy density in the radio lobes is
approximately the equipartition value and the lobes are in pressure equilbrium
with a surrounding intragroup/cluster medium. Selecting on radio size therefore
efficiently selects dense galactic sytems at these redshifts.


title:
Quantitative Morphology of the Intermediate-Redshift Galaxy Cluster
  Abell 2443 from Ground-Based Imaging -- Evidence for a galaxy concentration
  index correlation with cluster density

abstract:
  We present broad-band photometry and provide a quantitative analysis of the
structure of galaxies in the inner region of the Abell Cluster 2443 (z~0.1).
The galaxy parameters have been derived by fitting a two-component model
(Sersic r^{1/n} bulge and exponential disk) to a magnitude-limited sample.
Using a new method of analysis which takes into account the effects of seeing
on the structural parameters and considers the ellipticity as an active
parameter, we avoid systematic errors arising from assumptions of circular
symmetry. 76% of the sample galaxies were classified with these models, the
rest were morphologically peculiar. For the spiral galaxies, the relation
between n and B/D is consistent with the trend observed in nearby field galaxy
samples. The Sersic index n (which can be considered as a concentration index)
of the elliptical galaxies is correlated with the local surface density of the
cluster. Monte Carlo simulations were used to check the reliability of the
method and determine the magnitude selection criteria.


title:
86 GHz Very Long Baseline Polarimetry of 3C273 and 3C279 with the
  Coordinated Millimeter VLBI Array

abstract:
  86 GHz Very Long Baseline Polarimetry probes magnetic field structures within
the cores of Active Galactic Nuclei at higher angular resolutions and a
spectral octave higher than previously achievable. Observations of 3C273 and
3C279 taken in April 2000 with the Coordinated Millimeter VLBI Array have
resulted in the first total intensity (Stokes I) and linear polarization VLBI
images reported of any source at 86 GHz. These results reveal the 86 GHz
electric vector position angles within the jets of 3C273 and 3C279 to be
orthogonal to each other, and the core of 3C273 to be unpolarized. If this lack
of polarization is due to Faraday depolarization alone, the dispersion in
rotation measure is >=90000 rad/m^2 for the core of 3C273.


title: Cluster correlations in redshift space

abstract:
  We test an analytic model for the two-point correlations of galaxy clusters
in redshift space using the Hubble Volume N-body simulations. The correlation
function of clusters shows no enhancement along the line of sight, due to the
lack of any virialised structures in the cluster distribution. However, the
distortion of the clustering pattern due to coherent bulk motions is clearly
visible. The distribution of cluster peculiar motions is well described by a
Gaussian, except in the extreme high velocity tails. The simulations produce a
small but significant number of clusters with large peculiar motions. The form
of the redshift space power spectrum is strongly influenced by errors in
measured cluster redshifts in extant surveys. When these errors are taken into
account, the model reproduces the power spectrum recovered from the simulation
to an accuracy of 15% or better over a decade in wavenumber. We compare our
analytic predictions with the power spectrum measured from t! he!
  APM cluster redshift survey. The cluster power spectrum constrains the
amplitude of density fluctuations, as measured by the linear rms variance in
spheres of radius 8 h^(-1) Mpc, denoted by sigma_8. When combined with the
constraints on sigma_8 and the density parameter Omega derived from the local
abundance of clusters, we find a best fitting cold dark matter model with
sigma_8~1.25 and Omega~0.2, for a power spectrum shape that matches that
measured for galaxies. However, for the best fitting value of Omega and given
the value of Hubble's constant from recent measurements, the assumed shape of
the power spectrum is incompatible with the most readily motivated predictions
from the cold dark matter paradigm.


title: An inner ring and the micro lensing toward the Bulge

abstract:
  All current Bulge-Disk models for the inner Galaxy fall short of reproducing
self-consistently the observed micro-lensing optical depth by a factor of two
($> 2\sigma$). We show that the least mass-consuming way to increase the
optical depth is to add density roughly half-way the observer and the highest
micro-lensing-source density. We present evidence for the existence of such a
density structure in the Galaxy: an inner ring, a standard feature of barred
galaxies. Judging from data on similar rings in external galaxies, an inner
ring can contribute more than 50% of a pure Bulge-Disk model to the
micro-lensing optical depth. We may thus eliminate the need for a small viewing
angle of the Bar. The influence of an inner ring on the event-duration
distribution, for realistic viewing angles, would be to increase the fraction
of long-duration events toward Baade's window. The longest events are expected
toward the negative-longitude tangent point at $\ell\sim$ -22\degr . A properly
sampled event-duration distribution toward this tangent point would provide
essential information about viewing angle and elongation of the over-all
density distribution in the inner Galaxy.


title: Cosmological Reionization

abstract:
  The universe was reionized by redshift z ~ 6 by a small fraction of the
baryons in the universe, which released energy following their condensation out
of a cold, dark, and neutral IGM into the earliest galaxies. The theory of this
reionization is a critical missing link in the theory of galaxy formation. Its
numerous observable consequences include effects on the spectrum, anisotropy
and polarization of the cosmic microwave background and signatures of
high-redshift star and quasar formation. This energy release also created
feedback on galaxy formation which left its imprint on the mass spectrum and
internal characteristics of galaxies and on the gas between galaxies long after
reionization was complete. Recent work suggests that the photoevaporation of
dwarf galaxy minihalos may have consumed most of the photons required to
reionize the currently-favored Lambda-CDM universe. We will review recent
developments in our understanding of this process.


title:
The Obscuring Starburst of NGC 6221 and Implications for the Hard X-ray
  Background

abstract:
  We present NGC 6221 as a case study of ``X-ray loud composite galaxies,''
which appear similar to starbursts at optical wavelengths and resemble
traditional active galactic nuclei in X-rays. The net optical spectrum of NGC
6221 is properly characterized as a starburst galaxy, but in X-rays, NGC 6221
is similar to Seyfert 1 galaxies, exhibiting a power-law continuum spectrum, a
broad Fe K alpha line, and continuum variability on timescales of days and
years. High-resolution images reveal that the detected active nucleus is
relatively weak, not only at optical, but also at near-infrared wavelengths. An
obscuring starburst, in which the interstellar gas and dust associated with the
starburst conceal the active nucleus, accounts for these peculiar features. We
demonstrate quantitatively that obscuration by column density N_H = 10^{22}
cm^{-2} combined with relatively weak intrinsic nuclear activity can produce an
optical spectrum characteristic of the surrounding starburst alone. While
optical surveys would not identify the active nuclei that make these galaxies
significant X-ray sources, such galaxies may in fact be important contributors
to the X-ray background.


title: The Detection of a Cooling Flow Elliptical Galaxy from OVI Emission

abstract:
  Cooling flow models for the hot gas in elliptical galaxies predict that gas
is cooling at a rate of ~ 1 solar mass/yr, yet there is little evidence for
this phenomenon beyond the X-ray waveband. If hot gas is cooling, it will pass
through the 3x10^5 K regime and radiate in the OVI 1032,1038 ultraviolet lines,
which can be detected with the Far Ultraviolet Spectroscopic Explorer (FUSE)
and here we report on FUSE observations of the X-ray bright early-type galaxies
NGC 1404 and NGC 4636. In NGC 1404, the OVI doublet is not detected, implying a
cooling rate < 0.3 solar masses/yr, which is below the predicted values from
the cooling flow model of 0.4-0.9 solar masses/yr. In NGC 4636, both OVI lines
are clearly detected, indicating a cooling rate of 0.43 +/- 0.06 solar
masses/yr, which falls within the range of values from the cooling flow
prediction, 0.36-2.3 solar masses/yr and is closest to the model where the
production of the cooled gas is distributed through the galaxy. The emission
line widths, 44 +/- 15 km/s, are close to the Doppler broadening value (30
km/s), indicating that the flow is quiescent rather than turbulent, and that
the flow velocity is < 30 km/s.


title: Coronal structure geometries on pre-main sequence stars

abstract:
  We have re-analyzed using a hydrodynamic model large flaring events on three
different categories of pre-main sequence (PMS) stars: the young stellar object
(YSO) YLW 15, the classical T Tauri star (CTTS) LkHalpha92, the weak-line T
Tauri star (WTTS) V773 Tau, and the WTTS HD 283572 (the first three objects
were observed by ASCA, the last by ROSAT; all have been previously reported in
the literature). The first three flares were previously analyzed on the basis
of the quasi-static model mostly used up to now, consistently yielding large
loops (L >= R*) and no evidence of sustained heating. Our hydrodynamic modeling
approach, however, shows that the size of the flaring regions must be much
smaller (L <=R*) and moreover this method shows in all cases evidence of
vigorous sustained heating during the flare decay, so that the decay of the
observed light curve actually reflects the temporal profile of the heating
rather than that of the free decay of the heated loop(s). The events on the
protostar YLW 15 have durations comparable to the stellar rotation period, so
that their limited size and their lack of self-eclipses give evidence of a
polar location on the star. This is in contrast with the recently advanced
hypothesis that these flares are due to long loops spanning the region between
the star and the accretion disk. In general, the present analysis shows that
flaring coronae on PMS stars have a structure similar to the coronae on older
active stars.


title:
Comparative seismology of pre- and main sequence stars in the
  instability strip

abstract:
  Pulsational properties of 1.8 M$_{\odot}$ stellar models covering the latest
stages of contraction toward the main sequence up to early hydrogen burning
phases are investigated by means of linear nonadiabatic analyses. Results
confirm that pre-main sequence stars (pms) which cross the classical
instability strip on their way toward the main sequence are pulsationally
unstable with respect to the classical opacity mechanisms. For both pms and
main sequence types of models in the lower part of the instability strip, the
unstable frequency range is found to be roughly the same. Some non-radial
unstable modes are very sensitive to the deep internal structure of the star.
It is shown that discrimination between pms and main sequence stages is
possible using differences in their oscillation frequency distributions in the
low frequency range.


title:
An Empirical Ultraviolet Template for Iron Emission in Quasars as
  Derived from I Zw 1

abstract:
  We present an empirical template spectrum suitable for fitting/subtracting
and studying the FeII and FeIII line emission in the restframe UV spectra of
active galatic nuclei (AGNs), the first empirical UV iron template to cover the
full 1250 - 3090 A range. Iron emission is often a severe contaminant in
optical--UV spectra of AGNs. Its presence complicates and limits the accuracy
of measurements of both strong and weak emission lines and the continuum
emission, affecting studies of line and continuum interrelations, the
ionization structure, and elemental abundances in AGNs. Despite the wealth of
work on modeling the AGN FeII emission and the need to account for it in
observed AGN spectra, there is no UV template electronically available to aid
this process. The iron template we present is based on HST spectra of the
Narrow Line Seyfert 1, IZw1. Its intrinsic narrow lines (~900 km/s) and rich
iron spectrum make the template particularly suitable for use with most AGN
spectra. The iron emission spectrum and the line identifications and
measurements are presented and compared with the work of Laor et al. We
illustrate the application of the derived FeII and FeIII templates by fitting
and subtracting the iron emission from UV spectra of four high-z quasars and of
the nearby quasar, 3C273. We briefly discuss the small discrepancies between
this observed iron emission and the UV template, and compare the template with
previously published ones. We discuss the advantages and limitations of the
templates and of the template fitting method. We conclude that the templates
work sufficiently well to be a valuable and important tool for eliminating and
studying the iron emission in AGNs, at least until accurate theoretical iron
emission models are developed. (Abridged)


title: Detection of a multi-phase ISM at $z=0.2212$

abstract:
  We present sensitive Giant Metrewave Radio Telescope (GMRT) and
high-resolution Arecibo HI 21-cm observations of the damped Lyman-$\alpha$
absorber (DLA) at $z=0.2212$ towards OI 363. The spectra are in excellent
agreement and yield a spin temperature $T_s = 890 \pm 160$ K, far higher than
$T_s$ values measured in local spirals, but similar to values obtained in the
majority of DLAs ($T_s \ga 1000$ K).
  The high velocity resolution of the Arecibo spectra enables us to obtain
estimates of physical conditions in the absorbing clouds by fitting multiple
Gaussians to the absorption profile. The spectra are well fit by a
three-component model with two narrow and one wide components, with
temperatures ${T_{k_1}} = 308 \pm 24$ K, ${T_{k_2}} = 180 \pm 30$ K and
${T_{k_3}} = 7600 \pm 1250$ K, respectively. The last of these is in excellent
agreement with the expected temperatures for the WNM ($5000 - 8000$ K). The
mere fact that components are seen with lower temperatures than the estimated
$T_s$ implies that the absorber must have a multi-phase medium.
  We use the measured 21-cm optical depth and the above estimates of the
kinetic temperature to obtain the HI column density in the various components.
The total column density in the narrow components is found to be $\NHI (CNM)
\le 1.9 \pm 0.25 \times 10^{20}$ cm, while that in the wide component is $\NHI
(WNM) \ge 1.26 \pm 0.49 \times 10^{21}$ cm. Thus, the WNM contains at least 75%
of the total HI in the $z = 0.2212$ DLA, unlike our Galaxy, in which the CNM
and WNM have equitable contributions; this accounts for the difference in the
spin temperatures of the $z = 0.2212$ system and local spirals, suggesting that
the DLA is probably a dwarf or LSB type galaxy (abridged).


title: The blue straggler S1082: a triple system in the old open cluster M67

abstract:
  We present a photometric and spectroscopic study of the blue straggler S1082
in the open cluster M67. Our observations confirm the previously reported 1.07
day eclipse light curve and the absence of large radial-velocity variations of
the narrow-lined star. However, we find two more spectral components which do
vary on the 1.07 day period. We conclude that the system is triple. We fit the
light and radial-velocity curves and find that the total mass of the inner
binary is more than twice the turnoff mass and that the outer companion to the
binary is a blue straggler on its own account. We briefly discuss formation
scenarios for this multiple system.


title:
RXTE observations of the neutron star low-mass X-ray binary GX 17+2:
  correlated X-ray spectral and timing behavior

abstract:
  We have analyzed ~600 ks of Rossi X-ray Timing Explorer data of the neutron
star low-mass X-ray binary and Z source GX 17+2. A study was performed of the
properties of the noise components and quasi-periodic oscillations (QPOs) as a
function of the spectral properties, with the main goal to study the relation
between the frequencies of the horizontal branch and upper kHz QPOs. It was
found that when the upper kHz QPO frequency is below 1030 Hz these frequencies
correlate, whereas above 1030 Hz they anti-correlate. GX 17+2 is the first
source in which this is observed. We also found that the frequency difference
of the high frequency QPOs was not constant and that the quality factors (Q
values) of the HBO, its second harmonic, and the kHz QPOs are similar, and vary
almost hand in hand by a factor of more than three. Observations of the normal
branch oscillations during two type I X-ray bursts showed that their absolute
amplitude decreased as the flux from the neutron star became stronger. We
discuss these and other findings in terms of models that have been proposed for
these phenomena. We also compare the behavior of GX 17+2 and other Z sources
with that of black hole sources and consider the possibility that the mass
accretion rate might not be driving force behind all spectral and variability
changes.


title: The Fundamental Plane of Elliptical Galaxies in Compact Groups

abstract:
  We present a study focusing on the nature of compact groups through the study
of their elliptical galaxies. We determine velocity dispersions ($\sigma$) for
18 18 bright elliptical galaxies located in the core of Hickson compact groups
and a control sample of 12 bright bona fide ellipticals located in the field or
very loose groups. Several tests are carried out to avoid sources of systematic
effects in $\sigma$ measurements. We use these velocity dispersions to compare
the position of 11 compact group galaxies in the Fundamental Plane to that of a
large and homogeneous sample of elliptical galaxies (Burstein et al. 1987). We
find that little or no significant difference exists, as far as the Fundamental
Plane is concerned, between ellipticals in compact groups and their
counterparts in other environments.


title: Sco X-1: Energy Transfer from the Core to the Radio Lobes

abstract:
  The evolution of the radio emission from Sco X-1 is determined from a 56-hour
continuous VLBI observation and from shorter observations over a four-year
period. The radio source consists of a variable core near the binary, and two
variable compact radio lobes which form near the core, move diametrically
outward, then fade away. Subsequently, a new lobe-pair form near the core and
the behavior repeats. The differences in the radio properties of the two lobes
are consistent with the delay and Doppler-boosting associated with an average
space velocity of 0.45c at 44 deg to the line of sight. Four lobe speeds,
between 0.32c and 0.57c, were measured for several lobe-pairs on different
days. The speed during each epoch remained constant over many hours. The
direction of motion of the lobes over all epochs remained constant to a few
degrees.
  Two core flares are contemporaneous with two lobe flares after removal of the
delay associated with an energy burst moving with speed >0.95c in a twin-beam
from the core to each lobe. This is the first direct measurement of the speed
of energy flow within an astrophysical jet. The similarity of the core and lobe
flares suggests that the twin-beam flow is symmetric and that the core is
located near the base of the beam.


title:
The Fanaroff-Riley Transition and the Optical Luminosity of the Host
  Elliptical Galaxy

abstract:
  We show that a model for radio source dynamics we had earlier proposed can
readily reproduce the relationship between the radio power division separating
the two Fanaroff-Riley classes of extragalactic radio sources and the optical
luminosity of the host galaxy, as found by Owen and Ledlow (1994). In our
scenario, when less powerful jets eventually slow down to the point that the
advance of the working surface (i.e., hotspot) becomes subsonic with respect to
the external gas, the jet's collimation is severely weakened. This criterion
distinguishes the powerful and well collimated FR II sources from the weaker
sources producing the less collimated FR I type morphologies.


title:
Semi-analytic approximations for production of atmospheric muons and
  neutrinos

abstract:
  Simple approximations for fluxes of atmospheric muons and muon neutrinos are
developed which display explicitly how the fluxes depend on primary cosmic ray
energy and on features of pion production. For energies of approximately 10 GeV
and above the results are sufficiently accurate to calculate response functions
and to use for estimates of systematic uncertainties.


title:
The Contribution of Population III to the Enrichment and Preheating of
  the Intracluster Medium

abstract:
  Intracluster medium (ICM) abundances are higher than expected assuming
enrichment by supernovae with progenitors belonging to the simple stellar
population (SSP) observed in cluster galaxies, if stars formed with a standard
initial mass function (IMF). Moreover, new results on ICM oxygen abundances
imply that nucleosynthesis occurred with nonstandard yields. The hypothesis
that hypernovae (HN) in general, and HN associated with Population III (Pop
III) stars in particular, may significantly contribute to ICM enrichment is
presented and evaluated. The observed abundance anomalies can be explained by a
hypernovae-producing subpopulation of the SSP, but only if it accounts for half
of all supernova explosions and if Type Ia supernova rates are very low. Also,
the implied energy release may be excessive. However, an independent Pop III
contribution -- in the form of metal-free, very massive stars that evolve into
hypernovae -- can also account for all the observed abundances, while avoiding
these drawbacks and accommodating a normal IMF in subsequent stellar
generations. The required number of Pop III stars provides sufficient energy
injection (at high redshift) to explain the ICM ``entropy floor''. Pop III
hypernova pre-enrich the intergalactic medium, and can produce a significant
fraction of the metals observed in the Lyman-alpha forest. Several testable
predictions for ICM and IGM observations are made.


title:
Rapid UBVRI Follow-up of the Highly Collimated Optical Afterglow of
  GRB010222

abstract:
  (Abridged) We present the earliest optical observations of the optical
counterpart to the GRB 010222, obtained with the FLWO 1.2-m telescope in UBVRI
passbands, starting 3.64 hours after the burst. We also present late R-band
observations of the afterglow obtained with the 1.8-m VATT ~25 days after the
burst. The temporal analysis of our data joined with published data indicates a
steepening decay, independent of wavelength, asymptotically approaching
t^{-0.80+/-0.05} at early times (t << 1 day) and t^{-1.30+/-0.05} at late
times, with a sharp break at t_b=0.72+/-0.1 days. This is the second earliest
observed break of any afterglow (after GRB 980519), which clearly indicates the
importance of rapid multi-band follow-up for GRB afterglow research. The
optical spectral energy distribution, corrected for small Galactic reddening,
can be fit fairly well by a single power-law with a slope of -1.07+/-0.09.
However, when we fit using our BVRI data only, we obtain a shallower slope of
-0.88+/- 0.1, in excellent agreement with the slope derived from our
low-resolution spectrum (-0.89 +/- 0.03). The spectral slope and light curve
decay slopes we derive are not consistent with a jet model despite the presence
of a temporal break. Significant host dust extinction with a star-burst
reddening law would flatten the spectral index to match jet predictions and
still be consistent with the observed spectral energy distribution. We derive
an opening angle of 2.1 deg, smaller than any listed in the recent compilation
of Frail et al. The total beamed energy corrected for the jet geometry is
4x10^50 erg, very close to the ``standard'' value of 5x10^50 erg found by Frail
et al. for a number of other bursts with light-curve breaks.


title: Shocked ammonia in the WR nebula NGC 2359

abstract:
  We report the detection of the (1,1) and (2,2) metastable lines of ammonia
(NH_3) in the molecular cloud associated with the Wolf--Rayet (WR) nebula NGC
2359. Besides the CO and H_2, this is the first molecule detected in the
environs of a WR star. Width (3 km/s) and radial velocity (54 km/s) indicate
that the NH$_3$ lines arises from the molecular cloud which is interacting with
the WR star. The rotational temperature derived from the (1,1) and (2,2) line
intensity ratios is about 30 K, significantly larger than the typical kinetic
temperature of the ambient gas of 10 K. The derived NH_3 abundance is 10^{-8}.
Linewidth, abundance and kinetic temperature can be explained if NH_3 is
released from dust grain mantles to the gas phase by shocks produced by the
expansion of the bubble created by the WR stellar wind. We briefly discuss the
implications of the detection of warm NH_3 associated with a WR star in
connection to the hot NH_3 emission detected in the Galactic Center and in the
nuclei of external galaxies.


title: HI in Abell 3128

abstract:
  We discuss Australia Telescope Compact Array (ATCA) HI 21cm data for the
galaxy cluster A3128. Our observations are intentionally relatively shallow,
and a blind search through our data cube yields (tentative) detections of only
two galaxies, of which one is probably spurious.
  A3128 is part of the ESO Nearby Abell Cluster Survey (ENACS); redshifts are
available for 193 galaxies in the A3128 region. For 148 of these galaxies the
redshifts are such that the HI emission (if any) would lie within our data
cube. We use the known redshifts of these galaxies to coadd their spectra and
thus improve our sensitivity to HI emission. The technique is fairly successful
-- the coadded spectra allow detection of an average mass content of ~ 9x10^8
Msun, almost an order of magnitude lower than for direct detection of
individual objects.
  By dividing the total galaxy sample into subsamples we find that the gas
content of late type galaxies that lie outside the X-ray emitting core of the
cluster is substantially higher than that of those within the core. Even
outside the X-ray emitting region the distribution of gas-rich galaxies in the
cluster is not uniform, we find that gas-rich galaxies are concentrated in the
east of the cluster. This is consistent with earlier analyses of the kinematics
of the galaxies in A3128 which indicate the presence of subclustering.
  In summary we find that coadding spectra is a powerful tool for the study of
HI in cluster galaxies, and suggest that this technique could be applied to
substantially increase the redshift range over which such observations could be
carried out.