title:
The Evolution of Population Gradients in Galaxy Clusters: the
  Butcher-Oemler Effect and Cluster Infall

abstract:
  We present photometric and spectroscopic measurements of the galaxy
populations in clusters from the CNOC1 sample of rich, X-ray luminous clusters
at $0.18 < z < 0.55$. A classical measure of the galaxy blue fraction for
spectroscopically confirmed cluster members shows a significant Butcher-Oemler
effect for the sample, but only when radii larger than 0.5$r_{200}$ are
considered. We perform a principal component analysis of galaxy spectra to
divide the total cluster light into contributions from stellar populations of
different ages. Composite radial distributions of different stellar populations
show strong gradients as a function of cluster-centric radius. The composite
population is dominated by evolved populations in the core, and gradually
changes at radii greater than the virial radius to one which is similar to
coeval field galaxies. We do not see evidence at any radius within the clusters
for an excess of star formation over that seen in the coeval field. Within this
redshift range, significant evolution in the fractional population gradient is
seen. Both low and high redshift clusters have similar populations in the
cluster cores, but higher redshift clusters have steeper gradients and more
star forming galaxies at radii outside of the core region~-- in effect, a
restatement of the Butcher-Oemler effect. Luminosity density profiles are
consistent with a scenario where this phenomenon is due to a decline over time
in the infall rate of field galaxies into clusters. Depending on how long
galaxies reside in clusters before their star formation rates are diminished,
this suggests a decrease in the infall into clusters of a factor of $\sim 3$
between $z > 0.8$ and $z \sim 0.5$. We also discuss alternative scenarios for
the evolution of cluster populations.


title:
The Shape and Orientation of NGC 3379: Implications for Nuclear
  Decoupling

abstract:
  The intrinsic shape and orientation of the elliptical galaxy NGC 3379 are
estimated by dynamical modeling. The maximal ignorance shape estimate, an
average over the parameter space, is axisymmetric and oblate in the inner
parts, with an outward triaxiality gradient. The 1 sigma limits on total-mass
triaxiality T are T < 0.13 at 0.33 kpc and T = 0.08 +/- 0.07 at 3.5 kpc from
the center. The luminous short-to-long axis ratio c_L = 0.79 +0.05-0.1 inside
0.82 kpc, flattening to c_L = 0.66 +0.07-0.08 at 1.9 kpc. The results are
similar if the galaxy is assumed to rotate about its short axis. Estimates for
c_L are robust, but those for T are dependent on whether the internal rotation
field is disklike or spheroid-like. Short-axis inclinations between 30 and 50
degrees are preferred for nearly axisymmetric models; but triaxial models in
high inclination are also allowed, which can affect central black hole mass
estimates. The available constraints on orientation rule out the possibility
that the nuclear dust ring at R = 1.5" is in a stable equilibrium in one of the
galaxy's principal planes. The ring is thus a decoupled nuclear component not
linked to the main body of the galaxy. It may be connected with ionized gas
that extends to larger radii, since the projected gas rotation axis is near the
minor axis of the ring. The gas and dust may both be part of a strongly warped
disk; however, if caused by differential precession, the warp will wind up on
itself in a few 10^7 years. The decoupling with the stellar component suggests
that the gas has an external origin, but no obvious source is present.


title: A simple algorithm for orbit classification

abstract:
  We describe a simple algorithm for classifying orbits into orbit families.
This algorithm works by finding patterns in the sign changes of the principal
coordinates. Orbits in the logarithmic potential are studied as an application;
we classify orbits into boxlet families, and examine the influence of the core
radius on the set of stable orbit families.


title: Gas Dynamics in Galaxy Mergers

abstract:
  In interacting and merging galaxies, gas is subject to direct hydrodynamic
effects as well as tidal forces. One consequence of interactions is the rapid
inflows of gas which may fuel starbursts and AGN. But gas dynamics is not
limited to inflows; a small survey of equal-mass and unequal-mass encounters
produces a wide variety of features, including plumes between galaxies,
extended disks formed by infall of tidal debris, and counterrotating nuclear
disks. An even richer spectrum of behavior awaits better thermodynamic models
for gas in merging galaxies.


title: Dynamics of Stellar Collisions

abstract:
  I compare gas-dynamical and stellar-dynamical models of collisions. These two
models have distinctly different physics; for example, shocks introduce
irreversibility in gas systems, while stellar systems evolve in a completely
reversible fashion. Nonetheless, both models yield broadly similar results,
suggesting that analogies between gas and stellar dynamics have some heuristic
validity even applied to collisions.


title: Dwarf Spheroidal Galaxies in the M81 Group Imaged with WFPC2

abstract:
  We obtained HST/WFPC2 images of the dwarf spheroidal (dSph) galaxies K61,
K63, K64, DDO78, BK6N, and kk77 in the M81 group. Our color-magnitude diagrams
show red giant branches with tips (TRGB) falling within the range of I = [23.8
-24.0] mag. The derived true distance moduli (DM) of the 6 dSphs ranging from
27.71 to 27.93 mag are consistent with their membership in the group. Given
accurate distances of 5 other group members, which have been derived via TRGB
or cepheids, the mean DM of the M81 group is (27.84+-0.05) mag. We find the
difference of the mean distances to the M81 and NGC2403 groups to be D_M81 -
D_NGC2403 =(0.5+-0.2) Mpc, which yields a deprojected separation of 0.9 Mpc.
With respect to the Local group, M81 and NGC2403 have radial velocities of 106
and 267 km/s respectively, while the velocities of the group centroids are 142
and 281 km/s. The higher velocity of the closer system may indicate that these
groups are moving towards each other, similar to the Milky Way and M31 in the
Local group. Several globular cluster candidates have been identified in the
galaxies.


title: Australian Cosmic Ray Modulation Research

abstract:
  Australian research into variations of the cosmic ray flux arriving at the
Earth has played a pivotal role for more than 50 years. The work has been
largely led by the groups from the University of Tasmania and the Australian
Antarctic Division and has involved the operation of neutron monitors and muon
telescopes from many sites. In this paper the achievements of the Australian
researchers are reviewed and future experiments are described. Particular
highlights include: the determination of cosmic ray modulation parameters; the
development of modelling techniques of Ground Level Enhancements; the
confirmation of the Tail-In and Loss-Cone Sidereal anisotropies; the Space Ship
Earth collaboration; and the Solar Cycle latitude survey.


title: A group of galaxies around the giant spiral NGC 6946

abstract:
  We present large-scale CCD images of a dozen low surface brightness objects
in the vicinity of giant Sc galaxy NGC 6946, which have been found recently on
POSS-II copies. Six of them: UGC 11583, kk251, kk252, kkr55, kkr56, and kkr59
are dwarf irregular galaxies resolved into stars. We determined their distances
from the luminosity of the brightest blue stars. The mean estimated distance to
the NGC 6946 group is 5.9 +- 0.4 Mpc. Together with Cepheus 1, discovered
before, the group consist of 8 late type galaxies. Almost all of them are
detected or marginally detected in the HI line. The group is characterized by a
linear projected diameter of 420 kpc, a radial velocity dispersion of 95 km/s,
and a virial mass-to-luminosity ratio about 56 M_sun/L_sun.


title: Properties of Simulated Magnetized Galaxy Clusters

abstract:
  We study the evolution of magnetized clusters in a cosmological environment
using magneto-hydro dynamical simulations. Large scale flows and merging of
subclumps generate shear flows leading to Kelvin-Helmholtz instabilities,
which, in addition to the compression of the gas where the magnetic field is
frozen in, further amplify the magnetic field during the evolution of the
cluster. Therefore, well-motivated initial magnetic fields of
$<B^2>^{1/2}=10^{-9} {\rm G}$ reach the observed $\sim\mu{\rm G}$ field
strengths in the cluster cores at $z=0$. These magnetized clusters can be used
to study the final magnetic field structure, the dynamical importance of
magnetic fields for the interpretation of observed X-Ray properties, and help
to constrain further processes in galaxy clusters like the population of
relativistic particles giving rise to the observed radio halos or the behavior
of magnetized cooling flows.


title:
The Optical Gravitational Lensing Experiment. BVI Maps of Dense Stellar
  Regions. II. The Large Magellanic Cloud

abstract:
  We present the BVI photometric maps of the Large Magellanic Cloud. They
contain BVI photometry and astrometry of more than 7 million stars from the
central parts of the LMC. The data were collected during the second phase of
the OGLE microlensing project. We discuss the accuracy of the data and present
color-magnitude diagrams of all 26 fields observed by OGLE in the LMC.
  The BVI maps of the LMC are accessible electronically for the astronomical
community from the OGLE Internet archive.


title:
The Optical Gravitational Lensing Experiment. Stellar Distance
  Indicators in the Magellanic Clouds and Constraints on the Magellanic Cloud
  Distance Scale

abstract:
  BVI photometry of the Magellanic Clouds collected during the OGLE-II
microlensing experiment makes it possible to study in detail photometric
properties of the "major" stellar distance indicators in the Magellanic Clouds.
In addition to Cepheids, RR Lyr and red clump stars, which photometry was
presented in the earlier OGLE papers, we present the so far most accurate
determination of brightness of the tip of the red giant branch in the LMC and
SMC. We analyze the ratios of brightness of all four distance indicators in the
LMC and SMC. Additionally, we include in our analysis, when possible,
photometric data of the distance indicators in the metal poor Carina dwarf
galaxy for which photometry was also collected during the OGLE-II experiment.
The analysis is largely differential, free from zero point and extinction
uncertainties. The main conclusion is that the distance scales resulting from
all four stellar distance indicators are fully consistent with each other.
Therefore the distance scale problem is not a problem resulting from different
distances yielded by distance indicators, but rather a problem of the proper
zero point of this common distance scale. All four stellar distance indicators
have to be treated as an ensemble, and any determination of the zero point for
one of them must predict reasonable luminosities of the others. This puts
strong constraints on many proposed calibrations of distance indicators. For
example, very bright calibrations of Cepheids or RR Lyr can practically be
ruled out. (Abridged).


title: Why the Salpeter screening formula cannot be applied in the Sun

abstract:
  In a recent paper, Bahcall et al. (2000) list various new approaches to the
problem of screening of nuclear reactions in stellar plasma and assert that
they are all wrong or irrelevant. Except for two, all approaches mentioned by
Bahcall et al. assume the mean field approximation. The two exceptions are
Carraro et al. (1988) and Shaviv & Shaviv (2000a). While Carraro et al. (1988)
paper is discussed shortly and refuted by Bahcall et al. (2000) the Shaviv &
Shaviv (2000a) paper is not discussed and refuted only by association. However,
the association is totally unfounded because Shaviv & Shaviv (2000a) have shown
that kinetic equations must be used to solve the screening problem and that the
mean field approximation is inadequate for this problem. They also showed that
the Carraro et al. (1988) approach is erroneous.
  Therefore we summarize here the method of $S^{2}$ and their main result.
  We contrast the kinetic equations method with the mean field approximation
and expose the different assumptions and omissions in each method.


title:
Galaxy number counts - V. Ultra-deep counts: The Herschel and Hubble
  Deep Fields

abstract:
  We present u,b,r & i galaxy number counts and colours both from the North and
South Hubble Space Telescope Deep Fields and from the William Herschel Deep
Field. The latter comprises a 7'x7' area of sky reaching b~28.5 at its deepest.
We show that simple Bruzual & Charlot evolutionary models which assume
exponentially increasing star-formation rates with look-back time and q_0=0.05
continue to give excellent fits to galaxy counts and colours in the deep
imaging data. With q_0=0.5, an extra population of `disappearing dwarf'
galaxies is required to fit the optical counts. When we compare the observed
and predicted galaxy counts for UV dropouts in the range 2<z<3.5 we find
excellent agreement, indicating that the space density of galaxies may not have
changed much since z=0 and identifying the Lyman break galaxies with the bright
end of the evolved spiral luminosity function. Making the same comparison for B
dropout galaxies in the range 3.5<z<4.5 we find that the space density of
intrinsically bright galaxies remains the same but the space density of faint
galaxies drops by a factor of ~5 compared to the models, consistent with the
idea that L* galaxies were already in place at z~4 but that dwarf galaxies may
have formed later at 3<z<4.


title:
Unlocking the Keyhole - H2 and PAH emission from molecular clumps in the
  Keyhole Nebula

abstract:
  To better understand the environment surrounding CO emission clumps in the
Keyhole Nebula, we have made images of the region in H2 1-0 S(1) (2.122 um)
emission and polycyclic aromatic hydrocarbon (PAH) emission at 3.29 um. Our
results show that the H2 and PAH emission regions are morphologically similar,
existing as several clumps, all of which correspond to CO emission clumps and
dark optical features. The emission confirms the existence of photodissociation
regions (PDRs) on the surface of the clumps. By comparing the velocity range of
the CO emission with the optical appearance of the H2 and PAH emission, we
present a model of the Keyhole Nebula in which the most negative velocity
clumps are in front of the ionization region, the clumps at intermediate
velocities are in it, and those which have the least negative velocities are at
the far side. It may be that these clumps, which appear to have been swept up
from molecular gas by the stellar winds from eta Car, are now being over-run by
the ionization region and forming PDRs on their surfaces. These clumps comprise
the last remnants of the ambient molecular cloud around eta Car.


title: A Hard Tail in the Broad Band Spectrum of the Dipper XB 1254-690

abstract:
  We report on the results of spectral analysis of the dipping source XB
1254-690 observed by the BeppoSAX satellite. We find that the X-ray dips are
not present during the BeppoSAX observation, in line with recent RXTE results.
The broad band (0.1-100 keV) energy spectrum is well fitted by a
three-component model consisting of a multicolor disk blackbody with an inner
disk temperature of ~0.85 keV, a comptonized spectrum with an electron
temperature of ~2 keV, and a bremsstrahlung at a temperature of ~20 keV.
Adopting a distance of 10 kpc and taking into account a spectral hardening
factor of ~1.7 (because of electron scattering which modifies the blackbody
spectrum emitted by the disk) we estimated that the inner disk radius is
$R_{\rm in} \sqrt{\cos i} \sim 11$ km, where i is the inclination angle of the
system with respect to the line of sight. The comptonized component could
originate in a spherical corona or boundary layer, surrounding the neutron
star, with optical depth ~19. The bremsstrahlung emission, contributing ~4% of
the total luminosity, probably originates in an extended accretion disk corona
with radius $\sim 10^{10}$ cm. In this scenario we calculated that the optical
depth of this region is ~0.71 and its mean electron density is $N_e \sim 2.7
\times 10^{14}$ cm$^{-3}$. This last component might also be present in other
low mass X-ray binaries, but, because of its low intrinsic luminosity, it is
not easily observable. We also find an absorption edge at ~1.27 keV with an
optical depth of ~0.15. Its energy could correspond to the L-edge of Fe XVII,
or K-edge of Ne X or neutral Mg.


title:
Temperature of the central stars of planetary nebulae and the effect of
  the nebular optical depth

abstract:
  The effect of the nebula optical depth on the determination of the
temperature (T*) of the central stars in planetary nebulae is discussed. Based
on photoionization models for planetary nebulae with different optical depths,
we show, quantitatively, that the details of the distribution of the H and He
II Zanstra temperatures are mainly explained by an optical depth effect; in
particular, that the discrepancy is larger for low stellar temperatures. The
results also show that for high stellar temperatures the He II Zanstra
temperature underestimates the stellar temperature, even for high optical
depths. The stellar temperature, as well as the optical depth, can be obtained
from a Zanstra temperature ratio (ZR) plot ZR = Tz(He II)/Tz(H) versus Tz(He
II). The effects of departures from a blackbody spectrum, as well as of the He
abundance in the nebulae, are also discussed. For nebulae of very low optical
depth and/or high stellar temperature the distribution ZR versus Tz(He II) only
provides lower limits for T*. In order to obtain better values for the optical
depth and T*, we propose the use of the line intensity ratio He II/He I versus
Tz(He II) diagram.


title: Hard X-ray emission from IC443: evidence for a shocked molecular clump?

abstract:
  We report BeppoSAX observations of spatially resolved hard X-ray emission
from IC 443, a supernova remnant interacting with a molecular cloud. The
emission is shown to come from two localized features spatially correlated with
bright molecular emission regions. Both hard X-ray features have soft X-ray
counterparts, in one case shifted by ~ 2 arcmin toward the remnant interior.
The spectra of X-ray emission from both isolated features have photon index ~2
in the MECS regime. The emission detected from the remnant with PDS detector
extends up to 100 keV. We discuss the observed properties of the hard X-ray
features in relation to non-thermal emission from shocked molecular clumps and
pulsar wind nebula.


title:
Early Metal Enrichment of the Intergalactic Medium by Pregalactic
  Outflows

abstract:
  We assess supernova (SN)-driven pregalactic outflows as a mechanism for
distributing the product of stellar nucleosynthesis over large cosmological
volumes prior to the reionization epoch. SN ejecta will escape the grasp of
halos with virial temperatures T_vir>10^{4.3} K (corresponding to masses M>10^8
h^{-1} M_sun at redshift z=9 when they collapse from 2-sigma fluctuations) if
rapid cooling can take place, and a significant fraction of their baryonic mass
is converted into stars over a dynamical timescale. We study the evolution of
SN-driven bubbles as they blow out from subgalactic halos and propagate into
the intergalactic medium (IGM), and show that to lift the halo gas out of the
potential well the energy injection must continue at least until blow-away
occurs. The collective explosive output of about ten thousands SNe per halo at
these early epochs could pollute vast regions of intergalactic space to a mean
metallicity <Z>=Omega_Z/Omega_b>0.003 (comparable to the levels observed in the
Lyman-alpha forest at z=3) without much perturbing the IGM hydrodynamically,
i.e. producing large variations of the baryons relative to the dark matter. The
volume filling factor of the ejecta is higher than 20% if the star formation
efficiency is of order 10%. When the filling factor of the ejecta becomes
significant, enriched material will typically be at a higher adiabat than
expected from photoionization.


title: Size-Luminosity scaling and Inverse Compton Seed Photons in Blazars

abstract:
  We present preliminary results of our work on blazar unification. We assume
that all blazars have a broad line region (BLR) and that the size of the BLR
scales with the power of the source in a manner similar to that derived through
reverberation mapping in radio quiet active galactic nuclei (AGNs). Using a
self-consistent emission model that includes particle acceleration we show that
according to this scaling, in weak sources like MKN 421, the inverse Compton
(IC) scattering losses are dominated by synchrotron-self Compton scattering
(SSC), while in powerful sources, like 3C 279, they are dominated by external
Compton (EC) scattering of BLR photons. In agreement with other workers, we
show that even in the powerful sources that are dominated by EC scattering, the
hard X-ray emission is due to SSC. Finally, we show that this scaling
reproduces well the observed sequence of blazar properties with luminosity.


title: First Detection of CO in a Low Surface Brightness Galaxy

abstract:
  We report on the first attempts at searching for CO in red low surface
brightness galaxies, and the first detection of molecular gas in a low surface
brightness (mu_B(0)_{obs} > 23 mag arcsec^{-2}) galaxy. Using the IRAM 30m
telescope, CO(1-0) and CO(2-1) lines were searched for in four galaxies --
P06-1, P05-5, C05-3, & C04-2. In three of the galaxies no CO was detected, to
T_{MB} ~ 1.8mK (at the 3 sigma level). In the fourth galaxy, P06-1, both lines
were detected. Comparing our findings with previous studies shows P06-1 to have
a molecular-to-atomic mass ratio considerably lower than is predicted using
theoretical models based on high surface brightness galaxy studies. This
indicates the N(H_2)/(int{T(CO)dv}) conversion factor for low surface
brightness galaxies may currently be consistently underestimated by a factor of
3 - 20.


title: The recent history of the X-ray absorber in NGC3516

abstract:
  We present two BeppoSAX observations of the bright Seyfert 1 galaxy NGC3516,
performed four months apart (late 1996/early 1997). The earlier spectrum is
considerably weaker and harder in the whole 0.1-50 keV energy range. In
addition, the warm absorber oxygen features, which were already observed with
ROSAT (Mathur et al. 1997) and ASCA (Kriss et al. 1996), are much less
pronounced. The most straightforward explanation is that in 1996 November
NGC3516 was being seen through a substantial (N_H ~ 10^22 atoms/cm/cm) column
of cold material. This is the first confirmation with modern instrumentation
that NGC3516 indeed undergoes phases of strong cold X-ray absorption. We
speculate that these intervals may be connected to the onset of the remarkably
variable UV absorption system, making NGC3516 the best known example of
low-luminosity Broad Absorption Line (BAL) quasar. The absorbing matter could
be provided by clouds ablated from the rim of the circumnuclear molecular
torus, seen at a rather high inclination angle.


title:
BeppoSAX observations of three distant, highly luminous clusters of
  galaxies: RXJ1347-1145, Z3146 and A2390

abstract:
  We present an analysis of BeppoSAX observations of three clusters of galaxies
which are amongst the most luminous in the Universe: RXJ1347-1145, Zwicky 3146
and Abell 2390. Using data from both the Low Energy (LECS) and Medium Energy
(MECS) Concentrator Spectrometers, and a joint analysis with the Phoswich
Detection System (PDS) data above 10 keV, we constrain, with a relative
uncertainty of between 7 and 42 per cent (90 per cent confidence level), the
mean gas temperature in the three clusters. These measurements are checked
against any possible non-thermal contribution to the plasma emission and are
shown to be robust. We confirm that RXJ1347-1145 has a gas temperature that
lies in the range between 13.2 and 22.3 keV at the 90 per cent confidence
level, and is larger than 12.1 keV at 3 sigma level. The existence of such a
hot galaxy cluster at redshift of about 0.45 implies an upper limit on the mean
mass density in the Universe, Omega_m, of 0.5. Combining the BeppoSAX estimates
for gas temperature and luminosity of the three clusters presented in this work
with ASCA measurements available in the literature, we obtain a slope of 2.7 in
the L-T relation once the physical properties are corrected from the
contamination from the central cooling flows.


title: The Friedmann-Lemaitre models in perspective

abstract:
  I show that all FRW models (four dimensional pseudo-Riemannian manifolds with
maximally symmetric space) can be embedded in a flat Minkowski manifold with 5
dimensions. The pseudo Riemannian metric of space-time is induced by the flat
metric. This generalizes the usual embedding widely used for the \dS ~models. I
give the coordinate transformations for the embedding. Taking into account the
spatial isotropy, one can reduce space-time to a two-dimensional surface,
embedded in a three-dimensional Minkowski space. This allows to give exact
graphic representations of the FRW models, and in particular of their
curvature.


title:
The critical velocity effect as a cause for the H\alpha emission from
  the Magellanic stream

abstract:
  Observations show significant H\alpha-emissions in the Galactic halo near the
edges of cold gas clouds of the Magellanic Stream. The source for the
ionization of the cold gas is still a widely open question. In our paper we
discuss the critical velocity effect as a possible explanation for the observed
H\alpha-emission. The critical velocity effect can yield a fast ionization of
cold gas if this neutral gas passes through a magnetized plasma under suitable
conditions. We show that for parameters that are typical for the Magellanic
Stream the critical velocity effect has to be considered as a possible
ionization source of high relevance.


title:
Shear-Flow Driven Current Filamentation: Two-Dimensional
  Magnetohydrodynamic Simulations

abstract:
  The process of current filamentation in permanently externally driven,
initially globally ideal plasmas is investigated by means of two-dimensional
Magnetohydrodynamic (MHD)-simulations. This situation is typical for
astrophysical systems like jets, the interstellar and intergalactic medium
where the dynamics is dominated by external forces. Two different cases are
studied. In one case, the system is ideal permanently and dissipative processes
are excluded. In the second case, a system with a current density dependent
resistivity is considered. This resistivity is switched on self-consistently in
current filaments and allows for local dissipation due to magnetic
reconnection. Thus one finds tearing of current filaments and, besides, merging
of filaments due to coalescence instabilities. Energy input and dissipation
finally balance each other and the system reaches a state of constant magnetic
energy in time.


title:
Normal-Branch Quasi-Periodic Oscillations during the High Intensity
  State of Cygnus X-2

abstract:
  Using data obtained with the Rossi X-ray Timing Explorer, we report the
detection of a 5 Hz quasi-periodic oscillation (QPO) in the bright low-mass
X-ray binary and Z source Cygnus X-2 during high overall intensities (the high
intensity state). This QPO was detected on the so-called normal branch and can
be identified with the normal branch QPO or NBO. Our detection of the NBO is
the first one during times when Cygnus X-2 was in the high intensity state. The
rms amplitude of this QPO decreased from 2.8% between 2-3.1 keV to <1.9%
between 5.0-6.5 keV. Above 6.5 keV, its amplitude rapidly increased to ~12% rms
above 16 keV. The time lags of the QPO were consistent with being zero below 5
keV (compared to the 2-3.1 keV band), but they rapidly increased to ~70 ms
around 10 keV, above which the time lags remained approximately constant near
70 ms. The photon energy dependencies of the rms amplitude and the time lags
are very similar to those observed for the NBO with other satellites (Ginga,
EXOSAT) at different (i.e. lower) intensity states.


title:
Chandra X-ray Observatory Detection of Extended X-ray Emission from the
  Planetary Nebula BD+303639

abstract:
  We report the detection of well resolved, extended X-ray emission from the
young planetary nebula BD+303639 using the Advanced CCD Imaging Spectrometer
(ACIS) aboard the Chandra X-ray Observatory. The X-ray emission from BD+303639
appears to lie within, but is concentrated to one side of, the interior of the
shell of ionized gas seen in high-resolution optical and IR images. The
relatively low X-ray temperature (Tx ~ 3x10^6 K) and asymmetric morphology of
the X-ray emission suggests that conduction fronts are present and/or mixing of
shock-heated and photoionized gas has occurred and, furthermore, hints at the
presence of magnetic fields. The ACIS spectrum suggests that the X-ray emitting
region is enriched in the products of helium burning. Our detection of extended
X-ray emission from BD+303639 demonstrates the power and utility of Chandra
imaging as applied to the study of planetary nebulae.


title: WFPC2 Observations of Massive and Compact Young Star Clusters in M31

abstract:
  We present color magnitude diagrams of four blue massive and compact star
clusters in M31: G38, G44, G94, and G293. The diagrams of the four clusters
reveal a well-populated upper main sequence and various numbers of supergiants.
The U-B and B-V colors of the upper main sequence stars are used to determine
reddening estimates of the different lines of sight in the M31 disk. Reddening
values range from E(B-V) = 0.20 +/- 0.10 to 0.31 +/- 0.11. We statistically
remove field stars on the basis of completeness, magnitude and color. Isochrone
fits to the field-subtracted, reddening-corrected diagrams yield age estimates
ranging from 63 +/- 15 Myr to 160 +/- 60 Myr. Implications for the recent
evolution of the disk near NGC 206 are discussed.


title:
Formation and evolution of a 0.242 Msun helium white dwarf in presence
  of element diffusion

abstract:
  A 0.242 Msun object that finally becomes a helium white dwarf is evolved from
Roche lobe detachment down to very low luminosities. In doing so, we employ our
stellar code to which we have added a set of routines that compute the effects
due to gravitational settling, and chemical and thermal diffusion. Initial
model is constructed by abstracting mass to a 1 Msun red giant branch model up
to the moment at which the model begins to evolve bluewards. We find that
element diffusion introduces noticeable changes in the internal structure of
the star. In particular, models undergo three thermonuclear flashes instead of
one flash as we found with the standard treatment. This fact has a large impact
on the total mass fraction of hydrogen left in the star at entering the final
cooling track. As a result, at late stages of evolution models with diffusion
are characterized by a much smaller nuclear energy release, and they evolve
significantly faster compared to those found with the standard treatment.
  We find that models in which diffusion is considered predict evolutionary
ages for the white dwarf companion to the millisecond pulsar PSR B1855+09 in
good agreement with the spin-down age of the pulsar.


title: Cosmological Topology in Paris 1998

abstract:
  Quel est, ou pourrait \^etre, la topologie globale de la partie spatiale de
l'Univers ? L'Univers entier (pr\'ecis\'ement, l'hypersurface spatiale de
celui-ci) est-il observable ? Les math\'ematiciens, les physiciens et les
cosmologistes observationnels ont des approches diff\'erentes pour aborder ces
questions qui restent ouvertes.
  What is, or could be, the global topology of spatial sections of the
Universe? Is the entire Universe (spatial hypersurface thereof) observable ?
Mathematicians, physicists and observational cosmologists have different
strategies to approaching these questions which are not yet fully answered.


title: Big-Bang Nucleosynthesis Predictions for Precision Cosmology

abstract:
  The determination of the primeval deuterium abundance has opened a precision
era in big-bang nucleosynthesis (BBN), making accurate predictions more
important than ever before. We present in analytic form new, more precise
predictions for the light-element abundances and their error matrix. Using our
predictions and the primeval deuterium abundance we infer a baryon density of
Omega_B h^2 = 0.020 pm 0.002 (95% cl) and find no evidence for stellar
production (or destruction) of 3He beyond burning D to 3He. Conclusions about
4He and 7Li currently hinge upon possible systematic error in their
measurements.


title:
Evolution of Differentially Rotating Supermassive Stars to the Onset of
  Bar Instability

abstract:
  Thermal emission from a rotating, supermassive star will cause the
configuration to contract slowly and spin up. If internal viscosity and
magnetic fields are sufficiently weak, the contracting star will rotate
differentially. For each of the six initial angular momentum distributions
considered, a cooling supermassive star will likely encounter the dynamical bar
mode instability, which may trigger the growth of nonaxisymmetric bars. This
scenario will lead to the generation of long wavelength gravitational waves,
which could be detectable by future space-based laser interferometers like
LISA.


title: Two Highly Polarized QSOs Discovered by 2MASS

abstract:
  We present optical observations of two remarkable new AGN discovered by the
Two-Micron All Sky Survey. Both are classified as QSOs based on their optical
spectra, near-IR colors, and near-IR and [O III]5007 luminosities, but their
optical polarizations are among the highest seen for non-blazar AGN;
approaching 15% for 2MASSI J151653.2+190048. The polarized light spectrum for
each object exhibits broad Balmer emission lines, but lacks the narrow lines
that are evident in the total light spectrum. This is most pronounced for the
Type-1.5 object 2MASSI J165939.7+183436, where broad lines dominate only in
polarized light. The polarization properties of these AGN suggest that dust
near the nucleus at least partially obscures the AGN and that material probably
intermixed with the narrow line-emitting gas scatters nuclear light into our
line of sight. These QSOs illustrate the variety of highly polarized AGN that
have been missed by traditional optical search techniques, and demonstrate that
such objects are exposed by surveys in the near-IR.


title:
Infrared Spectra and Spectral Energy Distributions of Late-M- and
  L-Dwarfs

abstract:
  We have obtained 1.0-2.5um spectra at R~600 of 14 disk dwarfs with spectral
types M6 to L7. For four of the dwarfs we have also obtained infrared spectra
at R~3000 in narrow intervals. In addition, we present new L' photometry for
four of the dwarfs in the sample, which allows improved determinations of their
bolometric luminosities. We resolve the L-dwarf Denis-P J 0205-1159 into an
identical pair of objects separated by 0.35". The spectra, with the published
energy distribution for one other dwarf, are compared to synthetic spectra
generated by upgraded model atmospheres. Good matches are found for 2200> Teff
K>1900 (spectral types around M9 to L3), but discrepancies exist at Teff> 2300
K (M8) and for Teff<1800 K (L4-L7). At the higher temperatures the mismatches
are due to incompleteness in the water vapor linelist. At the lower
temperatures the disagreement is probably due to our treatment of dust: we
assume a photospheric distribution in equilibrium with the gas phase. We derive
effective temperatures for the sample from the comparison with synthetic
spectra and also by comparing our observed total intrinsic luminosities to
structural model calculations (which are mostly independent of the atmosphere
but are dependent on the unknown masses and ages of the targets). The two
derivations agree to ~200 K except for the faintest object in the sample where
the discrepancy is larger. Agreement with other temperature determinations is
also ~200 K, except for the L7 dwarf.


title: NICMOS Coronagraphic Observations of 55 Cancri

abstract:
  We present new near-infrared (1.1-micron) observations of the circumstellar
environment of the planet-bearing star 55 Cancri. With these Hubble Space
Telescope images we are unable to confirm the observation of bright scattered
radiation at longer NIR wavelengths previously reported by Trilling & Brown
(1998), and Trilling, Brown & Rivkin (2000). NICMOS coronagraphic images with
detection sensitivities to ~ 100 microJy arcsec$^{-2}$ at 1.1 microns in the
region 28 - 60 AU from the star fail to reveal any significant excess flux in
point- spread-function (PSF) subtracted images taken in two Hubble Space
Telescope orbits. These new observations place flux densities in the 19-28 AU
zone at a factor of ten or more below the reported ground-based observations.
Applying a suite of a dozen well-matched coronagraphic reference PSFs,
including one obtained in the same orbits as the observations of 55 Cnc,
yielded consistently null results in detecting a disk. We also searched for,
and failed to find, a suggested flux-excess anisotropy in the ratio of ~1.7:1
in the circumstellar background along and orthogonal to the plane of the
putative disk. We suggest that, if such a disk does exist, then the total
1.1-micron spectral flux density in an annular zone 28 - 42 AU from the star
must be no more than ~0.4mJy, at least ten times smaller than suggested by
Trilling and Brown, upon which their very large estimate for the total dust
mass (0.4~$M_{\earth}$) was based. Based on the far infrared and submillimeter
flux of this system and observations of scattered light and thermal emission
from other debris disks, we also expect the intensity of the scattered light to
be at least an order of magnitude below our upper limits.


title:
The Observed Offset Distribution of Gamma-Ray Bursts from Their Host
  Galaxies: A Robust Clue to the Nature of the Progenitors

abstract:
  We present a comprehensive study to measure the locations of gamma-ray bursts
(GRBs) relative to their host galaxies. In total, we find the offsets of 20
long-duration GRBs from their apparent host galaxy centers utilizing
ground-based images from Palomar and Keck and space-based images from the
Hubble Space Telescope (HST). The median projected angular (physical) offset is
0.17 arcsec (1.3 kpc). The median offset normalized by the individual host
half-light radii is 0.98 suggesting a strong connection of GRB locations with
the UV light of their hosts. This provides strong observational evidence for
the connection of GRBs to star formation. We compare the observed offset
distribution with an exponential disk, a model for the location of collapsars.
The statistical comparison shows good agreement, with the Kolmogorov-Smirnov
probability that the observed offsets derive from the model distribution of
P_KS = 0.45. We also compare the observed GRB offsets with the expected offset
distribution of delayed merging remnant progenitors (black hole--neutron star
and neutron star--neutron star binaries). We find that delayed merging remnant
progenitors, insofar as the predicted offset distributions from population
synthesis studies are representative, can be ruled out at the 2 x 10^{-3}
level. This is arguably the strongest observational constraint yet against
delayed merging remnants as the progenitors of long-duration GRBs. In the
course of this study, we have also discovered the putative host galaxies of GRB
990510 and GRB 990308 in archival HST data. (abstract abridged)


title: Identification of the companion stars of Type Ia supernovae

abstract:
  The nature of the binary systems giving rise to Type Ia supernovae (SNeIa)
remains an unsolved problem. In this {\it Letter} we calculate, from the
statistics of initial conditions (masses and binary separations), the mass,
luminosity, and velocity distributions of the possible binary companions
(main-sequence star, subgiant, red giant) following the explosion of the white
dwarf which gives rise to the SNeIa. Those companions could be detected from
either their proper or their radial motions, by means of high-precision
astrometric and radial-velocity measurements in young, nearby supernova
remnants. Peculiar velocities typically ranging from 100 to 450 km s$^{-1}$
should be expected, which places proper-motion measurements within reach of HST
instruments and makes radial-velocity ones feasible with 2.5-4m class
telescopes from the ground. Detections would solve the long-standing problem of
which kind of binaries do produce SNeIa and clear up the way to accurate
physical modeling of the explosions.


title:
Dynamical Instabilities in Extrasolar Planetary Systems Containing Two
  Giant Planets

abstract:
  Instabilities and strong dynamical interactions between several giant planets
have been proposed as a possible explanation for the surprising orbital
properties of extrasolar planetary systems. In particular, dynamical
instabilities would seem to provide a natural mechanism for producing the
highly eccentric orbits seen in many systems. Here we present results from
numerical integrations for the dynamical evolution of planetary systems
containing two identical giant planets in nearly circular orbits very close to
the dynamical stability limit. We determine the statistical properties of the
three main types of systems resulting from the development of an instability:
systems containing one planet, following either a collision between the two
initial planets, or the ejection of one of them to infinity, and systems
containing two planets in a new, quasi-stable configuration. We discuss the
implications of our results for the formation and evolution of observed
extrasolar planetary systems. We conclude that the distributions of
eccentricities and semimajor axes for observed systems cannot be explained
easily by invoking dynamical interactions between two planets initially on
circular orbits. While highly eccentric orbits can be produced naturally by
these interactions, collisions between the two planets, which occur frequently
in the range of observed semimajor axes, would result in many more nearly
circular orbits than in the observed sample.


title:
Prospects of observing pulsed radiation from gamma-ray pulsars with
  H.E.S.S

abstract:
  Observations and theoretical studies have demonstrated that the pulsed
spectra of all gamma-ray pulsars terminate at energies below a few hundred GeV.
In most cases we expect this cutoff energy E_o to be around 10 GeV. Only with
next generation ground-based gamma-ray telescopes, which are expected to have
non-zero trigger probabilities near 10 GeV, can we expect to detect pulsations.
The large gamma-ray fluxes below E_o, together with the associated sharp pulse
profiles, compensate for the lack of imaging capability near threshold. For
H.E.S.S. we find that the pulsed component of PSR B1706-44 should be detectable
near threshold, whereas the unidentified GeV EGRET sources should be detectable
if the superexponential cutoff energy is larger that approximately 30 GeV for
relatively hard pulsar photon spectra (~E^-1.5).


title: Calibrations of alpha Cen A & B

abstract:
  Detailed evolutionary models of the visual binary alpha Centauri, including
pre main-sequence evolution, have been performed using the masses recently
determined by Pourbaix et al. (1999). Models have been constructed using the
CEFF equation of state, OPAL opacities, NACRE thermonuclear reaction rates and
microscopic diffusion. A chi^2-minimization is performed to derive the most
reliable set of modeling parameters wp={t_alpha Cen, Yi, [Fe/H]i, alpha A,
alpha B}, where t alpha Cen is the age of the system, Yi the initial helium
content, [Fe/H]i the initial metallicity and, alpha A and alpha B the
convection parameters of the two components. Using the basic Bohm-Vitense
(1958) mixing-length theory of convection, we derive wp BV={2710 Myr,
0.284,0.257, 1.53, 1.57}. We obtain a noticeably smaller age than estimated
previously, in agreement with Pourbaix et al. (1999), mainly because of the
larger masses. If convective core overshoot is considered we get wp ov={3530
Myr, 0.279,0.264,1.64,1.66}. The use of Canuto & Mazitelli (1991, 1992)
convection theory leads to the set wp CM={4086 Myr, 0.271, 0.264, 0.964,
0.986}. Using the observational constraints adopted by Guenther & Demarque
(2000), and the basic mixing-length theory, we obtain wp GD={5640 Myr, 0.300,
0.296, 1.86, 1.97} and surface lithium depletions close to their observed
values.
  A seismological analysis of our calibrated models has been performed. The
determination of large and small spacings between the frequencies of acoustic
oscillations from seismic observations would help to discriminate between the
models of alpha Cen computed with different masses and to confirm or rules out
the new determination of masses.


title: Resolving Sirius-like binaries with the Hubble Space Telescope

abstract:
  We have imaged seventeen recently discovered Sirius-like binary systems with
HST/WFPC2 and resolved the white dwarf secondary in eight cases. Most of the
implied orbital periods are of order several hundred years, but in three cases
(56 Per, Zeta Cygni and REJ1925-566) the periods are short enough that it may
be possible to detect orbital motion within a few years. It will then be
possible to derive dynamically determined masses for the white dwarfs, and
potentially these stars could be used as stringent tests of the mass-radius
relation and initial-final mass relation.


title: Warm H2 in the Galactic center region

abstract:
  We present ISO observations of several H2 pure-rotational lines (from S(0) to
S(5)) towards a sample of 16 molecular clouds distributed along the central ~
500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1
observations of these sources made with the IRAM-30m telescope. With the CO
data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a
few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual
extinction using a self-consistent method. In every source, we find that the H2
emission exhibits a large temperature gradient. The S(0) and S(1) lines trace
temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures
of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is
predominantly gas with T=150 K. This is the first direct estimate of the total
column density of the warm molecular gas in the Galactic center region. These
warm H2 column densities represent a fraction of ~ 30 % of the gas traced by
the CO isotopes emission. The cooling by H2 in the warm component is comparable
to that by CO. Comparing our H2 and CO data with available ammonia NH3
observations from literature one obtains relatively high NH3 abundances of a
few 10e(-7) in both the warm and the cold gas. A single shock or
Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines.
Alternatives for the heating mechanisms are discussed.


title:
On the paper by M. Valluri and D. Merritt "Orbital Instability and
  Relaxation in Stellar Systems"

abstract:
  Some points related to the exponential instability of N-body gravitating
systems and which are misinterpreted in various papers are clarified.
Exponential instability is a phenomenon essentially responsible for the
relaxation, evolutionary, morphological and other properties of certain type of
galaxies and star clusters.


title: Theoretical expectations for clump red giants as distance indicators

abstract:
  Variations of ~0.4 mag are expected in the I-band absolute magnitude of red
clump giants, M_I^RC, as a function of both stellar age and metallicity. This
regardless of some potential theoretical uncertainties. Due to the quite large
differences in mean ages and metallicities of clump stars among galaxies,
systematic changes (also amounting up to ~0.4 mag) come out in their M_I^RC.
These numbers also indicate a distance to the LMC that is not necessarily
``short''.


title: The Topology of the Universe

abstract:
  The Hilbert-Einstein equations are insufficient to describe the geometry of
the Universe, as they only constrain a local geometrical property: curvature. A
global knowledge of the geometry of space, if possible, would require
measurement of the topology of the Universe. Since the subject was discussed in
1900 by Schwarzschild, observational attempts to measure global topology have
been rare for most of this century, but have accelerated in the 1990's due to
the rapidly increasing amount of observations of non-negligible fractions of
the observational sphere. A brief review of basic concepts of cosmic topology
and of the rapidly growing gamut of diverse and complementary observational
strategies for measuring the topology of the Universe is provided here.


title: The temperatures of dust-enshrouded AGNs

abstract:
  A high density of massive dark objects (MDOs), probably supermassive black
holes, in the centres of nearby galaxies has been inferred from recent
observations. There are various indications that much of the accretion
responsible for producing these objects took place in dust-enshrouded active
galactic nuclei (AGNs). If so, then measurements of the intensity of background
radiation and the source counts in the far-infrared and submillimetre wavebands
constrain the temperature of dust in these AGNs. An additional constraint comes
from the hard X-ray background, if this is produced by accretion. One
possibility is that the dust shrouds surrounding the accreting AGNs are cold,
about 30 K. In this event, the dusty AGNs could be some subset of the
population of luminous distant sources discovered at 850 microns using the
SCUBA array on the JCMT, as proposed by Almaini et al. (1999). An alternative
is that the dust shrouds surrounding the accreting AGNs are much hotter (> 60
K). These values are closer to the dust temperatures of a number of
well-studied low-redshift ultraluminous galaxies that are thought to derive
their power from accretion. If the local MDO density is close to the maximum
permitted, then cold sources cannot produce this density without the
submillimetre background being overproduced if they accrete at high radiative
efficiency, and thus a hot population is required. If the dust-enshrouded
accretion occurred at similar redshifts to that taking place in unobscured
optical quasars, then a significant fraction of the far-infrared background
radiation measured by COBE at 140 microns, but very little of the
submilllimetre background at 850 microns, may have been produced by hot
dust-enshrouded AGNs which may have already been seen in recent X-ray surveys.


title: Lensing in the Hercules Supercluster

abstract:
  We report Keck LRIS observations of an arc-like background galaxy near the
center of Abell 2152 (z=0.043), one of the three clusters comprising the
Hercules supercluster. The background object has a redshift z=0.1423 and is
situated 25 arcsec north of the primary component of the A2152 brightest
cluster galaxy (BCG). The object is about 15 arcsec in total length and has a
reddening-corrected R-band magnitude of $m_R = 18.55\pm0.03$. Its spectrum
shows numerous strong emission lines, as well as absorption features. The
strength of the H-alpha emission would imply a star formation rate $\SFR
\approx 3h^{-2} \msun $yr$^{-1}$ in the absence of any lensing. However, the
curved shaped of this object and its tangential orientation along the major
axis of the BCG suggest lensing. We model the A2152 core mass distribution
including the two BCG components and the cluster potential. We present velocity
and velocity dispersion profile measurements for the two BCG components and use
these to help constrain the potential. The lens modeling indicates a likely
magnification factor of $\sim1.9$ for the lensed galaxy, making A2152 the
nearest cluster in which such significant lensing of a background source has
been observed. Finally, we see evidence for a concentration of early-type
galaxies at $z=0.13$ near the centroid of the X-ray emission previously
attributed to A2152. We suggest that emission from this background
concentration is the cause of the offset of the X-ray center from the A2152
BCG. The background concentration and the dispersed mass of the Hercules
supercluster could add further to the lensing strength of the A2152 cluster.


title: The Unified model and the Seyfert 2 infrared dichotomy

abstract:
  An optical spectropolarimetric study has shown that the detectability of
polarised broad H-alpha in Seyfert 2 galaxies is correlated to the IRAS f60/f25
flux ratio where only those Seyfert 2s with "warm" IRAS colours show polarised
broad line emission. It was suggested that those Seyfert 2s with "cool" IRAS
colours have highly inclined tori which obscure the broad line scattering
screen.
  I present here hard X-ray observations inconsistent with this picture showing
that the derived column densities of warm and cool Seyfert 2 galaxies are
statistically the same. I classify the Bright Galaxy Sample to produce a
non-Seyfert comparison. The analysis of the properties of these galaxies with
the Seyfert 2s suggest that the IRAS f60/f25 flux ratio is consistent with
implying the relative strength of galactic to Seyfert emission. I show that
this new picture can account for the absence of polarised broad H-alpha in the
cool Seyfert 2s.


title: Observational Approaches to the Topology of the Universe

abstract:
  Many different and complementary strategies for translating the basic
principle of multiple topological imaging into observational analysis are now
available, both for three-dimensional and two-dimensional catalogues.


title: Details of the mass--temperature relation for clusters of galaxies

abstract:
  We present results on the total mass and temperature determination using two
samples of clusters of galaxies. One sample is constructed with emphasis on the
completeness of the sample, while the advantage of the other is the use of the
temperature profiles, derived with ASCA. We obtain remarkably similar fits to
the M-T relation for both samples, with the normalization and the slope
significantly different from both prediction of self-similar collapse and
hydrodynamical simulations. We discuss the origin of these discrepancies and
also combine the X-ray mass with velocity dispersion measurements to provide a
comparison with high-resolution dark matter simulations. Finally, we discuss
the importance of a cluster formation epoch in the observed M-T relation.


title: The Warm Ionized Medium in the Milky Way and Other Galaxies

abstract:
  Observations of the "Warm Ionized Medium" (or, equivalently, the "Diffuse
Ionized Gas") of the local ISM, the Perseus arm in the Milky Way, and also in
several other galaxies show strong [NII]6563 (~H-alpha in some cases) and
[SII]6717/[NII]6583 = 0.6 - 0.7 in all locations and objects. Other line ratios
(e.g., [O III]5007/H-beta) vary considerably. Simple photoionization models
reproduce the observed spectra, providing extra heating beyond that supplied by
photoionization is assumed (Reynolds, Haffner, & Tufte 1999). With observed
gas-phase abundances (not solar), the line ratios in the local arm at b = 0 deg
are fitted with no extra heating and (S/H) = 13 ppm (solar is 20 ppm). Local
gas observed at b = -35 deg requires extra heating of about gamma = 0.75, where
gamma is the extra heating in units of 10^{-25} erg H^{-1} s^{-1}. In the
Perseus arm, there are similar results, with a domposition consistent with the
Galactic abundance gradient. The requirements for NGC 891 are similar to the
Perseus arm: little or no extra heating at |z| = 1 kpc and gamma 3 at 2 kpc. In
NGC 891 there is also an increase of 5007/H-alpha with |z| that can only come
about if most of the ionizing radiation is supplied by stars with T~50000 K.
Either their radiation must propagate from the plane to high |z| through very
little intervening matter, or else the stars are located at high |z|. The total
power requirement of the extra heating is <15% of the photoionization power.
[O~II]3727/H-beta can serve as a useful diagnostic of extra heating, but
[S~III] 9065,9531/H-alpha is not useful in this regard.


title: On the Origin of Intrinsic Narrow Absorption Lines in z<1 QSOs

abstract:
  We present a statistical analysis of the associated, high ionization narrow
absorption line (NAL) systems in a sample of 59 QSOs defined from the HST QSO
Absorption Line Key Project. We have compiled the QSO luminosities at 2500 A, 5
GHz, and 2 keV, spectral indices at 2500 A and 5 GHz, the H-beta emission line
FWHM, the CIV emission line FWHM, and the radio core fraction. We find 17
associated NALs toward 15 QSOs, of which ~10 are statistically expected to be
intrinsic. From a multivariate clustering analysis, the QSOs seem to group
together based primarily on radio luminosity, followed by radio spectral index,
CIV emission line FWHM, and soft X-ray luminosity. Radio-loud QSOs which have
compact radio morphologies, flat radio spectra, and mediocre CIV FWHM (<6000
km/s) do not have detectable associated NALs, down to REW(CIV)=0.35 A. We find
that BALQSOs have an enhanced probability of hosting detectable NAL gas and
that the velocities of associated NALs are peaked around the emission
redshifts, rather than the systemic redshifts of the QSOs. There is only one
strong NAL [REW(CIV)>1.5 A] in our low redshift sample which implies evolution
in the number of strong NALs with redshift. We interpret these results in the
context of an accretion-disk model. In this framework, we suggest that the
observational clues result from differences in orientation and wind properties,
the latter presumably associated with the QSO radio properties. (abridged)


title: Is there a maximum observable redshift in an open universe?

abstract:
  An estimate of the maximum observable redshift is obtained using only t_{0}
approximately equal to (14+- 3)10^{9}years, H_{0} approximately equal to 65 +-
10Km\sec^{-1}Mpc^{-1}(t_{0}H_{0} approximately equal to 0.91 +- 0.08/0.18)
assuming \Lambda approximately equal to 0. The resulting maximum redshift z_{+}
approximately equal to 10 appears to give a reasonable upper limit to the
highest actually observed redshifts. Some implications are discussed.


title: Circumbinary disks and cataclysmic variable evolution

abstract:
  The influence of a circumbinary (CB) disk on the evolution of cataclysmic
variable (CV) binary systems is investigated. We show that CB mass surface
densities sufficient to influence the evolution rate are plausibly provided by
the outflows observed in CVs, if the net effect of these winds is to deliver
$10^{-4}$--$10^{-3}$ of the mass transfer rate to the CB disk. The torque
exerted by the CB disk provides a positive feedback between mass transfer rate
and CB disk mass which can lead to mass transfer rates of $\sim 10^{-8}
-10^{-7} \mpy$. This mechanism may be responsible for causing the range of
variation of mass transfer rates in CV's. In particular, it may explain rates
inferred for the novalike variables and the supersoft X-ray binary systems
observed near the upper edge of the period gap ($P \sim 3 - 4$ hr), as well as
the spread in mass transfer rates above and below the period gap. Consquences
and the possible observability of such disks are discussed.


title:
Uncovering the Intrinsic Shapes of Elliptical Galaxies. IV. Tests on
  Simulated Merger Remnants

abstract:
  We test the methods developed in previous papers for inferring the intrinsic
shapes of elliptical galaxies, using simulated objects from N-body experiments.
The shapes of individual objects are correctly reproduced to within the
statistical errors; close inspection of the results indicates a small
systematic bias in the sense of underestimating both the triaxiality and
short-to-long axis ratio. We also test the estimation of parent shape
distributions, using samples of independent, randomly oriented objects. The
best results are statistically accurate, but on average the parent
distributions are again slightly biased in the same sense. Using the posterior
probability densities for the individual objects, we estimate the magnitude of
the bias to be less than approximately 0.1 in both shape parameters. The
results support the continued use of these methods on real systems.


title: The Broken Light Curves of Gamma-Ray Bursts GRB 990123 and GRB 990510

abstract:
  We have collected all of the published photometry for GRB 990123 and GRB
990510, the first two gamma-ray bursts where breaks were seen in the light
curves of their optical afterglows, and determined the shapes of their light
curves and the break times. These parameters were used to investigate the
physical mechanisms responsible for the breaks and the nature of the ambient
medium that the bursts occurred in. The light curve for GRB 990123 is best fit
by a broken power law with a break 1.68 +/- 0.19 days after the burst, a slope
of alpha1 = -1.12 +/- 0.08 before the break, and a slope of alpha2 = -1.69 +/-
0.06 after the break. This is consistent with a collimated outflow with a fixed
opening angle (theta0) of approximately five degrees. In this case the break in
the light curve is due to the relativistic fireball slowing to a gamma factor
of approximately 1 / theta0. The light curve for GRB 990510 is best fit by a
continuous function with an early-time slope of alpha1 = -0.54 +/- 0.14, a
late-time slope of alpha2 = -1.98 +/- 0.19, and a slow transition between the
two regimes approximately one day after the burst. This is consistent with a
collimated outflow with an opening angle of approximately five degrees that is
initially radiative, but undergoes a sideways expansion that begins
approximately one day after the burst. This sideways expansion is responsible
for the slow break in the light curve.


title:
Parent Stars of Extrasolar Planets VI: Abundance Analyses of 20 New
  Systems

abstract:
  The results of new spectroscopic analyses of 20 recently reported extrasolar
planet parent stars are presented. The companion of one of these stars, HD
10697, has recently been shown to have a mass in the brown dwarf regime; we
find [Fe/H] $= +0.16$ for it. For the remaining sample, we derive [Fe/H]
estimates ranging from -0.41 to $+0.37$, with an average value of $+0.18 \pm
0.19$. If we add the 13 stars included in the previous papers of this series
and 6 other stars with companions below the 11 M$_{\rm Jup}$ limit from the
recent studies of Santos et al., we derive $<$[Fe/H]$> = +0.17 \pm 0.20$.
  Among the youngest stars with planets with F or G0 spectral types, [Fe/H] is
systematically larger than young field stars of the same Galactocentric
distance by 0.15 to 0.20 dex. This confirms the recent finding of Laughlin that
the most massive stars with planets are systematically more metal rich than
field stars of the same mass. We interpret these trends as supporting a
scenario in which these stars accreted high-Z material after their convective
envelopes shrunk to near their present masses. Correcting these young star
metallicities by 0.15 dex still does not fully account for the difference in
mean metallicity between the field stars and the full parent stars sample.
  The stars with planets appear to have smaller [Na/Fe], [Mg/Fe], and [Al/Fe]
values than field dwarfs of the same [Fe/H]. They do not appear to have
significantly different values of [O/Fe], [Si/Fe], [Ca/Fe], or [Ti/Fe], though.


title:
Galaxy Modelling - II. Multi-Wavelength Faint Counts from a
  Semi-Analytic Model of Galaxy Formation

abstract:
  (Abridged) This paper predicts self-consistent faint galaxy counts from the
UV to the submm wavelength range. The STARDUST spectral energy distributions
described in Devriendt et al. (1999) are embedded within the explicit
cosmological framework of a simple semi-analytic model of galaxy formation and
evolution. We build a class of models which capture the luminosity budget of
the universe through faint galaxy counts and redshift distributions in the
whole wavelength range spanned by our spectra. In contrast with a rather stable
behaviour in the optical and even in the far-IR, the submm counts are
dramatically sensitive to variations in the cosmological parameters and changes
in the star formation history. Faint submm counts are more easily accommodated
within an open universe with a low value of $\Omega_0$, or a flat universe with
a non-zero cosmological constant. This study illustrates the implementation of
multi-wavelength spectra into a semi-analytic model. In spite of its
simplicity, it already provides fair fits of the current data of faint counts,
and a physically motivated way of interpolating and extrapolating these data to
other wavelengths and fainter flux levels.


title: Models of Veritcal Disturbances in the Interstellar Medium

abstract:
  This paper describes some interesting properties of waves in, and
oscillations of, the interstellar medium in the direction normal to the plane
of the Galaxy. Our purpose is to examine possible reasons for four observed
phenomena: the falling sky in the northern hemisphere; the apparent presence of
clouds in absorption spectra when a sightline is occupied primarily only by
warm intercloud gas; the peculiar structuring of spiral arms involving clumps,
spurs, and feathering; and the existence of an abundance of high stage ions far
off the plane of the Galaxy.
  We explored the reaction of the interstellar medium - in the vertical
direction only - to large imposed disturbances (initial displacements,
expansive velocities, and compressions), and to the introduction of small
amplitude waves via oscillation of the midplane. Our findings included: 1) the
anticipated growth in amplitude of high frequency waves with height; 2) the
four lowest normal modes for the oscillation of the atmosphere as a whole, as
functions of the height of the outer boundary; 3) the time for material to
`bounce' from one unusually dense state to the next as a function of height;
and 4) the tendency for the disk to develop a hot outer halo, either after the
passage of a single shock from a large event, or in response to a continuous
stream of small amplitude waves.


title: Morphology and Evolution of LMC Planetary Nebulae

abstract:
  The LMC is ideal for studying the co-evolution of planetary nebulae (PNe) and
their central stars, in that the debilitating uncertainties of the Galactic PN
distance scale and selection biases from attenuation by interstellar dust do
not apply. We present images and analyze slit-less spectra which were obtained
in a survey of Large Magellanic Cloud PNe. These data on 29 targets were
obtained with HST using the Space Telescope Imaging Spectrograph. The data
permit us to determine the nebular dimensions and morphology in the
monochromatic light of several emission lines, including those that have
traditionally been used for morphological studies in the Galaxy: H-alpha, [N
II] 6583 and [O III] 5007, plus others of varying ionization, including [O I],
He I, and [S II]. Together with the 31 resolved LMC PNe for which monochromatic
images exist in the HST archive, these data show that the incidence of
non-symmetric nebulae, including bipolar nebulae (which is an indicator of
Population I ancestry in the Galaxy), is significantly higher than that
reported for the Galaxy. The onset of asymmetric features appears even in very
young nebulae (with dynamical ages of ~1400 yr), suggesting that at least the
gross features of the nebular morphology may be more closely tied to PN
formation, and that subsequent shaping of the expanding envelope by the
radiation field and wind from the central star may play the lesser role of
amplifying these gross features. There is some evidence of evolution between
two morphological types, in the sense that bipolar core (BC) nebulae may evolve
to pure bipolars late in the PN lifetime.


title:
Dynamical Bar Instability in Rotating Stars: Effect of General
  Relativity

abstract:
  We study the dynamical stability against bar-mode deformation of rapidly and
differentially rotating stars in the first post-Newtonian approximation of
general relativity. We vary the compaction of the star $M/R$ (where $M$ is the
gravitational mass and $R$ the equatorial circumferential radius) between 0.01
and 0.05 to isolate the influence of relativistic gravitation on the
instability. For compactions in this moderate range, the critical value of
$\beta \equiv T/W$ for the onset of the dynamical instability (where $T$ is the
rotational kinetic energy and $W$ the gravitational binding energy) slightly
decreases from $\sim 0.26$ to $\sim 0.25$ with increasing compaction for our
choice of the differential rotational law. Combined with our earlier findings
based on simulations in full general relativity for stars with higher
compaction, we conclude that relativistic gravitation {\em enhances} the
dynamical bar-mode instability, i.e. the onset of instability sets in for
smaller values of $\beta$ in relativistic gravity than in Newtonian gravity. We
also find that once a triaxial structure forms after the bar-mode perturbation
saturates in dynamically unstable stars, the triaxial shape is maintained, at
least for several rotational periods. To check the reliability of our numerical
integrations, we verify that the general relativistic Kelvin-Helmholtz
circulation is well-conserved, in addition to rest-mass energy, total
mass-energy, linear and angular momentum. Conservation of circulation indicates
that our code is not seriously affected by numerical viscosity. We determine
the amplitude and frequency of the quasi-periodic gravitational waves emitted
during the bar formation process using the quadrupole formula.


title: A search for L dwarf binary systems

abstract:
  We present analysis of HST Planetary Camera images of twenty L dwarfs
identified in the course of the Two Micron All-Sky Survey. Four of the targets
have faint, red companions at separations between 0.07 and 0.29 arcseconds (1.6
to 7.6 AU). In three cases, the bolometric magnitudes of the components differ
by less than 0.3 magnitudes. Since the cooling rate for brown dwarfs is a
strong function of mass, similarity in luminosities implies comparable masses.
The faint component in the 2M0850 system, however, is over 1.3 magnitudes
fainter than the primary in the I-band, and ~0.8 magnitudes fainter in M(bol).
Indeed, 2M0850B is ~0.8 magnitudes fainter in I than the lowest luminosity L
dwarf currently known, while the absolute magnitude we deduce at J is almost
identical with M_J for Gl 229B. Theoretical models indicate a mass ratio of
\~0.75. The mean separation of the L dwarf binaries in the current sample is
smaller by a factor of two than amongst M dwarfs. We discuss the implications
of these results for the temperature scale in the L/T transition region and for
the binary frequency amongst L dwarfs.


title: Beyond M: Ultracool dwarfs in the Solar Neighbourhood

abstract:
  I review the optical and near-infrared spectral characteristics of the
extremely cool dwarfs grouped under the new classification of type L. These
include both very low-mass stars and brown dwarfs, and we discuss the likely
temperature range. Finally, we consider the frequency of binaries amongst very
low-mass dwarfs, and the implications for formation mechanisms.


title:
Wide Field Imaging of the Hubble Deep Field South Region II: The
  Evolution of Galaxy Clustering at z<1

abstract:
  We present the galaxy-galaxy angular correlations as a function of
photometric redshift in a deep-wide galaxy survey centered on the Hubble Deep
Field South. Images were obtained with the Big Throughput Camera on the Blanco
4m telescope at CTIO, of 1/2 square degree in broad-band uBVRI, reaching ~24th
mag. Approximately 40,000 galaxies are detected in the survey. We determine
photometric redshifts using galaxy template fitting to the photometry. Monte
Carlo simulations show that redshifts from these data should be reliable out to
z~1, where the 4000 Angstrom break shifts into the I-band. The inferred
redshift distribution, n(z), shows good agreement with the distribution of
galaxies measured in the HDF North and the Canada-France Redshift Survey. After
assigning galaxies to redshift bins with width Delta_z=0.33, we determine the
two point angular correlation function in each bin. We find that the amplitude
of the correlation, A_w, drops across the three bins to redshift z~1. Simple
epsilon models of clustering evolution fit this result, with the best agreement
for epsilon=0. Hierarchical cold-dark-matter models best fit in a low density,
Lambda-dominated universe.


title:
Constraints on Galaxy Bias, Matter Density, and Primordial
  Non--Gausianity from the PSCz Galaxy Redshift Survey

abstract:
  We compute the bispectrum for the \IRAS PSCz catalog and find that the galaxy
distribution displays the characteristic signature of gravity. Assuming
Gaussian initial conditions, we obtain galaxy biasing parameters
$1/b_1=1.20^{+0.18}_{-0.19}$ and $b_2/b_1^2=-0.42\pm0.19$, with no sign of
scale-dependent bias for $k\leq 0.3$ h/Mpc. These results impose stringent
constraints on non-Gaussian initial conditions. For dimensional scaling models
with $\chi^2_N$ statistics, we find N>49, which implies a constraint on
primordial skewness $B_3<0.35$.


title: Evolution and Nucleosynthesis of Very Massive Primordial Stars

abstract:
  We investigate the evolution, final fate, and nucleosynthetic yields of
rotating and non-rotating very massive stars (VMS) of zero metallicity. First
we address the issue of mass loss during hydrogen burning due to vibrational
instabilities. We find that these objects are much more stable than what was
found in previous studies of VMS of solar composition, and expect only
negligible mass loss driven by the pulsations. As these stars thus reach the
end of their evolution with massive helium cores, they encounter the
pair-creation instability. We find that for helium core masses of ~64...133
solar masses these stars are completely disrupted with explosion energies of up
to ~1E53 erg and eject up to ~60 solar masses of Ni56 Stars with more massive
helium cores collapse into black holes. We present the first calculations that
follow the collapse of such a massive rotating star and predict that X-ray
burst and significant gravitational wave emission could result.


title: Morphological Properties of PPNs: Mid-IR and HST Imaging Surveys

abstract:
  We will review our mid-infrared and HST imaging surveys of the circumstellar
dust shells of proto-planetary nebulae. While optical imaging indirectly probes
the dust distribution via dust-scattered starlight, mid-IR imaging directly
maps the distribution of warm dust grains. Both imaging surveys revealed
preferencially axisymmetric nature of PPN dust shells, suggesting that
axisymmetry in planetary nebulae sets in by the end of the asymptotic giant
branch phase, most likely by axisymmetric superwind mass loss. Moreover, both
surveys yielded two morphological classes which have one-to-one correspondence
between the two surveys, indicating that the optical depth of circumstellar
dust shells plays an equally important role as the inclination angle in
determining the morphology of the PPN shells.


title:
Spatial Distributions of Multiple Dust Components in the PPN/PN Dust
  Shells

abstract:
  We investigate spatial distributions of specific dust components in the
circumstellar shells of a proto-planetary nebula candidate, HD 179821, and a
planetary nebula, BD$+30^{\circ}$3639, by means of spectral imaging. With
high-resolution ground-based images and ISO spectra in the mid-infrared, we can
derive ``dust feature only'' maps by subtracting synthesized continuum maps
from the observed images at the feature wavelength. Such spatially detailed
information will help to develop models for the evolution of dust grains around
evolved stars.


title: On the Small-Scale Clustering of Ly-alpha Forest Clouds

abstract:
  Recent measurements of the autocorrelation function of the Ly-alpha clouds
are analyzed from the point of view of a simple model with strong clustering on
the small scales. It is shown that this toy model reproduces fairly well the
important linear relation between amplitude of the absorber autocorrelation
function and neutral hydrogen column density. In addition, it predicts a
correct evolutionary trend of correlation amplitudes. Some possible
ramifications of these results are discussed.


title:
The Anthropic Argument against Infinite Past and the Eddington-Lemaitre
  Universe

abstract:
  This study in the philosophy of cosmology is a part of an ongoing effort to
investigate and reassess the importance of the anthropic (Davies-Tipler)
argument against cosmologies containing the past temporal infinity. Obviously,
the prime targets of this argument are cosmological models stationary on
sufficiently large scale, the classical steady state model of Bondi, Gold and
Hoyle being the best example. Here we investigate the extension of application
of this argument to infinitely old non-stationary models and discuss additional
constraints necessary to be imposed on such models for the edge of the
anthropic argument to be preserved. An illustrative counterexample is the
classical Eddington-Lemaitre model, in the analysis of which major such
constraints are presented. Consequences of such an approach for our
understanding of the nature of time are briefly discussed.


title: Correlated Sub-mm and X-ray Variability in Sgr A*

abstract:
  PAPER WITHDRAWN.
  The recent detection of Sgr A* in the X-ray band, together with the radio
polarization measurements conducted over the past few years, offer the best
constraints yet for understanding the nature of the emitting gas within several
Schwarzschild radii ($r_S$) of this supermassive black hole candidate at the
Galactic Center. Earlier, we showed that the sub-mm radiation from this source
may be associated with thermal synchrotron emission from an inner Keplerian
region within the circularization radius of the accreting plasma. In this
paper, we extend this analysis in a very important way, by calculating the
implied high-energy emission of Sgr A* associated with the orbiting, hot,
magnetized gas. We find that for the accretion rate inferred from the fits to
the sub-mm data, the dominant contribution to Sgr A*'s X-ray flux is due to
self-Comptonization of the radio photons, rather than from bremsstrahlung. The
latter is a two-body process, which would produce significant X-ray emission
only at much higher accretion rates. This picture leads to the testable
prediction that the physical conditions within the inner $\sim5r_S$ are
variable on a time scale of order a year. In particular, the accretion rate
$\dot M$ appears to have changed by about 15% between the sub-mm measurements
in 1996 and 1999. Given that the radio and self-Comptonized fluxes are strongly
correlated in this picture, the upcoming second generation Chandra observations
of Sgr A* may provide the direct evidence required to test this model.


title:
GRB 000926 and its optical afterglow: Another evidence for non-isotropic
  emission

abstract:
  CCD Johnsons BV and Cousins RI photometric magnitudes are determined for 20
stars in the field of GRB 000926. Using them as calibrators, B and R band light
curves are constructed of the afterglow from ours and other published
observations. They show a steepening of the flux decay, from a decay constant
of 1.4+/-0.1 to 2.6+/-0.06 at about 1.7 days after the burst. Attributing this
break to the onset of sideways expansion of a jet-like ejecta, we estimate an
initial jet opening angle of ~0.14 radian, and a burst energy of ~10^51 erg. We
construct the X-ray-optical-near infrared spectrum of the afterglow and derive
a spectral index of ~ -0.9. From the spectrum we also estimate that the source
has a rather large intrinsic extinction, amounting to E(B-V)=0.36+/-0.02.


title:
Helium white dwarf in PSR J0751+1807; too cool, in PSR J1012+5307; too
  hot?

abstract:
  We discuss the cooling history of the low-mass, helium core white dwarfs in
short orbital period millisecond pulsars PSR J0751+1807 and PSR J1012+5307. The
revised cooling age estimated by Alberts et al. agrees with the age estimation
for PSR J1012+5307, removing the discrepancy between the spin-down age and the
cooling age. However, if we accept this model then the helium white dwarf in
the binary pulsar system PSR J0751+1807 must be much hotter than is observed.
We propose that this discrepancy may be resolved if, after detachment of the
secondary star from its Roche lobe in PSR J0751+1807, the star loses its
hydrogen envelope due to pulsar irradiation. When hydrogen burning stops, the
white dwarf will cool down much more quickly than in the case of a thick
hydrogen envelope with a hydrogen burning shell. We discuss several
possibilities to explain different cooling histories of white dwarfs in both
systems.


title:
The Physics of Blazar Optical Emission Regions I: Alignment of Optical
  Polarization and the VLBI Jet

abstract:
  We collected optical and near IR linear polarization data obtained over
20--30 years for a sample of 51 blazars. For each object, we calculated the
probability that the distribution of position angles was isotropic. The
distribution of these probabilities was sharply peaked, with 27 blazars showing
a probability < 15% of an isotropic distribution of position angles.
  For these 27 objects we defined a preferred position angle. For those 17 out
of 27 blazars showing a well-defined radio structure angle (jet position angle)
on VLBI scales (1--3mas), we looked at the distribution of angle differences --
the optical polarization relative to the radio position angles. This
distribution is sharply peaked, especially for the BL Lac objects, with
alignment better than 15 degrees for half the sample. Those blazars with
preferred optical position angles were much less likely to have bent jets on
1--20mas scales. These results support a shock-in-jet hypothesis for the jet
optical emission regions.


title:
The Physics of Blazar Optical Emission Regions II: Magnetic Field
  Orientation, Viewing Angle and Beaming

abstract:
  For a sample of 51 blazars with extensive optical polarization data, we used
circular statistics to calculate the scatter among the polarization position
angles for each object. We found that this scatter is correlated with the radio
core dominance. We compared this relationship with the predictions of a simple
transverse shock model. The result suggests that blazar jets are likely to
cover a wide range of speeds consistent with those derived from observations of
superluminal motion.


title: A Polarization Flare in 3C 273: A Clue to Jet Physics

abstract:
  We present UBVRIJHK polarization and flux density observations of the quasar
3C 273 obtained during a time of outburst over two weeks in 1988 February. We
have modelled these data with two power law components, each with
wavelength-independent position angle. These components are roughly
perpendicular. The steeper-spectrum component has higher infrared polarized
flux density, with the electric vector approximately transverse to the
projected direction of the VLBI jet. The K-band polarized flux density and
position angle, and possibly the spectral index of the two components, are
correlated with a time lag of less than a day. We explain our results in terms
of a shocked jet model with two nearly co-spatial components: a shock component
with magnetic field approximately perpendicular to the jet and the other with
magnetic field approximately parallel to the jet.


title:
Dust disks around old Pre Main-Sequence stars: HST/NICMOS2 scattered
  light images and modeling

abstract:
  We present recent near-infrared detections of circumstellar disks around the
two old PMS Herbig stars HD 141569 and HD 100546 obtained with the HST/NICMOS2
camera. They reveal extended structures larger than 350-400 AU in radius. While
the HD 100546 disk appears as a continuous disk down to 40 AU, the HD 141569
environment seems more complex, splitted at least into two dust populations. As
a convincing example, the full modeling of the disk surrounding HR 4796,
another old PMS star, is detailed and confronted with more recent observations.


title:
Quiescent times in gamma-ray bursts: I. An observed correlation between
  the durations of subsequent emission episodes

abstract:
  Although more than 2000 astronomical gamma-ray bursts (GRBs) have been
detected, the precise progenitor responsible for these events is unknown. The
temporal phenomenology observed in GRBs can significantly constrain the
different models. Here we analyse the time histories of a sample of bright,
long GRBs, searching for the ones exhibiting relatively long (more than 5 per
cent of the total burst duration) quiescent times, defined as the intervals
between adjacent episodes of emission during which the gamma-rays count rate
drops to the background level. We find a quantitative relation between the
duration of an emission episode and the quiescent time elapsed since the
previous episode. We suggest here that the mechanism responsible for the
extraction and the dissipation of energy has to take place in a meta-stable
configuration, such that the longer the accumulation period, the higher is the
stored energy available for the next emission episode.


title:
Quiescent times in gamma-ray bursts: II. Dormant periods in the central
  engine?

abstract:
  Within the framework of the internal-external shocks model for gamma-ray
bursts, we study the various mechanisms that can give rise to quiescent times
in the observed gamma-ray light-curves. In particular, we look for the
signatures that can provide us with evidence as to whether or not the central
engine goes dormant for a period of time comparable to the duration of the
gaps. We show that the properties of the prompt gamma-ray and X-ray emission
can in principle determine whether the quiescent episodes are due to a
modulated relativistic wind or a switching off of the central engine. We
suggest that detailed observations of the prompt afterglow emission from the
reverse shock will strongly constrain the possible mechanisms for the
production of quiescent times in gamma-ray bursts.


title: News from the p-process: is the s-process a troublemaker?

abstract:
  The most detailed calculations of the p-process call for its development in
the O/Ne layers of Type II supernovae. In spite of their overall success in
reproducing the solar system content of p-nuclides, they suggest a significant
underproduction of the light Mo and Ru isotopes. On grounds of a model for the
explosion of a 25 solar mass star with solar metallicity, we demonstrate that
this failure might just be related to the uncertainties left in the rate of the
22Ne(alpha,n)25Mg neutron producing reaction. The latter indeed has a direct
impact on the distribution of the s-process seeds for the p-process.


title:
Discovery of a redshifted Iron K-line in the X-ray afterglow of GRB
  000214

abstract:
  We report the detection (3 sigma significance level) of a strong iron
emission line in the X-ray spectrum of the afterglow of GRB 000214
(``Valentine's Day Burst'') observed by BeppoSAX. An emission line feature was
observed with a centroid energy of 4.7+/-0.2 keV which, if interpreted as
K-alpha emission from hydrogen-like iron, corresponds to a redshift of z=0.47.
The observed intensity (EW ~2 keV) and duration (tens of hours) of the line
give information on the distance, from the burst region, of the emitting
material (R>3x10^15 cm) and its mass (M>1.4 solar mass). These results are not
easily reconciled with the binary merger and hypernova models for gamma ray
bursts.


title: Interaction rates of dark-matter halos and subhalos

abstract:
  [Abridged] The interaction rates of dark-matter halos and subhalos, are
computed using high-resolution cosmological N-body simulations of the
Lambda-CDM model. Although the number fraction of subhalos of mass
$>2x10^{11}\hsolmass$ is only ~10%, we find that the interaction rate of such
subhalos is relatively high because they reside in high density environments.
At low redshift, the subhalo collisions dominate the total collision rate, and
even at z=3 they are involved in more than 30% of all collisions. About 40% of
the "major" mergers are between subhalos. Therefore subhalo interactions must
be incorporated in models of structure formation. We find that the collision
rate between halos in physical density units, is $\propto (1+z)^\delta $, with
$\delta = 3-4$, in agreement with earlier simulations and most observational
data. We test previous analytic estimates of the interaction rates of subhalos
based on statistical models, which could be very inaccurate because of the
small number of subhalos and the variation of conditions within small host
halos. We find that, while such statistical estimates may severely overestimate
the rate within hosts $< 10^{13} \hsolmass$, typical of high redshifts, they
are valid for larger hosts regardless of the number of subhalos in them. We
find the Makino & Hut (1997) estimate of the subhalo merger rate to be valid
for hosts $ > 6x10^{11}\hsolmass$ at all redshifts. The collision rate between
subhalos and the central object of their host halo is approximated relatively
well using the timescale for dynamical friction in circular orbits. This
approximation fails in ~40% of the cases, partly because of deviations from the
assumption of circular orbits and partly because of the invalidity of the
assumption that the subhalo mass is negligible.


title:
Origin and Destiny of Dark Matter Halos: Cosmological Matter Exchange
  and Metal Enrichment

abstract:
  [Abridged] We analyze the exchange of dark matter between halos, subhalos,
and their environments in a high-resolution cosmological N-body simulation of a
Lambda CDM cosmology. At each analyzed redshift z we divide the dark matter
particles into 4 components: (i) isolated galactic halos, (ii) subhalos, (iii)
the diffuse medium of group and cluster halos, and (iv) the background outside
of virialized halos. We follow the time evolution of the mass distribution and
flows between these components and provide fitting functions for the exchange
rates. We use our derived exchange rates to gauge the importance of metal
redistribution in the universe due solely to gravity-induced interactions. The
diffuse metallicity in clusters is predicted to be ~40% that in isolated
galaxies (~55% of groups) at z=0, and should be lower only slightly by z=1,
consistent with observations. The metallicity of the diffuse media in poor
groups is expected to be lower by a factor of 5 by z~2, in agreement with the
observed metallicity of damped Ly$\alpha$ systems. The metallicity of the
background IGM is predicted to be (1-3)x10^{-4} that of z=0 clusters, also
consistent with observations. The agreement of predicted and observed trends
indicates that gravitational interaction alone may play an important role in
metal enrichment of the intra-cluster and intergalactic media.


title:
A High Resolution Study of the Hydra A Cluster with Chandra: Comparison
  of the Core Mass Distribution with Theoretical Predictions and Evidence for
  Feedback in the Cooling Flow

abstract:
  The cooling flow cluster Hydra A was observed during the orbital activation
and calibration phase of the Chandra Observatory. While the X-ray image of the
cluster exhibits complex structure in the central region as reported in
McNamara $etal$, the large scale X-ray morphology of the cluster is fairly
smooth. A spectroscopic analysis of the ACIS data shows that the gas
temperature in Hydra A increases outward, reaches a maximum temperature of 4
keV at 200 kpc, and then decreases slightly at larger radii. The distribution
of heavy elements is nonuniform, with a factor of two increase in the Fe and Si
abundances within the central 100 kpc. Beyond the central 100 kpc the Si-to-Fe
abundance ratio is twice solar, while the Si-to-Fe ratio of the central excess
is consistent with the solar value. One of the more surprising results is the
lack of spectroscopic evidence for multiphase gas within the bulk of the
cooling flow. Beyond the central 30 kpc, the ACIS spectra are adequately fit
with a single temperature model. The addition of a cooling flow component does
not significantly improve the fit. Only within the central 30 kpc (where the
cooling time is less than 1~Gyr), is there spectroscopic evidence for
multiphase gas. However, the spectroscopic mass deposition rate is more than a
factor of 10 less than the morphologically derived mass accretion rate at 30
kpc. We propose that the cooling flow region is convectively unstable due to
heating by the central radio source which significantly reduces the net
accretion rate.


title: Nature and Nurture: a model for soft gamma-ray repeaters

abstract:
  During supernova explosions, strange stars with almost bare quark surfaces
may be formed. Under certain conditions, these stars could be rapidly spun down
by the torque exerted by the fossil disks formed from the fall-back materials.
They may also receive large kicks and hence, have large proper motion
velocities. When these strange stars pass through the spherical ``Oort'' comet
cloud formed during the pre-supernova era, they will capture some small-scale
comet clouds and collide with some comet-like objects occasionally. These
impacts can account for the repeating bursts as observed from the soft gamma
repeaters (SGRs). According to this picture, it is expected that SGR 1900+14
will become active again during 2004-2005.


title: Scalar Field Dark Matter, Cross Section and Planck-Scale Physics

abstract:
  In recent papers we have proposed that the dark matter of the Universe could
be from scalar field origin. In this letter, we find that if the scale of
renormalization of the model is of order of the Planck Mass, then a scalar
field $\Phi $ endowed with the scalar potential $V=V_{o}[\cosh {(\lambda
\sqrt{\kappa_{0}}\Phi)}-1] can be a reliable model for dark matter in galaxies.
The predicted scattering cross section fits the value required for
self-interacting dark matter and the additional degree of freedom of the theory
is of order of hundreds of TeV.


title: Neutron Star Structure and the Neutron Radius of 208Pb

abstract:
  We study relationships between the neutron-rich skin of a heavy nucleus and
the properties of neutron-star crusts. Relativistic effective field theories
with a thicker neutron skin in $^{208}$Pb have a larger electron fraction and a
lower liquid-to-solid transition density for neutron-rich matter. These
properties are determined by the density dependence of the symmetry energy
which we vary by adding nonlinear couplings between isoscalar and isovector
mesons. An accurate measurement of the neutron radius in $^{208}$Pb---via
parity violating electron scattering---may have important implications for the
structure of neutron stars.


title: Constraining Cosmological Models by the Cluster Mass Function

abstract:
  We present a comparison between two observational and three theoretical mass
functions for eight cosmological models suggested by the data from the recently
completed BOOMERANG-98 and MAXIMA-1 cosmic microwave background (CMB)
anisotropy experiments as well as peculiar velocities (PVs) and type Ia
supernovae (SN) observations. The cosmological models have been proposed as the
best fit models by several groups. We show that no model is in agreement with
the abundances of X-ray clusters at $\sim 10^{14.7} h^{-1}M_{\odot}$.On the
other hand, we find that the BOOM+MAX+{\sl COBE}:I, Refined Concordance and
$\Lambda$MDM are in a good agreement with the abundances of optical clusters.
The P11 and especially Concordance models predict a slightly lower abundances
than observed at $\sim 10^{14.6} h^{-1}M_{\odot}$. The BOOM+MAX+{\sl COBE}:II
and PV+CMB+SN models predict a slightly higher abundances than observed at
$\sim 10^{14.9} h^{-1}M_{\odot}$. The nonflat MAXIMA-1 is in a fatal conflict
with the observational cluster abundances and can be safely ruled out.


title: Linear Analysis of the Hall Effect in Protostellar Disks

abstract:
  The effects of Hall electromotive forces (HEMFs) on the linear stability of
protostellar disks are examined. Earlier work on this topic focused on axial
field and perturbation wavenumbers. Here we treat the problem more generally.
Both axisymmetric and nonaxisymmetric cases are investigated. Though seldom
explicitly included in calculations, HEMFs appear to be important whenever
Ohmic dissipation is. They allow for the appearance of electron whistler waves,
and since these have right-handed polarization, a helicity factor is also
introduced into the stability problem. This factor is the product of the
components of the angular velocity and magnetic field along the perturbation
wavenumber, and it is destabilizing when negative. Unless the field and angular
velocity are exactly aligned, it is always possible to find destabilizing
wavenumbers. HEMFs can destabilize any differential rotation law, even those
with angular velocity increasing outward. Regardless of the sign of the angular
velocity gradient, the maximum growth rate is always given in magnitude by the
local Oort A value of the disk, as in the standard magnetorotational
instability. The role of Hall EMFs may prove crucial to understanding how
turbulence is maintained in the ``low state'' of eruptive disk systems.


title:
Keck Hires Observations of the QSO First J104459.6+365605: Evidence for
  a Large Scale Outflow

abstract:
  This paper presents an analysis of a Keck HIRES spectrum of the QSO FIRST
J104459.6+365605. The line of sight towards the QSO contains two clusters of
outflowing clouds that give rise to broad blue shifted absorption lines. The
outflow velocities of the clouds range from -200 to -1200 km/s and from -3400
to -5200 km/s, respectively. The width of the individual absorption lines
ranges from 50 to more than 1000 km/s. The most prominent absorption lines are
those of Mg II, Mg I, and Fe II. The low ionization absorption lines occur at
the same velocities as the most saturated Mg II lines, showing that the Fe II,
Mg I and Mg II line forming regions must be closely associated. Many absorption
lines from excited states of Fe II are present, allowing a determination of the
population of several low lying energy levels. From this we determine an
electron density in the Fe II line forming regions of 4000 per cubic cm.
Modelling the ionization state of the absorbing gas with this value of the
electron density as a constraint, we find that the distance between the Fe II
and Mg I line forming region and the continuum source is of order 700 parsec.
  From the correspondence in velocity between the Fe II, Mg I and Mg II lines
we infer that the Mg II lines must be formed at the same distance. The Mg II
absorption fulfills the criteria for Broad Absorption Lines defined by Weymann
et al. (1991). This large distance is surprising, since BALs are generally
thought to be formed in outflows at a much smaller distance from the nucleus.