title:
The Association of Jet Production with Geometrically Thick Accretion
  Flows and Black Hole Rotation

abstract:
  A model is presented in which the strongest radio-emitting jet outflows are
produced in black hole systems when the accretion is a geometrically thick
($H/R \sim 1$) inflow ({\it e.g.}, ADAF, CDAF) {\em and} if the black hole is
rotating. For galactic black hole candidates, the model naturally accounts for
the observed correlation of jet outflow with the black hole hard emission state
and predicts an association of strong jets with rapid black hole rotation. When
extended to the supermassive case, the model accounts for the highest radio
galaxy and quasar jet powers and provides additional theoretical support for
the ``spin paradigm'', which asserts that radio loud quasars are produced by
Kerr holes and radio quiet ones by Schwarzschild holes. In some cases, the
angular momentum and energy outflow in the jet may be large enough to
significantly alter the structure of the accretion flow from that predicted by
current models.


title:
Microlensing results from APO monitoring of the double quasar
  Q0957+561A,B between 1995 and 1998

abstract:
  If the halo of the lensing galaxy 0957+561 is made of massive compact objects
(MACHOs), they must affect the lightcurves of the quasar images Q0957+561 A and
B differently. We search for this microlensing effect in the double quasar by
comparing monitoring data for the two images A and B - obtained with the 3.5m
Apache Point Observatory from 1995 to 1998 - with intensive numerical
simulations. This way we test whether the halo of the lensing galaxy can be
made of MACHOs of various masses. We can exclude a halo entirely made out of
MACHOs with masses between 10e(-6) Msun and 10e(-2) Msun for quasar sizes of
less than 3*10e(14)cm (h = 0.6), hereby extending previous limits upwards by
one order of magnitude.


title: Pulsar spectra of radio emission

abstract:
  We have collected pulsar flux density observations and compiled spectra of
281 objects. The database of Lorimer et al. (\cite{lorimer}) has been extended
to frequencies higher than 1.4 GHz and lower than 300 MHz. Our results show
that above 100 MHz the spectra of the majority of pulsars can be described by a
simple power law with average value of spectral index $<\alpha> = -1.8\pm0.2$.
A rigorous analysis of spectral fitting revealed only about 5% of spectra which
can be modelled by the two power law. Thus, it seems that single power law is a
rule and the two power law spectrum is a rather rare exception, of an unknown
origin, to this rule. We have recognized a small number of pulsars with almost
flat spectrum ($\alpha \geq -1.0$) in the wide frequency range (from 300 MHz to
20 GHz) as well as few pulsars with a turn-over at unusually high frequency
($\sim$~1 GHz).


title: Broad band properties of medium and low Lx/Lb Early Type Galaxies

abstract:
  We have measured the spectral properties of five galaxies of low to
intermediate Lx/Lb ratios with BeppoSAX and ASCA. A hard component (kT ~4-10
keV) is observed in all galaxies. In NGC1553 the BeppoSAX data show that this
component is extended, and suggest an origin for the emission in the evolved
stellar population. In NGC3115, a point-like source appears embedded in an
extended component morphologically similar to the stellar body, suggesting an
almost equal contribution from the nuclear region and the binary population. A
large central mass concentration and low level optical activity in NGC3379
argue for a contribution from the nucleus also in this object. However, for
both NGC3115 and NGC3379 the nuclear emission is at a level well below that
observed in other galaxies who host similar nuclear black holes. A second soft
(kT ~0.3-0.7 keV) spectral component is needed to fit the data of NGC1407,
NGC1553 and NGC4125 over the entire energy range probed (about 0.2-10 keV),
best represented by a thermal component with line emission. We discuss possible
interpretations of the origin of this component, which however will be better
defined only with higher quality data.


title:
Correlation of Ultra High Energy Cosmic Rays with Compact Radio Loud
  Quasars

abstract:
  Angular correlations of ultra-high energy cosmic rays with cosmologically
distant sources may provide clues to these mysterious events. We compare cosmic
ray tracks with energies above $ 10^{20}eV$ to a compilation of radio-loud
compact QSO positions. The statistical method emphasizes invariant quantities
and a test of {\it statistical independence} of track and source distributions.
Statistical independence is ruled out by several independent statistics at
confidence levels of less that $10^{-3}$ ($99.9% $.), indicating that track
directions and QSO source positions are correlated at a highly significant
level.


title: A deep submillimetre survey of the Galactic Centre

abstract:
  We present first results from a submillimetre continuum survey of the
Galactic Centre `Central Molecular Zone' (CMZ), made with SCUBA on the James
Clerk Maxwell Telescope. SCUBA's scan-map mode has allowed us to make extremely
wide-field maps of thermal dust emission with unprecedented speed and
sensitivity. We also discuss some issues related to the elimination of
artefacts in scan-map data. Our simultaneous 850/450 micron maps have a total
size of approximately 2.8 x 0.5 degrees (400 x 75 pc) elongated along the
galactic plane. They cover the Sgr A region-including Sgr A*, the circumnuclear
disc, and the +20 km/s and +50 km/s clouds; the area around the Pistol; Sgr
B2-the brightest feature on the map; and at their Galactic Western and Eastern
edges the Sgr C and Sgr D regions. There are many striking features such as
filaments and shell-like structures, as well as point sources such as Sgr A*
itself. The total mass in the Central Molecular Zone is greater than that
revealed in previous optically-thin molecular line maps by a factor of ~3, and
new details are revealed on scales down to 0.33 pc across this 400 pc wide
region.


title: Planets and Axisymmetric Mass Loss

abstract:
  Bipolar planetary nebulae (PNe), as well as extreme elliptical PNe are formed
through the influence of a stellar companion. But half of all PN progenitors
are not influenced by any stellar companion, and, as I show here, are expected
to rotate very slowly on reaching the upper asymptotic giant branch; hence they
expect to form spherical PNe, unless they are spun-up. But since most PNe are
not spherical, I argue that about 50 percents of AGB stars are spun-up by
planets, even planets having a mass as low as 0.01 times the mass of Jupiter,
so they form elliptical PNe. The rotation by itself will not deform the AGB
wind, but may trigger another process that will lead to axisymmetric mass loss,
e.g., weak magnetic activity, as in the cool magnetic spots model. This model
also explains the transition from spherical to axisymmetric mass loss on the
upper AGB. For such low mass planets to substantially spin-up the stellar
envelope, they should enter the envelope when the star reaches the upper AGB.
This "fine-tuning" can be avoided if there are several planets on average
around each star, as is the case in the solar system, so that one of them is
engulfed when the star reaches the upper AGB.


title: K-band Spectroscopy of Clusters in NGC 4038/4039

abstract:
  Integral field spectroscopy in the K-band (1.9-2.4um) was performed on four
IR-bright star clusters and the two nuclei in NGC 4038/4039 (``The Antennae'').
Two of the clusters are located in the overlap region of the two galaxies, and
together comprise ~25% of the total 15um and ~10% of the total 4.8 GHz emission
from this pair of merging galaxies. The other two clusters, each of them
spatially resolved into two components, are located in the northern galaxy, one
in the western and one in the eastern loop of blue clusters. Comparing our
analysis of Brgamma, CO band-heads, He I (2.058um), Halpha (from archival HST
data), and V-K colors with stellar population synthesis models indicates that
the clusters are extincted (A_V ~ 0.7 - 4.3 mags) and young, displaying a
significant age spread (4-13 Myrs). The starbursts in the nuclei are much older
(65 Myrs), with the nucleus of NGC 4038 displaying a region of recent star
formation northward of its K-band peak. Using our derived age estimates and
assuming the parameters of the IMF (Salpeter slope, upper mass cut-off of 100
M_sun, Miller-Scalo between 1 M_sun and 0.1 M_sun), we find that the clusters
have masses between 0.5 and 5 * 10^6M_sun.


title:
Kinematics of Molecular Hydrogen Emission from Pre-planetary Nebulae:
  RAFGL 2688 and RAFGL 618

abstract:
  We present high spectral resolution maps of near-infrared molecular hydrogen
emission from the bipolar pre-planetary nebulae RAFGL 2688 and RAFGL 618,
obtained with the NOAO Phoenix spectrometer. The measured velocity gradients
along the polar axes of both nebulae indicate that the highest velocity gas
lies closest to the central stars. These results support the suggestion that
the polar H_2 emission regions of both nebulae contain shocked gas formed as
fast (~50-150 km/s), collimated, post-asymptotic giant branch (AGB) winds
collide with slower-moving (~10-20 km/s) material previously ejected while the
central stars were still on the AGB. The kinematics of H_2 emission
perpendicular to the polar axis of RAFGL 2688 are consistent with a model
combining expansion along the equator at 5-10 km/s with rotation about the
polar axis at 5-10 km/s. The rapid onset of the common envelope phase of a
close binary system may explain both the bipolar structure of RAFGL 2688 and
the presence and complex kinematics of its shocked H_2 emission.


title: SuperMassive Black Holes in Bulges

abstract:
  We present spatially extended gas kinematics at parsec-scale resolution for
the nuclear regions of four nearby disk galaxies, and model them as rotation of
a gas disk in the joint potential of the stellar bulge and a putative central
black hole. The targets were selected from a larger set of long-slit spectra
obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei
with STIS (SUNNS). They represents the 4 galaxies (of 24) that display
symmetric gas velocity curves consistent with a rotating disk. We derive the
stellar mass distribution from the STIS acquisition images adopting the stellar
mass-to-light ratio normalized so as to match ground-based velocity dispersion
measurements over a large aperture. Subsequently, we constrain the mass of a
putative black hole by matching the gas rotation curve, following two distinct
approaches. In the most general case we explore all the possible disk
orientations, alternatively we constrain the gas disk orientation from the
dust-lane morphology at similar radii. In the latter case the kinematic data
indicate the presence of a central black hole for three of the four objects,
with masses of 10^7 - 10^8 solar masses, representing up to 0.025 % of the host
bulge mass. For one object (NGC2787) the kinematic data alone provide clear
evidence for the presence of a central black hole even without external
constraints on the disk orientation. These results illustrate directly the need
to determine black-hole masses by differing methods for a large number of
objects, demonstrate that the variance in black hole/bulge mass is much larger
than previously claimed, and reinforce the recent finding that the black-hole
mass is tightly correlated with the bulge stellar velocity dispersion.


title: The "Twin Jet" Planetary Nebula M2-9

abstract:
  We present a model for the structure, temporal behavior, and evolutionary
status of the bipolar nebula M2-9. According to this model the system consists
of an AGB or post-AGB star and a hot white dwarf companion, with an orbital
period of about 120 years. The white dwarf has undergone a symbiotic nova
eruption about 1200 years ago, followed by a supersoft x-ray source phase. The
positional shift of the bright knots in the inner nebular lobes is explained in
terms of a revolving ionizing source. We show that the interaction between the
slow, AGB star's wind, and a collimated fast wind from the white dwarf clears a
path for the ionizing radiation in one direction, while the radiation is
attenuated in others. This results in the mirror-symmetric (as opposed to the
more common point-symmetric) shift in the knots. We show that M2-9 provides an
important evolutionary link among planetary nebulae with binary central stars,
symbiotic systems, and supersoft x-ray sources.


title: The AMS-01 Aerogel Threshold Cherenkov counter

abstract:
  The Alpha Magnetic Spectrometer in a precursor version (AMS-01), was flown in
June 1998 on a 51.6 degrees orbit and at altitudes ranging between 320 and 390
km, on board of the space shuttle Discovery (flight STS-91). AMS-01 included an
Aerogel Threshold Cherenkov counter (ATC) to separate antiprotons from
electrons and positrons from protons, for momenta below 3.5 GeV/c. This paper
presents a description of the ATC counter and reports on its performances
during the flight STS-91.


title: A Binary Millisecond Pulsar in Globular Cluster NGC6544

abstract:
  We report the detection of a new 3.06 ms binary pulsar in the globular
cluster NGC6544 using a Fourier-domain ``acceleration'' search. With an implied
companion mass of ~0.01 solar masses and an orbital period of only P_b~1.7
hours, it displays very similar orbital properties to many pulsars which are
eclipsed by their companion winds. The orbital period is the second shortest of
known binary pulsars after 47 Tuc R. The measured flux density of 1.3 +/- 0.4
mJy at 1332 MHz indicates that the pulsar is almost certainly the known
steep-spectrum point source near the core of NGC6544.


title: Gamma-ray burst afterglows from jetted shocks in wind environments

abstract:
  Gamma-ray bursts with long durations are widely thought to arise from the
collapse of massive stars, where the wind environment is unavoidable. It is
also believed that $\gamma$-ray bursts come from jets. Considering these two
points in this paper, we calculate the evolution of a highly collimated jet
that expands in a stellar wind environment and the expected afterglow from such
a jet. We use a set of refined dynamical equations and a realistic lateral
speed of the jet, and find: (1) There is no observable break at the time when
the Lorentz factor of the jet is equal to the inverse of its initial
half-opening angle. (2) No obvious break appears at the time when the blast
wave transits from the relativistic to the non-relativistic phase. (3) For the
wind case, there is no flattening tendency even up to $10^9$ s. (4) Compared
with the homogeneous medium case, our calculated flux is weaker in the stellar
wind case. Finally, we find that two kinds of GRB models (neutron star mergers
and massive star collapses) may be discriminated in our numerical results.


title: WFPC2 Observations of the Hubble Deep Field-South

abstract:
  The Hubble Deep Field-South observations targeted a high-galactic-latitude
field near QSO J2233-606. We present WFPC2 observations of the field in four
wide bandpasses centered at roughly 300, 450, 606, and 814 nm. Observations,
data reduction procedures, and noise properties of the final images are
discussed in detail. A catalog of sources is presented, and the number counts
and color distributions of the galaxies are compared to a new catalog of the
HDF-N that has been constructed in an identical manner. The two fields are
qualitatively similar, with the galaxy number counts for the two fields
agreeing to within 20%. The HDF-S has more candidate Lyman-break galaxies at z
> 2 than the HDF-N. The star-formation rate per unit volume computed from the
HDF-S, based on the UV luminosity of high-redshift candidates, is a factor of
1.9 higher than from the HDF-N at z ~ 2.7, and a factor of 1.3 higher at z ~ 4.


title: Modeling Observational Signatures of Disk-Driven Outflows

abstract:
  We present a self-consistent, semi-analytical dynamical model of disk driven
outflows in AGNs that are accelerated by a combination of magnetic stresses and
radiation pressure. This model will make it possible to examine scenarios in
which the wind is homogeneous as well as cases where it consists of dense
clouds embedded in a more tenuous, magnetized medium. The various ingredients
of this model will be tested through quantitative predictions from both
multiwavelength spectral observations and reverberation mapping of AGNs.


title: QSO Absorption Lines, Gaseous Infall and Star Formation

abstract:
  We investigate the impact of recent observational advances in research on the
gaseous content of the universe on our knowledge of star formation histories of
disk galaxies. Several contemporary surveys indicate that large quantities of
gas which has not been astrated or has been astrated only weakly are bound to
galaxies at later epochs ($z\le 1$).


title: Evolution of Mass Distribution in CDM Halos

abstract:
  On the basis of a new convergence study of high-resolution N-body
simulations, my colleagues and I now agree that the Navarro, Frenk, & White
(1996) density profile $\rho_{NFW}(r) \propto r^{-1} (r+r_s)^{-2}$ is a good
representation of typical dark matter halos of galactic mass. Comparing
simulations of the same halo with numbers of particles ranging from $\sim10^3$
to $\sim10^6$, we have also shown that $r_s$, the radius where the log-slope is
-2, can be determined accurately for halos with as few as $\sim10^3$ particles.
Based on a study of thousands of halos at many redshifts in an Adaptive
Refinement Tree (ART) simulation of a cosmological volume in a $\Lambda$CDM
cosmology, we have found that the concentration $\cvir \equiv \Rvir/\rs$ has a
log-normal distribution, with $1\sigma$ $\Delta (\log \cvir) = 0.18$ at a given
mass, corresponding to a scatter in maximum rotation velocities of $\Delta
\Vmax/\Vmax = 0.12$. The average concentration declines with redshift at fixed
mass as $\cvir(z) \propto (1+z)^{-1}$. This may have important implications for
galaxy rotation curves. Finally, we have found that the velocity function
determined from galaxy luminosity functions plus luminosity-velocity relations
agrees with the predictions from our $\Lambda$CDM simulations. But we also note
that the very limited evolution with redshift of the velocity function
predicted by $\Lambda$CDM conflicts with the data that is becoming available on
the number density of bright galaxies unless there is significant evolution of
the luminosity-velocity relation at $z>1$.


title: Interactions and mergers at higher redshift

abstract:
  In models of hierarchical structure formation, interactions and mergers at
high redshift play a key role in the formation and evolution of galaxies.
Numerical modeling and observations of nearby systems explore the detailed
physics of galaxy interactions, but applying these results to the high redshift
universe remains complicated. Evolution in the properties of galaxies over
cosmic history may lead to differences in interaction-induced star formation as
a function of redshift. High redshift interacting galaxies may be more prone to
extreme, disk-wide starburst activity than are low redshift systems. Also,
while activity in nearby clusters seems to be largely diminished, at higher
redshift we observe galaxies falling into clusters for the first time. A
variety of processes, including interactions and mergers, can drive strong
evolution in cluster galaxies at higher redshift.


title: H-alpha Velocity Fields of Normal Spiral Disks

abstract:
  We present H-alpha velocity fields for a sample of nearly face--on spiral
galaxies observed with DensePak on the WIYN telescope. We combine kinematic
inclinations and position angles measured from these data with photometric
inclinations and position angles measured from I-band images to show that
spiral disks are intrinsically non-circular.


title: Radio Halo and Relic Candidates from the Westerbork Northern Sky Survey

abstract:
  We have undertaken a systematic search for diffuse radio halos and relics in
all of the Abell clusters that are visible in the Westerbork Northern Sky
Survey (WENSS). In this survey we found 18 candidates, 11 of which are already
known from the literature, and 7 for which we provide the first evidence of
diffuse radio emission. All the clusters in this sample show other evidence for
a recent or ongoing merger. We also investigate the correlation between cluster
X-ray luminosity and radio power of halos. We develop a very simple model for
merger shocks that reproduces the sense of this correlation, although it is
probably not as steep as the correlation in the data. We discuss the
implications of X-ray--radio correlations for future detections of radio halos.


title: The Central Velocity Field in NGC 253 : Possible Indication of a Bar

abstract:
  We have investigated whether motion of gas in a bar-like potential can
account for the peculiar but systematic velocity field observed in the nuclear
region of the starburst galaxy NGC 253. This unusual velocity field with
gradients along both major and minor axes was revealed in a high resolution
($1.8^{\arcsec}\times 1.0^{\arcsec}$) H92$\alpha$ recombination line
observation by Anantharamiah and Goss (1996). A simple logarithmic potential is
used to model the bar. Assuming that the bulk of the gas flows along closed and
non-intersecting x1 (bar) and x2 (anti-bar) orbits of the bar potential, we
have computed the expected velocity field and position-velocity diagrams and
compared them with the observations. A comparison of the integrated CO
intensity maps with the spatial distribution of the x1 and x2 orbits in the
model indicates that the nuclear molecular gas in NGC 253 lies mainly on the x2
orbits. We also find that the velocity field observed in the central 100 pc
region in the H92$\alpha$ recombination line is well accounted for by the bar
model if most of the ionized gas resides in the inner x2 orbits. However, the
model is unable to explain the velocity field on a larger scale of $\sim 500$
pc observed using the OVRO interferometer with a resolution of
$5^{\arcsec}\times 3^{\arcsec}$. The direction of the observed CO velocity
field appears twisted compared to the model. We suggest that this perturbation
in the velocity field may be due to an accretion event that could have occurred
$10^7$ years ago.


title: Cosmic Mach Number as a Function of Overdensity and Galaxy Age

abstract:
  We carry out an extensive study of the cosmic Mach number (\mach) on scales
of R=5, 10 and 20h^-1Mpc using an LCDM hydrodynamical simulation. We
particularly put emphasis on the environmental dependence of \mach on
overdensity, galaxy mass, and galaxy age. We start by discussing the difference
in the resulting \mach according to different definitions of \mach and
different methods of calculation. The simulated Mach numbers are slightly lower
than the linear theory predictions even when a non-linear power spectrum was
used in the calculation, reflecting the non-linear evolution in the simulation.
We find that the observed \mach is higher than the simulated mean by more than
2-standard deviations, which suggests either that the Local Group is in a
relatively low-density region or that the true value of \Omega_m is ~ 0.2,
significantly lower than the simulated value of 0.37. We show from our
simulation that the Mach number is a weakly decreasing function of overdensity.
We also investigate the correlations between galaxy age, overdensity and \mach
for two different samples of galaxies --- DWARFs and GIANTs. Older systems
cluster in higher density regions with lower \mach, while younger ones tend to
reside in lower density regions with larger \mach, as expected from the
hierarchical structure formation scenario. However, for DWARFs, the correlation
is weakened by the fact that some of the oldest DWARFs are left over in
low-density regions during the structure formation history. For giant systems,
one expects blue-selected samples to have higher \mach than red-selected ones.
We briefly comment on the effect of the warm dark matter on the expected Mach
number.


title:
Mapping the Galactic Halo IV. Finding Distant Giants Reliably with the
  Washington System

abstract:
  We critically examine the use of the Washington photometric system (with the
51 filter) for identifying distant halo giants. While this is the most powerful
photometric technique for isolating G and K giant stars, spectroscopic
follow-up of giant candidates is vital. There are two situations in which
interlopers outnumber genuine giants in the diagnostic M-51/M-T2 plot, and are
indistinguishable photometrically from the giants. (1) In deep surveys covering
tens of square degrees, very metal-poor halo dwarfs are a significant
contaminant. An example is our survey of the outer halo (Morrison et al. 2000,
Dohm-Palmer et al. 2000), where these metal-poor dwarfs dominate the number of
photometric giant candidates at magnitudes fainter than V = 18 and cannot be
isolated photometrically. (2) In deep surveys of smaller areas with low
photometric precision, most objects in the giant region of the color-color plot
are dwarfs whose photometric errors have moved them there. Color errors in M-51
and M-T2 need to be smaller than 0.03 mag to avoid this problem. An example of
a survey whose photometric errors place the giant identifications under
question is the survey for extra-tidal giants around the Carina dwarf
spheroidal of Majewski et al. (2000a). Accurate photometry and spectroscopic
follow-up of giant candidates are essential when using the Washington system to
identify the rare outer-halo giants.


title: X-Ray Photoabsorption in KLL Resonances of O VI And Abundance Analysis

abstract:
  It is shown that photoabsorption via autoionizing resonances may be
appreciable and used for abundance analysis. Analogous to spectral lines, the
`resonance oscillator strength' f_r may be defined and evaluated in terms of
the differential oscillator strength df/d(epsilon) that relates bound and
continuum absorption. X-ray photoabsorption in KLL (1s2s2p) resonances of O VI
is investigated using highly resolved relativistic photoionization cross
sections with fine structure. It is found that f_r is comparable to that for UV
dipole transition in O VI (2s - 2p) and the X-ray (1s^2 ^1S_0 - 1s2p ^1P^o_1)
transition in O VII. The dominant O VI(KLL) components lie at 22.05 and 21.87
Angstroms. These predicted absorption features should be detectable by the
Chandra X-Ray Observatory (CXO) and the X-Ray Multi-Mirror Mission (XMM). The
combined UV/X-ray spectra of O VI/O VII should yield valuable information on
the ionization structure and abundances in sources such as the `warm absorber'
region of active galactic nuclei and the hot intergalactic medium. Some general
implications of resonant photoabsorption are addressed.


title: Fast CMB Analyses via Correlation Functions

abstract:
  We propose and implement a fast, universally applicable method for extracting
the angular power spectrum C_l from CMB temperature maps by first estimating
the correlation function \xi(\theta). Our procedure recovers the C_l's using
N^2 (but potentially N logN), operations, where N is the number of pixels. This
is in contrast with standard maximum likelihood techniques which require N^3
operations. Our method makes no special assumptions about the map, unlike
present fast techniques which rely on symmetries of the underlying noise
matrix, sky coverage, scanning strategy, and geometry. This enables for the
first time the analysis of megapixel maps without symmetries. The key element
of our technique is the accurate multipole decomposition of \xi(\theta). The
C_l error bars and cross-correlations are found by a Monte-Carlo approach. We
applied our technique to a large number of simulated maps with Boomerang sky
coverage in 81000 pixels. We used a diagonal noise matrix, with approximately
the same amplitude as Boomerang. These studies demonstrate that our technique
provides an unbiased estimator of the C_l's. Even though our method is
approximate, the error bars obtained are nearly optimal, and converged only
after few tens of Monte-Carlo realizations. Our method is directly applicable
for the non-diagonal noise matrix. This, and other generalizations, such as
minimum variance weighting schemes, polarization, and higher order statistics
are also discussed.


title:
Physical Parameters for the Afterglows of GRB 980703, 990123, 990510,
  and 991216 Determined from Modeling of Multi-Frequency Data

abstract:
  We model the radio, optical, and X-ray emission for the afterglows of GRB
980703, 990123, 990510, and 991216, within the framework of relativistic jets,
to determine their physical parameters. The models that yield acceptable fits
to the data have jet energies mostly between 10^{50} to 10^{51} erg and initial
opening angles between 1 deg and 4 deg. The external medium density is
uncertain by at least one order of magnitude in each case, being around
10^{-3}/cm^3 for GRB 980703 and 990123, ~0.1/cm^3 for GRB 990510, and ~3/cm^3
for GRB 991216. If the jets are uniform (i.e. there are no angular gradients of
the energy per solid angle) then the 20 keV -- 1 MeV radiative efficiency
during the GRB phase must have been at least 2-3% for GRB 990510, 20% for GRB
990123, and 30% for GRB 991216.


title: Iron K-alpha Emission from a Decaying Magnetar Model of Gamma-Ray Bursts

abstract:
  The recent report of X-ray Fe features in the afterglow of the gamma-ray
burst GRB 991216 may provide important clues for identifying the nature of its
progenitor and constraining the burst mechanism. We argue that the strong line
emission can be attributed to the interaction of a continuing (but decaying)
post-burst relativistic outflow from the central engine with the progenitor
stellar envelope at distances less than a light-hour. Only a small mass of Fe
is then required, which could have been readily produced by the star itself.


title: An evolutionary scenario for the U Scorpii

abstract:
  We perform evolutionary calculations of binary stars to find progenitors of
systems with parameters similar to the recurrent novae U Sco. We show that a U
Sco type-system may be formed starting with an initial binary system which has
a low-mass carbon-oxygen white dwarf as an accretor. Since the evolutionary
stage of the secondary is not well known, we calculate sequences with hydrogen
rich and helium rich secondaries. The evolution of the binary may be devided
into several observable stages as: classical nova, supersoft X-ray source with
hydrogen stable burning and strong wind phases, ending up with the formation of
a massive white dwarf near the Chandrasekhar mass limit. We follow the chemical
evolution of the secondary as well as of the matter lost from the system, and
we show that observed ^{12}C/^{13}C and N/C ratios may give some information
about the nature of the binary.


title: An X-ray flare from the Lindroos binary system HD 560

abstract:
  We report on a large X-ray flare from the Lindroos binary system HD 560
during a pointed ROSAT PSPC observation. Strong magnetic activity is expected
for the late-type component (a G dwarf star and post-T Tauri Star candidate).
The system is unresolved in the ROSAT image. Therefore, although not expected
from a theoretical point of view, it can not be excluded that the flare has
erupted on the primary, a late-B type star. We discuss the luminosity and
temperature evolution comparing the quiescent and the flare state of HD 560,
and present arguments for the G star as host of the flare.


title: GRB Afterglows from Anisotropic Jets

abstract:
  Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may
produce anisotropic jets in which the energy per unit solid angle is a
power-law function of the angle ($\propto\theta^{-k}$). We calculate light
curves and spectra for GRB afterglows when such jets expand either in the
interstellar medium or in the wind medium. In particular, we take into account
two kinds of wind: one ($n\propto r^{-3/2}$) possibly from a typical red
supergiant star and another ($n\propto r^{-2}$) possibly from a Wolf-Rayet
star. We find that in each type of medium, one break appears in the late-time
afterglow light curve for small $k$ but becomes weaker and smoother as $k$
increases. When $k\ge 2$, the break seems to disappear but the afterglow decays
rapidly. Thus, one expects that the emission from expanding, highly anisotropic
jets provides a plausible explanation for some rapidly fading afteglows whose
light curves have no break. We also present good fits to the optical afterglow
light curve of GRB 991208. Finally, we argue that this burst might arise from a
highly anisotropic jet expanding in the wind ($n\propto r^{-3/2}$) from a red
supergiant to interpret the observed radio-to-optical-band afterglow data
(spectrum and light curve).


title:
The eclipsing binary millisecond pulsar PSR B1744-24A - possible test
  for a magnetic braking mechanism

abstract:
  As presented by Nice et al. (2000), long-term timing of the eclipsing binary
PSR B1744-24A shows that the orbital period of this system decreases with a
time-scale of only ~ 200 Myr. To explain the much faster orbital period decay
than that predicted by only emission of the gravitational waves ~ 1000 Myr) we
propose that the orbital evolution of this system is also driven by magnetic
braking . If magnetic braking is to explain the rapid decay of the orbit, then
\lambda characterizing the effectiveness of the dynamo action in the stellar
convection zone in the magnetic stellar wind formula must be equal to 1.


title: UBVRI photometry of Southern Sky BL Lacs

abstract:
  Seventeen southern sky BL Lacs were observed in UBVRI using the CCD Camera on
the 1.0m telescope at the South African Astronomical Observatory (SAAO) in Aug
and Nov 1999. The analyses of all the seventeen sources are now complete, and
are available via anonymous ftp (ftp pukrs1.puk.ac.za/pub/Blazars). A few
examples of our results are however given in this paper. Whereas PKS 2005-489
and PKS 2155-304 apear to have been in a high state, PKS 0048-097 and PKS
0521-365 showed evidence of variability on a time-scale of a few days, with the
amplitude of variability increasing towards short wavelengths. This is
consistent with observations of gamma-ray BL Lacs, which show similar behaviour
in optical and X-rays.


title: Observations of Mkn 421 with the CELESTE Experiment

abstract:
  The CELESTE experiment uses the heliostats of an old solar farm in the French
Pyrenees to detect gamma ray air showers by the atmospheric Cerenkov technique.
Observations of the TeV blazar Markarian 421 have been made with the fully
instrumented CELESTE experiment since December 1999. The detection of gamma ray
emission from this source at energies greater than 50 GeV is presented here. A
comparison is made with the light curve from the CAT experiment at the same
site which shows correlation between the observed gamma ray fluxes and the
detection of short duration flaring episodes.


title: Status and Current Sensitivity of the CELESTE Experiment

abstract:
  The CELESTE experiment uses the heliostats of an old solar farm in the French
Pyrenees to detect gamma ray air showers by the atmospheric Cerenkov technique.
CELESTE has been operating since November 1999 with an array of 40 heliostats
fully instrumented with 1GHz flash ADCs.
  Significant advances have been made in the detector simulations and in the
data analysis techniques.
  We report here on results from recent observations of the Crab nebula above
an energy threshold of 50GeV.
  The results and simulations illustrate the current sensitivity of the
experiment.


title: The low surface brightness members of nearby groups

abstract:
  We are undertaking a large, wide-field survey of nearby groups and poor
clusters. The main goals are identifying and characterizing the properties of
low-surface-brightness dwarf galaxies and determining the galaxy luminosity
function for M$_R > -17$. Large areas (typically 0.3--0.5 degree$^2$ per system
but up to 7 degree$^2$) of the groups Telescopium, Leo I, Dorado, N5575, HCG
42, HCG 44, HCG 68 and the poor clusters IC 4765 and Hydra have so far been
surveyed in V and I. We present the preliminary results for the photometric
study of the groups HCG 42, Telescopium (or NGC6868) and IC4765. Hundreds of
new low surface brightness galaxies are catalogued. Their spatial
distributions, colors, types and sizes will be studied as a function of the
richness of their environments.


title: The thermal and non-thermal gaseous halo of NGC5775

abstract:
  In this letter we present first results from spectroscopic observations of
Diffuse Ionized Gas (DIG) in the halo of NGC5775 obtained with FORS1 attached
to UT1/Antu of the Very Large Telescope (VLT). At our slit position
perpendicular to the disk (41" SE of the nucleus) the emission of [NII]6583,
[OIII]5007, and [OII]3727 is detected out to 9 kpc into the halo, allowing
possible ionization mechanisms of the DIG to be examined. Photoionization
models which assume a dilute radiation field are able to fit the data for the
disk, but they cannot account for the line ratios measured in the halo (e.g.,
[OI]/Halpha or [OII]/Halpha). In particular they fail to predict the observed
increase of [OIII]5007/Halpha with increasing z. The most striking result
concerns the kinematics of the halo gas. Velocities at high galactic latitudes
drop from the midplane value to reach the systemic velocity at z ~ 9 kpc. An
analysis of VLA archive data of the polarized radio-continuum emission at 4.86
GHz and 1.49 GHz reveals that magnetic fields in the halo have a strong
component perpendicular to the disk and are aligned with the Halpha and
radio-continuum spurs in the halo. This can result either from a strong wind
action or, more likely, from the generation of dipolar magnetic fields. We
briefly discuss the interrelation of the magnetic field structure and gas
dynamics, in particular the role of magnetic fields in gas outflows, as well as
the possible heating of DIG by magnetic reconnection.


title:
Monte-Carlo simulations of thermal/nonthermal radiation from a
  neutron-star magnetospheric accretion shell

abstract:
  We discuss the space-and-time-dependent Monte Carlo code we have developed to
simulate the relativistic radiation output from compact astrophysical objects,
coupled to a Fokker-Planck code to determine the self-consistent lepton
populations. We have applied this code to model the emission from a magnetized
neutron star accretion shell near the Alfven radius, reprocessing the radiation
from the neutron sar surface. We explore the parameter space defined by the
accretion rate, stellar surface field and the level of wave turbulence in the
shell. Our results are relevant to the emission from atoll sources, soft-X-ray
transient X-ray binaries containing weakly magnetized neutron stars, and to
recently suggested models of accretion-powered emission from anomalous X-ray
pulsars.


title:
Low and intermediate-mass close binary evolution and the initial - final
  mass relation

abstract:
  Using Eggleton's stellar evolution code, we carry out 150 runs of Pop I
binary evolution calculations, with the initial primary mass between 1 and 8
solar masses the initial mass ratio between 1.1 and 4, and the onset of Roche
lobe overflow (RLOF) at an early, middle, or late Hertzsprung-gap stage. We
assume that RLOF is conservative in the calculations, and find that the remnant
mass of the primary may change by more than 40 per cent over the range of
initial mass ratio or orbital period, for a given primary mass. This is
contrary to the often-held belief that the remnant mass depends only on the
progenitor mass if mass transfer begins in the Hertzsprung gap. We fit a
formula, with an error less than 3.6 per cent, for the remnant (white dwarf)
mass as a function of the initial mass of the primary, the initial mass ratio,
and the radius of the primary at the onset of RLOF. We also find that a
carbon-oxygen white dwarf with mass as low as 0.33 solar masses may be formed
if the primary's initial mass is around 2.5 solar masses.


title: Probing the End of the Dark Ages

abstract:
  In currently popular cosmological scenarios -- all variants of the cold dark
matter (CDM) cosmogony -- some time beyond a redshift of 15, stars within the
numerous small halos that condense with virial temperatures ~1e4 K created the
first heavy elements; these protogalactic systems, together perhaps with an
early population of mini-quasars, generated the ultraviolet radiation and
mechanical energy that reheated and reionized the cosmos. The history of the
Universe during and soon after these crucial formative stages is recorded in
the thermal, ionization, and chemical state of the all-pervading intergalactic
medium (IGM), which contains most of the ordinary baryonic material left over
from the big bang. Throughout the epoch of structure formation, the IGM becomes
clumpy under the influence of gravity, and acts as a source for the gas that
gets accreted, cools, and forms stars within subgalactic fragments, and as a
sink for the metal enriched material, energy, and radiation which they eject.


title:
The Diffuse Interstellar Band at 8620 Ang: a good reddening tracer for
  GAIA

abstract:
  We report about a suprisingly good correlation between the equivalent width
of the diffuse interstellar band (DIB) at 8620 Ang and the interstellar
reddening. Such a correlation offers bright prospects in using the 8620 Ang DIB
as a tracer of the exctinction through the Galaxy in the context of GAIA,
approved Cornerstone mission by ESA.


title:
Discovery of short-period binary millisecond pulsars in four globular
  clusters

abstract:
  We report the discovery using the Parkes radio telescope of binary
millisecond pulsars in four clusters for which no associated pulsars were
previously known. The four pulsars have pulse periods lying between 3 and 6 ms.
All are in circular orbits with low-mass companions and have orbital periods of
a few days or less. One is in a 1.7-hour orbit with a companion of planetary
mass. Another is eclipsed by a wind from its companion for 40% of the binary
period despite being in a relatively wide orbit. These discoveries result from
the use of improved technologies and prove that many millisecond pulsars remain
to be found in globular clusters.


title: Rotation and lithium in single giant stars

abstract:
  In the present work, we study the link between rotation and lithium abundance
in giant stars of luminosity class III, on the basis of a large sample of
single stars of spectral type F, G and K. We shows that giant stars presenting
the highest lithium contents, typically stars earlier than G0III, are those
with the highest rotation rates, pointing for a dependence of lithium content
on rotation, as observed for other luminosity classes. Giant stars later than
G0III present, as a rule, the lowest rotation rates and lithium contents. A
large spread of about five magnitudes in lithium abundance is observed for the
slow rotators. Finally, single giant stars with masses 1.5 < M/Msun <= 2.5 show
a clearest trend for a correlation between rotational velocity and lithium
abundance.


title: Evidence for Rapid Iron K_alpha Line Flux Variability in MCG--6-30-15

abstract:
  This paper employs direct spectral fitting of individual orbital data in
order to measure rapid X-ray iron K_alpha line and continuum spectral slope
variations in Seyfert 1 galaxies with unprecedented temporal resolution.
Application of this technique to a long RXTE observation of MCG--6-30-15
indicates that the line flux does vary on short (~1d) timescales, but that
these variations are not correlated with changes in the continuum flux or
slope. These rapid variations indicate that the line does indeed originate
close to the black hole, confirming predictions based on its very broad
profile. However, the lack of a correlation with the continuum presents
problems for models in which the line variations are driven by those in the
continuum, modified only by light-travel time effects. Instead, it may be that
the line responds according to a physical process with a different time scale,
such as ionization instabilities in the disk, or perhaps that the geometry and
physical picture is more complex than implied by the simplest disk-corona
models.
  These data also indicate that the slope of the underlying power-law continuum
(Gamma) shows strong variability and is tightly correlated with the continuum
flux in the sense that the spectrum steepens as the source brightens. All of
these results have been checked with extensive simulations, which also
indicated that a spurious correlation between Gamma and Compton reflection
fraction (R) will result if these quantities are measured from the same
spectra. This casts serious doubts on previous claims of such a Gamma-R
correlation.


title:
Kinematics, ages, and metallicities of star clusters in NGC 1316: A 3
  Gyr old merger remnant

abstract:
  We report on multi-object spectroscopy of 37 candidate star clusters around
the giant early-type radio galaxy NGC 1316 (Fornax A), the brightest galaxy in
the Fornax cluster. For the star clusters in this sample, we measure a mean
radial velocity of 1698 +/- 46 km/s, a rotation velocity of 175 +/- 70 km/s,
and a velocity dispersion of 227 +/- 33 km/s within a galactocentric radius of
24 kpc. Using the projected mass estimator and assuming isotropic orbits, the
estimated total mass is (6.6 +/- 1.7) E+11 solar masses within a radius of 24
kpc. The mass is uncertain by about a factor of two, depending on the orbital
assumptions. The implied M/L_B ratio is in the range 3-6. Four star clusters in
our sample are exceptionally luminous (M_V < -12.3). This means that (1) at
least this many clusters in NGC 1316 are up to an order of magnitude more
luminous than the most luminous star cluster in our Galaxy or M31, and (2) that
the S/N ratio of their spectra allows us to measure line strengths with good
accuracy. By comparing the colours and equivalent widths of H-alpha and the Ca
II triplet absorption lines for those bright star clusters in our sample with
those of single-burst population models, we find that they are coeval with an
age of 3.0 +/- 0.5 Gyr. Their metallicities are found to be solar to within
0.15 dex. These results show once again that globular clusters with near-solar
metallicity do form during galactic mergers, and, moreover, that they can
survive disruption processes taking place during the merger (e.g., dynamical
friction, tidal disruption), as well as evaporation. In this respect, NGC 1316
provides a hitherto "missing" evolutionary link between young merger remnants
of age ~0.5 Gyr such as NGC 3597, NGC 3921 and NGC 7252 on one side, and older
giant ellipticals featuring bimodal colour distributions on the other side.


title:
Irregular Magnetic Fields in Interstellar Clouds and Variations in the
  Observed Circular Polarization of Spectral Lines

abstract:
  The strengths of magnetic fields in interstellar gas clouds are obtained
through observations of the circular polarization of spectral line radiation.
Irregularities in this magnetic field may be present due to turbulence, waves
or perhaps other causes, and may play an essential role in the structure and
evolution of the gas clouds. To infer information about these irregularities
from the observational data, we develop statistical relationships between the
rms values of the irregular component of the magnetic field and spatial
variations in the circular polarization of the spectral line radiation. The
irregularities are characterized in analogy with descriptions of
turbulence---by a sum of Fourier waves having a power spectrum with a slope
similar to that of Kolmogorov turbulence. For comparison, we also perform
computations in which turbulent magnetic and velocity fields from
representative MHD simulations by others are utilized. Although the effects of
the variations about the mean value of the magnetic field along the path of a
ray tend to cancel, a significant residual effect in the polarization of the
emergent radiation remains for typical values of the relevant parameters. A map
of observed spectra of the 21 cm line toward Orion A is analyzed and the
results are compared with our calculations in order to infer the strength of
the irregular component of the magnetic field. The rms of the irregular
component is found to be comparable in magnitude to the mean magnetic field
within the cloud. Hence, the turbulent and Alfven velocities should also be
comparable.


title: Accretion and Outflow in the Circinus AGN

abstract:
  The first VLBI images of water maser emission in the Circinus Galaxy AGN show
both an accretion disk and outflow 0.1 to 1 pc from the central engine. The
maser traces a warped, edge-on accretion disk between radii of about 0.1 and
0.4 pc that is bound by a 1.3x10^6 Msun central mass. The rotation curve is
somewhat flatter than a Keplerian rotation law and is consistent with a disk
mass on the order of 10^5 Msun. Clumpy substructure may display a kinematic
signature consistent with spiral arms. Away from the disk, a second population
of water masers traces broadly a bipolar, wide-angle outflow that contains
(bullet-like) clumps ejected from a region centered on the central engine and
<0.1 pc in radius. Out to a radius of about 0.4 pc, the warp of the accretion
disk appears to channel the outflow, the orientation of which on the sky
coincides with the orientation of the known kiloparsec-scale ionization cone.
Beyond this radius, the flow crosses the disk and truncates it. This suggests
that the current accretion event and associated disk-outflow geometry have a
lifetime on the order of 10^7 years.


title: The Galaxy Populations of X-Ray Detected, Poor Groups

abstract:
  (Abridged) We determine the quantitative morphology and star formation
properties of galaxies in six nearby X-ray detected, poor groups using
multi-object spectroscopy and wide-field R imaging. We measure structural
parameters for each galaxy by fitting a PSF-convolved, two component model to
their surface brightness profiles. To compare directly the samples, we fade,
smooth, and rebin each galaxy image so that we effectively observe each galaxy
at the same redshift (9000 km/s) and physical resolution (0.87h^(-1) kpc). We
compare results for the groups to a sample of field galaxies. We find that: 1)
Galaxies spanning a wide range in morphological type and luminosity are
well-fit by a de Vaucouleurs bulge with exponential disk profile. 2)
Morphologically classifying these nearby group galaxies by their bulge fraction
(B/T) is fairly robust on average, even when their redshift has increased by up
to a factor of four and the effective resolution of the images is degraded by
up to a factor of five. 3) The fraction of bulge-dominated systems in these
groups is higher than in the field (~50% vs. ~20%). 4) The fraction of
bulge-dominated systems in groups decreases with increasing radius, similar to
the morphology-radius (~density) relation observed in galaxy clusters. 5)
Current star formation in group galaxies is correlated with significant
morphological asymmetry for disk-dominated systems (B/T<0.4). 6) The group
galaxies that are most disk-dominated (B/T<0.2) are less star forming and
asymmetric on average than their counterparts in the field.


title: The Upsilon Andromedae System: Models and Stability

abstract:
  Radial velocity observations of the F8 V star Upsilon Andromedae taken at
Lick and at Whipple Observatories have revealed evidence of three periodicities
in the line-of-sight velocity of the star. These periodicities have been
interpreted as evidence for at least three low mass companions (LMCs) revolving
around Upsilon Andromedae. The mass and orbital parameters inferred for these
companions raise questions about the dynamical stability of the system. We
report here results from our independent analysis of the published radial
velocity data as well as new unpublished data taken at Lick Observatory. Our
results confirm the finding of three periods in the data. Our best fits to the
data, on the assumption that these periods arise from the gravitational
perturbations of companions in keplerian orbits, is also generally in
agreement, but with some differences, from the earlier findings. We find that
the available data do not constrain well the orbital eccentricity of the middle
companion in a three-companion model of the data. We also find that in order
for our best-fit model to the Lick data to be dynamically stable over the
lifetime of the star (~2 billion years), the system must have a mean
inclination to the plane of the sky greater than 13 degrees. The corresponding
minimum inclination for the best fit to the Whipple data set is 19 degrees.
These values imply that the maximum mass for the outer companion can be no
greater than about 20 Jupiter masses. Our analysis of the stability of the
putative systems also places constraints on the relative inclinations of the
orbital planes of the companions. We comment on global versus local (i.e.,
method of steepest descent) means of finding best-fit orbits from radial
velocity data sets.


title:
Disk-Jet Connection in the Microquasar GRS 1915+105 and IR and Radio
  Emission

abstract:
  We present evidence of a direct accretion disk-jet connection in the Galactic
microquasar GRS 1915+105 based on our analysis of RXTE/PCA data with a
``spike'' in X-ray light curves. We find that the radio emission increases as
the hardness ratio increases during the low hard state. We suggest that the
``spike'' which separates the dips with hard and soft spectra marks the
beginning of the burst phase when the luminosity of the soft X-rays (5-15 keV)
increases by a large factor (~ 10). This produces a major ejection episode of
the synchrotron - emitting plasma termed as ``baby jets'' which are associated
with infrared (IR) and radio flares of about half an hour period widely
reported in the literature. Subsequent short but frequent soft dips produce
overlapping faint flares which result in an enhanced level of quasi-steady
emission. We discuss the dissipation of accretion energy in the inner part of
the disk in terms of a comptonising cloud and the escaping mass which produces
synchrotron radiation. We discuss the differences between ``baby jets'' and
relativistic radio jets and especially investigate their signatures in X-rays.
Our results support the relativistic jet model with quasi-continuous mass
ejection and strong variations in the jet speeds required for the formation of
strong internal shocks to continuously accelerate the particles.


title: Are CSO host-galaxies undergoing merging?

abstract:
  The galaxy-type CSO (compact symmetric objects) rate from an intermediate
large sample PR+CJ1 is found to be consistent with the Sb-Sb merger rate of
4.6% in redshift z<1. The CSO detection rate seems going down when the sample
size is increasing, which could not reach the high merger rate in z>1, yet no
deep VLBI sample in z>1 is available now.


title:
Low-ionization structures in planetary nebulae: confronting models with
  observations

abstract:
  Around 50 PNe are presently known to possess "small-scale" low-ionization
structures (LISs). We consider here jets, jet-like, symmetrical and
non-symmetrical LISs and present a detailed comparison of the existing model
predictions with the observational morphological and kinematical properties. We
find that nebulae with LISs appear indistinctly spread among all morphological
classes of PNe, indicating that the processes leading to the formation of LISs
are not necessarily related to those responsible for the asphericity of the
large-scale morphological components of PNe. We show that both the observed
velocities and locations of most non-symmetrical LISs can be reasonably well
reproduced assuming either fossil condensations originated in the AGB wind or
in-situ instabilities. The jet models proposed to date (HD and MHD interacting
winds or accretion-disk collimated winds) appear unable to account
simultaneously for the kinematical ages and the angle between the jet and the
symmetry axes of the nebulae. The linear increase in velocity observed in
several jets favors MHD confinement compared to pure HD interacting wind
models. On the other hand, we find that the formation of jet-like systems
characterized by relatively low expansion velocities cannot be explained by any
of the existing models. Finally, the knots which appear in symmetrical and
opposite pairs of low velocity could be understood as the survival of fossil
(symmetrical) condensations formed during the AGB phase or as structures that
have experienced substantial slowing down by the ambient medium.


title:
Gamma-ray line emission from OB associations and young open clusters: I.
  Evolutionary synthesis models

abstract:
  We have developed a new diagnostic tool for the study of gamma-ray emission
lines from radioactive isotopes (such as 26Al and 60Fe in conjunction with
other multi-wavelength observables of Galactic clusters, associations, and
alike objects. Our evolutionary synthesis models are based on the code of
Cervino & Mas-Hesse (1994), which has been updated to include recent stellar
evolution tracks, new stellar atmospheres for OB and WR stars, and
nucleosynthetic yields from massive stars during hydrostatic burning phases and
explosive SN II and SN Ib events. The temporal evolution of 26Al and 60Fe
production, the equivalent yield of 26Al per ionising O7 V star (yal), and
other observables are predicted for a coeval population. We also employed our
model to estimate the steady state nucleosynthesis of a population of solar
metallicity. To allow for a fully quantitative analysis of existing and future
multi-wavelength observations, we propose a Bayesian approach that allows the
inclusion of IMF richness effects and observational uncertainties in the
analysis. In particular, a Monte Carlo technique is adopted to estimate
probability distributions for all observables of interest. We outline the
procedure of exploiting these distributions by applying our model to a fictive
massive star association. Applications to existing observations of the Cygnus
and Vela regions will be discussed in companion papers.


title:
Short-timescale Fluctuations in the Difference Light Curves of QSO
  0957+561A,B: Microlensing or Noise?

abstract:
  From optical R band data of the double quasar QSO 0957+561A,B, we made two
new difference light curves (about 330 days of overlap between the time-shifted
light curve for the A image and the magnitude-shifted light curve for the B
image). We observed noisy behaviours around the zero line and no
short-timescale events (with a duration of months), where the term event refers
to a prominent feature that may be due to microlensing or another source of
variability. Only one event lasting two weeks and rising - 33 mmag was found.
Measured constraints on the possible microlensing variability can be used to
obtain information on the granularity of the dark matter in the main lensing
galaxy and the size of the source. In addition, one can also test the ability
of the observational noise to cause the rms averages and the local features of
the difference signals. We focused on this last issue. The combined
photometries were related to a process consisting of an intrinsic signal plus a
Gaussian observational noise. The intrinsic signal has been assumed to be
either a smooth function (polynomial) or a smooth function plus a stationary
noise process or a correlated stationary process. Using these three pictures
without microlensing, we derived some models totally consistent with the
observations. We finally discussed the sensitivity of our telescope (at Teide
Observatory) to several classes of microlensing variability.


title: Dynamics of Fe-Ni Bubbles in Young Supernova Remnants

abstract:
  Observations of core-collapse supernovae (SNe) have revealed the presence of
extensive mixing of radioactive material in SN ejecta. The mixing of
radioactive material, mostly freshly synthesized Ni, is not complete, which
leads to a two-phase SN ejecta structure. The low-density phase consists of Fe
bubbles, created by the energy input from radioactive Co and Ni, surrounded by
compressed high-density metal-rich ejecta. We report on the theoretical
investigation of supernova remnant (SNR) dynamics with the two-phase SN ejecta.
We first present 3-dimensional hydrodynamic simulations of a single Fe bubble
immersed in an outer ejecta envelope, and compare the results with previous
work on shock-cloud interactions. We then consider randomly distributed Fe
bubbles with an average volume filling fraction of 1/2. We find that the
presence of Fe bubbles leads to vigorous turbulence and mixing of Fe with other
heavy elements and with the ambient normal-abundance gas. The turbulent energy
can be an order of magnitude larger than in the case of smooth ejecta. A
significant fraction of the shocked ejecta is found in narrow filaments and
clumps moving with radial velocities larger than the velocity of the forward
shock. Observational consequences of the two-phase ejecta on SNR X-ray spectra
and images are briefly mentioned.


title: On the Nonspherical Nature of Halo Formation

abstract:
  We present a new collapse condition to describe the formation of dark halos
via nonspherical gravitational clustering. This new nonspherical collapse
condition is obtained by the logical generalization of the spherical model to
the nonspherical one. By solving a diffusion-like random matrix equation with
the help of the Monte Carlo method, we show that this nonspherical collapse
condition yields the mass function derived by Sheth & Tormen (1999) which has
been shown to be in excellent agreement with the recent N-body results of high
resolution. We expect that this nonspherical collapse condition might provide
us a deeper insight into the structure formation, and suggest that it should be
widely applied to various cosmological issues such as the galaxy merging
history, the galaxy bias, and so forth.


title:
Two-point correlation functions on the light cone: testing theoretical
  predictions against N-body simulations

abstract:
  We examine the light-cone effect on the two-point correlation functions using
numerical simulations for the first time. Specifically, we generate several
sets of dark matter particle distributions on the light-cone up to z=0.4 and
z=2 over the field-of-view of \pi degree^2 from cosmological N-body
simulations. Then we apply the selection function to the dark matter
distribution according to the galaxy and QSO luminosity functions. Finally we
compute the two-point correlation functions on the light-cone both in real and
in redshift spaces using the pair-count estimator and compare with the
theoretical predictions. We find that the previous theoretical modeling for
nonlinear gravitational evolution, linear and nonlinear redshift-distortion,
and the light-cone effect including the selection function is in good agreement
with our numerical results, and thus is an accurate and reliable description of
the clustering in the universe on the light-cone.


title: Wind Interaction Models for the Afterglows of GRB 991208 and GRB 000301C

abstract:
  The simplest model of the afterglows of the gamma-ray bursts (GRBs) envisions
a spherical blast wave with a power-law distribution of electron energy above
some cutoff running into a constant density medium. A refinement involves a
narrow jet, often invoked to explain the steep decline and/or steepening of
light curves observed in some afterglows. The constant (ambient) density jet
model has been applied to GRBs 991208 and 000301C, based to a large extent on
radio observations. We show that, for these two sources, a spherical wind model
(with an $r^{-2}$ density ambient medium) can fit the radio data as well as the
jet model. %%We demonstrate that the The relatively steep decline and the
fairly abrupt steepening of the R-band light curves of, respectively, GRB
991208 and GRB 000301C can be accounted for with a non-standard, broken
power-law distribution of electron energy. Our model predicts a slower late
decline for the radio flux than does the jet model.


title: Hadronic Correlated Flares from Mrk 501

abstract:
  We discuss the possibility of explaining the extraordinary, correlated
X-ray/TeV flares observed during April 1997 from Mrk 501, by synchrotron-pair
cascades injected by synchrotron radiation from ultra-high energy protons and
muons. Evaluating the jet conditions required to explain the observed features
of the flares, the allowed region for this model in the parameter space of jet
magnetic field and Doppler factor is identified, and compared with the
parameter choices of other, both hadronic and leptonic models presented in the
literature. The present model requires magnetic fields similar to other
hadronic models for gamma-ray blazars (B >~ 30 G), but a lower Doppler factor
(D =~ 3).


title: Molecular QSO Absorption Line Systems

abstract:
  We review observations of molecular absorption line systems at high redshift
toward red quasars and gravitational lenses.


title:
Study of the Long-Term X-Ray Variability of a Possible Quasar RX
  J0957.9+6903 with ASCA

abstract:
  Long-term variability and spectral properties of a possible quasar, RX
J0957.9+6903, were studied, utilizing 16 ASCA observations spanning 5.5 years.
The average 0.7--10 keV spectrum of RX J0957.9+6903 is well represented by a
power-law continuum of photon index 1.58 $\pm$ 0.03, and an absorption column
of $\sim$ 1$\times{\rm 10}^{21}$ ${\rm cm}^{-2}$. The 2--10 keV flux of RX
J0957.9+6903 varied by a factor of four over the period of six years, around a
mean of $\sim$ 8.8$\times {\rm 10}^{-12}$ erg ${\rm s}^{-1}$ ${\rm cm}^{-2}$.
Peak to peak variability within each observation was less than 25% on $\sim$ 1
day time scale. These properties support the classification of RX J0957.9+6903
as a quasar. The power spectrum density (PSD) was estimated in a "forward"
manner, over a frequency range of $10^{-8.2}$--$10^{-4.3}$ Hz, by utilizing the
structure function method and Monte-Carlo simulation assuming a broken
power-law type PSD. Then, the break frequency $f_{\rm b}$ of the PSD of RX
J0957.9+6903 has been constrained as 1/$f_{\rm b}$ = $1600^{+\infty}_{-1100}$
days, and the logarithmic slope of the high-frequency region of the PSD as
$\alpha$ = $-$1.55 $\pm$ 0.2. A comparison of the estimated PSDs is made
between RX J0957.9+6903 and the M81 nucleus, observed in the same field of
view.


title:
Primary proton spectrum between 200 TeV and 1000 TeV observed with the
  Tibet burst detector and air shower array

abstract:
  Since 1996, a hybrid experiment consisting of the emulsion chamber and burst
detector array and the Tibet-II air-shower array has been operated at
Yangbajing (4300 m above sea level, 606 g/cm^2) in Tibet. This experiment can
detect air-shower cores, called as burst events, accompanied by air showers in
excess of about 100 TeV. We observed about 4300 burst events accompanied by air
showers during 690 days of operation and selected 820 proton-induced events
with its primary energy above 200 TeV using a neural network method. Using this
data set, we obtained the energy spectrum of primary protons in the energy
range from 200 to 1000 TeV. The differential energy spectrum obtained in this
energy region can be fitted by a power law with the index of -2.97 $\pm$ 0.06,
which is steeper than that obtained by direct measurements at lower energies.
We also obtained the energy spectrum of helium nuclei at particle energies
around 1000 TeV.


title: Searching for high-z field ellipticals: successes and problems

abstract:
  The most recent observational results on the search for high redshift field
ellipticals are reviewed in the context of galaxy formation scenarios. The
perspectives for Large Binocular Telescope (LBT) observations are also
discussed.


title:
The Optical Gravitational Lensing Experiment. Multiple Cluster
  Candidates in the Large Magellanic Cloud

abstract:
  We present 100 multiple cluster candidates selected from the OGLE catalog of
star clusters in the Large Magellanic Cloud. Statistical analysis shows that
the significant fraction of these objects may constitute physical systems.
Coeval ages of 102 components of multiple objects suggest their common origin.
53 components have very different ages. The comparison of the population of
multiple clusters candidates from the SMC and the LMC shows that: a)
distributions of sizes and ages of multiple and single clusters from the
Magellanic Clouds are very similar, b) the difference of sizes of components of
a given system is small, c) in both distributions of separation of multiple
clusters from the LMC and SMC two peaks are seen at about 9 pc and 15 pc, d)
both age distributions reveal peaks around 100 Myr, which may be connected with
the last encounter of the LMC and the SMC.


title:
Strings after D-term inflation: evolution and properties of chiral
  cosmic strings

abstract:
  We motivate the study of chiral cosmic strings through a scenario of
structure formation which mixes D-term inflation and strings. We then discuss
some properties of chiral cosmic strings, and results regarding their evolution
and possible cosmological consequences are presented.


title:
The effect of magnetic fields on the mass determination of clusters of
  galaxies

abstract:
  We test the effects of magnetic fields on the mass determination of galaxy
clusters with the X-ray method. As this method takes into account only thermal
pressure, additional non-thermal pressure support, e.g. caused by a magnetic
field, can result in an underestimation of the mass. SPH models of galaxy
clusters including magnetic fields of various strengths in two different
cosmologies are used for the test. We compare the true mass of the model
clusters with the mass determined from simulated X-ray images of the same
models. The effect of the magnetic field on the mass in relaxed clusters is
found to be negligible compared to other uncertainties, even at small radii,
where the strongest effects are expected. However in one model, in which the
cluster is in the process of merging, we find an effect of up to a factor of 2
caused by the magnetic field. In relaxed clusters the mass estimate, averaged
over many models, is at most 15% smaller in models with relatively high
magnetic field of $2\mu$G compared to models without a magnetic field with a
small dependence on the cosmology chosen. Therefore we conclude that the
presence of a magnetic field cannot be responsible for the discrepancies of a
factor of 2-3 found between X-ray and lensing mass in the central regions of
relaxed clusters.


title:
Probing the gravitational potential of a nearby lensing cluster Abell
  2104

abstract:
  The cluster Abell 2104 is one of the lowest redshift clusters (z=0.153) known
to have a gravitational lensing arc. We present detailed analysis of the
cluster properties such as the gravitational potential using the X-ray data
from ROSAT (HRI) and ASCA, as well as optical imaging and spectroscopic data
from the CFHT. The cluster is highly luminous in the X-ray with a bolometric
luminosity of Lx ~3x10^{45} ergs/s and a high gas temperature of ~10.4 keV. The
X-ray emission extending out to at least a radius of 1.46 Mpc, displays
significant substructure. The total mass deduced from the X-ray data under the
assumption of hydrostatic equilibrium and isothermal gas, is found to be
M_tot(r<1.46Mpc) ~(8.0 +/- 0.8)x10^{14} solar mass. The gas fraction within a
radius of 1.46 Mpc is ~5-10%. The cluster galaxy velocity distribution has a
dispersion of 1200 +/- 200 km/s with no obvious evidence for substructure. The
total mass within 1.46 Mpc, deduced from Jean's equation using the observed
galaxy number density distribution and velocity dispersion, is found to be
\~6.8x10^{14} solar mass to ~2.6x10^{15} solar mass marginally consistent with
the X-ray deduced total mass.


title: LMC and SMC Cepheids: combining OGLE with DENIS 2MASS infrared data

abstract:
  (Abridged) I cross-correlate the OGLE database of Cepheids in the Large and
Small Magellanic Clouds (MCs) with the second incremental release of the 2MASS
JHK survey, and the DENIS IJK point source catalog towards the MCs.
  Of the 3384 Cepheids in OGLE in both Clouds, 1745 have a counterpart in the
2MASS survey within an 1" radius and good-photometry in all three bands. Only
173 have a counterpart in the DENIS survey within an 1" radius and
good-photometry in all three bands. The reason for this difference is that the
limiting magnitudes of the 2MASS survey are considerably fainter than for
DENIS.
  To describe to first order the geometrical configuration of the MCs a plane
is fitted to the reddening-free Wesenheit (W) index. For the LMC an inclination
angle of 18 $\pm$ 3\degr is derived, for the SMC a value of $i = 68 \pm 2$\degr
is derived. These results are being used to take out the effect that some
Cepheids are closer to the observer than others.
  Using single-epoch 2MASS data for the OGLE Cepheids, PL-relations are fitted
in JHK for both Fundamental mode (FU) and First-overtone (FO) pulsators, taking
out the effect of the inclined disk.
  The derived PL-relations are used to derive a relative distance modulus of
0.50 \pm 0.02 between SMC and LMC assuming no metallicity correction.
  Combining the zero points of the Galactic and MCs PL-relations distance
moduli of 18.60 $\pm$ 0.11 (based on W) and 18.55 $\pm$ 0.17 (based on K) to
the LMC, and of 19.11 $\pm$ 0.11 (W) and 19.04 $\pm$ 0.17 (K) to the SMC are
derived (without taking into account possible metallicity corrections).


title: Cool helium-rich white dwarfs from the Hamburg/ESO Survey

abstract:
  We present an analysis of 40 cool helium-rich white dwarfs found in the
Hamburg/ESO survey. They were selected for follow-up spectroscopy because of
their U-B colour below -0.18, the absence of strong absorption lines, and a
continuum shape similar to that of a quasar. Effective temperatures for
individual stars were determined by fitting model atmospheres of nearly pure
helium with a small admixture of hydrogen. As a consequence of the selection
criteria all but one sample stars have Teff below 20000 K. Four stars clearly
show helium and hydrogen lines in their spectra. In the spectra of another
three, helium, hydrogen, and metal lines can be detected. For these stars
hydrogen and metal abundances were also determined by fitting appropriate model
atmospheres containing these elements. Seven sample stars most likely have
helium-rich atmospheres but do not show any helium lines. They either have
featureless spectra or show calcium lines.


title: NLTE effects on oxygen lines

abstract:
  The NLTE effects affecting oxygen-abundance determinations of solar-type
stars are discussed. LTE is perfectly safe for the forbidden lines. The
permitted triplet at 777 nm is expected to show NLTE effects on the order of a
few tenths of a dex (always in the sense that LTE overestimates the abundance),
but the magnitude of the effects is dependent on the still very uncertain cross
sections of collisional excitation by collisions with neutral hydrogen atoms.
Little is known about the NLTE effects on molecular line formation.


title: Model Spectra of Rotation Powered Pulsars in the INTEGRAL Range

abstract:
  The energy range of IBIS is a promising ground for testing mutual relations
of distinct components expected in the spectra of high-energy radiation from
rotation powered pulsars. According to some polar-cap models two such
components - due to curvature and synchrotron emission - may contribute
comparable amounts of power between 15 keV and 10 MeV (Rudak & Dyks 1999).
Zhang & Harding (2000) argued recently for the inclusion of a third possible
component, due to inverse Compton scattering (ICS) of soft thermal photons on
secondary $\epm$-pairs. Here we present the results of Monte Carlo calculations
of all three spectral components within a polar-cap model which allows for
interactions of relativistic particles with the soft photons coming from the
pulsar surface. For teragauss pulsars with the surface temperature of a few
times 10^5 K the ICS component dominates the spectrum in the energy range below
10 MeV, and thus its presence increases the ratio of X-ray to gamma-ray
luminosity (in comparison to the models ignoring the ICS on secondary
$\epm$-pairs) to a level observed in the Vela pulsar.


title: Evolution of the Internal Dynamics of Galaxy Clusters

abstract:
  We consider a sample of 51 distant galaxy clusters at 0.15<z<0.9 (<z> about
0.3), each cluster having at least 10 galaxies with available redshift in the
literature. We select member galaxies, analyze the velocity dispersion
profiles, and evaluate in a homogeneous way cluster velocity dispersions and
virial masses. We apply the same procedures already recently applied on a
sample of nearby clusters (z<0.15, Girardi et al. 1998b) in order to properly
analyze the possible dynamical evolution of galaxy clusters. We remark problems
induced by the poor sampling and the small spatial extension of the sampled
cluster region in the computation of velocity dispersion. We do not find any
significant difference between nearby and distant clusters. In particular, we
consider the galaxy spatial distribution, the shape of the velocity dispersion
profile, and the relations between velocity dispersion and X-ray luminosity and
temperature. Our results imply little dynamical evolution in the range of
redshift spanned by our cluster sample, and suggest that the typical redshift
of cluster formation is higher than that of the sample we analyze.


title:
Image-Subtraction Photometry of Variable Stars in the Field of the
  Globular Cluster NGC 6934

abstract:
  (Abridged) We present CCD BVI photometry of 85 variable stars from the field
of the globular cluster NGC 6934. The photometry was obtained with the image
subtraction package ISIS. 35 variables are new identifications: 24 RRab stars,
5 RRc stars, 2 eclipsing binaries of W UMa-type, one SX Phe star, and 3
variables of other types. Both detected contact binaries are foreground stars.
The SX Phe variable belongs most likely to the group of cluster blue
stragglers. Large number of newly found RR Lyr variables in this cluster, as
well as in other clusters recently observed by us, indicates that total RR Lyr
population identified up to date in nearby galactic globular clusters is
significantly (>30%) incomplete.
  Fourier decomposition of the light curves of RRc variables was used to
estimate the basic properties of these stars: M=0.63 M_odot,
log(L/L_odot)=1.72, T_eff=7300 and Y=0.27. Using RRab variables we obtain:
M_V=0.81, [Fe/H]=-1.53 and T_eff=6450. From the B-V color at minimum light of
the RRab variables we obtained E(B-V)=0.09+-0.01. Different calibrations of
absolute magnitudes of RRab and RRc available in literature were used to
estimate apparent distance modulus of the cluster: (m-M)_V= 16.09+-0.06. We
note a likely error in the zero point of the HST-based V-band photometry of NGC
6934 recently presented by Piotto et al. Among analyzed sample of RR Lyr stars
we have detected a short period and low amplitude variable which possibly
belongs to the group of second overtone pulsators (RRe subtype variables).
  The BVI photometry of all variables is available electronically via anonymous
ftp. The complete set of the CCD frames is available upon request.


title: Scientific impact of large telescopes

abstract:
  The scientific impacts of telescopes worldwide have been compared on the
basis of their contributions to (a) the 1000 most-cited astronomy papers
published 1991-8 (125 from each year), and (b) the 452 astronomy papers
published in Nature 1989-98. 1-m and 2-m ground-based telescopes account for ~
5% of the citations to the top-cited papers, 4-m telescopes 10%, Keck I/II 4%,
sub-mm and radio telescopes 4%, HST 8%, other space telescopes 23%. The
remaining citations are mainly to theoretical and review papers. The strong
showing by 1-m and 2-m telescopes in the 1990s augurs well for the continued
scientific impact of 4-m telescopes in the era of 8-m telescopes. The impact of
individual ground-based optical telescopes is proportional to collecting area
(and approximately proportional to capital cost). The impacts of the various
4-m telescopes are similar, with CFHT leading in citation counts, and WHT in
Nature papers. HST has about 15 times the citation impact of a 4-m ground-based
telescope, but cost > 100 times as much. Citation counts are proportional to
counts of papers published in Nature, but for radio telescopes the ratio is a
factor ~ 3 smaller than for optical telescopes, highlighting the danger of
using either metric alone to compare the impacts of different types of
telescope. Breakdowns of citation counts by subject (52% extragalactic), and
journal (ApJ 44%, Nature 11%, MNRAS 9%, A&A 6%) are also presented.


title: Clustering of Galaxies and Groups in the NOG Sample

abstract:
  We use the two-point correlation function in redshift space, $\xi(s)$, to
study the clustering of the galaxies and groups of the Nearby Optical Galaxy
(NOG) Sample, which is a nearly all-sky, complete, magnitude-limited sample of
\~7000 bright and nearby optical galaxies. The correlation function of galaxies
is well-described by a power-law, $\xi(s)= (s/s_0)^{-\gamma}$, with
$\gamma\sim1.5$ and $s_0\sim 6.4 h^{-1}$ Mpc. We find evidence of morphological
segregation between early- and late-type galaxies, with a gradual decreasing of
the strength of clustering from the S0 to the late-type spirals, on
intermediate scales. Furthermore, luminous galaxies (with $M_B\leq -19.5 + 5
\log h$) are more clustered than dim galaxies. The groups show an excess of
clustering with respect to galaxies. Groups with greater velocity dispersions,
sizes, and masses are more clustered than those with lower values of these
quantities.


title: Absolute Proper Motions of Open Clusters: I. Observational data

abstract:
  Mean proper motions and parallaxes of 205 open clusters were determined from
their member stars found in the Hipparcos Catalogue. 360 clusters were searched
for possible members, excluding nearby clusters with distances D < 200 pc.
Members were selected using ground based information (photometry, radial
velocity, proper motion, distance from the cluster centre) and information
provided by Hipparcos (proper motion, parallax). Altogether 630 certain and 100
possible members were found. A comparison of the Hipparcos parallaxes with
photometric distances of open clusters shows good agreement. The Hipparcos data
confirm or reject the membership of several Cepheids in the studied clusters.


title: Parameterisation of Galaxy Spectra in the 2dF Galaxy Redshift Survey

abstract:
  The 2dF Galaxy Redshift Survey has already yielded over 130,000 galaxy
spectra (out of the planned 250,000). We present a method for the
parameterisation of galaxy spectra in the 2dF and other large redshift surveys
using a new continuous parameter eta=0.44pc1 - pc2, based on a Principal
Components Analysis (where pc1 and pc2 are the projections onto the first and
second principal components respectively). This parameter is designed to be
robust to instrumental uncertainties and it represents the absorption/emission
strength of a galaxy.


title: Early observations of the Afterglow of GRB000301c

abstract:
  We report multiband observations of the Optical Transient (OT) associated
with GRB000301c carried out between 2--4 March 2000 using the 2.34-m Vainu
Bappu Telescope (VBT) at Kavalur, India. When combined with other reported
data, the initial decline in the R-band magnitude with log ($t-t_0$), the time
since the burst is fit with a slope $\alpha_{1}$ = -0.70 $\pm$ 0.07 which
steepens after about 6.0 days to a slope of $\alpha_{2}$ = -2.44 $\pm$ 0.29.
This change in slope does not occur smoothly but there is an indication for a
bimodal distribution. The available measurements of the evolution of (B--R)
color do not show any discernible evolution in the first 12 days.


title: Constraints on Omega_B, Omega_m and h from MAXIMA and BOOMERANG

abstract:
  We analyse the BOOMERANG and MAXIMA results in the context of models with
$\Omega_{\rm Total} = 1$ and $n_s = 1$. We attempt to constrain three other
parameters---$h$, $\Omega_B$, and $\Omega_m$---from these observations. We show
that: (a) the value of $\Omega_B h^2$ is too high to be compatible with
primordial nucleosynthesis observations at 95% confidence level (b) universe
with age greater than $ 12 \rm Gyr$ is ruled out at 95% confidence level and
(c) the value of $\Omega_m h$ is too high to be compatible with the shape of
the power spectrum of gravitational clustering. In effect, our analysis shows
that models with $\Omega_{\rm Total} = 1$ and $n_s = 1$ are ruled out by
BOOMERANG and MAXIMA observations.


title: X-ray Line Emission from the Hot Stellar Wind of theta 1 Ori C

abstract:
  We present a first emission line analysis of a high resolution X-ray spectrum
of the stellar wind of theta 1 Ori C obtained with the High Energy Transmission
grating Spectrometer onboard the Chandra X-ray Observatory. The spectra are
resolved into a large number of emission lines from H- and He-like O, Ne, Mg,
Si, S, Ar and Fe ions. The He-like Fe XXV and Li-like Fe XXIV appear quite
strong indicating very hot emitting regions. From H/He flux ratios, as well as
from Fe He/Li emission measure ratios we deduce temperatures ranging from 0.5
to 6.1 x 10^7 K. The He-triplets are very sensitive to density as well. At
these temperatures the relative strengths of the intercombination and forbidden
lines indicate electron densities well above 10^12 cm^-3. The lines appear
significantly broadened from which we deduce a mean velocity of 770 km/s with a
spread between 400 and 2000 km/s. Along with results of the deduced emission
measure we conclude that the X-ray emission could originate in dense and hot
regions with a characteristic size of less then 4 x 10^10 cm.


title:
General Relativistic Collapse of Neutron Stars to Strange Stars: A
  Mechanism for Gamma Ray Bursts

abstract:
  It is known that Neutron Stars may be converted into more compact Strange
Stars (SS) on capture/formation of a ``seed'' of strange matter. It is also
known that the binding energy of the nascent hot SS is likely to be radiated as
$\nu -\bar\nu$s so that an electromagnetic pair fireball (FB) may be created by
neutrino annihilation. And already this process has been considered as a
probable source of Gamma Ray Bursts (GRB) by several authors, the most notable
of which is due to Cheng & Dai (PRL, 77, 1210, 1996). They mentioned that this
process may generate a FB luminosity (outside the neutrinosphere) of 10**(49)
erg. But we point out here that a SS is a relativistic object with surface
gravitation z=0.5. Accordingly, we present here a Relativistic Model of this
process, and show that, for a massive SS with M=2 solar mass, one may explain a
GRB energy of ~ 10**(53) erg., which is 4 orders higher than previous
non-relativistic estimate. Further since the baryonic mass of the thin crust of
a strange star is negligible, this FB will generate an extremely relativistic
blast wave. Thus this process may be one of the viable routes for the genesis
of hitherto unexplained cosmic Gamma Ray Bursts. While some of the GRBs might
be beamed, many of them do not show any long term spectral break in their
afterglows and are likely to be quasi-spherical. A particular example is GRB
971214 for which estimated GRB energy is 3. 10**(53) erg. Our model may be
useful for understanding the genesis of such powerful and unexplained bursts.


title:
The Peak Brightness and Spatial Distribution of AGB Stars Near the
  Nucleus of M32

abstract:
  The bright stellar content near the center of the Local Group elliptical
galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with
the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving
near the tip of the asymptotic giant branch (AGB), are resolved to within 2
arcsec of the nucleus, and it is concluded that the peak stellar brightness
near the center of M32 is similar to that in the outer regions of the galaxy.
Moreover, the projected density of bright AGB stars follows the visible light
profile to within 2 arcsec of the nucleus, indicating that the brightest stars
are well mixed throughout the galaxy. Thus, there is no evidence for an age
gradient, and the radial variations in spectroscopic indices and ultraviolet
colors that have been detected previously must be due to metallicity and/or
some other parameter. We suggest that either the bright AGB stars formed as
part of a highly uniform and coherent galaxy-wide episode of star formation, or
they originated in a separate system that merged with M32.


title: STIS observations of five hot white dwarfs

abstract:
  We present some early results from a study of five hot DA white dwarf stars,
based on spectra obtained using STIS. All show multiple components in one or
more of the strong resonance absorption lines typically associated with the
stellar photosphere (e.g. C IV, Si IV, N V and O V). Possible relationships
between the non-photospheric velocity components and the interstellar medium or
local stellar environment, are investigated, including contributions from
gravitational redshifting.


title: Chandra Observations of the X-ray Narrow-Line Region in NGC 4151

abstract:
  We present the first high-resolution X-ray spectrum of the Seyfert 1.5 galaxy
NGC 4151. Observations with the Chandra High Energy Transmission Grating
Spectrometer reveal a spectrum dominated by narrow emission lines from a
spatially resolved (1.6 kpc), highly ionized nebula. The X-ray narrow-line
region is composite, consisting of both photoionized and collisionally ionized
components. The X-ray emission lines have similar velocities, widths, and
spatial extent to the optical emission lines, showing that they arise in the
same region. The clouds in the narrow-line region must contain a large range of
ionization states in order to explain both the optical and X-ray photoionized
emission. Chandra data give the first direct evidence of X-ray line emission
from a hot plasma (T~1e7 K) which may provide pressure confinement for the
cooler (T=3e4 K) photoionized clouds.


title: Probing the Substellar Regime with SIRTF

abstract:
  One of the main scientific drivers of the Space InfraRed Telescope Facility
(SIRTF) is the search for brown dwarfs and extrasolar superplanets. We discuss
observational strategies for identification of these objects, and conclude that
an optimal strategy is a wide IRAC survey (18 deg$^2$) with a 5 $\sigma$
sensitivity of 3.9 $\mu$Jy in channel 2 (M$\sim19.1^m$). For this sensitivity,
we provide estimates of the number of low mass brown dwarfs and isolated
planets detected per square degree for power-law mass functions with
$\alpha$=1.5, and 1.0. Shallower surveys covering a larger area are inefficient
because of large overheads and detector noise. Deeper surveys covering a
smaller area become more and more affected by crowding with galaxies. A survey
like the one that we propose would determine the field mass function down to a
few Jupiter masses through the identification of a large sample of brown dwarfs
and isolated planets. The proposed SIRTF survey would also allow the first
detection of ultracool substellar objects with temperatures between 700 K and
200 K. The cooling curves of substellar objects with masses less than 20
Jupiters imply that they should spend most of their lifetimes at temperatures
below 700 K. Preliminary models indicate that their atmospheres could be
dominated by water clouds, which would diminish their optical and near-infrared
fluxes. The properties of those objects are still completely unexplored.


title: Hydrodynamic Effects on the QPO Frequencies of Accreting Compact Objects

abstract:
  The variability properties of accreting compact objects predicted by
dynamical models are characterized by a number of distinct frequencies that are
specific to each model. Because the accretion disks around neutron stars and
black holes are hydrodynamic flows, these modulation frequencies cannot be
strictly equal to test particle frequencies. I discuss several implications of
hydrodynamic corrections to the modulation frequencies predicted by dynamical
models. Finally, I show that the recent detection of yet a third kHz QPO in
three neutron-star systems favors a previously developed model that attributes
the various QPO frequencies to fundamental general relativistic frequencies in
the accretion flow.


title:
The Nuclear Stellar Cluster in the Seyfert~1 Galaxy NGC 3227: High
  Angular Resolution NIR Imaging and Spectroscopy

abstract:
  NIR high angular resolution speckle imaging and imaging spectroscopy of the
nuclear region (10'' ~ 840pc) of the Seyfert1 galaxy NGC3227 are presented. A
nuclear stellar cluster is slightly resolved in the J and H band with
increasing contribution to the NIR continuum from the K to the J band. The
stellar absorption lines are extended compared to the neighboring continuum
suggesting a cluster size of ~ 70pc FWHM. Analysis of those lines suggests that
the stars are contributing about 65% (40%) of the total continuum emission in
the H (K) band in a 3.6'' aperture. Population synthesis in conjunction with
NIR spectral synthesis indicates an age of 25 to 50 Myr when red supergiants
contribute most to the NIR light. This is supported by published optical data
on the MgIb line and the CaII triplet. Although a higher age of ~ 0.5 Gyr where
AGB stars dominate the NIR light can not be excluded, the observed parameters
are at the limit of those expected for a cluster dominated by AGB stars.
However, in either case the resolved stellar cluster contributes only about ~
15 % of the total dynamical mass in the inner 300pc implying another much older
stellar population. Pure constant star formation over the last 10 Gyr can be
excluded. Therefore, at least two star formation/starburst events took place in
the nucleus of NGC3227. Since such sequences in the nuclear star formation
history are also observed in the nuclei of other galaxies a link between the
activity of the star formation and the AGN itself seems likely.


title:
Estimation of relativistic accretion disk parameters from iron line
  emission

abstract:
  The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the emission. Here we present an analysis of the geometrical and
kinematic properties of the disk based on the extreme frequency shifts of a
line profile as determined by measurable flux in both the red and blue wings.
The edges of the line are insensitive to the distribution of the X-ray flux
over the disk, and hence provide a robust alternative to profile fitting of
disk parameters. Our approach yields new, strong bounds on the inclination
angle of the disk and the location of the emitting region. We apply our method
to interpret observational data from MCG-6-30-15 and find that the commonly
assumed inclination 30 deg for the accretion disk in MCG-6-30-15 is
inconsistent with the position of the blue edge of the line at a 3 sigma level.
A thick turbulent disk model or the presence of highly ionized iron may
reconcile the bounds on inclination from the line edges with the full line
profile fits based on simple, geometrically thin disk models. The bounds on the
innermost radius of disk emission indicate that the black hole in MCG-6-30-15
is rotating faster than 30 % of theoretical maximum. When applied to data from
NGC 4151, our method gives bounds on the inclination angle of the X-ray
emitting inner disk of 50\pm 10 deg, consistent with the presence of an
ionization cone grazing the disk as proposed by Pedlar et al. (1993). The
frequency extrema analysis also provides limits to the innermost disk radius in
another Seyfert-1 galaxy, NGC 3516.


title: Clusters of Galaxies: Diagnostic Tools for Cosmology

abstract:
  Clusters of galaxies can be used for very different kinds of cosmological
tests. I review a few of the methods: determination of cluster masses and dark
matter content, metal enrichment and its connection to the origin of the
intra-cluster gas, and the dynamical state of clusters. The progress in the
different fields expected from observations with the new X-ray satellites XMM
and Chandra is pointed out.


title:
Prompt High Energy $\gamma$-Ray Emission From the Synchrotron
  Self-Compton Process in the Reverse Shocks of $\gamma$-Ray Bursts

abstract:
  In the standard scenario of the fireball model of gamma-ray bursts(GRBs), the
huge initial energy release produces a relativistic blast wave expanding into
the external medium and a reverse shock moving into and heating the fireball
ejecta. We calculate the high energy gamma-ray emission due to inverse Compton
scattering of the synchrotron photons from relativistic electrons in the
reverse shock. Under the favorable values of the physical parameters of the
GRBs and the interstellar medium, our result shows that during the prompt
phase, this emission dominates over the component from the forward shock at
high energy gamma-ray bands. This mechanism can excellently account for the
observations of the prompt high energy gamma-rays detected by EGRET, such as
from GRB930131.


title:
Polarization and Extent of Maser Emission from Late-Type Stars: Support
  for a Plasma Turbulence Model of Maser Production

abstract:
  The integrated spectrum of OH emission from late-type stars is often
circularly polarized, by as much as 50% in some cases. While the spectra are
partially polarized, the individual maser components revealed by VLBI are much
more so. Using VLBI observations of late-type stars from the literature, we
show that the difference in circular polarization between main lines correlates
with a difference in angular extent for a given object. This is a natural
result if turbulent magnetic fields are causing the masers to be polarized via
the Cook mechanism, and might serve as a good diagnostic for determining which
objects should be investigated in the search for magnetic fields around evolved
stars.