title:
Detection of mid-infrared Aromatic Hydrocarbon Emission Features from
  the Small Magellanic Cloud

abstract:
  The mid-infrared (5-16 micron) spectral energy distribution for an individual
quiescent molecular cloud in the Small Magellanic Cloud (SMC) was observed
using ISOCAM. The spectrum is dominated by broad emission bands at 6.2, 7.7,
and 11.3 microns, with weaker bands at 8.6 and 12.7 microns. As these are the
same bands, with similar shape and relative strengths, as observed in the ISM
of our Galaxy, the same carriers must exist in both galaxies. The carriers are
widely consider to be large molecules or clusters of aromatic hydrocarbons,
which absorb ultraviolet and visible photons and emit mid-infrared photons
during high-temperature pulses. Based on the brightness of the mid-infrared
emission and the estimated strength of the radiation field in the SMC, the
absorption by aromatic hydrocarbons is of order 10% of total dust absorption,
comparable to the case for Galactic dust. Ultraviolet observations of
extinction of most SMC stars have shown that dust in the SMC does not absorb in
the 2175 Angstrom feature that is so prominent in Milky Way extinction. If
aromatic hydrocarbons and featureless extinction curves were ubiquitous in the
SMC, then we would conclude that aromatic hydrocarbons are not the carriers of
the 2175 Angstrom feature. However, SMC extinction curve measurements are
biased toward hot, luminous stars, where aromatic hydrocarbons are destroyed,
so that the absence of the 2175 Angstrom bump may not be typical of SMC dust.
The presence of aromatic hydrocarbons in the SMC further demonstrate that these
molecules exist even in an interstellar medium with an order-of-magnitude lower
metallicity than in the disk of the Milky Way.


title: Efficiencies of Low-Mass Star and Star Cluster Formation

abstract:
  Using a quantitative model for bipolar outflows driven by hydromagnetic
protostellar winds, we calculate the efficiency of star formation assuming that
available gas is either converted into stars or ejected in outflows. We
estimate the efficiency of a single star formation event in a protostellar
core, finding 25%-70% for cores with various possible degrees of flattening.
The core mass function and the stellar initial mass function have similar
slopes, because the efficiency is not sensitive to its parameters. We then
consider the disruption of gas from a dense molecular clump in which a cluster
of young stars is being born. In both cases, we present analytical formulae for
the efficiencies that compare favorably against observations and, for clusters,
against numerical simulations. We predict efficiencies in the range 30%-50% for
the regions that form clusters of low-mass stars. In our model, star formation
and gas dispersal happen concurrently. We neglect the destructive effects of
massive stars: our results are therefore upper limits to the efficiency in
regions more massive than about 3000 Msun.


title: Heavy X-ray Absorption in Soft X-ray Weak Active Galactic Nuclei

abstract:
  Recent ROSAT studies have identified a significant population of Active
Galactic Nuclei (AGN) that are notably faint in soft X-rays relative to their
optical fluxes. Are these AGN intrinsically X-ray weak or are they just highly
absorbed? Brandt, Laor & Wills have systematically examined the optical and UV
spectral properties of a well-defined sample of these soft X-ray weak (SXW) AGN
drawn from the Boroson & Green sample of all the Palomar Green AGN with z<0.5.
We present ASCA observations of three of these SXW AGN: PG 1011-040, PG
1535+547 (Mrk 486), and PG 2112+059. In general, our ASCA observations support
the intrinsic absorption scenario for explaining soft X-ray weakness; both PG
1535+547 and PG 2112+059 show significant column densities (N_H~10^22-10^23
cm^-2) of absorbing gas. Interestingly, PG 1011-040 shows no spectral evidence
for X-ray absorption. The weak X-ray emission may result from very strong
absorption of a partially covered source, or this AGN may be intrinsically
X-ray weak. PG 2112+059 is a Broad Absorption Line (BAL) QSO, and we find it to
have the highest X-ray flux known of this class. It shows a typical power-law
X-ray continuum above 3 keV; this is the first direct evidence that BAL QSOs
indeed have normal X-ray continua underlying their intrinsic absorption.
Finally, marked variability between the ROSAT and ASCA observations of PG
1535+547 and PG 2112+059 suggests that the soft X-ray weak designation may be
transient, and multi-epoch 0.1-10.0 keV X-ray observations are required to
constrain variability of the absorber and continuum.


title: A Morphological Diagnostic for Dynamical Evolution of Wolf-Rayet Bubbles

abstract:
  We have observed H-alpha and [OIII] emission from eight of the most well
defined Wolf-Rayet ring nebulae in the Galaxy. We find that in many cases the
outermost edge of the [OIII] emission leads the H-alpha emission. We suggest
that these offsets, when present, are due to the shock from the Wolf-Rayet
bubble expanding into the circumstellar envelope. Thus, the details of the WR
bubble morphology at H-alpha and [OIII] can then be used to better understand
the physical condition and evolutionary stage of the nebulae around Wolf-Rayet
stars, as well as place constraints on the nature of the stellar progenitor and
its mass loss history.


title:
New constraints from Haverah Park data on the photon and iron fluxes of
  UHE cosmic rays

abstract:
  Using data from inclined events ($60^{\circ}<\theta<80^{\circ}$) recorded by
the Haverah Park shower detector, we show that above 10^19 eV less than 30% of
the primary cosmic rays can be photons or iron nuclei at the 95% confidence
level. Above 4 10^19 eV less than 55% of the cosmic rays can be photonic at the
same confidence level. These limits place important constraints on some models
of the origin of ultra high energy cosmic rays. Details of two new events above
10^20 eV are reported.


title: Magnetic deformation of the white dwarf surface structure

abstract:
  The influence of strong, large-scale magnetic fields on the structure and
temperature distribution in white dwarf atmospheres is investigated. Magnetic
fields may provide an additional component of pressure support, thus possibly
inflating the atmosphere compared to the non-magnetic case. Since the magnetic
forces are not isotropic, atmospheric properties may significantly deviate from
spherical symmetry. In this paper the magnetohydrostatic equilibrium is
calculated numerically in the radial direction for either for small deviations
from different assumptions for the poloidal current distribution. We generally
find indication that the scale height of the magnetic white dwarf atmosphere
enlarges with magnetic field strength and/or poloidal current strength. This is
in qualitative agreement with recent spectropolarimetric observations of
Grw$+10\degr8247$. Quantitatively, we find for e.g. a mean surface poloidal
field strength of 100 MG and a toroidal field strength of 2-10 MG an increase
of scale height by a factor of 10. This is indicating that already a small
deviation from the initial force-free dipolar magnetic field may lead to
observable effects. We further propose the method of finite elements for the
solution of the two-dimensional magnetohydrostatic equilibrium including
radiation transport in the diffusive approximation. We present and discuss
preliminary solutions, again indicating on an expansion of the magnetized
atmosphere.


title: Evolutionary tracks and isochrones for alpha-enhanced stars

abstract:
  We present four large sets of evolutionary tracks for stars with initial
chemical compositions [Y=0.250, Z=0.008], [Y=0.273, Z=0.019], [Y=0.320,
Z=0.040] and [Y=0.390, Z=0.070] and enhancement of alpha elements with respect
to the solar pattern. The major improvement with respect to previous similar
calculations is that we use consistent opacities - i.e. computed with the same
chemical composition as adopted in the stellar models - over the whole relevant
range of temperatures. For the same initial chemical compositions [Y,Z] and
otherwise identical input physics we present also new evolutionary sequences
with solar-scaled mixtures of abundances. Based on these stellar models we
calculate the corresponding sets of isochrones both in the Johnson-Cousins
UBVRIJHK and HST/WFPC2 photometric systems. Furthermore, we derive integrated
magnitudes, colours and mass-to-light ratios for ideal single stellar
populations with total mass equal to 1 Msun. Finally, the major changes in the
tracks, isochrones, and integrated magnitudes and colours passing from
solar-scaled to alpha-enhanced mixtures are briefly outlined. Retrieval of the
complete data set is possible via http://pleiadi.pd.astro.it .


title: The constant density region of the dark halos of spiral galaxies

abstract:
  From the kinematics of 137 spirals we find that the DM halo density profiles
are self-similar at least out to R_opt and show core radii much larger that the
corresponding disk scale-lenghts. The luminous regions of spirals consist of
stellar disks embedded in dark halos with roughly constant density. With
respect to previous work, the present evidence is obtained by means of a robust
method and for a large and complete sample of normal spirals.


title:
Formation of the Neutron Donor C13 in AGB Stars by Overshoot and
  Rotation

abstract:
  (abridged) Observations clearly show that low-mass AGB stars can provide a
nucleosynthesis site of the s-process. Recent stellar evolution models indicate
that radiative burning of C13 between thermal pulses in low-mass AGB stars may
indeed provide the needed neutrons. Some mixing between the proton-rich
envelope and the carbon-rich core may lead to the production of C13. However,
the responsible physical mechanism is not yet unambiguously identified. We
present stellar model calculations with overshoot and rotation. Overshoot, with
a time-dependent and exponentially decaying efficiency, leads to a partial
mixture of protons and C12 during the third dredge-up. According to the
depth-dependent ratio of protons and C12, a small C13-pocket forms underneath a
N14-rich layer. Overshoot does not allow for any mixing during the interpulse
phase. Rotation introduces mixing driven by large angular velocity gradients
which form at the envelope-core interface in AGB stars, in particular after a
thermal pulse. This leads to partial mixing after a pulse, as in the case of
overshoot. However, rotation continues to mix the region of the C13-pocket with
a diffusion coefficient of log D ~ 2...3 [cm**2 / s]. This does not only spread
the C13-pocket, but also the more massive N14-rich layer, and finally leads to
mixture of both layers. By the time when the temperature there has risen to
about 9*10E7K and neutron production sets in, the N14 abundance exceeds the C13
abundance by a factor of 5...10. We analyze the role of N14 as a neutron poison
by considering the recycling of neutrons via N14(n,p)C14 and
C12(p,gamma)N13(beta+)C13 qualitatively. We find that as long as X(N14) <<
X(C12), the s-process will still be possible to occur under radiative
conditions.


title:
Space-VLBI observations of OH maser OH34.26+0.15: low interstellar
  scattering

abstract:
  We report on the first space-VLBI observations of the OH34.26+0.15 maser in
two main line OH transitions at 1665 and 1667 MHz. The observations involved
the space radiotelescope on board the Japanese satellite HALCA and an array of
ground radio telescopes. The map of the maser region and images of individual
maser spots were produced with an angular resolution of 1 milliarcsec which is
several times higher than the angular resolution available on the ground. The
maser spots were only partly resolved and a lower limit to the brightness
temperature 6x10^{12} K was obtained. The maser seems to be located in the
direction of low interstellar scattering, an order of magnitude lower than the
scattering of a nearby extragalactic source and pulsar.


title:
Multiple BeppoSAX Observations of IC 4329A to Probe the Origin of the
  Compton Reflection Component in Seyfert 1 Galaxies

abstract:
  IC 4329A is the brightest known Seyfert galaxy in hard (~2-30 keV) X-rays and
is likely to be representative of Seyfert 1 galaxies as a class. A recent 100
ks BeppoSAX observation (Perola et al. 1999) clearly confirmed the presence of
a warm absorber, a reflection component (R~0.6), and a high-energy cut-off in
the power law at Ec~270 keV. Its richness in spectral features, combined with
its large flux (~1.6 x 10**-10 erg/cm2s between 2-10 keV), make this target
ideal for multiple observations (in particular with BeppoSAX) to search for
spectral variations. Results obtained from 3 follow-up observations (40 ks
each) are presented here. The first and most important goal of this study was
to probe the origin of the Compton reflection component observed in Seyfert
galaxies by monitoring the variability of the reflection continuum and FeK line
in response to primary continuum variations. The second goal was to search for
variability in the high energy cutoff. We obtain however no conclusive results
on any of these issues. In fact, all four observations unfortunately caught the
source at almost the same flux, showing only little, and marginal, spectral
changes between different observations.


title:
Mid-infrared interferometry on spectral lines: II. Continuum (dust)
  emission around IRC+10216 and VY CMa

abstract:
  The U. C. Berkeley Infrared Spatial Interferometer has measured the
mid-infrared visibilities of the carbon star IRC+10216 and the red supergiant
VY CMa. The dust shells around these sources have been previously shown to be
time-variable, and these new data are used to probe the evolution of the dust
shells on a decade time-scale, complementing contemporaneous studies at other
wavelengths. Self-consistent, spherically-symmetric models at maximum and
minimum light both show the inner radius of the IRC+10216 dust shell to be much
larger (150 mas) than that expected from the dust condensation temperature,
implying that dust production has slowed or stopped in recent years.
Apparently, dust does not form every pulsational cycle (638 days), and these
mid-infrared results are consistent with recent near-IR imaging which indicates
little or no new dust production in the last three years (Tuthill et al 2000).
Spherically symmetric models failed to fit recent VY CMa data, implying that
emission from the inner dust shell is highly asymmetric and/or time-variable.


title: The Galactic disk: study of four low latitude Galactic Fields

abstract:
  We present deep V and I photometry of stellar field in four previously
unstudied low latitude regions of the Galactic disk. All observed fields are
located at the western side of the Galactic Center in the direction of the
Coalsack-Carina region. Two of them are suspected to cross the inner spiral
arm. More than $10^5$ stars are detected in total, down to a magnitude of V
$\sim 23.5$.
  Due to the low galactic latitude of the studied fields, the scale length and
mainly the scale height of the thick disk are not strongly constrained by the
observations. However a thin disk scale height of about
  250$\pm 60$ pc seems to be favored. The data are very sensitive to the star
formation rate of the thin disk. A decreasing star formation rate (SFR) is
necessary to reproduce the distribution of the stars in the colour-magnitude
diagrams (CMDs) as well as the luminosity functions. A constant or a strongly
increasing SFR as derived using Hipparcos data for the solar neighborhood
(Bertelli et al 1999) are marginally in agreement with the luminosity functions
but they are at odds with the CMDs. The analysis of these data suggest that the
solar neighborhood SFR cannot be considered as representative for the whole
thin disk. From the star-counts the local neighborhood mass density in stars
more massive than 0.1 M$_\odot$ is found to be 0.036--0.02 M$_\odot$ pc$^{-3}$.
Finally, the location of inner spiral arm is discussed. We find evidence of a
population younger than 10$^8$ yr distributed in a spiral arm at a distance of
1.3$\pm 0.2$ and 1.5 $\pm 0.2$\,Kpc in the directions l$\sim$\,292 and
l$\sim$\,305 respectively. This result is consistent with the spiral arm
pattern defined on the basis of pulsars and young associations.


title:
The ATESP Radio Survey I. Survey Description, Observations and Data
  Reduction

abstract:
  This paper is the first of a series reporting the results of the Australia
Telescope ESO Slice Project (ATESP) radio survey obtained at 1400 MHz with the
Australia Telescope Compact Array (ATCA) close to the South Galactic Pole (SGP)
over the region covered by the ESO Slice Project (ESP) galaxy redshift survey
(decl. ~ -40 degr). The survey consists of 16 radio mosaics with ~8"x14"
resolution and uniform sensitivity (1sigma noise level ~79 microJy) over an
area of ~26 sq.degrees. Here we present the design of the survey, we describe
the mosaic observing technique which was used to obtain an optimal combination
of uniform and high sensitivity over the whole area, and the data reduction:
the problems encountered and the solutions adopted.


title:
Detection and Measurement of Parsec-Scale Circular Polarization in Four
  AGN

abstract:
  We present five epochs of 15 GHz VLBA observations of 13 AGN. These
observations were specially calibrated to detect parsec scale circular
polarization and our calibration techniques are discussed and analyzed in
detail. We obtained reliable detections of parsec scale circular polarization
in the radio jets of 4 AGN: 3C84, PKS 0528+134, 3C273, and 3C279. For each of
these objects our detections are at the level of ~ 0.3-1% local fractional
circular polarization. Our observations are consistent across multiple epochs
(and different calibration techniques) in the sign and magnitude of the
circular polarization observed. 3C273 and 3C279 both undergo core outbursts
during our observations and changes in the circular polarization of both
sources are correlated with these outbursts. In general, we observe the
circular polarization to be nearly coincident with the strong VLBI cores of
these objects; however, in 3C84 the circular polarization is located a full
milli-arcsecond south of the source peak, and in the 1996.73 epoch of 3C273 the
circular polarization is predominately associated with the newly emerging jet
component. Circular polarization may be produced as an intrinsic component of
synchrotron radiation or by the Faraday conversion of linear to circular
polarization. Our single frequency observations do not easily distinguish
between these possible mechanisms, but independent of mechanism, the remarkable
consistency across epoch of the sign of the observed circular polarization
suggests the existence of a long term, stable, uni-directional magnetic field.
Single dish observations of 3C273 and 3C279 at 8 GHz by Hodge and Aller suggest
that this stability may persist for decades in our frame of observation.


title:
Chandra constraints on the thermal conduction in the intracluster plasma
  of A2142

abstract:
  In this Letter, we use the recent Chandra observation of A2142 reported by
Markevitch et al. to put constraints on thermal conduction in the intracluster
plasma. We show that the observed sharp temperature gradient requires that
classical conductivity has to be reduced at least by a factor of between 250
and 2500. The result provides a direct constraint on an important physical
process relevant to the gas in the cores of clusters of galaxies.


title: The ATESP Radio Survey II. The Source Catalogue

abstract:
  This paper is part of a series reporting the results of the Australia
Telescope ESO Slice Project (ATESP) radio survey obtained at 1400 MHz with the
Australia Telescope Compact Array (ATCA) over the region covered by the ESO
Slice Project (ESP) galaxy redshift survey. The survey consists of 16 radio
mosaics with ~8"x14" resolution and uniform sensitivity (1sigma noise level ~79
microJy) over the whole area of the ESP redshift survey (~26 sq. degrees at
decl. -40 degr). Here we present the catalogue derived from the ATESP survey.
We detected 2960 distinct radio sources down to a flux density limit of ~0.5
mJy (6sigma), 1402 being sub-mJy sources. We describe in detail the procedure
followed for the source extraction and parameterization. The internal accuracy
of the source parameters was tested with Monte Carlo simulations and possible
systematic effects (e.g. bandwidth smearing) have been quantified.


title: Are passive protostellar disks stable to self-shadowing?

abstract:
  The uniqueness and stability of irradiated flaring passive protostellar disks
is investigated in the context of a simplified set of equations for the
vertical height H as a function of radius R. It is found that the well-known
flaring disk solution with H ~ R^{9/7} is not unique. Diverging solutions and
asymptotically conical (H ~ R) solutions are also found. Moreover, using
time-dependent linear perturbation analysis, it is found that the flaring disk
solution may become unstable to self-shadowing. A local enhancement in the
vertical height alters the functional form of irradiation grazing angle, and
causes the 'sunny side' of the enhancement to grow and the 'shadow side' to
collapse in a run-away fashion. This instability operates in regions of the
disk in which the cooling time is much shorter than the vertical sound crossing
time, which may occur in the outer regions of the passive irradiated disk if
dust and gas are sufficiently strongly thermally coupled. Processes that may
stabilize the disk, which include active accretion, irradiation from above
(e.g. a scattering corona) and low disk optical depth, are likely to operate
only at small or at large radius. The simple analysis of this Letter therefore
suggests that the instability may alter the flaring disk structure at
intermediate radii (between the actively accreting and fast rotating inner
regions and the optically thin outer regions).


title: Monsters at the Heart of Galaxy Formation

abstract:
  Surveys with the Hubble Space Telescope find supermassive black holes (BHs)
of 10**6 to 10**9.5 solar masses in every galaxy that has an
elliptical-galaxy-like "bulge" component. BHs appear to be standard equipment
in galaxy bulges. At the 2000 June meeting of the American Astronomical
Society, two groups announced the discovery of a tight correlation between BH
mass and the global velocity dispersion of stars in the host galaxy bulge
(astro-ph/0006053 and astro-ph/0006289). Also, BH mass scales with bulge mass;
the median BH mass fraction is 0.15 % of the bulge mass. In contrast, BH mass
does not scale with disk properties. Pure disk galaxies - ones that lack a
bulge component - must have BH mass fractions that are much smaller than the BH
mass fractions in bulges. These results imply a close connection between BH
growth and bulge (but not disk) formation. Observations increasingly support
the hypothesis that the major events that form a bulge or elliptical galaxy and
the main growth phases of its BH - when it shone like a quasar - were the same
events. These and other results are leading to a profound change in how
astronomers view BHs. More than just exotica needed to explain rare active
galactic nuclei, they are becoming an integral part of our understanding of
galaxy formation.


title: Supermassive Black Holes in Disk Galaxies

abstract:
  Dynamical searches find central dark objects - candidate supermassive black
holes (BHs) - in at least 34 galaxies. The demographics of these objects lead
to the following conclusions: (1) BH mass correlates with the luminosity of the
bulge component of the host galaxy, albeit with considerable scatter. The
median BH mass fraction is 0.15 % of the mass of the bulge. The quartiles are
0.09 % and 0.7 %. (2) BH mass correlates with the mean velocity dispersion of
the bulge component inside its effective radius, i.e., with how strongly the
bulge stars are gravitationally bound to each other. For the best BH mass
determinations, the scatter is consistent with the measurement errors. (3) BH
mass correlates with the luminosity of the high-density central component in
disk galaxies independent of whether this is a real bulge (a mini-elliptical)
or a "pseudobulge" (believed to form via inward transport of disk material).
(4) BH mass does not correlate with the luminosity of galaxy disks. If pure
disks contain BHs (and observations of active nuclei argue that some do), then
their BH mass fractions are much smaller than the canonical 0.15 % for bulges.
  These results lead to the following conclusions: (A) Present observations
show no dependence of BH mass on the details of whether BH feeding happens
rapidly during a collapse or slowly via secular evolution of the disk. (B) The
above results increasingly support the hypothesis that the major events that
form a bulge or elliptical galaxy and the main growth phases of its BH -- when
it shone as an active nucleus -- were the same events.


title: The HI Content of Spirals. II. Gas Deficiency in Cluster Galaxies

abstract:
  We derive the atomic hydrogen content for a total of 1900 spirals in the
fields of eighteen nearby clusters. By comparing the HI deficiency
distributions of the galaxies inside of and outside of one Abell radius of each
cluster, we find that two thirds of the clusters in our sample show a dearth of
neutral gas in their interiors. Possible connections between the gaseous
deficiency and the characteristics of both the underlying galaxies and their
environment are investigated in order to gain insight into the mechanisms
responsible for HI depletion. While we do not find a statistically significant
variation of the fraction of HI-deficient spirals in a cluster with its global
properties, a number of correlations emerge that argue in favor of the
interplay between spiral disks and their environment. In the clusters in which
neutral gas deficiency is pronounced, we see clear indications that the degree
of HI depletion is related to the morphology of the galaxies and not to their
optical size: early-type and, probably, dwarf spirals are more easily emptied
of gas than the intermediate Sbc--Sc types. The wealth of 21-cm data collected
for the Virgo region has made it possible to study the 2D pattern of HI
deficiency in that cluster. We also find evidence that gas-poor spirals in
HI-deficient clusters move on orbits more radial than those of the gas-rich
objects. The implications of all these results on models of how galaxies
interact with their environment are reviewed. Hydrodynamic effects appear as
the most plausible cause of HI removal. (abridged)


title:
On the nature of cosmic rays above the Greisen--Zatsepin--Kuz'min cut
  off

abstract:
  A re-examination of the atmospheric cascade profile of the highest energy
cosmic ray is presented. The study includes air-shower simulations considering
different cross sections, particle multiplicity and variation of the
hadronic-event-generator to model interactions above 200 GeV. The analysis
provides evidence that a medium mass nucleus primary reproduces the shower
profile quite well. This result does not support the idea, increasingly popular
at present, that the highest energy particles are protons, derived from the
decay of supermassive relic particles. On the other hand, we show that debris
of relativistic super-heavy nuclei, which can survive a 100 Mpc journey through
the primeval radiation are likely to generate such a kind of cascade.


title:
Computational AstroStatistics: Fast Algorithms and Efficient Statistics
  for Density Estimation in Large Astronomical Datasets

abstract:
  We present initial results on the use of Mixture Models for density
estimation in large astronomical databases. We provide herein both the
theoretical and experimental background for using a mixture model of Gaussians
based on the Expectation Maximization (EM) Algorithm. Applying these analyses
to simulated data sets we show that the EM algorithm - using the both the AIC &
BIC penalized likelihood to score the fit - can out-perform the best kernel
density estimate of the distribution while requiring no ``fine-tuning'' of the
input algorithm parameters. We find that EM can accurately recover the
underlying density distribution from point processes thus providing an
efficient adaptive smoothing method for astronomical source catalogs. To
demonstrate the general application of this statistic to astrophysical problems
we consider two cases of density estimation; the clustering of galaxies in
redshift space and the clustering of stars in color space. From these data we
show that EM provides an adaptive smoothing of the distribution of galaxies in
redshift space (describing accurately both the small and large-scale features
within the data) and a means of identifying outliers in multi-dimensional
color-color space (e.g. for the identification of high redshift QSOs).
Automated tools such as those based on the EM algorithm will be needed in the
analysis of the next generation of astronomical catalogs (2MASS, FIRST, PLANCK,
SDSS) and ultimately in the development of the National Virtual Observatory.


title:
X-ray spectral components in the hard state of GRS 1915+105: origin of
  the 0.5 - 10 Hz QPO

abstract:
  We investigate the origin of the ubiquitous 0.5 - 10 Hz QPO in the Galactic
microquasar GRS 1915+105. Using the archival X-ray data from RXTE, we make a
wide band X-ray spectral fitting to the source during a low-hard state observed
in 1999 June. We resolve the X-ray spectra into three components, namely a
multi-color disk component, a Comptonised component and a power-law at higher
energies. This spectral description is favored compared to other normally used
spectra like a cut-off power law, hard components with reflection etc. We find
that the 0.5 - 10 Hz QPO is predominantly due to variations in the Comptonised
component. We use this result to constrain the location of the various spectral
components in the source.


title:
The Discovery of Broad P Cygni X-ray Lines from Circinus X-1 with the
  Chandra High Energy Transmission Grating Spectrometer

abstract:
  We present the first grating-resolution X-ray spectra of the X-ray binary Cir
X-1, obtained with the High Energy Transmission Grating Spectrometer on
Chandra. These reveal a rich set of lines from H-like and/or He-like Ne, Mg,
Si, S and Fe detected with a high signal-to-noise ratio. The lines are broad
(+/- 2000 km/s) and show P Cygni profiles. The absorption components of the
lines extend to low velocity, and they have about the same widths and strengths
as the corresponding emission components. The widths of the X-ray P Cygni lines
are comparable to that of the broad component of the strong, asymmetric H-alpha
line from Cir X-1, suggesting that the two phenomena may be related. We discuss
outflow models and propose that the P Cygni profiles may arise in the moderate
temperature (5 x 10^6 K) region of the wind from an X-ray heated accretion
disk. This basic picture strengthens the idea that the accretion disk in Cir
X-1 is viewed in a relatively edge-on manner, and it suggests that Cir X-1 is
the X-ray binary analog of a Broad Absorption Line quasar.


title: Extreme intranight variability in the BL Lacertae object AO 0235+164

abstract:
  We present results of two-colour photometry with high time resolution of the
violently variable BL Lac object AO 0235+164. We have found extreme intranight
variability with amplitudes of ~ 100 % over time scales of 24 hours. Changes of
0.5 magnitudes in both R and V bands were measured within a single night, and
variations up to 1.2 magnitudes occurred from night to night. A complete
outburst with an amplitude ~ 30 % was observed during one of the nights, while
the spectrum remained unchanged. This seems to support an origin based on a
thin relativistic shock propagating in such a way that it changes the viewing
angle, as recently suggested by Kraus et al. (1999) and Qian et al. (2000).


title: The X-ray Faint Early-Type Galaxy NGC4697

abstract:
  We analyze archival ROSAT HRI, ROSAT PSPC, and ASCA data of the X-ray faint
early-type galaxy NGC4697. The joint ROSAT PSPC + ASCA spectrum is fit by a
two-component thermal model, a MEKAL model with kT_{MEKAL}=0.26^{+0.04}_{-0.03}
keV with low metallicity and a bremsstrahlung model with
kT_{BREM}=5.2^{+3.0}_{-1.6} keV. A similar model was found to fit the spectra
of another faint early-type galaxy (NGC4382) and the bulge of M31. We interpret
this soft emission as a combination of emission from a soft component of low
mass X-ray binaries (LMXBs) and from a low temperature interstellar medium,
although the relative contributions of the two components could not be
determined. Twelve point sources were identified within 4' of NGC4697, of which
11 are most likely LMXBs associated with the galaxy. The soft X-ray colors of
four of the LMXBs in NGC4697 support the claim that LMXBs possess a soft
spectral component. Finally, we present a simulation of what we believe the
Chandra data of NGC4697 will look like.


title: The Onset of Cluster Formation around Intermediate Mass Stars

abstract:
  We present the results of a recent near-infrared survey of the fields
surrounding a large sample of intermediate-mass pre-main-sequence (Herbig
Ae/Be) stars. While late-type Be and Ae stars are never associated with
conspicuous groups of young lower mass stars, early-type Be stars are usually
found within rich clusters. This finding has been tested against possible
biases due to different mass sensitivity of the observations or dynamical
dissipation of the clusters around older stars. Our results suggest that
massive stars are preferentially produced in dense stellar clusters, possibly
by dynamical interaction rather than by standard gas accretion as in the case
of lower mass stars. The possibility that the observed correlation between
maximum stellar mass and cluster richness could be the results of random
sampling the cluster size spectrum and the stellar IMF is also discussed.
Future observational tests capable of discriminating between these two
competitive models are outlined.


title:
A comparison of central temperatures of the intracluster gas determined
  from X-ray and SZ measurements

abstract:
  A combination of the X-ray imaging and SZ measurements of clusters permits an
indirect determination of the radial temperature profiles of intracluster gas,
which requires no assumption about the dynamical properties and the equation of
state for clusters. A comparison of such a derived gas temperature with that
given by the X-ray spectral analysis constitutes an effective probe of
properties of intracluster gas. Using the available data of 31 clusters in
literature, we have performed the first comparison of the central gas
temperatures provided by the two methods. The good agreement between these two
independent temperature estimates suggests that the distribution of
intracluster gas is essentially consistent with isothermality characterized by
a mean polytropic index of gamma=0.9+-0.1.


title:
1.65 micron (H-band) surface photometry of galaxies. V: Profile
  decomposition of 1157 galaxies

abstract:
  We present near-infrared H-band (1.65 micron) surface brightness profile
decomposition for 1157 galaxies in five nearby clusters of galaxies: Coma,
A1367, Virgo, A262 and Cancer, and in the bridge between Coma and A1367 in the
"Great Wall". The optically selected (m_p<=16.0) sample is representative of
all Hubble types, from E to Irr+BCD, except dE and of significantly different
environments, spanning from isolated regions to rich clusters of galaxies. We
model the surface brightness profiles with a de Vaucouleurs r^(1/4) law (dV),
with an exponential disk law (E), or with a combination of the two (B+D). From
the fitted quantities we derive the H band effective surface brightness (mu_e)
and radius (r_e) of each component, the asymptotic magnitude H_T and the light
concentration index C_31. We find that: i) Less than 50% of the Elliptical
galaxies have pure dV profiles. The majority of E to Sb galaxies is best
represented by a B+D profile. All Scd to BCD galaxies have pure exponential
profiles. ii) The type of decomposition is a strong function of the total H
band luminosity (mass), independent of the Hubble classification: the fraction
of pure exponential decompositions decreases with increasing luminosity, that
of B+D increases with luminosity. Pure dV profiles are absent in the low
luminosity range L_H<10^(10) L_solar and become dominant above 10^(11) L_solar.


title: Presupernova collapse models with improved weak-interaction rates

abstract:
  Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.


title: The Effect of Gas Loss on the Formation of Bound Stellar Clusters

abstract:
  The effect of gas ejection on the structure and binding energy of newly
formed stellar clusters is investigated. The star formation efficiency (SFE),
necessary for forming a gravitationally bound stellar cluster, is determined.
  Two sets of numerical N-body simulations are presented: As a first simplified
approach we treat the residual gas as an external potential. The gas expulsion
is approximated by reducing the gas mass to zero on a given timescale, which is
treated as a free parameter. In a second set of simulations we use smoothed
particle hydrodynamics (SPH) to follow the dynamics of the outflowing residual
gas self-consistently. We investigate cases where gas outflow is induced by an
outwards propagating shock front and where the whole gas cloud is heated
homogeneously, leading to ejection.
  If the stars are in virial equilibrium with the gaseous environment
initially, bound clusters only form in regions where the local SFE is larger
than 50% or where the gas expulsion timescale is long compared to the dynamical
timescale. A small initial velocity dispersion of the stars leads to a
compaction of the cluster during the expulsion phase and reduces the SFE needed
to form bound clusters to less than 10%.


title: Compact star clusters of the LMC HII region N11C

abstract:
  Based on imaging and spectroscopy obtained at the ESO NTT telescope and using
an efficient image analysis algorithm, we study the core of the LMC OB
association LH13, particularly the two compact stellar clusters Sk-6641 and HNT
in the HII, region N11C. We resolve Sk-6641 into 15 components and for the
first time the HNT cluster into 70 stars, and derive photometry for the
members. Moreover, from medium resolution spectroscopy we determine the
spectral types for sixteen stars in N11C. We compare the color-magnitude
diagrams of the clusters with that of the field stars and discuss the cluster
ages. With an age of ~100 Myr, the HNT cluster appears significantly older than
the very young (< 5 Myr) Sk-6641 starburst. We suggest that most of the `field'
O-stars in the core of N11C have actually been ejected from Sk-6641 through
dynamical interactions in the compact cluster. The properties of the Sk-6641
and HNT clusters suggest that we are viewing different star formation regions
lying at different distances along the same line of sight.


title:
Wide-Field Thermal Imaging of the Orion Nebula at High Spatial
  Resolution

abstract:
  We describe multi-wavelength (8--20 micron), diffraction-limited,
mid-infrared images of the OMC-1 cloud core in Orion, covering an approximately
two arcminute area around the Trapezium and BN/KL regions. We have detected
mid-infrared emission at the locations of a subset of the previously identified
proplyds in the Orion Nebula along with two new infrared sources. The Ney-Allen
nebula surrounding the OB star theta 1 Ori d exhibits a ring or toroidal
structure at the longest wavelengths. The BN/KL complex appears as an extended,
butterfly-shaped structure with significant bipolar symmetry which is
bifurcated by a dust lane at the longer wavelengths. The infrared sources IRc3,
4, and 5 give the appearance of a ring-like structure with a possible jet-like
protrusion from its center along a line from IRc2. Derived color temperature
and dust opacity maps suggest that IRc3, 4, and 5 may not be self-luminous
objects.


title:
A test of arm induced star formation in spiral galaxies from near-IR and
  H\alpha imaging

abstract:
  We have imaged a sample of 20 face-on spiral galaxies in H\alpha and in the
near-IR K-band (2.2 microns), in order to determine the location and strength
of star formation with respect to perturbations in the old stellar population.
We found that star formation rates are significantly enhanced near K-band arms.
If K-band light is dominated by emission from old stars, this shows that
density waves trigger star formation. However, no significant correlation was
found between the global underlying disk properties of spiral galaxies and
their total star formation rates.


title:
A Near-Infrared Stellar Census of Blue Compact Dwarf Galaxies: NICMOS
  Detection of Red Giant Stars in the Wolf-Rayet Galaxy Mrk 178

abstract:
  We observed the Blue Compact Dwarf/Wolf-Rayet galaxy Mrk 178 with the NICMOS
camera aboard HST. The galaxy is well resolved into individual stars in the
near-IR; photometry in J and H yields color-magnitude diagrams containing 791
individual point sources. We discuss the stellar content, drawing particular
attention to the intermediate age and/or old stars. Mrk 178 is only the second
Blue Compact Dwarf galaxy in which the red giant branch has been resolved,
indicating stars with ages of at least 1-2Gyr. This allows us to derive a
distance of $\geq$4.2($\pm$0.5)Mpc. The near-IR color-magnitude diagram also
exhibits an abundance of luminous, asymptotic giant branch stars. We find that
this requires vigorous star formation several hundred Myr ago. Some candidate
carbon stars are identified via their extreme near-IR color. We argue that
Mrk178 is fundamentally an old galaxy, based on the NICMOS detection of red
giants underlying the blue, starburst core, and its extended, faint halo of
redder color.


title:
Mid-infrared interferometry on spectral lines: III. Ammonia and Silane
  around IRC+10216 and VY CMa

abstract:
  Using the U.C. Berkeley Infrared Spatial Interferometer with an RF
filterbank, the first interferometric observations of mid-infrared molecular
absorption features of ammonia (NH_3) and silane (SiH_4) with very high
spectral resolution (R ~ 100000) were made. Under the assumptions of spherical
symmetry and uniform outflow, these new data permitted the molecular
stratification around carbon star IRC+10216 and red supergiant VY CMa to be
investigated. For IRC+10216, both ammonia and silane were found to form in the
dusty outflow significantly beyond both the dust formation and gas acceleration
zones. Specifically, ammonia was found to form before silane in a region of
decaying gas turbulence (>~ 20 R_star), while the silane is produced in a
region of relatively smooth gas flow much further from the star (>~ 80 R_star).
The depletion of gas-phase SiS onto grains soon after dust formation may fuel
silane-producing reactions on the grain surfaces. For VY CMa, a combination of
interferometric and spectral observations suggest that NH_3 is forming near the
termination of the gas acceleration phase in a region of high gas turbulence (~
40 R_star).


title:
Testing Standard and Degenerate Big Bang Nucleosynthesis with BOOMERanG
  and MAXIMA-1

abstract:
  We test different Big Bang Nucleosynthesis scenarios using the recent results
on the Cosmic Microwave Background Anisotropies provided by the BOOMERanG and
MAXIMA-1 experiments versus the observed abundances of 4He, D and 7Li. The
likelihood analysis, based on Bayesian approach, shows that, in the case of
high deuterium abundance, Y_D=(2.0\pm0.5) 10^{-4}, both standard and degenerate
BBN are inconsistent with the CMBR measurement at more than 3 \sigma. Assuming
low deuterium abundance, Y_D=(3.4\pm0.3) 10^{-5}, the standard BBN model is
still inconsistent with present observations at 2 \sigma level, while the
degenerate BBN results to be compatible. Unless systematics effects will be
found in nuclide abundances and/or in CMBR data analysis this result may be a
signal in favour of new physics like a large chemical potential of the relic
neutrino-antineutrino background.


title:
Simulated Extragalactic Observations with a Cryogenic Imaging
  Spectrophotometer

abstract:
  In this paper we explore the application of cryogenic imaging
spectrophotometers. Prototypes of this new class of detector, such as
superconducting tunnel junctions (STJs) and transition edge sensors (TESs),
currently deliver low resolution imaging spectrophotometry with high quantum
efficiency (70-100%) and no read noise over a wide bandpass in the visible to
near-infrared. In order to demonstrate their utility and the differences in
observing strategy needed to maximize their scientific return, we present
simulated observations of a deep extragalactic field. Using a simple analytic
technique, we can estimate both the galaxy redshift and spectral type more
accurately than is possible with current broadband techniques. From our
simulated observations and a subsequent discussion of the expected migration
path for this new technology, we illustrate the power and promise of these
devices.


title: Modeling the X-ray - UV Correlations in NGC 3516

abstract:
  We test the "reprocessing paradigm" of the optical -- UV AGN variability by
detailed modeling of the correlated X-ray -- optical (3590 and 5510 Angstrem)
variability of the recent multiwavelength campaign of NGC 3516. We produce
model optical light curves by convolving the observed X-ray flux with the
response function of an infinite, thin accretion disk, illuminated by a
point-like X-ray source at a given height above the compact object (the
lamp-post model) and compare their properties to those observed. Special
attention is given to the correct computation of the X-ray albedo of the disk.
We further compute the X-ray reflection response at two energies (E = 1 and 20
keV) and argue for the possibility of hard lags in their cross spectra. We also
compute the continuum Optical -- UV and the X-ray reflection spectra as well as
the Fe K-alpha fluorescent line profiles which we also compare to observations.
Despite the large (~50 percent) amplitude excursions of the X-ray flux, the
model optical light curves exhibit variability amplitudes of 3 -- 4 percent,
not unlike those observed. However, the model CCF between the X-ray and the
model optical variations show clear lags of 0.1 and 0.25 days for black hole
masses M = 10^7 and 10^8 Solar masses, respectively, not apparent in the data.
The synchrony of X-ray - optical variations points toward the BH mass < 10^7
Solar masses, whereas the X-ray spectra prefer BH mass > 10^8. Our conclusion
is that the combination of the observed optical/UV/X-ray spectral and timing
observations are inconsistent with the lamp-post model geometry for NGC 3516.


title: Dust Distribution in Gas Disks. A Model for the Ring Around HR 4796A

abstract:
  There have been several model analyses of the near and mid IR flux from the
circumstellar ring around HR4796A. In the vicinity of a young star, the
possibility that the dust ring is embedded within a residual protostellar gas
disk cannot be ruled out. In a gas-rich environment, larger sizes ($>100 \mu
m$) are needed for the particles to survive the radiative blow out. The total
dust mass required to account for the IR flux is $< 10^{-1} M_\oplus$. The
combined influence of gas and stellar radiation may also account for the
observed sharp inner boundary and rapidly fading outer boundary of the ring.
The pressure gradient induced by a small (10%) amplitude variation in the
surface density distribution of a low-mass gaseous disk would be sufficient to
modify the rotation speed of the gas.


title: Neutron Stars and Quantum Billiards

abstract:
  Homogeneous neutron matter at subnuclear densities becomes unstable towards
the formation of inhomogeneities. Depending on the average value of the neutron
density one can observe the appearance of either bubbles, rods, tubes or plates
embeded in a neutron gas. We estimate the quantum corrections to the ground
state energy (which could be termed either shell correction or Casimir energy)
of such phases of neutron matter. The calculations are performed by evaluating
the contribution of the shortest periodic orbits in the Gutzwiller trace
formula for the density of states. The magnitude of the quantum corrections to
the ground state energy of neutron matter are of the same order as the energy
differences between various phases.


title:
HST/NICMOS observations of the host galaxies of powerful radio sources:
  Does size matter?

abstract:
  We present near-infrared J and K band imaging of a sample of powerful radio
source host galaxies with the Hubble Space Telescope NICMOS2 camera. These
sources have been selected on their double lobed radio structure, and include a
wide range of projected radio source sizes. The largest projected linear sizes
range from the compact Gigahertz Peaked Spectrum (GPS, <1 kpc) and Compact
Steep Spectrum (CSS, < 20 kpc) radio sources, up to the large-scale (> 20 kpc)
classical doubles (FR II radio sources). We investigate the dependence of host
galaxy properties (including near-IR surface brightness profiles) on radio
source size, using both our own and published data. The absolute magnitudes and
surface brightness profiles are consistent with the host galaxies being regular
giant elliptical galaxies rather than Brightest Cluster Galaxies (BCGs). We
find that the GPS, CSS, and FR II host galaxies are a uniform class of objects,
consistent with a scenario in which a powerful radio source evolves along this
size sequence.


title: The W4 chimney/superbubble

abstract:
  A conical void in the Galactic HI within the Perseus arm has been proposed to
be a chimney. However, Halpha data suggest that the structure may be closed at
higher latitudes and therefore is a superbubble rather than a chimney. Recent
observations have extended our view of the HI and the radio continuum emission
to higher latitudes, up to 8 degrees;. The new images show the HI structure to
be open at the top and a small filament suggests recent breakout. The conical
shape of the structure is not easily explained by superbubble models.


title: On Galaxy-Cluster Sizes and Temperatures

abstract:
  We show that the distribution of the sizes and temperatures of clusters can
be used to constrain cosmological models. The size-temperature (ST)
distribution predicted in a flat Gaussian cluster-abundance-normalized
Omega_0=0.3 model agrees well with the fairly tight ST relation observed. A
larger power-spectrum amplitude sigma_8 would give rise to a larger scatter
about the ST relation as would a larger value of Omega_0 and/or long
non-Gaussian high-density tails in the probability density function. For
Gaussian initial conditions, the ST distribution suggests a constraint sigma_8
Omega_0^{0.26} \simeq 0.76. The ST relation is expected to get tighter at high
redshifts. In the process, we derive a simple formula for the halo
formation-redshift distribution for non-Gaussian models. We also suggest that
the discrepancy between the naive zero-redshift ST relation and that observed
may be due, at least in part, to the fact that lower-mass clusters form over a
wider range of redshifts. An Appendix derives an equation for the
formation-redshift distribution of halos.


title: PG 2131+066: A Test of Pre-White Dwarf Asteroseismology

abstract:
  PG 2131+066 is a composite-spectrum binary with a hot pulsating PG 1159-type
pre-white dwarf and an early M-type main sequence star. Analysis of Whole Earth
Telescope observations of the pulsating pre-white dwarf component provided an
asteroseismological determination of its mass, luminosity, and effective
temperature. These determinations allowed Kawaler et al. (1995) to determine
the distance to this star. In this paper, we refine the asteroseismological
distance determination, and confirm the distance by an independent measurement
to the system via the spectroscopic parallax of the M star. PG 2131+066 was
observed by the HST using the original PC in September 1993. Exposures with
filters F785LP and F555W both showed the companion at a distance of 0.3 arc
seconds. Photometry of the images provides an apparent magnitude for the main
sequence companion of v=18.97+/-0.15, from which we find a distance of 560
(+200 -134) pc. We also recalculated the asteroseismological distance to the
pre-white dwarf using updated models and new spectroscopic constraints from UV
spectra. The new seismological distance is 668 (+78 -83) pc, in satisfactory
agreement with the distance of the secondary star. These results suggest that
this is indeed a physical binary, and that seismological distance determination
may be the best way to determine the distance to the pulsating hot pre-white
dwarf stars.


title: Inferences from the dark sky: Olbers' paradox revisited

abstract:
  The classical formulation of "Olbers' Paradox" consists in looking for an
explanation of the fact that the sky at night is dark. We use the experimental
datum of the nocturnal darkness in order to put constraints on a Newtonian
cosmological model. We infer then that the Universe, in such a model, should
have had an origin at a finite time in the past.


title: Scintillation-Induced Circular Polarization in Pulsars and Quasars

abstract:
  We present a physical interpretation for the generation of circular
polarization resulting from the propagation of radiation through a magnetized
plasma in terms of a rotation measure gradient, or `Faraday wedges'. Criteria
for the observability of scintillation-induced circular polarization are
identified. Application of the theory to the circular polarization in pulsars
and compact extragalactic sources is discussed.


title: The ASCA Hard Serendipitous Survey

abstract:
  We summarize here some of the main results we have reached so far in the ASCA
Hard Serendipitous Survey, a survey program conducted in the 2-10 keV energy
band. In particular we will discuss the number-flux relationship, the 2-10 keV
spectral properties of the sources, and the cross-correlation of the ASCA
sources with ROSAT sources.


title: The ASCA HSS: Looking for Type 2 AGN

abstract:
  We will briefly discuss the X-ray spectral properties of the objects in the
ASCA Hard Serendipitous Survey (HSS) sample and its present AGN contents. We
also present X-ray spectroscopy of two Type 2 AGN belonging to the sample.


title: Comptonization in X-ray bright neutron star globular cluster systems

abstract:
  A BeppoSAX survey of bright Neutron Star (NS) systems in globular clusters,
together with the results obtained on Galactic LMXRB, suggests that a
two-component model is a reasonable description of the 0.1-200 keV spectra. At
energies lower than a few keV, a thermal component dominates, either due to the
direct view of the NS surface, to the emission of the boundary layer, or to a
multi-temperature blackbody originating in the accretion disk. At higher
energies, a power-law with an high-energy cutoff is the likely signature of
thermal Comptonization of soft X-ray photons, probably produced by the same
thermal mechanisms as above. The plasma optical depth (electron temperature)
correlates (anticorrelates) with the X-ray luminosity, suggesting that the
properties of the Comptonizing plasma are driven by the X-ray energy output.


title: Disk Galaxies and Galaxy Disks

abstract:
  The conference Galaxy Disks and Disk Galaxies, sponsored by the Vatican
Observatory, was held in June 12-16, 2000 at the Pontifical Gregorian
University, in Rome (Italy). The meeting hosted about 230 participants coming
from 30 countries. The very full program consisted of 29 review papers, 34
invited talks, and more than 180 posters. The meeting covered topics regarding
the structure, formation and evolution of galaxies with disks. Particular
attention was dedicated to the stellar and gaseous disk of the Milky Way, the
global characteristics of galaxy disks, their structure, morphology and
dynamics, the gaseous components, star formation, and chemical evolution, the
interactions, accretion, mergers and starbursts, the dark and luminous matter,
the establishment of the scaling laws, and the formation and evolution of disk
galaxies from a theoretical and observational point of view.


title:
Spectral energy distributions of FRI nuclei and the FRI/BL Lac unifying
  model

abstract:
  We consider archival ROSAT and HST observations of five FRI radio galaxies
and isolate their nuclear emission from that of the host galaxy. This enable us
to determine the Spectral Energy Distributions (SED) of their nuclei spanning
from the radio to the X-ray band. They cannot be described as single power-laws
but require the presence of an emission peak located between the IR and soft
X-ray band. We found consistency between the SED peak position and the values
of the broad band spectral indices of radio galaxies with those of BL Lac, once
the effects of beaming are properly taken into account. FRI SED are thus
qualitatively similar to those of BL Lacs supporting the identification of FRI
sources as their mis-oriented counterparts. No dependence of the shape of the
SED on the FR~I orientation is found.


title:
HST infrared imaging polarimetry of Centaurus A: implications for the
  unified scheme and the existence of a mis-directed BL Lac nucleus

abstract:
  We report results from HST/NICMOS 2 micron imaging polarimetry of the central
region of Centaurus A. In the vicinity of the nucleus we observe a complex
polarization structure which we explain by a combination of scattering of
nuclear light and dichroic polarization associated with the dust lane. The
scattered nuclear radiation is found in an angular region which extends over ~
70 degrees and thus it does not originate from a highly collimated beam, but is
associated with more omni-directional nuclear illumination. These observations
also show the presence of an unresolved, highly polarized (P = 11.1 %) nuclear
source whose polarization angle PA = 148.2 degrees is perpendicular to the jet
axis. We set an upper limit of 0.04'' (~0.8 pc) to its extent. The observed
nuclear polarization is naturally accounted for if we are observing scattered
light from an otherwise obscured nucleus provided that both the scattering
region and the occulting torus are extremely compact, with an outer radius of
less than ~ 1 pc. Alternatively, we might be directly seeing the infrared
counterpart of the radio core, similar to those found in other low luminosity
radio-galaxies observed with HST. We discuss these results in the framework of
the FRI / BL Lac unifying model.


title: The origin of runaway stars

abstract:
  Milli-arcsecond astrometry provided by Hipparcos and by radio observations
makes it possible to retrace the orbits of some of the nearest runaway stars
and pulsars to determine their site of origin. The orbits of the runaways AE
Aurigae and mu Columbae and of the eccentric binary iota Orionis intersect each
other about 2.5 Myr ago in the nascent Trapezium cluster, confirming that these
runaways were formed in a binary-binary encounter. The path of the runaway star
zeta Ophiuchi intersects that of the nearby pulsar PSR J1932+1059, about 1 Myr
ago, in the young stellar group Upper Scorpius. We propose that this neutron
star is the remnant of a supernova that occurred in a binary system which also
contained zeta Oph, and deduce that the pulsar received a kick velocity of
about 350 km/s in the explosion. These two cases provide the first specific
kinematic evidence that both mechanisms proposed for the production of runaway
stars, the dynamical ejection scenario and the binary-supernova scenario,
operate in nature.


title: An excess of mid-IR luminous galaxies in Abell 1689?

abstract:
  We present the results of infrared observations of Abell 1689 which was
observed with ISOCAM, at 6.7\mic and 15\mic, and ISOPHOT at 200\mic. The
cluster galaxies detected above a sensitivity limit of 0.15 mJy in the 6.7\mic
band, whose emission is mostly dominated by their stellar component, show
optical colors similar to the overall cluster population and are gathered in
the center of the cluster, following the distribution of the cluster
early-types. In the 15\mic band, above a sensitivity limit of 0.3 mJy, the
galaxies spectroscopically confirmed to be cluster members are blue outliers of
the cluster color-magnitude relation and become brighter going from the center
to the outer parts of the cluster.
  We compare the 6.7\mic and 15\mic fluxes and the cumulative distributions of
the B-[6.75] and B-[15] colors of the A1689 galaxies, above our 90%
completeness limits of 0.2 and 0.4 mJy for 6.7\mic and 15\mic respectively, to
the galaxies of two nearby clusters, Virgo and Coma, and to the field galaxies
at the same redshift of the cluster. Although the B-[6.7] color distributions
of the three clusters are compatible, we find a systematic excess of B-[15]
color distribution for the galaxies located in Abell 1689 with respect to Coma
or Virgo galaxies. This result proves the existence of a mid-infrared
equivalent of the Butcher-Oemler effect measured in the optical. The comparison
of 15\mic flux and B-[15] color distributions of A1689 and field galaxies does
not show strong differences between the population of starburst galaxies in the
cluster and in the field.


title:
A Possible Intrinsic Fluence-Duration Power-Law Relation in Gamma-ray
  Bursts

abstract:
  We argue that the distributions of both the intrinsic fluence and the
intrinsic duration of the gamma-ray emission in gamma-ray bursts from the BATSE
sample are well represented by log-normal distributions, in which the intrinsic
dispersion is much larger than the cosmological time dilatation and redshift
effects. We perform separate bivariate log-normal distribution fits to the
BATSE short and long burst samples. The bivariate log-normal behavior results
in an ellipsoidal distribution, whose major axis determines an overall
statistical relation between the fluence and the duration. We show that this
fit provides evidence for a power-law dependence between the fluence and the
duration, with a statistically significant different index for the long and
short groups. We discuss possible biases which might affect this result, and
argue that the effect is probably real. This may provide a potentially useful
constraint for models of long and short bursts.


title: Young Stellar Groups, Runaway Stars, and Pulsars

abstract:
  Milli-arcsecond astrometry provided by Hipparcos and by radio observations
makes it possible to retrace the orbits of nearby runaway stars and pulsars
with sufficient accuracy to identify their parent stellar cluster or
association. For two cases it is even possible to deduce the specific formation
scenario. The runaway star zeta Oph and PSR J1932+1059 are the result of a
supernova explosion which took place 1 Myr ago in a massive binary in the Upper
Scorpius association. The pulsar received a kick velocity of about 350 km/s in
this event. The runaway stars mu Col and AE Aur and the isolated eccentric
massive binary iota Ori result from a binary-binary encounter, most likely
inside the Trapezium cluster, 2.5 Myr ago. Future astrometric missions such as
DIVA, FAME and in particular GAIA will allow extension of these studies to a
significant fraction of the Galactic disk, and will provide new constraints on
the formation and evolution of massive stars.


title:
Metal Enrichment of The Primordial Interstellar Medium through 3-D
  Hydrodynamical Evolution of The First Supernova Remnant

abstract:
  The long-term evolution of supernova remnants (SNRs) in the primordial
interstellar medium (ISM) with an inhomogeneous structure is calculated to
investigate metal enrichment of the primordial gas. For this purpose, we have
constructed a parallel 3-D hydrodynamics code incorporating the radiative
cooling and self-gravity. The self-gravity and radiative cooling develop the
inhomogeneous structure of the ISM from a small perturbation with a power-law
spectrum. The resultant density ranges from 0.5 cm$^{-3}$ to 10$^6$ cm $^{-3}$.
Calculations for a supernova (SN) with the progenitor mass of 20 solar mass are
performed as the first step of a series of our study. It is found from the
results that a single SN distributes some of newly synthesized heavy elements
into a dense filament of the ISM with densities ranging from 100 to 10$^4$
cm$^{-3}$ depending on where the SN explodes. Thus, the metallicity [Mg/H] of
the dense filaments polluted by the SN ejecta becomes $-2.7\pm0.5$. From these
filaments, the first Population II stars will form. This value is in accordance
with the previous analytical work (Shigeyama & Tsujimoto 1998) with accuracy of
$\sim 0.3$ dex.


title:
The X-Ray Jet of PKS 0637-752: Inverse Compton Radiation from the Cosmic
  Microwave Background ?

abstract:
  We propose that the X-ray emission detected by Chandra from the
100-kiloparsec-scale jet of PKS 0637-752 is produced through inverse Compton
scattering of the Cosmic Microwave Background (CMB). We analyze the physical
state of the jet and show that inverse Compton scattering from the CMB is
consistent with equipartition for a moderate beaming of the emission, with
Doppler factor delta~10. The power transported by the jet is then similar to
that of other powerful blazars, Lj~10^{48} erg/s and the jet has low radiative
efficiency. The radiative cooling times of the electrons are a few thousand
years, compatible with the size of the knot. The low-energy cutoff of the
electron distribution is constrained to be gamma_min~10, the first such
constraint from spectral considerations. A parallel analysis for the SSC model
yields far less reasonable physical conditions.


title: Cygnus OB2 - a young globular cluster in the Milky Way

abstract:
  The morphology and stellar content of the Cygnus OB2 association has been
determined using 2MASS infrared observations in the J, H, and K bands. The
analysis reveals a spherically symmetric association of 2 deg in diameter with
a half light radius of 13', corresponding to Rh = 6.4 pc at an assumed distance
of 1.7 kpc. The interstellar extinction for member stars ranges from Av = 5m to
20m, which led to a considerable underestimation of the association size and
richness in former optical studies. From the infrared colour-magnitude diagram,
the number of OB member stars is estimated to 2600 +/- 400, while the number of
O stars amounts to 120 +/- 20. This is the largest number of O stars ever found
in a galactic massive star association. The slope of the initial mass function
has been determined from the colour-magnitude diagram to Gamma=-1.6 +/- 0.1.
The total mass of Cyg OB2 is estimated to (4-10)*10^4 Msol, where the primary
uncertainty comes from the unknown lower mass cut-off. Using the radial density
profile of the association, the central mass density is determined to
rho0=40-150 Msol pc^-3. Considering the mass, density, and size of Cyg OB2 it
seems untenable to classify this object still as OB association. Cygnus OB2
more closely resembles a young globular cluster like those observed in the
Large Magellanic Cloud or in extragalactic star forming regions. It is
therefore suggested to re-classify Cygnus OB2 as young globular cluster - an
idea which goes back to Reddish et al. (1966). Cygnus OB2 would then be the
first object of this class in the Milky Way.


title: Sources of CMB Spots in Closed Hyperbolic Universes

abstract:
  The results of a previous paper by the author [Astrophys. J. 470, 43 (1996)
are improved, through a more reliable technique for finding the location of
sources whose images are on the surface of last scattering.


title: Death of Stellar Baryonic Dark Matter Candidates

abstract:
  The nature of the dark matter in the Universe is one of the outstanding
questions in astrophysics. In this talk, I address possible stellar baryonic
contributions to the 50-90% of our Galaxy that is made of unknown dark matter.
First I show that faint stars and brown dwarfs constitute only a few percent of
the mass of the Galaxy. Next, I show that stellar remnants, including white
dwarfs and neutron stars, are also insufficient in abundance to explain all the
dark matter of the Galaxy. High energy gamma-rays observed in HEGRA data place
the most robust constraints, $\Omega_{WD} < 3 \times 10^{-3} h^{-1}$, where $h$
is the Hubble constant in units of 100 km s$^{-1}$ Mpc$^{-1}$. Overproduction
of chemical abundances (carbon, nitrogen, and helium) provide the most
stringent constraints, $\Omega_{WD} < 2 \times 10^{-4} h^{-1}$. Comparison with
recent updates of microlensing data are also made. According to the gamma-ray
limit, all Massive Compact Halo Objects seen by the experiments (Machos) can be
white dwarfs if one takes the extreme numbers; however, from chemical
overproduction limits, NOT all Machos can be white dwarfs. Comments on recent
observations of the infrared background and of white dwarfs are also made. In
conclusion, a nonbaryonic component in the Halo seems to be required.


title: An excess of very bright stars in the inner bulge

abstract:
  From an analysis of the stars remaining in central regions of the Galaxy
after subtracting those belonging to the disc and the bulge, we deduce that the
inner bulge must have an extra young population with respect the rest of the
bulge. It is shown that there is a higher ratio of very bright stars in the
central bulge than than there is in the outer bulge. This is interpreted as
being an additional young component due to the presence of star formation
regions near the Galactic Centre which is absent in the outer bulge.


title: The Stability of Radiatively Cooled Jets in Three Dimensions

abstract:
  The effect of optically thin radiative cooling on the Kelvin-Helmholtz
instability of three dimensional jets is investigated via linear stability
theory and nonlinear hydrodynamical simulation. Two different cooling functions
are considered: radiative cooling is found to have a significant effect on the
stability of the jet in each case. The wavelengths and growth rates of unstable
modes in the numerical simulations are found to be in good agreement with
theoretical predictions. Disruption of the jet is found to be sensitive to the
precessional frequency at the origin with lower frequencies leading to more
rapid disruption. Strong nonlinear effects are observed as the result of the
large number of normal modes in three dimensions which provide rich mode-mode
interactions. These mode-mode interactions provide new mechanisms for the
formation of knots in the flows. Significant structural features found in the
numerical simulations appear similar to structures observed on protostellar
jets.


title:
On the compared accuracy and reliability of spectroscopic and
  photometric redshift measurements

abstract:
  We present a comparison between the spectroscopic catalog of the HDF recently
published by Cohen et al and the redshifts measured by our group for those
objects using photometric techniques. We perform this comparison in order to
characterize the errors associated to the photometric redshift technique. The
spectroscopic sample includes over 140 objects in the HDF proper, representing
the deepest, cleanest, most complete spectroscopic catalog ever compiled. We
study each object for which our redshift and the one measured by Cohen et al
disagree. In most cases the photometric evidence is strong enough to call for a
careful review of the spectroscopic values, as they seem to be in error. It is
possible to characterize the systematic errors associated to our technique,
which when combined with the photometric errors allow us to obtain complete
information on the redshift of each galaxy and its associated confidence
interval, regardless of apparent magnitude. One of the main conclusions of this
study is that, to date, all the redshifts from our published catalog that have
been checked have been shown to be correct (within the stated confidence
limits). This implies that our galaxy template set is a fair representation of
the galaxy population at all redshifts (0<z<6) and magnitudes (R<24) explored
to date. On the other hand, spectroscopy of faint sources is subject to unknown
and uncharacterized systematic errors, that will in turn be transmitted to any
photometric redshift technique which uses spectroscopic samples as calibration.
Our analysis proves that photometric redshift techniques can and must be used
to extend the range of applicability of the spectroscopic redshift
measurements. (Abridged)


title: MBM12: A Younger Version of the TW Hydrae Association?

abstract:
  We present optical and infrared photometry of stars near the MBM 12 dark
cloud. The optical data indicate that there are about 40 candidate pre-main
sequence stars in the same area currently occupied by eight bona-fide PMS
stars. Analysis of 2MASS data suggests that 60% of PMS candidates have been
detected. Some of these show evidence of optically thick disks. Our study of
the 2MASS data also reveals a large enhancement of sources within the MBM 12
cloud, indicating that it is still actively forming stars.


title: The Onset of Methane in L Dwarfs

abstract:
  We have detected weak absorption features produced by the strong nu(3)
methane band at 3.3 microns in two L dwarfs, 2MASSW J1507476-162738 and 2MASSI
J0825196+211552, classified by Kirkpatrick et al. (2000) as spectral types L5
and L7.5 respectively. These absorptions occur in objects warmer than any in
which methane previously has been detected, and mark the first appearance of
methane in the ultracool star-to-brown dwarf spectral sequence.


title: Large scale dynamos with ambipolar diffusion nonlinearity

abstract:
  It is shown that ambipolar diffusion as a toy nonlinearity leads to very
similar behaviour of large scale turbulent dynamos as full MHD. This is
demonstrated using both direct simulations in a periodic box and a closure
model for the magnetic correlation functions applicable to infinite space.
Large scale fields develop via a nonlocal inverse cascade as described by the
alpha-effect. However, because magnetic helicity can only change on a resistive
timescale, the time it takes to organize the field into large scales increases
with magnetic Reynolds number.


title: X-Ray Bursts and Proton Captures Close to the Dripline

abstract:
  The hydrogen-rich accreted envelopes of neutron stars in binary systems are
the site for the rapid proton capture nucleosynthethic process (rp-process),
which involves nuclei close to the proton dripline. An overview of the relevant
reactions and nuclear properties for rp-process studies is given, along with
motivations for further experimental nuclear physics studies.


title:
Probing large-distance higher-dimensional gravity with Cosmic Microwave
  Background measurements

abstract:
  It has been recently argued that higher dimensional gravity theories may
manifest themselves not only at short microscopic distances but also at large
cosmological scales. We study the constraints that cosmic microwave background
measurements set on such large distance modifications of the gravitational
potential.


title:
Prediction of Astrophysical Reaction Rates: Methods, Data Needs, and
  Consequences for Nucleosynthesis Studies

abstract:
  The majority of nuclear reactions in astrophysics involve unstable nuclei
which are not fully accessible by experiments yet. Therefore, there is high
demand for reliable predictions of cross sections and reaction rates by
theoretical means. The majority of reactions can be treated in the framework of
the statistical model (Hauser-Feshbach). The global parametrizations of the
nuclear properties needed for predictions far off stability probe our
understanding of the strong force and take it to its limit.
  The sensitivity of astrophysical scenarios to nuclear inputs is illustrated
in the framework of a detailed nucleosynthesis study in type II supernovae.
Abundances resulting from calculations in the same explosion model with two
different sets of reaction rates are compared. Key reactions and required
nuclear information are identified.


title: Precision Cosmology and the Density of Baryons in the Universe

abstract:
  Big-bang Nucleosynthesis (BBN) and Cosmic Microwave Background (CMB)
anisotropy measurements give independent, accurate measurements of the baryon
density and can test the framework of the standard cosmology. Early CMB data
are consistent with the longstanding conclusion from BBN that baryons
constitute a small fraction of matter in the Universe, but may indicate a
slightly higher value for the baryon density. We clarify precisely what the two
methods determine, and point out that differing values for the baryon density
can indicate either an inconsistency or physics beyond the standard models of
cosmology and particle physics. We discuss other signatures of the new physics
in CMB anisotropy.


title: Formation of an Infalling Disklike Envelope around a Protostar

abstract:
  We examined the gravitational contraction of isothermal molecular cloud cores
with slow rotation by means of two-dimensional numerical simulations. Applying
a sink-cell method, we followed the evolution of the cloud cores up to the
stages at which most of the matter accretes onto the central region (i.e., a
protostar and a rotationally-supported circumstellar disk). We show that both
an infalling disklike envelope and a rotationally-supported disk around the
central star are natural outcome of the gravitational contraction of a {\it
prolate} cloud core with slow rotation. The early evolution of the infalling
envelopes resembles sheet models recently proposed by Hartmann and coworkers.
In the infalling disklike envelope, the radial profiles of the density, radial
velocity, and azimuthal velocity can be approximated by $\rho\propto r^{-1.5}$,
$v_r\propto r^{-0.5}$, and $v_\phi\propto r^{-1}$, respectively. The fate of
the infalling envelopes is also discussed.


title: Chandra imaging of the complex X-ray core of the Perseus cluster

abstract:
  We report subarcsec-resolution X-ray imaging of the core of the Perseus
cluster around the galaxy NGC 1275 with the Chandra X-ray Observatory. The
ROSAT-discovered holes associated with the radio lobes have X-ray bright rims
which are cooler than the surrounding gas and not due to shocks. The holes
themselves may contain some hotter gas. We map strong photoelectric absorption
across the Northern lobe and rim due to a small infalling irregular galaxy,
known as the high velocity system. Two outer holes, one of which was previously
known, are identified with recently found spurs of low-frequency radio
emission. The spiral appearance of the X-ray cooler gas and the outer optical
parts of NGC 1275 may be due to angular momentum in the cooling flow.


title:
Magnification cross-sections of gravitational lensing by galaxies in
  general FLRW cosmologies

abstract:
  For a wide variety of cosmological models characterized by the cosmic mass
density $\Omega_M$ and the normalized cosmological constant $\Omega_{\Lambda}$,
we derive an analytic expression for the estimate of magnification
cross-sections by an ensemble of isothermal spheres as models of galactic mass
distributions. This provides a simple approach to demonstrate how the lensing
probability by galaxies depends on the cosmological parameters. An immediate
consequence is that, while a non-zero cosmological constant indeed leads to a
significant increase of the lensing probability as it has been shown in the
literature, only a small fraction of sky to $z\sim 3$ can be moderately ($\mu
\sim 1.3$) lensed by galaxies even in a $\Lambda$-dominated flat universe.
Therefore, whether or not there is a nonzero cosmological constant, it is
unlikely that the overall quasar counts have been seriously contaminated by the
presence of galactic lenses.


title: An Efficient Search for Gravitationally-Lensed Radio Lobes

abstract:
  We performed an automated comparison of the FIRST radio survey with the APM
optical catalog to find radio lobes with optical counterparts. Based on an
initial survey covering ~3000 square degrees, we selected a sample of 33 lens
candidates for VLA confirmation. VLA and optical observations of these
candidates yielded two lens systems, one a new discovery (J0816+5003), and one
of which was previously known (J1549+3047). Two other candidates have radio
lobes with galaxies superposed, but lack evidence of multiple imaging. One of
our targets (J0958+2947) is a projected close pair of quasars (8'' separation
at redshifts 2.064 and 2.744). Our search method is highly efficient, with >5%
of our observing targets being lensed, compared to the usual success rate of
<1%. Using the whole FIRST survey, we expect to find 5--10 lenses in short
order using this approach, and the sample could increase to hundreds of lensed
lobes in the Northern sky, using deeper optical surveys and planned upgrades to
the VLA. Such a sample would be a powerful probe of galaxy structure and
evolution.


title: Morphometry of Spatial Patterns

abstract:
  Minkowski functionals constitute a family of order parameters which
discriminate spatial patterns according to size, shape and connectivity. Here
we point out, that these scalar descriptors can be complemented by
vector-valued curvature measures also known as Quermass vectors. Using examples
of galaxy clusters, we demonstrate that the Quermass vectors provide additional
morphological information on directional features and symmetries displayed by
spatial data.


title:
Changing Frequency Separation of Kilohertz Quasi-Periodic Oscillations
  in the Sonic-Point Beat-Frequency Model

abstract:
  Previous work on the sonic-point beat-frequency (SPBF) model of the kilohertz
quasi-periodic oscillations (QPOs) observed in the X-ray flux from neutron
stars in low-mass binary systems has shown that it naturally explains many
properties of these QPOs. These include the existence of just two principal
QPOs in a given source, the commensurability of the frequency separation \Dnu\
of the two kilohertz QPOs and the spin frequency \nus inferred from burst
oscillations, and the high frequencies, coherence, and amplitudes of these
QPOs. Here we show that the SPBF model predicts that \Dnu is less than but
close to \nus, consistent with the observed differences between \Dnu and \nus.
It also explains naturally the decrease in \Dnu with increasing QPO frequency
seen in some sources and the plateau in the QPO frequency--X-ray flux observed
in 4U 1820-30. The model fits well the QPO frequency behavior observed in Sco
X-1, 4U 1608-52, 4U 1728-34, and 4U 1820-30 (chisqdof = 0.4-2.1), giving masses
ranging from 1.59 to 2.0 Msun and spin rates ranging from 279 to 364 Hz. In the
SPBF model, the kilohertz QPOs are effects of strong-field gravity. Thus, if
the model is validated, the kilohertz QPOs can be used not only to determine
the properties of neutron stars but also to explore quantitatively general
relativistic effects in the strong-field regime.


title: The Young Starburst Nucleus of the Wolf-Rayet LINER Galaxy NGC 6764

abstract:
  NIR K band imaging spectroscopy of the central 8'' (1.3 kpc) in the
Wolf-Rayet LINER galaxy NGC6764 shows that the most recent star formation is
most likely still unresolved at sub-arcsecond resolution (< 100pc). The
continuum source has a size of about 1.5''. The H_2 emission is resolved
showing a rotating ring/disk of ~1.2'' diameter. An analysis of the nuclear
stellar light using population synthesis models in conjunction with NIR
spectral synthesis models suggests following star formation history: Two
starbursts with decay times of 3 Myr occurred 3 - 5 Myr and 15 - ~50 Myr ago.
Continuous star formation over at least 1Gyr can also explain the observed
parameter. However, the mass relocation and consumption involved as well as the
different spatial distribution of the lines associated with the star formation
strongly favor the 'two starburst' scenario. In that scenario, up to 35% of the
total observed Br_gamma flux could still be due to the AGN. In contrast to
other starburst galaxies (e.g. M82, NGC 7552), the younger starburst in NGC6764
is very likely located closer to the nucleus and surrounded by the older
starburst. One possible explanation can be that the stellar bar still
transports gas down to radii close to the nucleus. This suggests that the
massive star formation activity is directly competing with the AGN for the
fuel. The 44 ksec. HRI ROSAT data show the presence of a X-ray source (probably
an AGN) which varies by more than a factor of 2 over a time scale of 7 days.
This implies the presence of a compact source with a discrete or at most 1000
AU source size. In addition, we find an extended X-ray component which looks
similar to published VLA radio continuum maps.


title: Deprojecting Sunyaev-Zeldovich statistics

abstract:
  We apply the hierarchical clustering model and non-linear perturbation theory
to the cosmological density and temperature fields. This allows us to calculate
the intergalactic gas pressure power spectrum, SZ anisotropy power spectrum,
skewness and related statistics. Then we show the effect of the
non-gravitational heating. Our model confirms recent simulations yielding mass
weighted gas temperature \bar{T}_g \sim 0.35 keV and reproduces the power
spectra found in these simulations.
  While the SZ effect contains only angular information, we show that it is
possible to extract the full time resolved gas pressure power spectrum when
combined with galaxy photometric redshift surveys by using a variation on the
cross-correlation. This method further allows the disentanglement of the
gravitational and the non-gravitational heating.


title:
Cosmological density perturbation on spin dominated universes and COBE
  data

abstract:
  The evolution equation of cosmic density perturbation of a spin dominated
universe in Einstein-Cartan gravity is obtained.Examples of inflationary
cosmologies with spin are also given where we compute the density perturbations
in terms of redshift and show that spin-torsion effects produce a growth on the
inhomogeneity of the universe as the redshift is high.Spin-torsion effects are
redshifted out when the Universe expands and would be rarely of importance in
present day phase of the universe.Comparison with the primordial density
fluctuation of COBE data is also obtained.From our computations a COBE type
experiment could be proposed to measure the spin-torsion effects in large scale
structure objects such as spiral Galaxies and globular clusters.


title: A variability analysis of low-latitude unidentified gamma-ray sources

abstract:
  We present a study of 40 low-latitude unidentified 3EG gamma-ray sources
which were found to be not positionally coincident with any known class of
potential gamma-ray emitters in the Galaxy (Romero, Benaglia & Torres, 1999).
We have performed a variability analysis which reveals that many of these 40
sources are variable. These sources have, in addition, a steep mean value of
the gamma-ray spectral index, $<\Gamma> = 2.41 \pm 0.2$, which, combined with
the high level of variability seems to rule out a pulsar origin. The positional
coincidences with uncatalogued candidates to supernova remnants were also
studied. Only 7 sources in the sample are spatially coincident with these
candidates, a result that is shown to be consistent with the expected level of
pure chance association. A complementary search for weak radio counterparts was
also conducted and the results are presented as an extensive table containing
all significant point-like radio sources within the 40 EGRET fields. We argue
that in order to produce the high variability, steep gamma-ray spectra, and
absence of strong radio counterparts observed in some of the gamma-ray sources
of our sample a new class of objects should be postulated, and we analyze a
viable candidate.


title: An inquiry into the nature of the gamma-ray source 3EG J1828+0142

abstract:
  The unidentified, low-latitude, $\gamma$-ray source 3EG J1828+0142 presents
high levels of variability and a steep spectral index $\Gamma\sim2.7$. Here we
propose a model for this source where the high-energy emission is produced by a
galactic Kerr-Newman black hole. The model takes into account electron-positron
annihilation losses in the calculation of the expected spectral energy
distribution and can reproduce the observational features, including the
absence of a strong radio counterpart. We also report the discovery of a nearby
supernova remnant that could be associated with the original supernova
explosion that created the black hole. Several faint radio sources were also
detected in the radio field within the inner $\gamma$-ray confidence contour
and their spectral index estimated. Some of these sources could be the expected
weak radio counterpart.


title: On the Structure of the Magnetic Field in a Kinematic ABC Flow Dynamo

abstract:
  The kinematic induction equation of MHD is solved numerically in the case of
the normal ``111'' ABC flow using a general staggered mesh method. Careful 3-D
visualizations of the topology of the magnetic field reveal that previous
conclusions about the modes of operation of this type of kinematic dynamo must
be revised. The two known windows of dynamo action at low and high magnetic
Reynolds number, correspond to two distinct modes, both relying crucially on
the replenishing of the magnetic field near a discontinuity at the beta-type
stagnation points in the flow. One of these modes display double magnetic
structures that were previously found only to obscure the physics of the
dynamo: They turn out, however, to play an important part in the process of
amplifying the magnetic field. Invariant properties of the mode in the second
magnetic Reynolds number window support the case for the normal ABC flow as a
fast dynamo.


title: Radio - Optical Correlations in High-z Radio Galaxies and Quasars

abstract:
  The narrow emission line luminosities of radio-loud AGN are well-correlated
with their low-frequency radio luminosities. This correlation is linear and
extends over at least four orders of magnitude. The correlation is discussed in
terms of a linear relationship between the power in the radio-emitting jets and
the photoionizing (i.e. accretion) luminosity. Support for this theory comes
from a direct correlation between the optical continuum and radio luminosities
of steep-spectrum quasars. We discuss the dependence of the observed
luminosities of AGN on their black hole masses and how close to the Eddington
limit they are accreting.


title:
X-ray Source Populations in the Region of the Open Clusters NGC 6633 and
  IC 4756

abstract:
  Using deep exposures (~10^5 s) with the ROSAT HRI, we have performed
flux-limited surveys for X-ray sources in the vicinity of the Hyades-age open
stellar clusters NGC 6633 and IC 4756, detecting 31 and 13 sources
respectively. Our primary aim is to search for so-far unrecognised cluster
members. We propose identifications or classifications (cluster member, field
star, extragalactic field object) for the X-ray sources, based on published
membership lists, and on X-ray:optical flux ratios and optical
colour--magnitude diagrams. Results of simulating the expected X-ray-emitting
source populations are compared with the ROSAT measurements and with the
expected capabilities of the XMM mission. The simulations provide a novel
method of comparing the activity levels of NGC 6633 and IC 4756 with that of
the Hyades. The measurements and simulations confirm that cluster members are
the major class of X-ray emitter in these fields at flux levels f_x >
10^{-14}/erg/cm^2/s (0.1--2.4 keV), contributing ~40 per cent of the total
X-ray sources. We find 6 possible new members in NGC 6633, and 4 candidates in
IC 4756; all require further observation to establish membership probability.


title: A Search for Extended Line Emission from BALQSOs

abstract:
  We have obtained HST NICMOS snapshot images of ten BALQSOs and four
radio-quiet non-BALQSOs to search for extended narrow-line regions. For each
object we obtained an image in the F160W filter and a narrow-band image in
either H-alpha, H-beta, or [O ii]. We find no detections of extended
narrow-line emission in the sample. We generate simulated images of QSOs with
extended narrow-line regions to place limits on the amount of flux that may be
present, and we conclude that BALQSOs cannot be more than several times
brighter in extended emission than typical type 2 AGN. Spatially unresolved
H-alpha emission is detected in all nine objects observed in this line, which
we attribute to broad H-alpha emission. The five detected BALQSOs have derived
H-alpha equivalent widths that are larger than those of the four non-BALQSOs by
a factor of around 2 on average. This may point to an intrinsic difference
between the rest-frame optical spectral properties of BALQSOs and non-BALQSOs.


title:
Search for interactions between ejections of GRS 1915+105 and its
  environment

abstract:
  To unravel the effect of likely interactions between the energetic ejections
of the galactic superluminal source GRS 1915+105 and its surrounding
interstellar medium, we observed its environment. Two IRAS sources are
symmetrically placed with respect to GRS 1915+105, and are aligned with the
sub-arcsec ejections of this source.
  We analyzed these two sources IRAS 19124+1106 and IRAS 19132+1035 through
near-infrared, millimeter and centimeter wavelengths. The evidence for these
regions being interaction zones seems inconclusive.


title: Distance to the Large Magellanic Cloud: the RR Lyrae stars

abstract:
  (abridged) New photometry and spectroscopy for more than a hundred RR Lyrae
stars in two fields located close to the bar of the Large Magellanic Cloud
(LMC) are used to derive new accurate estimates of the average magnitude, the
local reddening, the luminosity-metallicity relation, and of the distance to
the LMC. Metallicities from low resolution spectra obtained with the VLT have
been derived for 101 RR Lyr's; the average value is [Fe/H]=-1.48+/-0.03
(sigma=0.29). An estimate of the reddening was obtained with 2 different
methods, and we found: E(B-V)=0.116+/-0.017, and 0.086+/-0.017 mag in field A
and B, respectively. The dereddened apparent average luminosity of the RR
Lyrae's defined by the present photometry is <V(RR)>_0=19.064+/-0.064 at
[Fe/H]=-1.5. We determine the slope of the luminosity-metallicity relation for
the RR Lyrae stars: Delta[M_V(RR)]/ Delta[Fe/H]=0.214+/-0.047, with no clear
evidence for a change in slope at [Fe/H]=-1.5. When coupled with the absolute
magnitude derived from the Baade-Wesselink and the statistical parallaxes
methods, both methods known to favour the short distance scale, this value
leads mod(LMC)=18.38+/- 0.16 and 18.30+/-0.14, respectively. If we use instead
the absolute magnitude from the new Main Sequence Fitting of Galactic globular
clusters [M_V(RR)=0.61+/-0.07 mag at [Fe/H]=-1.5] we derive
mod(LMC)=18.45+/-0.09. The average I luminosity of the clump stars is
I_0=18.12+/-0.06, and moves the clump distance modulus to the LMC to
mod(LMC)=18.42+/-0.07 and 18.45+/-0.07 when two metallicity-I relations are
used. All these values are only 1 sigma shorter than provided by the Pop I
distance indicators, and allow to reconcile the short and long distance scale
on a common value for the distance modulus of the LMC of 18.515+/-0.085 mag.


title:
UVES observations of QSO 0000-2620: Molecular hydrogen abundance in the
  damped Ly-alpha system at z_abs = 3.3901

abstract:
  We have discovered molecular hydrogen in a fourth quasar damped Ly-alpha
system (hereafter DLA). The UVES spectrograph on the 8.2m ESO Kueyen telescope
has allowed the detection of H2 in gas with low metallicity, Z/Z_solar ~=
10^{-2}, and high neutral hydrogen column density, N(HI) ~= 2.6*10^{21}
cm^{-2}, at redshift z_abs = 3.3901 toward QSO 0000-2620. The measured H2
fractional abundance of f(H2) ~= 4*10^{-8} is lower than a typical value for
Galactic interstellar clouds of high N(HI) column density by a factor of
(2-3)*10^6. Since H2 molecules are formed efficiently on dust grains, it
implies that the dust condensation in this DLA is negligible, and hence the
abundances derived from metal absorption lines are the actual ones. The
obtained f(H2) value leads to an estimate of the dust number density of < n_d
>_DLA ~= 10^{-3}*< n_d >_ISM, which is consistent with the dust-to-gas ratio k
<= 1.6 10^{-3} derived independently from the [Cr/Zn] and [Fe/Zn] ratios.


title: Obscured Active Galactic Nuclei

abstract:
  The properties of the absorption in type 2, narrow line AGNs are reviewed by
focusing on the X-ray indicators. I discuss the properties of the cold
absorbing medium (the putative torus) and of the reprocessed components, as
well as their implications for the unified model. The relation between optical
classification and X-ray absorption is examined. The case of "fossil" AGNs,
whose type 2 classification is not due to absorption effects, is also
discussed. Although this review is mainly focused on nearby Seyfert 2 galaxies,
I also shortly discuss the effects of absorption at higher luminosities and
higher redshift and the implications for the X-ray background.