title:
Metal Abundances in the Hot Interstellar Medium in Early-Type Galaxies
  Observed with ASCA

abstract:
  We have analyzed ASCA data of 27 early-type galaxies, and studied the
properties of their X-ray emitting ISM (Inter Stellar
  Medium) in detail. We found that overlapping lines and free-bound continuum
cause strong coupling in the derived abundances of various elements. The
abundance determination is also difficult due to the uncertainties in the Fe-L
atomic physics, because Fe-L lines couple with O and Ne K-lines. However, when
abundances of $\alpha$-elements are fixed, all the plasma codes give similar Fe
abundances with a scattering of only 20-30%. To relax the strong coupling among
the elements, we included 20% systematic errors in the Fe-L region of the
spectra. Then, in X-ray luminous galaxies, the derived abundance of Fe and
$\alpha$-elements both became $\sim 1$ solar within a factor of 2. This result
relaxes the previous severe discrepancy between the ISM and stellar
metallicities. The ISM metallicity in X-ray fainter galaxies is still
uncertain, but we can at least constrain that contribution from type-Ia SN to
the ISM abundance is lower than in X-ray luminous systems. These results
strongly suggest that a large fraction of SN Ia products have escaped into
intergalactic space.


title: Hard X-ray emission from the galaxy cluster A2256

abstract:
  After the positive detection by BeppoSAX of hard X-ray radiation up to ~80
keV in the Coma cluster spectrum, we present evidence for nonthermal emission
from A2256 in excess of thermal emission at a 4.6sigma confidence level. In
addition to this power law component, a second nonthermal component already
detected by ASCA could be present in the X-ray spectrum of the cluster, not
surprisingly given the complex radio morphology of the cluster central region.
The spectral index of the hard tail detected by the PDS onboard BeppoSAX is
marginally consistent with that expected by the inverse Compton model. A value
of ~0.05 microG is derived for the intracluster magnetic field of the extended
radio emission in the northern regions of the cluster, while a higher value of
\~0.5 microG could be present in the central radio halo, likely related to the
hard tail detected by ASCA.


title: Modelling the Infrared continuum of Centaurus A

abstract:
  We present ISOPHOT_S, ISOSWS and 8 to 13 micron ground based observations of
Centaurus A that show prominent PAH and silicate features. These and other data
are used to construct a model for the infrared continuum. We find that in a
nuclear sized aperture (~4 arcsec, ~60 pc) the SED is characteristic of
emission from a starburst and AGN torus; in larger apertures an additional
component of cirrus emission is required. Based on our model, the torus
diameter is estimated to be 3.6 pc and the best fitting inclination angle of
the torus is 45 degrees. This result has implications for the detectability of
tori in low power AGN and in particular for the use of the IRAS 60/25 micron
flux ratio as an indicator of the torus inclination.


title: Radiatively inefficient accretion disks

abstract:
  Radiatively inefficient (or advection dominated) disks are discussed at an
introductory level. Ion supported and radiation supported flows are discussed,
the different consequences of advection dominated flows onto black holes vs.
solid surfaces (neutron stars, white dwarfs), hydrodynamics, the role of the
ratio of specific heats, and the possible connection between ADAFs and
outflows.


title: Accretion Disks

abstract:
  In this lecture the basic theory of accretion disks is introduced, with
emphasis on aspects relevant for X-ray binaries and Cataclysmic Variables.


title:
Near and mid-IR sub-arcsecond structure of the dusty symbiotic star R
  Aqr

abstract:
  The results of a high-resolution interferometric campaign targeting the
symbiotic long-period variable (LPV) R~Aqr are reported. With both
near-infrared measurements on baselines out to 10m and mid-infrared data
extending to 32m, we have been able to measure the characteristic sizes of
regions from the photosphere of the LPV and its extended molecular atmosphere,
out to the cooler circumstellar dust shell. The near-infrared data were taken
using aperture masking interferometry on the Keck-I telescope and show R~Aqr to
be partially resolved for wavelengths out to 2.2 microns but with a marked
enlargement, possibly due to molecular opacity, at 3.1 microns. Mid-infrared
interferometric measurements were obtained with the U.C. Berkeley Infrared
Spatial Interferometer (ISI) operating at 11.15 microns from 1992 to 1999.
Although this dataset is somewhat heterogeneous with incomplete coverage of the
Fourier plane and sampling of the pulsation cycle, clear changes in the
mid-infrared brightness distribution were observed, both as a function of
position angle on the sky and as a function of pulsation phase. Spherically
symmetric radiative transfer calculations of uniform-outflow dust shell models
produce brightness distributions and spectra which partially explain the data,
however limitations to this approximation are noted. Evidence for significant
deviation from circular symmetry was found in the mid-infrared and more
tentatively at 3.08 microns in the near-infrared, however no clear detection of
binarity or of non-LPV elements in the symbiotic system is reported.


title: Michelson Interferometry with the Keck I Telescope

abstract:
  We report the first use of Michelson interferometry on the Keck I telescope
for diffraction-limited imaging in the near infrared JHK and L bands. By using
an aperture mask located close to the f/25 secondary, the 10 m Keck primary
mirror was transformed into a separate-element, multiple aperture
interferometer. This has allowed diffraction-limited imaging of a large number
of bright astrophysical targets, including the geometrically complex dust
envelopes around a number of evolved stars. The successful restoration of these
images, with dynamic ranges in excess of 200:1, highlights the significant
capabilities of sparse aperture imaging as compared with more conventional
filled-pupil speckle imaging for the class of bright targets considered here.
In particular the enhancement of the signal-to-noise ratio of the Fourier data,
precipitated by the reduction in atmospheric noise, allows high fidelity
imaging of complex sources with small numbers of short-exposure images relative
to speckle. Multi-epoch measurements confirm the reliability of this imaging
technique and our whole dataset provides a powerful demonstration of the
capabilities of aperture masking methods when utilized with the current
generation of large-aperture telescopes. The relationship between these new
results and recent advances in interferometry and adaptive optics is briefly
discussed.


title: Turbulence and diffusion: fossil turbulence

abstract:
  Fossil turbulence processes are central to turbulence, turbulent mixing, and
turbulent diffusion in the ocean and atmosphere, in astrophysics and cosmology,
and in most other natural flows. George Gamov suggested in 1954 that galaxies
might be fossils of primordial turbulence produced by the Big Bang. John Woods
showed that breaking internal waves on horizontal dye sheets in the interior of
the stratified ocean form highly persistent remnants of these turbulent events,
which he called fossil turbulence. The dark mixing paradox of the ocean refers
to undetected mixing that must exist somewhere to explain why oceanic scalar
fields like temperature and salinity are so well mixed, just as the dark matter
paradox of galaxies refers to undetected matter that must exist to explain why
rotating galaxies don't fly apart by centrifugal forces. Both paradoxes result
from sampling techniques that fail to account for the extreme intermittency of
random variables involved in self-similar, nonlinear, cascades over a wide
range of scales; turbulent vorticity for dark mixing, and accreting
small-planetary-mass MACHO number density for dark matter.


title: Fundamental properties of the open cluster NGC 2355

abstract:
  NGC 2355 is an old open cluster in the outer part of the galactic disk (l=203
deg, b=+11 deg) which has been little studied until now. This paper presents
the first astrometric and spectroscopic investigation of this cluster. We have
measured precise absolute proper motions from old Carte du Ciel plates, POSS-I
plates and recent CCD observations obtained with the Bordeaux meridian circle.
The proper motion data reveal 38 highly probable cluster members down to B=15
mag within 7' of the cluster center. We have also obtained ELODIE high
resolution spectra for 24 stars. Seventeen of them are confirmed to be members
of the cluster on the basis of radial velocity. Eight of them are fast rotating
turnoff stars for which the projected rotational velocity has been determined.
The spectroscopic observations have also provided estimates of the physical
parameters Teff, log(g), [Fe/H], Mv of the 24 target stars. Two stragglers have
been identified in the cluster. Combining our astrometric and spectroscopic
results with previous UBV photometry and recent JHKs photometry from the 2MASS
survey we have derived the fundamental properties of the cluster : metallicity,
age, distance, size, spatial velocity and orbit.


title: Massive star clusters in dwarf starburst galaxies

abstract:
  I will discuss the presence of massive star clusters in starburst galaxies
with an emphasis on low mass galaxies outside the local group. I will show that
such galaxies, with respect to their mass and luminosity, may be very rich in
young luminous clusters.


title: Where Do We Stand?

abstract:
  I review the understanding of bulges that emerged from the lively discussions
and presentations during the meeting, and emphasize areas for future work. The
evidence is for a diversity of `bulges', and of formation mechanisms.


title: Observations of Eddington-limited type-I X-ray bursts from 4U 1812-12

abstract:
  During more than 3 years (August 1996-October 1999) monitoring of a 40x40
degree sky region around the Galactic Centre by the Wide Field Cameras on board
BeppoSAX, a total of 8 type-I bursts have been detected from a sky position
consistent with that of 4U 1812-12, a likely neutron-star low-mass X-ray
binary. We present the results of a detailed study of the bursts of 4U 1812-12,
about 15 years after the last reported observations of X-ray bursts from this
source (Murakami et al. 1983). Clear evidence for photospheric radius expansion
due to Eddington-limited burst luminosity is present in most of the observed
events, allowing an accurate estimate of the source distance (~4 kpc) and its
burst parameters.


title: Bound systems in an expanding universe

abstract:
  The Schwarzchild solution insertion in an expanding universe, the so-called
"Swiss cheese model," is shown to possess a very unphysical property.
Specifically, in this model some trajectories are discontinuous functions of
their initial conditions. An alternate metric is proposed as a remedy. It goes
smoothly between the Schwarzchild exterior solution and the Friedmann-Lemaitre,
expanding universe metric. It is further shown that the effects of the
expansion on planetary motions in the solar system are too small to be
currently observed for this alternate metric.


title: A Rapid X-ray Flare from Markarian 501

abstract:
  We present X-ray observations of the BL Lacertae (BL Lac) object Markarian
501 (Mrk 501), taken with the Rossi X-ray Timing Explorer in 1998 May as part
of a multi-wavelength campaign. The X-ray light curve shows a very rapid flare
in which the 2-10 keV flux increased by ~60% in <200 seconds. This rapid rise
is followed by a drop-off in the 2-10 keV flux of ~40% in <600 seconds. The
10-15 keV variation in this flare is roughly a factor of two on similar
time-scales. During the rise of the flare, the 3-15 keV spectral index hardened
from 2.02 +/- 0.03 to 1.87 +/- 0.04, where it remained during the decay of the
flare. This is the fastest variation ever seen in X-rays from Mrk 501 and among
the fastest seen at any wavelength for this object. The shift in the energy at
which the spectral power peaks (from <3 keV before the flare to >30 keV during
the flare) is also among the most rapid shifts seen from this object. This
flare occurs during an emission state (2-10 keV flux approximately 1.2e-10 erg
cm^-2 s^-1) that is approximately 25% of the peak flux observed in 1997 April
from this object but which is still high compared to its historical average
X-ray flux. The variations in the hardness ratio are consistent with the low
energy variations leading those at high energies during the development and
decay of the flare. This pattern is rare among high frequency peaked BL Lac
objects like Mrk 501, but has been seen recently in two other TeV emitting BL
Lacs, Mrk 421 and PKS 2155-304. The hard lag is consistent with a flare
dominated by the acceleration time-scale for a simple relativistic shock model
of flaring.


title:
The Chemical Composition of the Small Magellanic Cloud H II Region NGC
  346 and the Primordial Helium Abundance

abstract:
  Spectrophotometry in the 3400-7400 range is presented for 13 areas of the
brightest H II region in the SMC: NGC 346. The observations were obtained at
CTIO with the 4-m telescope. Based on these observations its chemical
composition is derived. The helium and oxygen abundances by mass are given by:
Y(SMC)=0.2405+-0.0018 and O(SMC)=0.00171+-0.00025. From models and observations
of irregular and blue compact galaxies it is found that dY/dO=3.5+-0.9 and
consequently that the primordial helium abundance by mass is given by:
Yp=0.2345+-0.0026 (1-sigma). This result is compared with values derived from
Big Bang nucleosynthesis, and with other determinations of Yp.


title: The Morphologies of the Small Magellanic Cloud

abstract:
  We compare the distribution of stars of different spectral types, and hence
mean age, within the central SMC and find that the asymmetric structures are
almost exclusively composed of young main sequence stars. Because of the
relative lack of older stars in these features, and the extremely regular
distribution of red giant and clump stars in the SMC central body, we conclude
that tides alone are not responsible for the irregular appearance of the
central SMC. The dominant physical mechanism in determining the current-day
appearance of the SMC must be star formation triggered by a hydrodynamic
interaction between gaseous components. These results extend the results of
population studies (cf. Gardiner and Hatzidimitriou) inward in radius and also
confirm the suggestion of the spheroidal nature of the central SMC based on
kinematic arguments (Dopita et al; Hardy, Suntzeff & Azzopardi). Finally, we
find no evidence in the underlying older stellar population for a ``bar'' or
``outer arm'', again supporting our classification of the central SMC as a
spheroidal body with highly irregular recent star formation.


title: Photometry of Nova V 1493 Aql

abstract:
  We report on photometric observations of V 1493 Aql during the early decline
and highlight some uncommon aspects of the light curve. V 1493 Aql was hotter
at maximum light than in the following phases, and was characterized by the
presence of a long lasting secondary maximum, that, unlike in other novae, was
quite red in color. The mean of three distance estimates yields d ~ 18.8 +/-
3.6 Kpc. Such a large distance would place V 1493 Aql at the extreme outskirts
of our Galaxy or even in an external Local Group galaxy.


title:
Very-high-energy gamma radiation associated with the unshocked wind of
  the Crab pulsar

abstract:
  We show that the relativistic wind in the Crab pulsar, which is commonly
thought to be invisible in the region upstream of the termination shock at R <
0.1 pc, in fact could be directly observed through its inverse Compton gamm-ray
emission. The search for such specific component of radiation in the gamma-ray
spectrum of the Crab can provide unique information about the unshocked pulsar
wind that is not accessible at other wavelengths.


title: Two Large HI Shells in the Outer Galaxy near l=279 degrees

abstract:
  As part of a survey of HI 21-cm emission in the Southern Milky Way, we have
detected two large shells in the interstellar neutral hydrogen near l=279 deg.
The center velocities are +36 and +59 km/s, which puts the shells at kinematic
distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and
very empty, with density contrast of at least 15 between the middle and the
shell walls. It has expansion velocity of about 20 km/s and swept up mass of
several million solar masses. The energy indicated by the expansion may be as
high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The
smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s
and swept up mass of about 10^6 solar masses.
  Morphologically both regions appear to be shells, with high density regions
mostly surrounding the voids, although the first appears to have channels of
low density which connect with the halo above and below the HI layer. They lie
on the edge of the Carina arm, which suggests that they may be expanding
horizontally into the interarm region as well as vertically out of the disk. If
this interpretation is correct, this is the first detection of an HI chimney
which has blown out of both sides of the disk.


title:
TeV Gamma Rays from BL Lac Objects due to Synchrotron Radiation of
  Extremely High Energy Protons

abstract:
  One of remarkable features of the gamma ray blazar Markarian 501 is the
reported shape of the TeV spectrum, which during strong flares of the source
remains essentially stable despite dramatic variations of the absolute
gamma-ray flux. I argue that this (to a large extent unexpected) behavior of
the source could be explained assuming that the TeV gamma-ray emission is a
result of synchrotron radiation of extremely high energy protons in highly
magnetized compact regions of the jet.


title: Large Scale Structure in the weakly non-linear regime

abstract:
  Is gravitational growth responsible for the observed large scale structure in
the universe? Do we need non-gaussian initial conditions or non-gravitational
physics to explain the large scale features traced by galaxy surveys? I will
briefly revise the basic ideas of non-linear perturbation theory (PT) as a tool
to understand structure formation, in particular through the study of higher
order statistics, like the skewness and the 3-point function. Contrary to what
happens with the second order statistics (the variance or power-spectrum), this
test of gravitational instability is independent of the overall amplitude of
fluctuations and of cosmic evolution, so that it does not require comparing the
clustering at different redshifts. Predictions from weakly non-linear PT have
been compared with observations to place constraints on our assumptions about
structure formation, the initial conditions and how galaxies trace the mass.


title: Star Clusters and the Duration of Starbursts

abstract:
  The duration of starbursts is important for determining how they are
regulated and the impact they have on their environment. Starbursts contain
numerous prominent star clusters which typically comprise in total ~20% of the
ultraviolet light, embedded in a diffuse glow of recently formed stars
responsible for the dominant ~80%. Hubble Space Telescope images have been
obtained for four starburst galaxies in order to determine their burst duration
from the ages of their star clusters. Preliminary results for NGC3310 are
reported here. The UBI colors of its clusters and diffuse light give consistent
results. The clusters have a broad range of colors, consistent with a
population of instantaneous bursts with ages ranging from 0 to a few 100 Myr.
The diffuse light has a narrow range of color consistent with continuous star
formation over timescales ranging from ~ 10 to 100 Myr. Hence NGC3310's
starburst has lasted ~100 Myr, or about 10 times the crossing time. Other
results noted in the literature also indicate bursts lasting several times
longer than the crossing time. These results suggest that starbursts are not
self-extinguishing flashes. Rather they are sustainable, perhaps
self-regulated.


title:
GADGET: A code for collisionless and gasdynamical cosmological
  simulations

abstract:
  We describe the newly written code GADGET which is suitable both for
cosmological simulations of structure formation and for the simulation of
interacting galaxies. GADGET evolves self-gravitating collisionless fluids with
the traditional N-body approach, and a collisional gas by smoothed particle
hydrodynamics. Along with the serial version of the code, we discuss a parallel
version that has been designed to run on massively parallel supercomputers with
distributed memory. While both versions use a tree algorithm to compute
gravitational forces, the serial version of GADGET can optionally employ the
special-purpose hardware GRAPE instead of the tree. Periodic boundary
conditions are supported by means of an Ewald summation technique. The code
uses individual and adaptive timesteps for all particles, and it combines this
with a scheme for dynamic tree updates. Due to its Lagrangian nature, GADGET
thus allows a very large dynamic range to be bridged, both in space and time.
So far, GADGET has been successfully used to run simulations with up to 7.5e7
particles, including cosmological studies of large-scale structure formation,
high-resolution simulations of the formation of clusters of galaxies, as well
as workstation-sized problems of interacting galaxies. In this study, we detail
the numerical algorithms employed, and show various tests of the code. We
publically release both the serial and the massively parallel version of the
code.


title:
Gravity-Modes in ZZ Ceti Stars: IV. Amplitude Saturation by Parametric
  Instability

abstract:
  ZZ Ceti stars exhibit small amplitude photometric pulsations in multiple
gravity-modes. We demonstrate that parametric instability, a form of resonant
3-mode coupling, limits overstable modes to amplitudes similar to those
observed. In particular, it reproduces the observed trend that longer period
modes have larger amplitudes. Parametric instability involves the
destabilization of a pair of stable daughter modes by an overstable parent
mode. The 3-modes must satisfy exact angular selection rules and approximate
frequency resonance. The lowest instability threshold for each parent mode is
provided by the daughter pair that minimizes
$(\delta\omega^2+\gamma_d^2)/\kappa^2$, where $\kappa$ is the nonlinear
coupling constant, $\delta\omega$ is the frequency mismatch, and $\gamma_d$ is
the energy damping rate of the daughter modes. The overstable mode's amplitude
is maintained at close to the instability threshold value.
  Although parametric instability defines an upper envelope for the amplitudes
of overstable modes in ZZ Ceti stars, other nonlinear mechanisms are required
to account for the irregular distribution of amplitudes of similar modes and
the non-detection of modes with periods longer than $1,200\s$. Resonant 3-mode
interactions involving more than one excited mode may account for the former.
Our leading candidate for the latter is Kelvin-Helmholtz instability of the
mode-driven shear layer below the convection zone.


title:
Primordial Lithium Abundance as a Stringent Constraint on the Baryonic
  Content of the Universe

abstract:
  We have refined the estimate of the primordial level of $^7$Li abundance to
an accuracy better than 10%, based on high-precision Li abundances for
metal-poor halo stars, and a recent model of post-BBN (Big Bang
Nucleosynthesis) chemical evolution that provides a quantitative explanation of
the detected gentle ascent of the Spite Plateau for stars with metallicities
[Fe/H]$ > -3$. Our maximum likelihood analysis obtains an estimate for the
primordial Li abundance of $A({\rm Li})_p=2.07^{+0.16}_{-0.04}$, after taking
into account possible systematic errors in the estimation of Li abundances,
with the exception of a still-controversial issue regarding stellar depletion.
The inferred value of $\eta$ (the baryon-to-photon number-density ratio in the
universe) based on this estimate is more consistent with that derived from the
set of reported ``low He'' + ``high D'' from extragalactic sites than that
derived from reported ``high He'' + ``low D'' measurements. Since, within
current models of stellar depletion processes, it is difficult to account for
the observed very small scatter of Li abundance in metal-poor stars, our
estimate of $A({\rm Li})_p$ should be taken as an independent constraint on the
baryonic mass density parameter in the universe, giving $\Omega_b
h^2=(0.64-1.4)\times 10^{-2}$ with $h=H_0/100$ km s$^{-1}$Mpc$^{-1}$.


title: The vertical extent and kinematics of the HI in NGC 2403

abstract:
  The neutral hydrogen line profiles along the major axis of the nearby spiral
galaxy NGC 2403 show a wing towards the sytemic velocity. This asymmetry can be
explained with the presence of an abnormally thick HI disk (FWHM ~ 5 kpc) or
with a two-component structure: a thin disk and a slowly rotating, thicker (1-3
kpc) HI layer. The latter model gives a better representation of the
observations. These results throw a new light on the disk-halo connection. In
particular, the decrease of rotational velocity with height above the plane may
be the result of a galactic fountain flow. A vertically extended, slowly
rotating HI layer may be common among spiral galaxies with high levels of star
formation.


title: Influence of new reaction rates on 18F production in novae

abstract:
  Gamma-ray emission from classical novae is dominated, during the first hours,
by positron annihilation resulting from the beta decay of radioactive nuclei.
The main contribution comes from the decay of 18F and hence is directly related
to 18F formation during the outburst. A good knowledge of the nuclear reaction
rates of production and destruction of 18F is required to study 18F synthesis
in novae and the resulting gamma-ray emission. The rates relevant for the main
mode of 18F destruction (i.e, through proton captures) have been the object of
many recent experiments. However, subsequent analyses were focused on providing
rates for X-ray burst nucleosynthesis not valid at nova temperatures (lower
than 3.5 10^8 K). Accordingly, it is crucial to propose and discuss new
reaction rates, incorporating all new experimental results, down to the domain
of nova nucleosynthesis. We show that in this temperature regime, the
18F(p,gamma) and (p,alpha) reaction rates remain uncertain and deserve further
experimental and theoretical efforts. Our hydrodynamic calculations including
the new nuclear rates demonstrate that their impact on 18F synthesis in nova
explosions is quite large and, consequently, the early gamma-ray emission from
classical novae is also affected.


title:
The Sombrero Galaxy III. Ionised gas and dust in the central 200 pc: a
  nuclear bar?

abstract:
  We present the results of new 3D TIGER spectroscopic observations and
archived HST/WFPC2 and NICMOS images of the central region of M 104. The
[NII]+Ha images reveal the presence of a nuclear spiral structure, and the
gaseous kinematics in the central arcsecond shows evidence for kinematical
decoupling of the central peak. A straight nuclear dust lane, with a weak
symmetric counterpart, is seen in the V-I and V-H colour maps. These results
hint for the presence of a strong nuclear bar, that would be located inside the
inner Linblad resonance of the large-scale bar discussed by Emsellem (1995).


title: X-ray emitting-absorbing media in Seyfert 1 galaxies

abstract:
  Seyfert galaxies have been shown to exhibit a large variety of features in
their X-ray spectra from which the environment of the central engine can be
deduced. We focus on the two following aspects: the Warm Absorber, mainly
responsible of the soft X-ray properties, and the reprocessing/reflecting
plasma medium at the origin of the iron Kalpha fluorescent line. The physical
parameters and the location of the so-called Warm Absorber (WA), a photoionized
medium along the line of sight to the nuclear region, are more strongly
constrained by optical coronal lines than by the oxygen edges observed in the
soft X-rays and produced by the WA. The photoionization models also predict the
intensities of the X-ray emission lines which are going to be detected with the
new generation of X-ray satellites. An alternative model to the relativistic
accretion disc is proposed to explain the profile of the X-ray iron Kalpha line
observed in the Seyfert 1. This line can be formed in the framework of a
quasi-spherical accretion of optically thick clouds. An optically thick
photoionization code coupled with a Monte-Carlo code has been developed to
compute the entire spectrum from the IR to the hard X-rays for a close geometry
with a large covering factor. The multiple Compton reflections allow to
reproduce the redshifted broad iron line as detected in several Seyfert 1
galaxies and in MCG-6-30-15.


title: Hydrodynamical Models of Accretion Disks in SU UMa Systems

abstract:
  We numerically test the mode-coupling model (Lubow 1991a) of tidal
instability in SU UMa systems. So far, all numercial models confirming it have
been based on SPH codes and isothermal equation of state. In our paper we
present Eulerian models, using both isothermal approximation and the full
energy equation. We also investigate influence of different ways of mass
transfer. While isothermal models behave similarly to SPH simulations, the
behaviour of models with full energy equation is quite different, and the
mode-coupling model is not confirmed in this case.


title: Grain Survival in Supernova Remnants and Herbig-Haro Objects

abstract:
  By using the flux ratio [FeII]8617/[OI]6300, we demonstrate that most of the
interstellar dust grains survive in shocks associated with supernova remnants
and Herbig-Haro objects. The [FeII]/[OI] flux ratio is sensitive to the
gas-phase Fe/O abundance ratio, but is insensitive to the ionization state,
temperature, and density of the gas. We calculate the [FeII]/[OI] flux ratio in
shocks, and compare the results with the observational data. When only 20% of
iron is in the gas phase, the models reproduce most successfully the
observations. This finding is in conflict with the current consensus that
shocks destroy almost all the grains and 100% of metals are in the gas phase.
We comment on previous works on grain destruction, and discuss why grains are
not destroyed in shocks.


title:
Ekman layers and the damping of inertial r-modes in a spherical shell:
  application to neutron stars

abstract:
  Recently, eigenmodes of rotating fluids, namely inertial modes, have received
much attention in relation to their destabilization when coupled to
gravitational radiation within neutron stars. However, these modes have been
known for a long time by fluid dynamicists. We give a short account of their
history and review our present understanding of their properties. Considering
the case of a spherical container, we then give the exact solution of the
boundary (Ekman) layer flow associated with inertial r-modes and show that
previous estimations all underestimated the dissipation by these layers. We
also show that the presence of an inner core has little influence on this
dissipation. As a conclusion, we compute the window of instability in the
Temperature/rotation plane for a crusted neutron star when it is modeled by an
incompressible fluid.


title:
Rearrangement of the Fermi Surface of Dense Neutron Matter and Direct
  Urca Cooling of Neutron Stars

abstract:
  It is proposed that a rearrangement of single-particle degrees of freedom may
occur in a portion of the quantum fluid interior of a neutron star. Such a
rearrangement is associated with the pronounced softening of the spin-isospin
collective mode which, under increasing density, leads to pion condensation.
Arguments and estimates based on fundamental relations of many-body theory show
that one realization of this phenomenon could produce very rapid cooling of the
star via a direct nucelon Urca process displaying a $T^5$ dependence on
temperature.


title:
High resolution near-infrared polarimetry of Eta Carinae and the
  Homunculus Nebula

abstract:
  High resolution near-infrared adaptive optics imaging and polarimetry have
been obtained of the Homunculus Nebula and its central star Eta Carinae (HD
93308). Maps of the linear polarization at a resolution of <0.2'' have been
made in the J, H and K bands and in a narrow 2.15micron continuum band (K_c).
The overall appearance of the nebula in the near-IR is similar in all bands and
to that at V, with specific features less sharp to longer wavelengths. The
large-scale pattern of polarization vectors is centro-symmetric demonstrating
that single scattering dominates everywhere except perhaps in the central
bright core. Polarization maps at K_c in the near vicinity of Eta Carinae were
restored using a PSF derived from blind deconvolution. An extension of the flux
in the direction of the optically visible speckle knots was detected, with 18%
polarization determined for one knot.
  The most remarkable result of the linear polarization mapping is the level of
similarity in the spatial structure, and the degree of polarization, between
the near-IR and optical data. Such a polarization pattern cannot arise in Mie
scattering from a single power law distribution of grain sizes unless the
particles are very small compared with the wavelength, in disagreement with
mid-IR observations. The colour dependence of the extinction was found to be
shallower than the typical ISM, indicating the presence of large grains.
Optical depth effects and a broad distribution in grain sizes are suggested to
explain these contradictory results, perhaps with a dependence of the grain
size with depth into the small obscuring clouds in the Homunculus lobes.
However the presence of aligned grains, previously inferred from mid-IR
polarization, may also affect the dust-scattered radiation. (Abridged abstract)


title: Structured outflow from a dynamo active accretion disc

abstract:
  We present an axisymmetric numerical model of a dynamo active accretion disc.
If the dynamo-generated magnetic field in the disc is sufficiently strong
(close to equipartition with thermal energy), a fast magneto-centrifugally
driven outflow develops within a conical shell near the rotation axis, together
with a slower pressure driven outflow from the outer parts of the disc as well
as around the axis. Our results show that a dynamo active accretion disc can
contribute to driving an outflow even without any external magnetic field. The
fast outflow in the conical shell is confined by the azimuthal field which is
produced by the dynamo in the disc and advected to the disc corona. This part
of the outflow has high angular momentum and is cooler and less dense than its
surroundings. The conical shell's half-opening angle is typically about 30
degrees near the disc and decreases slightly with height. The slow outflow is
hotter and denser.


title: First Detection of Molecular Gas in the Shells of CenA

abstract:
  Shells are faint arc-like stellar structures, which have been observed around
early type galaxies and are thought to be the result of an interaction. HI gas
has recently been detected in shells, a surprising result in view of the
theoretical predictions that most of the gas should decouple from stars and
fall into the nucleus in such interactions. Here we report the first detection
of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128
(CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The
ratio between CO and HI emission in the shells is the same as that found in the
central regions, which is unexpected given the metallicity gradient usually
observed in galaxies. We propose that the dynamics of the gas can be understood
within the standard picture of shell formation if one takes into account that
the interstellar medium is clumpy and hence not highly dissipative. The
observed metal enrichment could be due to star formation induced by the AGN jet
in the shells. Furthermore our observations provide evidence that molecular gas
in mergers may be spread out far from the nuclear regions.


title:
Reionization Constraints on the Contribution of Primordial Compact
  Objects to Dark Matter

abstract:
  Many lines of evidence suggest that nonbaryonic dark matter constitutes
roughly 30% of the critical closure density, but the composition of this dark
matter is unknown. One class of candidates for the dark matter is compact
objects formed in the early universe, with typical masses M between 0.1 and 1
solar masses to correspond to the mass scale of objects found with microlensing
observing projects. Specific candidates of this type include black holes formed
at the epoch of the QCD phase transition, quark stars, and boson stars. Here we
show that accretion onto these objects produces substantial ionization in the
early universe, with an optical depth to Thomson scattering out to z=1100 of
approximately tau=2-4 [f_CO\epsilon_{-1}(M/Msun)]^{1/2} (H_0/65)^{-1}, where
\epsilon_{-1} is the accretion efficiency \epsilon\equiv L/{\dot M}c^2 divided
by 0.1 and f_CO is the fraction of matter in the compact objects. The current
upper limit to the scattering optical depth, based on the anisotropy of the
microwave background, is approximately 0.4. Therefore, if accretion onto these
objects is relatively efficient, they cannot be the main component of
nonbaryonic dark matter.


title: The DENIS Point Source Catalogue towards the Magellanic Clouds

abstract:
  We have compiled the near infrared Point Source Catalogue (PSC) towards the
Magellanic Clouds (MCs) extracted from the data obtained with the Deep Near
Infrared Survey of the Southern Sky - DENIS (Epchtein et al. 1997). The
catalogue covers an area of of 19.87* 16 square degrees centered on (RA,
DEC)=(5h27m20s, -69o00'00'') for the Large Magellanic Cloud (LMC) and 14.7* 10
square degrees centered on (RA, DEC)=(h02m40s, -73o00'00'' for the Small
Magellanic Cloud (SMC) at the epoch J2000. It contains about 1300000 sources
towards the LMC and 300000 sources towards the SMC each detected in at least 2
of the 3 photometric bands involved in the survey (I, J, Ks). 70% of the
detected sources are true members of the Magellanic Clouds, respectively and
consist mainly of red giants, asymptotic giant branch stars and super-giants.
The observations have all been made with the same instrument and the data have
been calibrated and reduced uniformly. The catalogue provides a homogeneous set
of photometric data.


title:
Phase space transport in cuspy triaxial potentials: Can they be used to
  construct self-consistent equilibria?

abstract:
  (Abridged) This paper studies chaotic orbit ensembles evolved in triaxial
generalisations of the Dehnen potential which have been proposed to model
ellipticals with a strong density cusp that manifest significant deviations
from axisymmetry. Allowance is made for a possible supermassive black hole, as
well as low amplitude friction, noise, and periodic driving which can mimic
irregularities associated with discreteness effects and/or an external
environment. The degree of chaos is quantified by determining how (1) the
relative number of chaotic orbits and (2) the size of the largest Lyapunov
exponent depend on the steepness of the cusp and the black hole mass, and (3)
the extent to which Arnold webs significantly impede phase space transport,
both with and without perturbations. In the absence of irregularities, chaotic
orbits tend to be extremely `sticky,' so that different pieces of the same
chaotic orbit can behave very differently for 10000 dynamical times or longer,
but even very low amplitude perturbations can prove efficient in erasing many
-- albeit not all -- these differences. The implications thereof are discussed
both for the structure and evolution of real galaxies and for the possibility
of constructing approximate near-equilibrium models using Schwarzschild's
method. Much of the observed qualitative behaviour can be reproduced with a toy
potential given as the sum of an anisotropic harmonic oscillator and a
spherical Plummer potential, which suggests that the results may be generic.


title: Extreme Energy nu_tau Propagation Through the Earth

abstract:
  The propagation of extremely energetic nu_tau's through the Earth is studied
by means of a detailed Monte Carlo simulation. All major mechanisms of nu_tau
interactions and tau energy loss as well as all its relevant decay modes are
properly taken into account. The detection possibility of nu_tau's emerging
from Earth in forthcoming neutrino telescopes is addressed


title: The first radius-expansion X-ray burst from GX 3+1

abstract:
  During several observations in 1999 August with RXTE of the low-mass X-ray
binary GX 3+1, we found a single short and strong X-ray burst. This is the
first burst from GX 3+1 which clearly shows evidence for radius expansion of
the neutron-star photosphere during the thermo-nuclear runaway. We show that
the cooling phase of the neutron star photosphere starts already just before
the end of the contraction phase. Considering the fact that the radius
expansion is due to the burst luminosity being at the Eddington luminosity,
assuming standard burst parameters and accounting for gravitational redshift
effects we derive a distance to the source of ~4.5 kpc, although relaxing these
assumptions may lead to uncertainties up to ~30%. By comparing the persistent
flux with that observed at the peak of the burst we infer that near the time of
the X-ray burst the persistent luminosity of GX 3+1 is ~0.17*L_edd, confirming
predictions from theoretical modeling of X-ray spectra of bright sources like
GX 3+1.


title: Cosmic magnetism, curvature and the expansion dynamics

abstract:
  We discuss how a cosmological magnetic field could affect the expansion of
the universe, through its interaction with the spacetime geometry. The tension
of the field lines means that the magneto-curvature coupling tends to
accelerate positively curved regions and decelerate those with negative
curvature. Depending on the equation of state of the matter, the effect varies
from mimicking that of a cosmological constant to resembling a time-decaying
quintessence. Most interestingly, the coupling between magnetism and geometry
implies that even weak fields have a significant impact if the curvature
contribution is strong. This leads to kinematical complications that can
inhibit the onset of the accelerated phase in spatially open inflationary
models. We employ a simple cosmological model to illustrate these effects and
examine the conditions necessary for them to have a appreciable impact.


title:
The Energy Spectrum of TeV Gamma-Rays from the Crab Nebula as measured
  by the HEGRA system of imaging air Cherenkov telescopes

abstract:
  The Crab Nebula has been observed by the HEGRA (High-Energy Gamma-Ray
Astronomy) stereoscopic system of imaging air Cherenkov telescopes (IACTs) for
a total of about 200 hrs during two observational campaigns: from September
1997 to March 1998 and from August 1998 to April 1999. The recent detailed
studies of system performance give an energy threshold and an energy resolution
for gamma-rays of 500 GeV and ~ 18%, respectively. The Crab energy spectrum was
measured with the HEGRA IACT system in a very broad energy range up to 20 TeV,
using observations at zenith angles up to 65 degrees. The Crab data can be
fitted in the energy range from 1 to 20 TeV by a simple power-law, which yields
dJg/dE = (2.79+/-0.02 +/- 0.5) 10^{-7} E^{-2.59 +/- 0.03 +/- 0.05}, ph m^{-2}
s^{-1} TeV^{-1} The Crab Nebula energy spectrum, as measured with the HEGRA
IACT system, agrees within 15% in the absolute scale and within 0.1 units in
the power law index with the latest measurements by the Whipple, CANGAROO and
CAT groups, consistent within the statistical and systematic errors quoted by
the experiments. The pure power-law spectrum of TeV gamma-rays from the Crab
Nebula constrains the physics parameters of the nebula environment as well as
the models of photon emission.


title: A Prediction of Observable Rotation in the ICM of Abell 3266

abstract:
  We present a numerical Hydro+N-body model of A3266 whose X-ray surface
brightness, temperature distribution, and galaxy spatial and velocity
distribution data are consistent with the A3266 data. The model is an old (~3
Gyr), off-axis merger having a mass ratio of ~2.5:1. The less massive
subcluster in the model is moving on a trajectory from southwest to northeast
passing on the western side of the dominant cluster while moving into the plane
of the sky at ~45 degrees. Off-axis mergers such as this one are an effective
mechanism for transferring angular momentum to the intracluster medium (ICM),
making possible a large scale rotation of the ICM. We demonstrate here that the
ICM rotation predicted by our fully 3-dimensional model of A3266 is observable
with current technology. As an example, we present simulated observations
assuming the capabilities of the high resolution X-ray spectrometer (XRS) which
was to have flown on Astro-E.


title:
The pattern speed of the nuclear disk of M31 using a variant of the
  Tremaine-Weinberg method

abstract:
  The twin peaks in the nucleus of M31 have been interpreted by Tremaine as a
thick, eccentric, disk of stars orbiting a massive dark object; the required
alignment of the apoapsides of the stellar orbits could be maintained by
self-gravity, and the whole structure might be a discrete, nonlinear eigenmode.
The pattern speed of this mode could, in principle, be determined by the
Tremaine-Weinberg (TW) method, which requires measurements of the surface
brightness, and radial velocity along a strip parallel to the line of nodes.
However, spectroscopic observations along the line of nodes are not available.
We propose a variant of the TW method, which exploits a basic feature of the
eccentric disk model, to extract estimates of the pattern speed from {\it
Hubble Space Telescope} spectroscopic data, taken along the line joining the
two peaks. Within limitations imposed by the data, we estimate that the pattern
rotates in a prograde manner and, for an assumed disk inclination of
$77^{\circ}$, the pattern speed $|\Omega_p| < 30 {\rm km s^{-1} pc^{-1}}$, or
period more than $200,000$ years.


title: On the Radii of Close-in Giant Planets

abstract:
  The recent discovery that the close-in extrasolar giant planet, HD209458b,
transits its star has provided a first-of-its-kind measurement of the planet's
radius and mass. In addition, there is a provocative detection of the light
reflected off of the giant planet, $\tau$ Boo b. Including the effects of
stellar irradiation, we estimate the general behavior of radius/age
trajectories for such planets and interpret the large measured radii of
HD209458b and $\tau$ Boo b in that context. We find that HD209458b must be a
hydrogen-rich gas giant. Furthermore, the large radius of close-in gas giant is
not due to the thermal expansion of its atmosphere, but to the high residual
entropy that remains throughout its bulk by dint of its early proximity to a
luminous primary. The large stellar flux does not inflate the planet, but
retards its otherwise inexorable contraction from a more extended configuration
at birth. This implies either that such a planet was formed near its current
orbital distance or that it migrated in from larger distances ($\geq$0.5 A.U.),
no later than a few times $10^7$ years of birth.


title:
Accretion disc-stellar magnetosphere interaction: field line inflation
  and the effect on the spin-down torque

abstract:
  We calculate the structure of a force-free magnetosphere which is assumed to
corotate with a central star and which interacts with an embedded
differentially rotating accretion disc. The magnetic and rotation axes are
aligned and the stellar field is assumed to be a dipole. We concentrate on the
case when the amount of field line twisting through the disc-magnetosphere
interaction is large and consider different outer boundary conditions. In
general the field line twisting produces field line inflation (eg. Bardou &
Heyvaerts 1996) and in some cases with large twisting many field lines can
become open. We calculate the spin-down torque acting between the star and the
disc and we find that it decreases significantly for cases with large field
line twisting. This suggests that the oscillating torques observed for some
accreting neutron stars could be due to the magnetosphere varying between
states with low and high field line inflation. Calculations of the spin
evolution of T Tauri stars may also have to be revised in light of the
significant effect that field line twisting has on the magnetic torque
resulting from star-disc interactions.


title: Extracting Energy from Accretion into Kerr Black Hole

abstract:
  The highest efficiency of converting rest mass into energy by accreting
matter into a Kerr black hole is ~ 31% (Thorne 1974). We propose a new process
in which periods of accretion from a thin disk, and the associated spin-up of
the black hole, alternate with the periods of no accretion and magnetic
transfer of energy from the black hole to the disk. These cycles can repeat
indefinitely, at least in principle, with the black hole mass increasing by ~
66% per cycle, and up to ~ 43% of accreted rest mass radiated away by the disk.


title: Nonlinear Shock Acceleration and Photon Emission in Supernova Remnants

abstract:
  We have extended a simple model of nonlinear diffusive shock acceleration
(Berezhko and Ellison 1999) to include the injection and acceleration of
electrons and the production of photons from bremsstrahlung, synchrotron,
inverse-Compton, and pion-decay processes. We argue that the results of this
model, which is simpler to use than more elaborate ones, offer a significant
improvement over test-particle, power-law spectra which are often used in
astrophysical applications of diffusive shock acceleration. With an
evolutionary supernova remnant (SNR) model to obtain shock parameters as
functions of ambient interstellar medium parameters and time, we predict
broad-band continuum photon emission from supernova remnants in general, and
SN1006 in particular, showing that our results compare well with the more
complete time-dependent and spherically symmetric nonlinear model of Berezhko,
Ksenofontov, and Petukhov (1999). We discuss the implications nonlinear shock
acceleration has for X-ray line emission, and use our model to describe how
ambient conditions determine the TeV/radio flux ratio, an important parameter
for gamma-ray observations of radio SNRs.


title: Angular size in quintessence cosmology

abstract:
  We investigate the influence of an exotic fluid component ("quintessence") on
the angular size-redshift relation for distant extragalactic sources.
Particular emphasis is given for the redshif $z_{m}$ at which the angular size
takes its minimal value. We derive an analytical closed form which determines
how $z_m$ depends on the parameter of the equation of state describing the
exotic component. The results for a flat model dominated by a "quintessence"
are compared in detail with the ones for the standard open model dominated by
cold dark matter. Some consequences of systematic evolutionary effects on the
values of $z_{m}$ are also briefly discussed. It is argued that the critical
redshift, for all practical purposes, may completely be removed if such effects
are taken into account.


title: A Measurement of the Cosmic Ray Spectrum and Composition at the Knee

abstract:
  The energy spectrum and primary composition of cosmic rays with energy
between $3\times 10^{14}$ and $3\times10^{16}\unit{eV}$ have been studied using
the CASA-BLANCA detector. CASA measured the charged particle distribution of
air showers, while BLANCA measured the lateral distribution of Cherenkov light.
The data are interpreted using the predictions of the CORSIKA air shower
simulation coupled with four different hadronic interaction codes.
  The differential flux of cosmic rays measured by BLANCA exhibits a knee in
the range of 2--3 PeV with a width of approximately 0.5 decades in primary
energy. The power law indices of the differential flux below and above the knee
are $-2.72\pm0.02$ and $ -2.95\pm0.02$.
  We present our data both as a mean depth of shower maximum and as a mean
nuclear mass. A multi-component fit using four elemental species shows the same
composition trends given by the mean quantities, and also indicates that QGSJET
and VENUS are the preferred hadronic interaction models. We find that an
initially mixed composition turns lighter between 1 and 3 PeV, and then becomes
heavier with increasing energy above 3 PeV.


title: The ROSAT Brightest Cluster Sample (BCS) - IV. The extended sample

abstract:
  We present a low-flux extension of the X-ray selected ROSAT Brightest Cluster
Sample (BCS) published in Paper I of this series. Like the original BCS and
employing an identical selection procedure, the BCS extension is compiled from
ROSAT All-Sky Survey (RASS) data in the northern hemisphere (dec > 0 deg) and
at high Galactic latitudes (|b| > 20 deg). It comprises 100 X-ray selected
clusters of galaxies with measured redshifts z < 0.3 (as well as seven more at
z > 0.3) and total fluxes between 2.8 x 10-12 erg cm-2 s-1 and 4.4 x 10-12 erg
cm-2 s-1 in the 0.1-2.4 keV band (the latter value being the flux limit of the
original BCS). The extension can be combined with the main sample published in
1998 to form the homogeneously selected extended BCS (eBCS), the largest and
statistically best understood cluster sample to emerge from the ROSAT All-Sky
Survey to date.
  The nominal completeness of the combined sample (defined with respect to a
power law fit to the bright end of the BCS log N-log S distribution) is
relatively low at 75 per cent (compared to 90 per cent for the high-flux sample
of Paper I). However, just as for the original BCS, this incompleteness can be
accurately quantified, and thus statistically corrected for, as a function of
X-ray luminosity and redshift.
  In addition to its importance for improved statistical studies of the
properties of clusters in the local Universe, the low-flux extension of the BCS
is also intended to serve as a finding list for X-ray bright clusters in the
northern hemisphere which we hope will prove useful in the preparation of
cluster observations with the next generation of X-ray telescopes such as
Chandra or XMM-Newton.


title: The only non-contradictory model of universe

abstract:
  The Friedmann equations of universe dynamics describe the infinite number of
the Friedmannian models of universe. The consistent and distinguished
relativistic, classical-mechanical, quantum-mechanical and formal-logical
analysis of the Friedmannian universe models leads to a surprising and
unexpected conclusion: The Friedmannian model of the flat expansive homogeneous
and isotropic universe with the zero gravitational force state equation is the
only model of universe, which does not contradict the: 1st Einstein general
theory of relativity (and its special partial solutions: the Einstein special
theory of relativity and the Newton theory of gravitation); 2nd quantum
mechanics; 3rd fundamental formal principles of logical thinking; and 4th
observations.


title:
Small Deviations from Gaussianity and The Galaxy Cluster Abundance
  Evolution

abstract:
  We raise the hypothesis that the density fluctuations field which originates
the growth of large scale structures is a combination of two or more
distributions. By applying the statistical analysis of finite mixture
distributions to a specific combination of Gaussian plus non-Gaussian random
fields, we studied the case where just a small departure from Gaussianity is
allowed. Our results suggest that even a very small level of non-Gaussianity
may introduce significant changes in the cluster abundance evolution rate.


title: Disk heating and stellar migration in galaxies

abstract:
  The paper claimed that significant radial migration of stars in a stellar
disk like that of the Milky Way could not occur. We now think that while the
treatment of the effects of molecular clouds was correct, the paper seriously
underestimated the ability of spiral arms to shift the radii of stars that
corotate with them. Consequently, it is likely that significant radial
migration_is_ possible.


title: Accretion by Galaxies

abstract:
  Both theory and observation indicate that galaxies like the Milky Way accrete
matter at the rate of a few Msun per year.


title:
Detection and Mapping of Decoupled Stellar and Ionized Gas Structures in
  the Ultraluminous Infrared Galaxy IRAS 12112+0305

abstract:
  Integral field optical spectroscopy with the INTEGRAL fiber-fed system and
HST optical imaging are used to map the complex stellar and warm ionized gas
structure in the ultraluminous infrared galaxy IRAS 12112+0305. Images
reconstructed from wavelength-delimited extractions of the integral field
spectra reveal that the observed ionized gas distribution is decoupled from the
stellar main body of the galaxy, with the dominant continuum and emission-line
regions separated by projected distances of up to 7.5 kpc. The two optical
nuclei are detected as apparently faint emission-line regions, and their
optical properties are consistent with being dust-enshrouded weak-[OI] LINERs.
The brightest emission-line region is associated with a faint (m_{I}= 20.4),
giant HII region of 600 pc diameter, where a young (about 5 Myr) massive
cluster of about 2 $\times$ 10$^7$ $M_{\odot}$ dominates the ionization.
Internal reddening towards the line-emitting regions and the optical nuclei
ranges from 1 to 8 magnitudes, in the visual. Taken the reddening into aacount,
the overall star formation in IRAS 12112+0305 is dominated by starbursts
associated with the two nuclei and corresponding to a star formation rate of 80
$M_{\odot}$ yr$^{-1}$.


title:
Does the unification of BL Lac and FR I radio galaxies require jet
  velocity structures?

abstract:
  We explore the viability of the unification of BL Lacs and FR I radio
galaxies by comparing the core emission of radio galaxies with those of BL Lacs
of similar extended radio power, taking advantage of the newly measured optical
nuclear luminosity of FR I sources. The spectral properties of complete samples
are also studied in the radio-optical luminosity plane: starting from the
Spectral Energy Distribution (SED) of BL Lacs, we calculate the predicted
luminosity of FR I nuclei in the frame of a simple one--zone model, by properly
taking into account the relativistic transformations. We find that the bulk
Lorentz factors required by the spread in the observed luminosities in all
bands are significantly smaller than those implied by other, both observational
and theoretical, considerations. This discrepancy is also reflected in the fact
that FR I nuclei are over-luminous by a factor of 10-10^4, with respect to the
predictions, both in the radio and in the optical band.
  In order to reconcile these results with the unification scheme, velocity
structures in the jet are suggested, where a fast spine is surrounded by a slow
(but still relativistic) layer so that the emission at different angles is
dominated by different velocity components: the fast one dominates the emission
in BL Lacs while the slow layer dominates the emission in misaligned objects.
Furthermore for the lowest luminosity BL Lacs it has to be also postulated that
their beaming factor in the radio band is lower than in the optical (and
X-ray), as would result from deceleration of the jet.
  The self-consistency of the unification model therefore requires that both
intrinsic differences in the SED and different beaming properties play a
substantial role in characterizing the phenomenology of these sources.


title:
The European Large Area ISO Survey II: mid-infrared extragalactic source
  counts

abstract:
  We present preliminary source counts at 6.7um and 15um from the Preliminary
Analysis of the European Large Area ISO survey, with limiting flux densities of
\~2mJy at 15um & ~1mJy at 6.7um. We separate the stellar contribution from the
extragalactic using identifications with APM sources made with the likelihood
ratio technique. We quantify the completeness & reliability of our source
extraction using (a) repeated observations over small areas, (b) cross-IDs with
stars of known spectral type, (c) detections of the PSF wings around bright
sources, (d) comparison with independent algorithms. Flux calibration at 15um
was performed using stellar IDs; the calibration does not agree with the
pre-flight estimates, probably due to effects of detector hysteresis and
photometric aperture correction. The 6.7um extragalactic counts are broadly
reproduced in the Pearson & Rowan-Robinson model, but the Franceschini et al.
(1997) model underpredicts the observed source density by ~0.5-1 dex, though
the photometry at 6.7um is still preliminary. At 15um the extragalactic counts
are in excellent agreement with the predictions of the Pearson & Rowan-Robinson
(1996), Franceschini et al. (1994), Guiderdoni et al. (1997) and the evolving
models of Xu et al. (1998), over 7 orders of magnitude in 15um flux density.
The counts agree with other estimates from the ISOCAM instrument at overlapping
flux densities (Elbaz et al. 1999), provided a consistent flux calibration is
used. Luminosity evolution at a rate of (1+z)^3, incorporating mid-IR spectral
features, provides a better fit to the 15um differential counts than (1+z)^4
density evolution. No-evolution models are excluded, and implying that below
around 10mJy at 15um the source counts become dominated by an evolving
cosmological population of dust-shrouded starbursts and/or active galaxies.


title: Dark Matter Problem in Disk Galaxies

abstract:
  In the generic CDM cosmogony, dark-matter halos emerge too lumpy and
centrally concentrated to host observed galactic disks. Moreover, disks are
predicted to be smaller than those observed. We argue that the resolution of
these problems may lie with a combination of the effects of protogalactic
disks, which would have had a mass comparable to that of the inner dark halo
and be plausibly non-axisymmetric, and of massive galactic winds, which at
early times may have carried off as many baryons as a galaxy now contains. A
host of observational phenomena, from quasar absorption lines and intracluster
gas through the G-dwarf problem point to the existence of such winds. Dynamical
interactions will homogenize and smooth the inner halo, and the observed disk
will be the relic of a massive outflow. The inner halo expanded after absorbing
energy and angular momentum from the ejected material. Observed disks formed at
the very end of the galaxy formation process, after the halo had been reduced
to a minor contributor to the central mass budget and strong radial streaming
of the gas had died down.


title: Using HI to probe large scale structures at z ~ 3

abstract:
  The redshifted 1420 MHz emission from the HI in unresolved damped
Lyman-\alpha clouds at high z will appear as a background radiation in low
frequency radio observations. This holds the possibility of a new tool for
studying the universe at high-z, using the mean brightness temperature to probe
the HI content and its fluctuations to probe the power spectrum. Existing
estimates of the HI density at z~3 imply a mean brightness temperature of 1 mK
at 320 Mhz. The cross-correlation between the temperature fluctuations across
different frequencies and sight lines is predicted to vary from 10^{-7} K^2 to
10^{-8} K^2 over intervals corresponding to spatial scales from 10 Mpc to 40
Mpc for some of the currently favoured cosmological models. Comparing this with
the expected sensitivity of the GMRT, we find that this can be detected with
\~10 hrs of integration, provided we can distinguish it from the galactic and
extragalactic foregrounds which will swamp this signal. We discuss a strategy
based on the very distinct spectral properties of the foregrounds as against
the HI emission, possibly allowing the removal of the foregrounds from the
observed maps.


title: Pre-Supernova Evolution of Massive Stars

abstract:
  We present the preliminary results of a detailed theoretical investigation on
the hydrodynamical properties of Red Supergiant (RSG) stars at solar chemical
composition and for stellar masses ranging from 10 to 20 M_sun. We find that
the main parameter governing their hydrodynamical behavior is the effective
temperature, and indeed when moving from higher to lower effective temperatures
the models show an increase in the dynamical perturbations. Also, we find that
RSGs are pulsationally unstable for a substantial portion of their lifetimes.
These dynamical instabilities play a key role in driving mass loss, thus
inducing high mass loss rates (up to several 1e-4 M_sun/yr) and considerable
variations of the mass loss activity over timescales of the order of 10,000
years. Our results are able to account for the variable mmass loss rates as
implied by radio observations of type II supernovae, and we anticipate that
comparisons of model predictions with observed circumstellar phenomena around
SNII will provide valuable diagnostics about their progenitors and their
evolutionary histories.


title:
SN Ia Enrichment in Virgo Early-Type Galaxies from ROSAT and ASCA
  Observations

abstract:
  We analyzed nine X-ray bright Virgo early-type galaxies observed by both ASCA
and ROSAT. Through spatially resolved spectroscopy, we determined the radial
temperature profile and abundances of Mg, Si and Fe for six galaxies. The
temperature profiles are consistent with isothermal temperatures outside of a
cooler region at the galaxy center. We present new evidence for iron abundance
gradients in NGC4472 and NGC4649 and confirm the previous results on NGC4636.
Mg and Si abundance gradients on average are flatter compared to those of iron
and correspond to an underabundance of alpha-process elements at high Fe
values, while at low iron, the element ratios favor enrichment by type II SNe.
We explain the observed trend by the metallicity dependence of SN Ia metal
production and present constraints on the available theoretical modeling for
low-metallicity inhibition of SNe Ia (Kobayashi et al. 1998). Our results imply
a cut-off metallicity in the range 0.07-0.3 solar and require a lower limit of
0.3 solar on the Fe contribution of SN Ia. We estimate an SN Ia rate at the
centers of the brightest galaxies in our sample of ~0.08 (h_75)^3 SNu
(supernova units). The rates inferred from optical searches should be corrected
for the presence of `faint' SN Ia events, since these release limited metals
and therefore do not contribute significantly to the measured metallicity in
the X-ray gas. With this correction the present-epoch SN Ia rate in early-type
galaxies is 0.10+/-0.06 (h_75)^2 SNu (Cappellaro et al. 1997) and is therefore
comparable with the X-ray estimates. Abridged.


title: QSOs and Absorption Line Systems Surrounding the Hubble Deep Field

abstract:
  We have imaged a 45x45 sq. arcmin. area centered on the Hubble Deep Field
(HDF) in UBVRI passbands, down to respective limiting magnitudes of
approximately 21.5, 22.5, 22.2, 22.2, and 21.2. The principal goals of the
survey are to identify QSOs and to map structure traced by luminous galaxies
and QSO absorption line systems in a wide volume containing the HDF. We have
selected QSO candidates from color space, and identified 4 QSOs and 2 narrow
emission-line galaxies (NELGs) which have not previously been discovered,
bringing the total number of known QSOs in the area to 19. The bright z=1.305
QSO only 12 arcmin. away from the HDF raises the northern HDF to nearly the
same status as the HDF-S, which was selected to be proximate to a bright QSO.
About half of the QSO candidates remain for spectroscopic verification.
Absorption line spectroscopy has been obtained for 3 bright QSOs in the field,
using the Keck 10m, ARC 3.5m, and MDM 2.4m telescopes. Five heavy-element
absorption line systems have been identified, 4 of which overlap the
well-explored redshift range covered by deep galaxy redshift surveys towards
the HDF. The two absorbers at z=0.5565 and z=0.5621 occur at the same redshift
as the second most populated redshift peak in the galaxy distribution, but each
is more than 7Mpc/h (comoving, Omega_M=1, Omega_L=0) away from the HDF line of
sight in the transverse dimension. This supports more indirect evidence that
the galaxy redshift peaks are contained within large sheet-like structures
which traverse the HDF, and may be precursors to large-scale ``pancake''
structures seen in the present-day galaxy distribution.


title: Structure of the Large Magellanic Cloud from 2MASS

abstract:
  We derive structural parameters and evidence for extended tidal debris from
star count and preliminary standard candle analyses of the Large Magellanic
Cloud based on Two Micron All Sky Survey (2MASS) data. The full-sky coverage
and low extinction in K_s presents an ideal sample for structural analysis of
the LMC.
  The star count surface densities and deprojected inclination for both young
and older populations are consistent with previous work. We use the full areal
coverage and large LMC diameter to Galactrocentric distance ratio to infer the
same value for the disk inclination based on perspective.
  A standard candle analysis based on a sample of carbon long-period variables
(LPV) in a narrow color range, 1.6<J-K_s<1.7 allows us to probe the
three-dimensional structure of the LMC along the line of sight. The intrinsic
brightness distribution of carbon LPVs in selected fields implies that
$\sigma_M\simlt 0.2^m$ for this color cut. The sample provides a {\it direct}
determination of the LMC disk inclination: $42.3^\circ\pm 7.2^\circ$.
  Distinct features in the photometric distribution suggest several distinct
populations. We interpret this as the presence of an extended stellar component
of the LMC, which may be as thick as 14 kpc, and intervening tidal debris at
roughly 15 kpc from the LMC.


title: Weak Lensing by Large-Scale Structure: A Dark Matter Halo Approach

abstract:
  Weak gravitational lensing observations probe the spectrum and evolution of
density fluctuations and the cosmological parameters which govern them but are
currently limited to small fields and subject to selection biases. We show how
the expected signal from large-scale structure arises from the contributions
from and correlations between individual halos. We determine the convergence
power spectrum as a function of the maximum halo mass and so provide the means
to interpret results from surveys that lack high mass halos either through
selection criteria or small fields. Since shot noise from rare massive halos is
mainly responsible for the sample variance below 10', our method should aid our
ability to extract cosmological information from small fields.


title: 2dF QSO Redshift Survey

abstract:
  With approximately 6000 QSO redshifts,the 2dF QSO redshift survey is already
the biggest complete QSO survey. The aim for the survey is to have 25000 QSO
redshifts, providing an order of magnitude increase in QSO clustering
statistics. We first describe the observational parameters of the 2dF QSO
survey. We then describe several highlights of the survey so far, including new
estimates of the QSO luminosity function and its evolution. We also review the
current status of QSO clustering analyses from the 2dF data. Finally, we
discuss how the complete QSO survey will be able to constrain the value of
Omega_o by measuring the evolution of QSO clustering, place limits on the
cosmological constant via a direct geometrical test and determine the form of
the fluctuation power-spectrum out to the approximately 1000 Mpc scales only
previously probed by COBE.


title:
Detection of new sources of methanol emission at 95 GHz with the Mopra
  telescope

abstract:
  A southern hemisphere survey of methanol emission sources has been carried
out using the ATNF Mopra millimetre telescope. 85 sources, the majority of them
masers, have been detected in the 8(0)-7(1)A+ transition of methanol at 95 GHz.
Together with a similar northern hemisphere survey this completes the search
for 95-GHz methanol emission from the Galactic Plane. The previously found
correlation between intensity of methanol emission at 44 and 95 GHz is
confirmed here with the larger sample of sources. The results of LVG
statistical equilibrium calculations confirm the classification of these
sources as class I methanol masers pumped through collisional excitation.


title:
Reanalyses of Voyager Ultraviolet Spectrometer Limits to the EUV and FUV
  Diffuse Astronomical Flux

abstract:
  We re-examine {\em Voyager} Ultraviolet Spectrograph data used to establish
upper limits to the $\lambda$ 500-900 \AA and $\lambda$ 900-1100 \AA cosmic
diffuse background. The measurement of diffuse flux with the {\em Voyager} UVS
data requires complex corrections for noise sources which are far larger than
the astronomical signal. In the analyses carried out to date, the upper limits
obtained on the diffuse background show statistical anomalies which indicate
that substantial systematic errors are present. We detail these anomalies and
identify specific problems with the analysis. We derive statistically robust
2~$\sigma$ upper limits for continuum flux of 570~photons~s$^{-1}$
cm$^{-2}$~ster$^{-1}$~\AA$^{-1}$ and for the 1000 \AA~ diffuse line flux of
11,790~photons~s$^{-1}$ cm$^{-2}$~ster$^{-1}$. The true limits may be
substantially higher because of unknown systematic uncertainties. The new
statistical limits alone are insufficient to support previous conclusions based
on the {\em Voyager} data including work on the character of interstellar dust
and estimates of the diffuse extragalactic far UV background as absorbed by
intergalactic dust.


title: A Very Fast and Momentum-Conserving Tree Code

abstract:
  The tree code for the approximate evaluation of gravitational forces is
extended and substantially accelerated by including mutual cell-cell
interactions. These are computed by a Taylor series in Cartesian coordinates
and in a completely symmetric fashion, such that Newton's third law is
satisfied by construction and hence momentum exactly conserved. The
computational effort is further reduced by exploiting the mutual symmetry of
the interactions. For typical astrophysical problems with N=10^5 and at the
same level of accuracy, the new code is about four times faster than the tree
code. For large N, the computational costs are found to scale almost linearly
with N, which can also be supported by a theoretical argument, and the
advantage over the tree code increases with ever larger N.


title:
Kilohertz quasi-periodic oscillations in low mass X-ray binary sources
  and their relation with the neutron star magnetic field

abstract:
  Starting from the observation that kilohertz Quasi Period Oscillations (kHz
QPO) occur in a very narrow range of X-ray luminosities in neutron star low
mass X-ray binaries, we try to link the kHz QPO observability to variations of
the neutron star magnetospheric radius, in response to changing mass inflow
rate. At low luminosities, the drop off of kHz QPOs activity may be explained
by the onset of the centrifugal barrier, when the magnetospheric radius reaches
the corotation radius. At the opposite side, at higher luminosities, the
magnetospheric radius may reach the neutron star and the vanishing of the
magnetosphere may led to the stopping of the kHz QPOs activity. If we apply
these constraints, the magnetic fields of atoll (B ~ 0.3-1 10^8 G for Aql X-1)
and Z (B ~ 1-8 10^8 G for Cyg X-2) sources can be derived. These limits
naturally apply in the framework of beat frequency models but can also work in
the case of general relativistic models.


title: The HII Region KR 140: Spontaneous Formation of a High Mass Star

abstract:
  We have used a multiwavelength data set from the Canadian Galactic Plane
Survey (CGPS) to study the Galactic HII region KR 140, both on the scale of the
nebula itself and in the context of the star forming activity in the nearby
W3/W4/W5 complex of molecular clouds and HII regions. From both radio and
infrared data we have found a covering factor of about 0.5 for KR 140 and we
interpret the nebula as a bowl-shaped region viewed close to face on.
Extinction measurements place the region on the near side of its parent
molecular cloud. The nebula is kept ionized by one O8.5 V(e) star, VES 735,
which is less than a few million years old. CO data show that VES 735 has
disrupted much of the original molecular cloud for which the estimated mass and
density are about 5000 $M_{\odot}$ and 100 cm$^{-3}$, respectively. KR 140 is
isolated from the nearest star forming activity, in W3. Our data suggest that
KR 140 is an example of spontaneous (i.e., non-triggered) formation of,
unusually, a high mass star.


title: Protostellar Collapse with Various Metallicities

abstract:
  The thermal and chemical evolution of gravitationally collapsing protostellar
clouds is investigated, focusing attention on their dependence on metallicity.
Calculations are carried out for a range of metallicities spanning the local
interstellar value to zero. During the time when clouds are transparent to
continuous radiation, the temperatures are higher for those with lower
metallicity, reflecting lower radiative ability. However, once the clouds
become opaque, in the course of the adiabatic contraction of the transient
cores, their evolutionary trajectories in the density-temperature plane
converge to a unique curve that is determined by only physical constants. The
trajectories coincide with each other thereafter. Consequently, the size of the
stellar core at the formation is the same regardless of the gas composition of
the parent cloud.


title:
A Direct Distance to the LMC Cepheid HV 12198 from the Infrared Surface
  Brightness Technique

abstract:
  We report on a first application of the infrared surface brightness technique
on a Cepheid in the Large Magellanic Cloud, the variable HV 12198 in the young
globular cluster NGC 1866. From this one star, we determine a distance modulus
of 18.42 +- 0.10 (random and systematic uncertainty) to the cluster. When the
results on further member Cepheids in NGC 1866 become available, we expect to
derive the distance to the LMC with a +- 3-4 percent accuracy, including
systematic errors, from this technique.


title:
The Cosmic Ray - X-ray Connection: Effects of Nonlinear Shock
  Acceleration on Photon Production in SNRs

abstract:
  Cosmic-ray production in young supernova remnant (SNR) shocks is expected to
be efficient and strongly nonlinear. In nonlinear, diffusive shock
acceleration, compression ratios will be higher and the shocked temperature
lower than test-particle, Rankine-Hugoniot relations predict. Furthermore, the
heating of the gas to X-ray emitting temperatures is strongly coupled to the
acceleration of cosmic-ray electrons and ions, thus nonlinear processes which
modify the shock, influence the emission over the entire band from radio to
gamma-rays and may have a strong impact on X-ray line models. Here we apply an
algebraic model of nonlinear acceleration, combined with SNR evolution, to
model the radio and X-ray continuum of Kepler's SNR.


title: Persistent fluctuations and scaling properties in galaxy number counts

abstract:
  Counts of galaxies as a function of apparent magnitude are among the most
time-honored observations in cosmology. In this Letter, we focus on some
statistical properties of these counts which are fundamental in order to
characterize the large scale correlations in the galaxy spatial distribution.
There are, in fact, no longer doubts since two decades of the existence of very
large scale structures. The still remaining problem concerns the correct
characterization of their statistical properties. We propose to study two
properties of galaxy counts data, in order to discriminate between a small
scale (~ 5 - 20 Mpc/h) homogeneous distribution, and a fractal structure on
large scales (~ 20 - 300 Mpc/h). Firstly, the average slope of the counts which
can be associated to an eventual fractal dimension in real space by simple
arguments. Secondly, we propose to study fluctuations of counts around the
average behavior as a function of apparent magnitude in the whole magnitude
range B_j > 11. These fluctuations can discriminate between a genuine fractal
distribution and a homogeneous one. In fact, they are related to the very
statistical properties of the spatial distribution, independently on
cosmological corrections. More specifically, in a fractal distribution one
expects to find persistent scale-invariant fluctuations around the average
behavior, which do not decay with apparent magnitude. On the other hand, in an
homogeneous distribution, on large enough scales, the relative variance of the
counts should decrease exponentially with apparent magnitude. Smooth
cosmological corrections cannot change such a behavior.


title:
Discovery of Spatial and Spectral Structure in the X-Ray Emission from
  the Crab Nebula

abstract:
  The Chandra X-ray Observatory observed the Crab Nebula and Pulsar during
orbital calibration. Zeroth-order images with the High-Energy Transmission
Grating (HETG) read-out by the Advanced CCD Imaging Spectrometer spectroscopy
array (ACIS-S) show a striking richness of x-ray structure, at a resolution
comparable to that of the best ground-based visible-light observations. The
HETG--ACIS-S images reveal, for the first time, an x-ray inner ring within the
x-ray torus, the suggestion of a hollow-tube structure for the torus, and x-ray
knots along the inner ring and (perhaps) along the inward extension of the
x-ray jet. Although complicated by instrumental effects and the brightness of
the Crab Nebula, the spectrometric analysis shows systematic variations of the
x-ray spectrum throughout the Nebula.


title:
Another Faint UV Object Associated with a Globular Cluster X-Ray Source:
  The Case of M92

abstract:
  The core of the metal poor Galactic Globular Cluster M92 (NGC 6341) has been
observed with WFPC2 on the Hubble Space Telescope through visual, blue and
mid-UV filters in a program devoted to study the evolved stellar population in
a selected sample of Galactic Globular Clusters. In the UV $(m_{255},
m_{255}-U)$ color magnitude diagram we have discovered a faint `UV-dominant'
object. This star lies within the error box of a Low Luminosity Globular
Cluster X-ray source (LLGCX) recently found in the core of M92. The properties
of the UV star discovered in M92 are very similar to those of other UV stars
found in the core of some clusters (M13, 47 Tuc, M80, etc)---all of them are
brighter in the UV than in the visible and are located in the vicinity of a
LLGCX. We suggest that these stars are a new sub-class of cataclysmic
variables.


title: Radio and X-ray Signatures of Merging Neutron Stars

abstract:
  We describe the possible electromagnetic signals expected from the
magnetospheric interactions of a neutron star binary prior to merger. We find
that both radio and X-ray signals of detectable strength are possible. We
discuss possible links with the phenomenon of gamma-ray bursts and describe the
prospects for direct detection of these signals in searches for radio and X-ray
transients.


title:
The Northern ROSAT All-Sky (NORAS) Galaxy Cluster Survey I: X-ray
  Properties of Clusters Detected as Extended X-ray Sources

abstract:
  In the construction of an X-ray selected sample of galaxy clusters for
cosmological studies, we have assembled a sample of 495 X-ray sources found to
show extended X-ray emission in the first processing of the ROSAT All-Sky
Survey. The sample covers the celestial region with declination $\delta \ge
0\deg $ and galactic latitude $|b_{II}| \ge 20\deg $ and comprises sources with
a count rate $\ge 0.06$ counts s$^{-1}$ and a source extent likelihood of 7. In
an optical follow-up identification program we find 378 (76%) of these sources
to be clusters of galaxies. ...


title: A code for optically thick and hot photoionized media

abstract:
  We describe a code designed for hot media {(T $\ge$} a few 10$^4$ K),
optically thick to Compton scattering. It computes the structure of a
plane-parallel slab of gas in thermal and ionization equilibrium, illuminated
on one or on both sides by a given spectrum. Contrary to the other
photoionization codes, it solves the transfer of the continuum and of the lines
in a two stream approximation, without using the local escape probability
formalism to approximate the line transfer. We stress the importance of taking
into account the returning flux even for small column densities (10$^{22}$
cm$^{-2}$), and we show that the escape probability approximation can lead to
strong errors in the thermal and ionization structure, as well as in the
emitted spectrum, for a Thomson thickness larger than a few tenths. The
transfer code is coupled with a Monte Carlo code which allows to take into
account Compton and inverse Compton diffusions, and to compute the spectrum
emitted up to MeV energies, in any geometry. Comparisons with {\sc{cloudy}}
show that it gives similar results for small column densities. Several
applications are mentioned.


title: The reality of old moving groups - the case of HR 1614

abstract:
  We prove the existence of the old and metal-rich moving group HR1614. This is
done using the new Hipparcos parallaxes in combination with metallicities
derived from Stromgren photometry, supported by dynamical simulations of the
evolution of old moving groups in the galactic potential. A new selection
criterion for this moving group is presented as well as a new, extended sample
of probable member stars. In particular we find that the HR 1614 moving group
has an age of about 2 Gyr (using Bertelli et al. 1994 isochrones) and a [Fe/H]
= 0.19 +/- 0.06 dex. We also revisit and apply our new selection criterion to
the samples in Eggen (1992) and Eggen (1998). It is found that, when binaries
and stars with too low metallicity have been removed, 15 of his stars fulfill
our criteria.


title: New evidence for the complex struscture of the RGB of Omega Cen

abstract:
  We report on the complex structure of the red giant branch (RGB) of omega
Cen, based on a new wide field and wide color baseline B and I photometry. Our
color magnitude diagram (CMD) shows the presence of multiple populations along
this branch, in particular we discovered an anomalous branch (RGB-a), which
appears to be well separated from the bulk of the RGB stars. On the basis of
our CMD and from the previous literature we conclude that (1) these stars,
clearly identified as a separate population in our CMD, represent the extreme
metal rich extension ([Ca/H]>-0.3) of the stellar content of omega Cen, and
show anomalous abundances of s-process elements (as Ba and Zr) as well; (2)
they are physical members of the omega Cen system; (3) they comprise ~ 5% of
the stars of the whole system; (4) this component and the metal-intermediate
one (-0.4>[Ca/H]>-1) have been found to share the same spatial distribution,
both of them differing significantly from the most metal poor one ([Ca/H]<-1).
This last evidence supports the hypothesis that metal rich components could
belong to an independent (proto?) stellar system captured in the past by omega
Cen.


title:
The Tip of the Red Giant Branch and Distance of the Magellanic Clouds:
  results from the DENIS survey

abstract:
  We present a precise determination of the apparent magnitude of the tip of
the red giant branch (TRGB) in the I (0.8 micron), J (1.25 micron), and K_S
(2.15 micron) bands from the luminosity function of a sample of data extracted
from the DENIS catalogue towards the Magellanic Clouds (Cioni et al. 2000).
  From the J and Ks magnitudes we derive bolometric magnitudes m_bol. We
present a new algorithm for the determination of the TRGB magnitude, which we
describe in detail and test extensively using Monte-Carlo simulations. We note
that any method that searches for a peak in the first derivative (used by most
authors) or the second derivative (used by us) of the observed luminosity
function does not yield an unbiased estimate for the actual magnitude of the
TRGB discontinuity. We stress the importance of correcting for this bias, which
is not generally done. We combine the results of our algorithm with theoretical
predictions to derive the distance modulus of the Magellanic Clouds. We obtain
m-M = 18.55 (0.04 formal, 0.08 systematic) for the Large Magellanic Cloud
(LMC), and m-M = 18.99 (0.03 formal, 0.08 systematic) for the Small Magellanic
Cloud (SMC). These are among the most accurate determinations of these
quantities currently available, which is a direct consequence of the large size
of our sample and the insensitivity of near infrared observations to dust
extinction.


title:
The evolution of the stellar populations in low surface brightness
  galaxies

abstract:
  We investigate the star formation history and chemical evolution of low
surface brightness (LSB) disk galaxies by modelling their observed
spectro-photometric and chemical properties using a galactic chemical and
photometric evolution model incorporating a detailed metallicity depen dent set
of stellar input data. For a large fraction of the LSB galaxies in our sample,
observed properties are best explained by models incorporating an exponentially
decreasing global star formation rate (SFR) ending at a present-day gas
fraction (M_{gas}/(M_{gas}+M_{stars}) = 0.5 for a galaxy age of 14 Gyr. For
some galaxies small amplitude star formation bursts are required to explain the
contribution of the young (5-50 Myr old) stellar population to the galaxy
integrated luminosity. This suggests that star formation has proceeded in a
stochastic manner.
  The presence of an old stellar population in many late-type LSB galaxies
suggests that LSB galaxies roughly follow the same evolutionary history as HSB
galaxies, except at a much lower rate. In particular, our results imply that
LSB galaxies do not form late, nor have a delayed onset of star formation, but
simply evolve slowly.


title: The Mid-Infrared Spectra of Interacting Galaxies: From ISO to SIRTF

abstract:
  We present mid-infrared (5--16micron) images and spectra of a sequence of
interacting galaxies, observed by ISOCAM. The galaxies were selected as being
at progressive stages in the time evolution of a merging event, following what
is known as Toomre's ``merger sequence'', and having no detected contribution
from an active galactic nucleus (AGN) in their mid-infrared spectrum. To trace
the intensity of the global star formation in those galaxies, we use the ratio
of the 15micron to 7micron flux. Our analysis indicates that this ratio
increases from ~1 to ~5 as galaxies move from the pre-starburst to the
merging/starburst phase only to decrease to ~1 again in the post-starburst
phase of the evolved merger remnants. Moreover, we find that the variation of
this ratio is well correlated with the one of the IRAS 25/12 and 60/100 flux
ratios. The potential to test and improve upon these results using the Infrared
Spectrograph (IRS) on board SIRTF is discussed.


title: IRS: the Infrared Spectrograph on SIRTF

abstract:
  The Infrared Spectrograph (IRS)is one of the three instruments on board the
Space Infrared Telescope Facility (SIRTF) to be launched in December 2001. The
IRS will provide high resolution spectra R~600 from 10--37microns and low
resolution spectra R~60 from 5.3--40microns Its high sensitivity and ``spectral
mapping-mode'' make it a powerful instrument for observing both faint
point-like and extended sources.


title: Cosmic Microwave Background at low frequencies

abstract:
  The next generation low-frequency radio telescopes may probe cosmological
models by means of observations of the cosmic microwave background (CMB). I
discuss the prospects for observations of CMB imprints - recombination lines
from the epoch of recombination, \mu distortions and angular temperature
anisotropies - at low frequencies. A future low-frequency radio telescope, like
the proposed SKA, may be capable of attempting some difficult CMB measurements
because of the large collecting area and large element numbers; however, this
will require a telescope design that will allow specialized calibration
strategies and will give emphasis to the control of spurious responses.


title: How Stochastic is the Relative Bias Between Galaxy Types?

abstract:
  Examining the nature of the relative clustering of different galaxy types can
help tell us how galaxies formed. To measure this relative clustering, I
perform a joint counts-in-cells analysis of galaxies of different spectral
types in the Las Campanas Redshift Survey (LCRS). I develop a
maximum-likelihood technique to fit for the relationship between the density
fields of early- and late-type galaxies. This technique can directly measure
nonlinearity and stochasticity in the biasing relation. At high significance, a
small amount of stochasticity is measured, corresponding to a correlation
coefficient of about 0.87 on scales corresponding to 15 Mpc/h spheres. A large
proportion of this signal appears to derive from errors in the selection
function, and a more realistic estimate finds a correlation coefficient of
about 0.95. These selection function errors probably account for the large
stochasticity measured by Tegmark & Bromley (1999), and may have affected
measurements of very large-scale structure in the LCRS. Analysis of the data
and of mock catalogs shows that the peculiar geometry, variable flux limits,
and central surface-brightness selection effects of the LCRS do not seem to
cause the effect.


title: Nearly Coherent Oscillations in Type I X-Ray Bursts from KS 1731-260

abstract:
  We present an analysis of the nine type I X-ray bursts that were observed
from KS 1731-260 with RXTE. We find that the bursts divide naturally into two
populations: ``fast bursts'' occur on the Banana Branch when the accretion rate
is high and exhibit short decay times, high peak fluxes, and radius expansion
episodes. ``Slow bursts'' occur in the Island State at lower accretion rates,
have lower peak fluxes, higher fluences, longer decay times, and show no
evidence of radius expansion. All five of the fast bursts, and none of the four
slow bursts, show coherent oscillations near 524 Hz.
  We perform in-burst phase connection of the burst pulsations, which allows us
to unambiguously characterize their frequency evolution. That evolution
exhibits a variety of behaviors, including a sharp spin-down during one burst.
Applying our phase models, we find that the pulsations are spectrally harder
than the burst emission, with the strength of the pulsations increasing
monotonically with photon energy. Coherently summing all of the burst signals,
we find upper limits of ~5% relative to the strength of the main pulse on any
modulation at 1/2 or 3/2 the main pulse frequency. We discuss our results in
the context of models in which the burst pulsations are produced by a hot spot
on the surface of a spinning neutron star.


title: Non-existence of radiation damping of gravitational motions

abstract:
  A rigorous, non-perturbative proof that there is no radiation damping of
gravitational motions.


title: Topological defects: fossils of an anisotropic era?

abstract:
  We consider the evolution of domain walls produced during an anisotropic
phase in the very early universe, showing that the resulting network can be
very anisotropic. If the domain walls are produced during an inflationary era,
the network will soon freeze out in comoving coordinates retaining the imprints
of the anisotropic regime, even though inflation makes the universe isotropic.
Only at late times, when the typical size of the major axis of the domain walls
becomes smaller than the Hubble radius, does the network evolve rapidly towards
isotropy. Hence, we may hope to see imprints of the anisotropic era if by today
the typical size of the major axis of the domain walls is of the order of the
Hubble radius, or if the walls re-entered it only very recently. Depending on
the mass scale of the domain walls, there is also the possibility that they
re-enter at earlier times, but their evolution remains friction-dominated until
recently, in which case the signatures of the anisotropic era will be much
better preserved. These effects are expected to occur in generic topological
defect models.


title: Cosmic Velocities 2000: A Review

abstract:
  I review the status of cosmic velocity analysis as of January 2000, with an
emphasis on two key questions: (1) What is the scale of the largest bulk flows
in the universe? and (2) What is the value of beta=Omega_m^{0.6}/b indicated by
cosmic velocities, and what does this tell us about Omega_m itself? These are
the most important issues for cosmic flow analysis, and each has been
controversial in recent years. I argue that a preponderance of the evidence at
present argues against very large scale (> 100 Mpc) bulk flows, and favors beta
= 0.4-0.5, corresponding to a low-density (Omega_m=0.2-0.3) universe.


title:
Gravitational potential energy of simple bodies: the homogeneous
  bispherical concavo-convex lens

abstract:
  Gravitational potential energy of the homogeneous bispherical concavo-convex
lens, with the equal absolute values of radii of curvature of both surfaces is
studied.


title: Blue compact galaxies and the primordial 4Helium abundance

abstract:
  Blue compact galaxies (BCG) are ideal objects in which to derive the
primordial 4He abundance because they are chemically young and have not had a
significant stellar He contribution. We discuss a self-consistent method which
makes use of all the brightest He I emission lines in the optical range and
solves consistently for the electron density of the He II zone. We pay
particular attention to electron collision and radiative transfer as well as
underlying stellar absorption effects which may make the He I emission lines
deviate from their recombination values. Using a large homogeneous sample of 45
low-metallicity H II regions in BCGs, and extrapolating the Y-O/H and Y-N/H
linear regressions to O/H = N/H = 0, we obtain Yp = 0.2443+/-0.0015, in
excellent agreement with the weighted mean value Yp = 0.2452+/-0.0015 obtained
from the detailed analysis of the two most metal-deficient BCGs known, I Zw 18
and SBS 0335-052. The derived slope dY/dZ = 2.4+/-1.0 is in agreement with the
value derived for the Milky Way and with simple chemical evolution models with
homogeneous outflows. Adopting Yp = 0.2452+/-0.0015 leads to a baryon-to-photon
ratio of (4.7+/-1.0)x10^{-10} and to a baryon mass fraction in the Universe
Omega_b h^2_{50} = 0.068+/-0.015, consistent with the value derived from the
primordial D abundance of Burles & Tytler (1998).