title:
The Optical/Infrared Astronomical Quality of High Atacama Sites. II.
  Infrared Characteristics

abstract:
  We discuss properties of the atmospheric water vapor above the high Andean
plateau region known as the Llano de Chajnantor, in the Atacama Desert of
northern Chile. A combination of radiometric and radiosonde measurements
indicate that the median column of precipitable water vapor (PWV) above the
plateau at an elevation of 5000 m is approximately 1.2 mm. The exponential
scaleheight of the water vapor density in the median Chajnantor atmosphere is
1.13 km; the median PWV is 0.5 mm above an elevation of 5750 m. Both of these
numbers appear to be lower at night. Annual, diurnal and other dependences of
PWV and its scaleheight are discussed, as well as the occurrence of temperature
inversion layers below the elevation of peaks surrounding the plateau. We
estimate the background for infrared observations and sensitivities for broad
band and high resolution spectroscopy. The results suggest that exceptional
atmospheric conditions are present in the region, yielding high infrared
transparency and high sensitivity for future ground-based infrared telescopes.


title: Origin of quasi-periodic shells in dust forming AGB winds

abstract:
  We have combined time dependent hydrodynamics with a two-fluid model for dust
driven AGB winds. Our calculations include self-consistent gas chemistry, grain
formation and growth, and a new implementation of the viscous momentum transfer
between grains and gas. This allows us to perform calculations in which no
assumptions about the completeness of momentum coupling are made. We derive new
expressions to treat time dependent and non-equilibrium drift in a hydro code.
Using a stationary state calculation for IRC +10216 as initial model, the time
dependent integration leads to a quasi-periodic mass loss in the case where
dust drift is taken into account. The time scale of the variation is of the
order of a few hundred years, which corresponds to the time scale needed to
explain the shell structure of the envelope of IRC +10216 and other AGB and
post-AGB stars, which has been a puzzle since its discovery. No such
periodicity is observed in comparison models without drift between dust and
gas.


title: The BMW X-ray Cluster Survey

abstract:
  We describe the main features of the BMW survey of serendipitous X-ray
clusters, based on the still unexploited ROSAT-HRI archival observations. The
sky coverage, surface density and first deep optical CCD images of the
candidates indicate that this sample can represent an excellent complement to
the existing PSPC deep cluster surveys and will provide us with a fully
independent probe of the evolution of the cluster abundance, in addition to
significantly increasing the number of clusters known at z>0.6.


title:
The Stellar Composition of the Star Formation Region CMa R1. II.
  Spectroscopic and Photometric Observations of 9 Young Stars

abstract:
  We present new high and low resolution spectroscopic and photometric data of
nine members of the young association CMa R1. All the stars have circumstellar
dust at some distance as could be expected from their association with
reflection nebulosity. Four stars (HD 52721, HD 53367, LkHalpha 220 and
LkHalpha 218) show Halpha emission and we argue that they are Herbig Be stars
with discs. Our photometric and spectroscopic observations on these stars
reveal new characteristics of their variability. We present first
interpretations of the variability of HD 52721, HD 53367 and the two LkHalpha
stars in terms of a partially eclipsing binary, a magnetic activity cycle and
circumstellar dust variations, respectively. The remaining five stars show no
clear indications of Halpha emission in their spectra, although their spectral
types and ages are comparable with those of HD 52721 and HD 53367. This
indicates that the presence of a disc around a star in CMa R1 may depend on the
environment of the star. In particular we find that all Halpha emission stars
are located at or outside the arc-shaped border of the H II region, which
suggests that the stars inside the arc have lost their discs through
evaporation by UV photons from nearby O stars, or from the nearby (< 25 pc)
supernova, about 1 Myr ago.


title: Formation of Galactic Nuclei

abstract:
  We investigate a model in which galactic nuclei form via the coalescence of
pre-existing stellar systems containing supermassive black holes. Merger
simulations are carried out using N-body algorithms that can follow the
formation and decay of a black-hole binary and its effect on the surrounding
stars down to sub-parsec scales. Our initial stellar systems have steep central
density cusps similar to those in low-luminosity elliptical galaxies. Formation
of a black-hole binary transfers energy to the stars and lowers the central
density; continued decay of the binary creates a ~1/r density cusp similar to
those observed in bright elliptical galaxies, with a break radius that extends
well beyond the sphere of gravitational influence of the black holes. The decay
of the black hole binary is followed over a factor of ~20 in separation after
formation of a hard binary, considerably farther than in previous simulations.
We see almost no dependence of the binary's decay rate on number of particles
in the simulation, contrary to earlier studies in which a lower initial density
of stars led to a more rapid depletion of the binary's loss cone. We
nevertheless argue that the decay of a black hole binary in a real galaxy would
be expected to stall at separations of 0.01-1 pc unless some additional
mechanism is able to extract energy from the binary. Our results support a
picture in which the observed dependence of nuclear cusp slope on galaxy
luminosity is a consequence of galaxy interactions. We also discuss the
implications of our results for the survivability of dark-matter cusps.


title:
The Evolution of Balmer Absorption Line Strengths in E/S0 Galaxies from
  z=0 to z=0.83

abstract:
  We present new results from a systematic study of absorption line strengths
of galaxies in clusters approaching redshifts of unity. In this paper, we
specifically compare the strengths of the high-order Balmer absorption features
of Hgamma and Hdelta in E/S0s in the four clusters Abell 2256 (z=0.06),
CL1358+62 (z=0.33), MS2053--04 (z=0.58), and MS1054--03 (z=0.83). By comparing
the correlation of Balmer line strength with velocity dispersion for E/S0s in
the three clusters, we find moderate evolution in the zero-point of the
(Hdelta_A+Hgamma_A)-sigma relation with redshift. The trend is consistent with
passive evolution of old stellar populations. Under the assumption that the
samples can be compared directly, we use single-burst stellar population
synthesis models to constrain the last major occurrences of star-formation in
the observed E/S0s to be z_f > 2.5 (95% confidence). We have compared the
evolution of the Balmer absorption with the evolution of the B-band fundamental
plane and find that simple stellar population models agree very well with the
data. While the best agreement occurs with a low value for Omega_m, the data
provide strong confirmation of the time-evolution in recent stellar population
models.


title:
Keck Adaptive Optics Observations of the Radio Galaxy 3C294: A Merging
  System at z = 1.786?

abstract:
  We present seeing-limited and adaptive optics (AO) images of the z = 1.786
radio galaxy 3C294 in the H and K' infrared bands obtained at Keck Observatory.
The infrared emission of 3C294 is dominated by two distinct components
separated by ~1" (9 kpc). The eastern knot contains an unresolved core that
contributes ~4% of the K'-band light; we identify this core with the active
nucleus. The western component is about 2.5 times brighter. The most plausible
interpretation of the near-infrared morphology is an ongoing merger event, with
the active nucleus located in the less massive of the two galaxies.


title: A Survey of the Northern Sky for TeV Point Sources

abstract:
  A search for steady TeV point sources anywhere in the northern sky has been
made with data from the Milagrito air-shower-particle detector. Over 3 x 10**9
events collected from 1997 February to 1998 May have been used in this study.
No statistically significant excess above the background from the isotropic
flux of cosmic rays was found for any direction of the sky with declination
between -5 degrees and 71.7 degrees. Upper limits are derived for the photon
flux above 1 TeV from any steady point source in the northern sky.


title:
VLT observations of the peculiar globular cluster NGC 6712, II:
  luminosity and mass functions

abstract:
  We have carried out extensive VLT-FORS1 observations covering a fair fraction
of the intermediate metallicity globular cluster NGC 6712 in the V and R bands.
We derive accurate colour-magnitude diagrammes (CMD) and luminosity functions
(LFs) of the cluster main sequence (MS) for four overlapping fields extending
from the centre of the cluster out to a radius of ~10', well beyond the nominal
tidal radius, and for a control field at ~42' distance. The LFs extend from the
cluster turn-off (TO) at M_R 4 to the point at which the incompleteness drops
below 50% (corresponding to R 23 or M_R 7.5) for most fields studied. Cluster
stars become indistinguishable from field stars at r 5'. The shape of the
cluster's LF and its variation with distance from the centre in these ranges
are well described by a standard multi-mass static model having the following
parameters: core radius r_c=1', half-light radius r_hl=1.8', tidal radius
r_t=5.2', concentration ratio c=0.7, and a power-law global mass function (MF)
with index alpha 0.9 for masses smaller than 0.8 Msolar, i.e. for all detected
MS stars, and alpha -2.35 for evolved objects. The MF obtained in this way is
consistent with that found in a preliminary investigation of this cluster with
the VLT Test Camera and confirms that this is the only globular cluster known
so far for which the global MF drops with decreasing mass below the TO.
Possible reasons for this unique characteristic are discussed with the most
likely associated with its extreme vulnerability to tidal disruption.


title: CE-315 : A New Interacting Double-degenerate Binary Star

abstract:
  We present spectroscopic observations of object CE-315 revealing a blue
continuum with strong emission lines. Most of the detected lines are identified
with He I or He II in emission with a handful of faint lines of nitrogen.
Notable is the complete absence of hydrogen lines. The He lines exhibit
triple-peaked profiles with remakably broad widths, of about 2000 km/s (FWZP).
  The observations show that CE-315 is an interacting binary system with an
orbital period of 65.1 +/- 0.7 minutes, and a mass ratio of 0.022. We conclude
that the most likely scenario for this object is that of an accreting 0.77
solar mass white dwarf with a 0.017 solar mass helium white dwarf as mass
donor.


title:
A Fast Technique for the Creation of Large-Scale High-Resolution IRAS
  (HIRES) Beam-matched Images

abstract:
  HIRES processing provides a significant improvement in both resolution and
image quality over previous IRAS image products, but the characteristics of the
HIRES beam make accurate comparisons between the various IRAS bandpasses and
between HIRES data and data at other wavelengths non-trivial. We present a new,
fast technique for the construction of HIRES beam-matched images that is
especially well suited for the creation of large-scale (several square degrees)
ratio maps. Other techniques for the construction of ratio maps are discussed
and compared with the new algorithm. Examples of the large-scale ratio maps
that can be constructed using this new technique are presented. The algorithm's
application to the construction of multiwavelength difference images and
multi-colour images is also demonstrated.


title:
X-Ray Light Curves of Gamma-ray Bursts Detected with the All-Sky Monitor
  on RXTE

abstract:
  We present X-ray light curves (1.5-12 keV) for fifteen gamma-ray bursts
(GRBs) detected by the All-Sky Monitor on the Rossi X-ray Timing Explorer. We
compare these soft X-ray light curves with count rate histories obtained by the
high-energy (>12 keV) experiments BATSE, Konus-Wind, the BeppoSAX Gamma-Ray
Burst Monitor, and the burst monitor on Ulysses. We discuss these light curves
within the context of a simple relativistic fireball and synchrotron shock
paradigm, and we address the possibility of having observed the transition
between a GRB and its afterglow. The light curves show diverse morphologies,
with striking differences between energy bands. In several bursts, intervals of
significant emission are evident in the ASM energy range with little or no
corresponding emission apparent in the high-energy light curves. For example,
the final peak of GRB 970815 as recorded by the ASM is only detected in the
softest BATSE energy bands. We also study the duration of bursts as a function
of energy. Simple, singly-peaked bursts seem consistent with the E^{-0.5} power
law expected from an origin in synchrotron radiation, but durations of bursts
that exhibit complex temporal structure are not consistent with this
prediction. Bursts such as GRB 970828 that show many short spikes of emission
at high energies last significantly longer at low energies than the synchrotron
cooling law would predict.


title:
ASCA Temperature Maps of Three Clusters of Galaxies Abell 1060, AWM7,
  and the Centaurus Cluster

abstract:
  We present two-dimensional temperature maps of three bright clusters of
galaxies Abell 1060, AWM7, and the Centaurus cluster, based on multi-pointing
observations with the ASCA GIS. The temperatures are derived from hardness
ratios by taking into account the XRT response. For the Centaurus cluster, we
subtracted the central cool component using the previous ASCA and ROSAT
results, and the metallicity gradients observed in AWM7 and the Centaurus
cluster were included in deriving the temperatures. The intracluster medium in
Abell 1060 and AWM7 is almost isothermal from the center to outer regions with
a temperature of 3.3 and 3.9 keV, respectively. The Centaurus cluster exhibits
remarkable hot regions within about 30' from the cluster center showing a
temperature increase of +0.8 keV from the surrounding level of 3.5 keV, and
outer cool regions with lower temperatures by -1.3 keV. These results imply
that a strong merger has occurred in the Centaurus in the recent 2-3 Gyr, and
the central cool component has survived it. In contrast, the gas in Abell 1060
was well-mixed in an early period, which probably has prevented the development
of the central cool component. In AWM7, mixing of the gas should have occurred
in a period earlier than the epoch of metal enrichment.


title:
The Expected Mass Function for Low Mass Galaxies in a CDM Cosmology: Is
  There a Problem?

abstract:
  It is well known that the mass function for_halos_ in CDM cosmology is a
relatively steep power law for low masses, possibly too steep to be consistent
with observations. But how steep is the_galaxy_ mass function? We have analyzed
the stellar and gas mass functions of the first massive luminous objects formed
in a \Lambda CDM universe, as calculated in the numerical simulation described
in Gnedin (2000ab). We found that while the dark matter mass function is steep,
the stellar and gas mass functions are flatter for low mass objects. The
stellar mass function is consistently flat at the low mass end. Moreover, while
the gas mass function follows the dark matter mass function until reionization
at z~7, between z=7 and z=4, the gas mass function also flattens considerably
at the low mass end. At z=4, the gas and stellar mass functions are fit by a
Schechter function with \alpha ~ -1.2 +/- 0.1, significantly shallower than the
dark matter halo mass function and consistent with some recent observations.
The baryonic mass functions are shallower because (a) the dark matter halo mass
function is consistent with the Press-Schechter formulation at low masses n(M)
  M^-2 and (b) heating/cooling and ionization processes appear to cause baryons
to collect in halos with the relationship M_b M_d^4 at low masses. Combining
(a) and (b) gives n(M_b) M_b^-5/4, comparable to the simulation results. Thus,
the well known observational fact that low mass galaxies are underabundant as
compared to expectations from numerical dark matter simulations or
Press-Schechter modeling of CDM universes emerges naturally from these results,
implying that perhaps no ``new physics'' beyond the standard model is needed.


title: Accretion Models of Gamma-Ray Bursts

abstract:
  Many models of gamma-ray bursts (GRBs) involve accretion onto a compact
object, usually a black hole, at a mass accretion rate of order a fraction of a
solar mass per second. If the accretion disk is larger than a few tens or
hundreds of Schwarzschild radii, the accretion will proceed via a
convection-dominated accretion flow (CDAF) in which most of the matter escapes
to infinity rather than falling onto the black hole. Models involving the
mergers of black hole white dwarf binaries and black hole helium star binaries
fall in this category. These models are unlikely to produce GRBs since very
little mass reaches the black hole. If the accretion disk is smaller, then
accretion will proceed via neutrino cooling in a neutrino-dominated accretion
disk (NDAF) and most of the mass will reach the center. Models involving the
mergers of double neutron star binaries and black hole neutron star binaries
fall in this category and are capable of producing bright GRBs. If the
viscosity parameter $\alpha$ in the NDAF has a standard value $\sim0.1$, these
mergers can explain short GRBs with durations under a second, but they are
unlikely to produce long GRBs with durations of tens or hundred of seconds. If
the accretion disk is fed by fallback of material after a supernova explosion,
as in the collapsar model, then the time scale of the burst is determined by
fallback, not accretion. Such a model can produce long GRBs. Fallback models
again require that the accretion should proceed via an NDAF rather than a CDAF
in order for a significant amount of mass to reach the black hole. This
condition imposes an upper limit on the radius of injection of the gas.


title: Thermal emission from bare quark matter surfaces of hot strange stars

abstract:
  We consider the thermal emission of photons and electron-positron pairs from
the bare quark surface of a hot strange star. The radiation of high-energy (>
20 MeV) equilibrium photons prevails at the surface temperature T_S > 5 x
10^{10} K, while below this temperature, 8 x 10^8 < T_S < 5 x 10^{10} K,
emission of electron-positron pairs created by the Coulomb barrier at the quark
surface dominates. The thermal luminosity of a hot strange star in both photons
and pairs is estimated.


title:
Application of a relativistic accretion disc model to X-ray spectra of
  LMC X-1 and GRO J1655-40

abstract:
  We present a general relativistic accretion disc model and its application to
the soft-state X-ray spectra of black hole binaries. The model assumes a flat,
optically thick disc around a rotating Kerr black hole. The disc locally
radiates away the dissipated energy as a blackbody. Special and general
relativistic effects influencing photons emitted by the disc are taken into
account. The emerging spectrum, as seen by a distant observer, is parametrized
by the black hole mass and spin, the accretion rate, the disc inclination angle
and the inner disc radius. We fit the ASCA soft state X-ray spectra of LMC X-1
and GRO J1655-40 by this model. We find that having additional limits on the
black hole mass and inclination angle from optical/UV observations, we can
constrain the black hole spin from X-ray data. In LMC X-1 the constrain is
weak, we can only rule out the maximally rotating black hole. In GRO J1655-40
we can limit the spin much better, and we find 0.68 < a < 0.88. Accretion discs
in both sources are radiation pressure dominated. We don't find Compton
reflection features in the spectra of any of these objects.


title: Myon Density Measurements with the KASCADE Central Detector

abstract:
  Frequency distributions of local muon densities in high-energy extensive
air-showers (EAS) are presented as signature of the primary cosmic ray energy
spectrum in the knee region. Together with the gross shower variables like
shower core position, angle of incidence, and the shower sizes, the KASCADE
experiment is able to measure local muon densities for two different muon
energy thresholds. The spectra have been reconstructed for various core
distances, as well as for particular subsamples, classified on the basis of the
shower size ratio N_mu/N_e. The measured density spectra of the total sample
exhibit clear kinks reflecting the knee of the primary energy spectrum. While
relatively sharp changes of the slopes are observed in the spectrum of EAS with
small values of the shower size ratio, no such feature is detected at EAS of
large N_mu/N_e ratio in the energy range of 1--10 PeV. Comparing the spectra
for various thresholds and core distances with detailed Monte Carlo simulations
the validity of EAS simulations is discussed.


title: The radio galaxy K-z relation to z ~ 4.5

abstract:
  Using a new radio sample, 6C* designed to find radio galaxies at z > 4 along
with the complete 3CRR and 6CE sample we extend the radio galaxy K-z relation
to z~4.5. The 6C* K-z data significantly improve delineation of the K-z
relation for radio galaxies at high redshift (z > 2). Accounting for
non-stellar contamination, and for correlations between radio luminosity and
estimates of stellar mass, we find little support for previous claims that the
underlying scatter in the stellar luminosity of radio galaxies increases
significantly at z > 2. This indicates that we are not probing into the
formation epoch until at least z > 3.


title: The Cross-Spectra of Cir X-1: Evolution of Time Lags

abstract:
  Earlier work showed that the track in the X-ray hardness-intensity diagram of
Cir X-1 corresponds to a Z track in its color-color diagram. In this paper, we
study the cross spectrum of Cir X-1 in different regions of the
hardness-intensity diagram with $RXTE$/PCA data. Comparing the light curves of
Cir X-1 for the energy band 1.8-5.1 keV to those for 5.1-13.1 keV, we find that
Cir X-1 exhibits a hard time lag on the horizontal branch, and a soft time lag
on both the normal and the flaring branch. This indicates that Cir X-1 is
similar to GX 5-1 and Cyg X-2 on the horizontal branch, but is different from
them on the normal branch. We briefly discuss the mechanism of the time lags in
the context of Comptonization models.


title: General-Relativistic Curvature of Pulsar Vortex Structure

abstract:
  The motion of a neutron superfluid condensate in a pulsar is studied. Several
theorems of general-relativistic hydrodynamics are proved for a superfluid. The
average density distribution of vortex lines in pulsars and their
general-relativistic curvature are derived.


title:
The flux-dependent amplitude of broadband noise variability in X-ray
  binaries and active galaxies

abstract:
  Standard shot-noise models, which seek to explain the broadband noise
variability that characterises the X-ray lightcurves of X-ray binaries and
active galaxies, predict that the power spectrum of the X-ray lightcurve is
stationary (i.e. constant amplitude and shape) on short time-scales. We show
that the broadband noise power spectra of the black hole candidate Cyg X-1 and
the accreting millisecond pulsar SAX J1808.4-3658 are intrinsically
non-stationary, in that RMS variability scales linearly with flux.
Flux-selected power spectra confirm that this effect is due to changes in
power-spectral amplitude and not shape. The lightcurves of three Seyfert
galaxies are also consistent with a linear relationship between RMS variability
and flux, suggesting that it is an intrinsic feature of the broadband noise
variability in compact accreting systems over more than 6 decades of central
object mass. The RMS variability responds to flux variations on all measured
time-scales, raising fundamental difficulties for shot-noise models which seek
to explain this result by invoking variations in the shot parameters. We
suggest that models should be explored where the longest time-scale variations
are fundamental and precede the variations on shorter time-scales. Possible
models which can explain the linear RMS-flux relation include the fractal
break-up of large coronal flares, or the propagation of fluctuations in mass
accretion rate through the accretion disk. The linear relationship between RMS
variability and flux in Cyg X-1 and SAX J1808.4-3658 is offset on the flux
axis, suggesting the presence of a second, constant-flux component to the
lightcurve which contributes 25% of the total flux. The spectrum of this
constant component is similar to the total spectrum [abridged].


title:
The ROSAT Deep Survey: VI. X-ray sources and Optical identifications of
  the Ultra Deep Survey

abstract:
  We describe in this paper the ROSAT Ultra Deep Survey (UDS), an extension of
the ROSAT Deep Survey (RDS) in the Lockman Hole. The UDS reaches a flux level
of 1.2 x 10E-15 erg/cm2/s in 0.5-2.0 keV energy band, a level ~4.6 times
fainter than the RDS. We present nearly complete spectroscopic identifications
(90%) of the sample of 94 X-ray sources based on low-resolution Keck spectra.
The majority of the sources (57) are broad emission line AGNs (type I), whereas
a further 13 AGNs show only narrow emission lines or broad Balmer emission
lines with a large Balmer decrement (type II AGNs) indicating significant
optical absorption. The second most abundant class of objects (10) are groups
and clusters of galaxies (~11%). Further we found five galactic stars and one
''normal'' emission line galaxy. Eight X-ray sources remain spectroscopically
unidentified. The photometric redshift determination indicates in three out of
the eight sources the presence of an obscured AGN in the range of 1.2 < z <
2.7. These objects could belong to the long-sought population of type 2 QSOs,
which are predicted by the AGN synthesis models of the X-ray background.
Finally, we discuss the optical and soft X-ray properties of the type I AGN,
type II AGN, and groups and clusters of galaxies, and the implications to the
X-ray backround.


title: Calculations of radio pulses from High Energy Showers

abstract:
  In this article we review the progress made in understanding the main
characteristics of coherent \v{C}erenkov radiation induced by high energy
showers in dense media. A specific code developed for this purpose is described
because it took a significant part in this process. Subsequent approximations
developed for the calculation of radio pulses from EeV showers are reviewed.
Emphasis is given to the relation between the shower characteristics and
different features of the corresponding radio emission.


title: Astroparticle Physics: The High Energy Tail of the Cosmic Ray Spectrum

abstract:
  In this article I review the main theoretical problems that are posed by the
highest energy end of the observed cosmic ray spectrum, stressing the
importance of establishing their composition in order to decide between
proposed scenarios. I then discuss the possibilities that are opened by the
detection of inclined showers with extensive air shower arrays. Recent progress
in modelling magnetic deviations for these showers has allowed the analysis of
inclined showers that were detected by the Haverah Park experiment. This
analysis disfavours models that predict a large proportion of photons in the
highest energy cosmic rays and open up new possibilities for future shower
array detectors particularly those, like the Pierre Auger Observatory, using
water Cherenkov detectors.


title: ZeV Air Showers: The View from Auger

abstract:
  In this article I briefly discuss the characteristics of the Auger
observatories paying particular attention to the role of inclined showers, both
in the search for high energy neutrino interactions deep in the atmosphere and
as an alternative tool for the study of cosmic rays, particulartly their
composition.


title: Timing the millisecond pulsars in 47 Tucanae

abstract:
  In the last 10 years 20 millisecond pulsars have been discovered in the
globular cluster 47 Tucanae. Hitherto, only 3 of these had published timing
solutions. Here we improve upon these 3 and present 12 new solutions. These
measurements can be used to determine a variety of physical properties of the
pulsars and of the cluster. The 15 pulsars have positions determined with
typical uncertianties of only a few milliarcsec and they are all located within
1.2 arcmin of the cluster centre. We have also measured the proper motions of 5
of the pulsars, which are consistent with the proper motion of 47 Tuc based on
Hipparcos data. The period derivatives measured for many of the pulsars are
dominated by the dynamical effects of the cluster gravitational field, and are
used to constrain the surface mass density of the cluster. All pulsars have
characteristic ages T > 170 Myr and magnetic fields B < 2.4e9 Gauss, and the
average T > 1 Gyr. We have measured the rate of advance of periastron for the
binary pulsar J0024-7204H, implying a total system mass 1.4+-0.8 solar masses.


title:
ISOCAM observations of the rho Ophiuchi cloud: Luminosity and mass
  functions of the pre-main sequence embedded cluster

abstract:
  We present the results of the first extensive mid-infrared (IR) imaging
survey of the rho Ophiuchi embedded cluster, performed with the ISOCAM camera
on board the ISO satellite. The main molecular cloud L1688, as well as L1689N
and L1689S, have been completely surveyed for point sources at 6.7 and 14.3
micron. A total of 425 sources are detected including 16 Class I, 123 Class II,
and 77 Class III young stellar objects (YSOs). Essentially all of the mid-IR
sources coincide with near-IR sources, but a large proportion of them are
recognized for the first time as YSOs. Our dual-wavelength survey allows us to
identify essentially all the YSOs with IR excess in the embedded cluster down
to Fnu ~ 10 - 15 mJy. It more than doubles the known population of Class II
YSOs and represents the most complete census to date of newly formed stars in
the rho Ophiuchi central region. The stellar luminosity function of the
complete sample of Class II YSOs is derived with a good accuracy down to L=
0.03 Lsun. A modeling of this lumino- sity function, using available pre-main
sequence tracks and plausible star for- mation histories, allows us to derive
the mass distribution of the Class II YSOs which arguably reflects the IMF of
the embedded cluster. We estimate that the IMF in rho Ophiuchi is well
described by a two-component power law with a low- mass index of -0.35+/-0.25,
a high-mass index of -1.7 (to be compared with the Salpeter value of -1.35),
and a break occurring at M = 0.55+/-0.25 Msun. This IMF is flat with no
evidence for a low-mass cutoff down to at least 0.06 Msun.


title: A Catalog of OB Associations in the spiral galaxy NGC 300

abstract:
  We present results of a search for OB associations in NGC 300. Using an
automatic and objective method (PLC technique) 117 objects were found.
Statistical tests indicate that our sample is contaminated by less than 10 % of
spurious detections due to random concentrations of blue stars. Spatial
distributions of detected associations and H II regions are strongly
correlated. The size distribution reveals a significant peak at about 60
$\mu$rad which corresponds to 125 parsecs if a distance modulus of 26.66 mag is
assumed. Besides the objects with sizes corresponding to typical associations
we also found several much larger objects. A second level application of our
detection method revealed that most of these are composed of smaller subgroups,
with sizes of about 100 pc.


title:
Implications of modes of star formation for the overall dynamics of
  galactic disks

abstract:
  One of the present concepts for the onset of massive star formation is the
Kennicutt criterion. This relates the onset of massive star formation to a
general gravitational instability of the gas disks of spiral galaxies. It is
often overlooked, however that such gravitational instabilities of the gas
disks have severe implications for the overall stability of the gas and star
disks of spiral galaxies. I show by numerical simulations of the evolution of a
combined gas and star disk that the violation of the stability condition
induces violent dynamical evolution of the combined system. In particular the
star disk heats up on time scales less than a Gyr to unrealistic high values of
the Toomre stability parameter Q. The morphologies of both the star and gas
disk resemble then no longer observed morphologies of spiral galaxies. Star
formation of stars on low velocity dispersion orbits would lead to dynamical
cooling of the disks to more realistic states. However, the required star
formation rate is extremely high.


title: A Submillimeter View of Star Formation Near the HII Region KR 140

abstract:
  We present the results of 450 and 850 micron continuum mapping of the HII
region KR 140 using the SCUBA instrument on the JCMT. KR 140 is a small (5.7 pc
diameter) HII region at a distance of 2.3+/-0.3 kpc. Five of the six IRAS point
sources near KR 140 were mapped in this study. Our analysis shows that two of
these IRAS sources are embedded late B type stars lying well outside the HII
region, two are a part of the dust shell surrounding the HII region, and one is
the combined emission from an ensemble of smaller sources unresolved by IRAS.
We have discovered a number of relatively cold submillimeter sources not
visible in the IRAS data, ranging in size from 0.2 to 0.7 pc and in mass from
0.5 to 130 Msun. The distribution of masses for all sources is well
characterized by a power law N(>M) ~ M^{-alpha} with alpha = 0.5 +/- 0.04, in
agreement with the typical mass function for clumped structures of this scale
in molecular clouds. Several of the submillimeter sources are found at the
HII-molecular gas interface and have probably been formed as the result of the
expansion of the HII region. Many of the submillimeter sources we detect are
gravitationally bound and most of these follow a mass-size relationship
expected for objects in virial equilibrium with non-thermal pressure support.
Upon the loss of non-thermal support they could be sites of star formation.
Along with the two B stars that we have identified as possible cluster members
along with VES 735, we argue that five nearby highly-reddened stars are in a
pre-main-sequence stage of evolution.


title:
Broadband Observations of the Afterglow of GRB 000926: Observing the
  Effect of Inverse Compton Scattering and Evidence for a High-Density
  Environment

abstract:
  GRB 000926 has one of the best-studied afterglows to-date, with multiple
X-ray observations, as well as extensive multi-frequency optical and radio
coverage. Broadband afterglow observations, spanning from X-ray to radio
frequencies, provide a probe of the density structure of the circumburst
medium, as well as of the ejecta energetics, geometry, and the physical
parameters of the relativistic blastwave resulting from the explosion. We
present an analysis of {\em Chandra X-ray Observatory} observations of this
event, along with {\em Hubble Space Telescope} and radio monitoring. We combine
these data with ground-based optical and IR observations and fit the
synthesized afterglow lightcurve using models where collimated ejecta expand
into a surrounding medium. We find that we can explain the broadband lightcurve
with reasonable physical parameters only if the cooling is dominated by inverse
Compton scattering. Excess X-ray emission in the broadband spectrum indicates
that we are directly observing a contribution from inverse Compton scattering.
It is the first time this has been observed in a GRB afterglow, and it implies
that the GRB exploded in a reasonably dense (n~30 cm^{-3}) medium, consistent
with a diffuse interstellar cloud environment.


title: The Power Spectrum of Flux Limited X-Ray Galaxy Cluster Surveys

abstract:
  We compute the redshift space power spectrum of two X-ray cluster samples:
the X-ray Brightest Abell Cluster Sample (XBACS) and the Brightest Cluster
Sample (BCS) using the method developed by Feldman, Kaiser & Peacock. The power
spectrums derived for these samples are in agreement with determinations of
other optical and X-ray cluster samples. For XBACS we find the largest power
spectrum amplitude expected given the high richness of this sample ($R \ge 2$).
In the range $0.05 \uk < k < 0.4 \uk$ the power spectrum shows a power law
behavior $P(k)\propto k^{n}$ with an index $n\simeq-1.2$. In a similar range
$0.04 \uk < k < 0.3 \uk $ BCS power spectrum has a smaller amplitude with index
$n\simeq-1.0$. We do not find significant evidence for a peak at $k \simeq 0.05
\uk$ suggesting that claims such of feature detections in some cluster samples
could relay on artificial inhomogeneities of the data. We compare our results
with power spectrum predictions derived by Moscardini et al. within current
cosmological models (LCDM and OCDM). For XBACS we find that both models
underestimate the amplitude of the power spectrum but for BCS there is
reasonably good agreement at $k\gsim 0.03 \uk$ for both models.


title:
The Structure and Motions of the 3C~120 Radio Jet on Scales of 0.6 to
  300 Parsecs

abstract:
  Results are presented from long term VLBI monitoring of the parsec-scale
radio jet in 3C120, primarily at 5 and 1.7 GHz. Numerous superluminal features
are followed, some over large fractional distances from the core. The speeds of
the features are not all the same, but vary by less than a factor of 2. The
superluminal features extend to core distances of over 0.2 arcseconds and the
region of slowing has not been found. The 1.7 GHz images show evidence for
stationary features in addition to the superluminal features. The structure of
the jet in the vicinity of the most likely stationary feature is suggestive of
a helical pattern seen in projection. The deprojected wavelength of the pattern
is very long relative to the jet radius, unlike the situation in sources such
as M87. If the 3C120 jet does contain a slowly-moving, helical structure, then
theory suggests that the jet resides in a relatively cool medium, not in a
relativistically hot cocoon or lobe.


title: Coalescence of a strange star with a black hole

abstract:
  We present the first numerical results on the binary coalescence of a quark
star with a black hole, obtained with a 3-D Newtonian smooth particle hydro
(SPH) code. The star is initially represented by 17,000 particles modeling a
self-gravitating fluid with the equation of state P= (rho - rho0)c**2/3, and
the black hole by a point mass with an absorbing boundary at the Schwarzschild
radius. As in similar calculations carried out for a stiff polytrope, the
stellar core survives the initial episode of mass transfer, but here an
accretion disk is clearly formed as well.


title: A Transition to the Soft State in GRS 1758-258

abstract:
  Near the end of 2001 February, the black-hole candidate (BHC) GRS 1758-258
made an abrupt transition from a standard hard (low) state to a soft state.
Unlike Cyg X-1 and other BHCs, whose luminosity increases during this
transition, GRS 1758-258 was dimmer after the transition. We present
observations with the Proportional Counter Array on the Rossi X-ray Timing
Explorer and interpret the phenomenon in the context of a ``dynamical'' soft
state model. Using this model we predicted that mass transfer from the
companion had ceased, and that the luminosity should decay on a timescale of a
few weeks. The most recent data support this prediction, being consistent with
a decay time of 34 dy. The current state is consistent with the ``off'' state
of GRS 1758-258 reported by GRANAT/Sigma in 1991-1992.


title: Towards Population III: The Collapse and Fragmentation of Primordial Gas

abstract:
  We briefly review the motivations for studying the formation of the first
``Population III'' stars and present recent results from our numerical
simulations in this area. We discuss the new questions raised as a result of
the simulations presented by us and others at this meeting.


title: The Theory of Brown Dwarfs and Extrasolar Giant Planets

abstract:
  Straddling the traditional realms of the planets and the stars, objects below
the edge of the main sequence have such unique properties, and are being
discovered in such quantities, that one can rightly claim that a new field at
the interface of planetary science and and astronomy is being born. In this
review, we explore the essential elements of the theory of brown dwarfs and
giant planets, as well as of the new spectroscopic classes L and T. To this
end, we describe their evolution, spectra, atmospheric compositions, chemistry,
physics, and nuclear phases and explain the basic systematics of
substellar-mass objects across three orders of magnitude in both mass and age
and a factor of 30 in effective temperature. Moreover, we discuss the
distinctive features of those extrasolar giant planets that are irradiated by a
central primary, in particular their reflection spectra, albedos, and transits.
Aspects of the latest theory of Jupiter and Saturn are also presented.
Throughout, we highlight the effects of condensates, clouds, molecular
abundances, and molecular/atomic opacities in brown dwarf and giant planet
atmospheres and summarize the resulting spectral diagnostics. Where possible,
the theory is put in its current observational context.


title: Gamma-Ray Bursts at Low Redshift

abstract:
  Long duration gamma-ray bursts (GRB) are at cosmological distance, they
appear to be located near star forming regions, and are likely associated with
some type of supernovae. They are also likely to be strongly beamed, which
lowers their energetics by several orders of magnitude, and increases their
rate by the same factor. Therefore, it is likely that one out of 100 - 1000
core collapse supernovae generates ultra-relativistic jets, which beam
gamma-rays and all other early emission into two narrow cones. After a year, or
so, the jets are decelerated and become non-relativistic, and their emission
becomes more or less isotropic. At least two GRB: 970508 (z = 0.835) and 980703
(z = 0.966) show strong radio emission from late, and therefore
non-relativistic afterglows. Such events should be readily detectable at low
redshift, say z = 0.03. A search for strong radio emitters among recent nearby
supernovae should be done $ \sim 1 $ year after the explosion. If some of these
explosions generated GRB and their gamma-ray beam missed us, the bipolar
structure of the radio remnant should be resolvable with VLBA.


title: The Flattened, Rotating Molecular Gas Core of Protostellar Jet HH 212

abstract:
  The recently discovered protostellar jet known as HH212 is beautifully
symmetric, with a series of paired shock knots and bow shocks on either side of
the exciting source region, IRAS 05413-0104 (Zinnecker et al. 1998). We present
VLA ammonia maps of the IRAS 05413-0104 molecular gas envelope in which the
protostellar jet source is embedded. We find that the envelope, with mass of
0.2 M(sun) detected by the interferometer, is flattened perpendicular to the
jet axis with a FWHM diameter of 12000 AU and an axis ratio of 2:1, as seen in
NH3 (1,1) emission. There is a velocity gradient of about 4-5 km sec^-1 pc^-1
across the flattened disk-like core, suggestive of rotation around an axis
aligned with the jet. Flux-weighted mean velocities increase smoothly with
radius with a roughly constant velocity gradient. In young (Class 0) systems
such as HH212, a significant amount of material is still distributed in a large
surrounding envelope, and thus the observable kinematics of the system may
reflect the less centrally condensed, youthful state of the source and
obscuration of central dynamics. The angular momentum of this envelope material
may be released from infalling gas through rotation in the HH212 jet, as recent
observations suggest (Davis et al. 2000). A blue-shifted wisp or bowl of
emitting gas appears to be swept up along the blue side of the outflow,
possibly lining the cavity of a wider angle wind around the more collimated
shock jet axis. Our ammonia (2,2)/(1,1) ratio map indicates that this very cold
core is heated to 14 Kelvin degrees in a centrally condensed area surrounding
the jet source. This edge-on core and jet system appears to be young and deeply
embedded. This environment, however, is apparently not disrupting the pristine
symmetry and collimation of the jet.


title: Dark Halos: The Flattening of the Density Cusp by Dynamical Friction

abstract:
  N-body simulations and analytical calculations of the gravitational collapse
in an expanding universe predict that halos should form with a diverging inner
density profile, the cusp. There are some observational indications that the
dark matter distribution in galaxies might be characterized by a finite core.
This `core catastrophe' has prompted a search for alternatives to the CDM
cosmogony. It is shown here that the discrepancy between theory and
observations can be very naturally resolved within the standard CDM model,
provided that gas is not initially smoothly distributed in the dark matter
halo, but rather is concentrated in clumps of mass $\geq 0.01 %$ the total mass
of the system. Dynamical friction acting on these lumps moving in the
background of the dark matter particles, dissipates the clumps orbital energy
and deposits it in the dark matter. Using Monte-Carlo simulations, it is shown
that the dynamical friction provides a strong enough drag, and that with
realistic baryonic mass fractions, the available orbital energy of the clumps
is sufficient to heat the halo and turn the primordial cusp into a finite,
non-diverging core --- overcoming the competing effect of adiabatic contraction
due to gravitational influence of the shrinking baryonic component. Depending
on the initial conditions, the total density distribution may either become
more or less centrally concentrated. Possible consequences of the proposed
mechanism for other problems in the CDM model and for the formation and early
evolution of the baryonic component of galaxies are also briefly discussed.


title:
First Investigation of the Clustering Environment of Damped Lyman Alpha
  Absorbers at z=4

abstract:
  We report the first observations of the clustering environment of damped
Lyman alpha absorption systems at z=4. Color selection and photometric
redshifts were used to select 44 candidate Lyman-break galaxies brighter than
I_AB=25.5 from deep BRI images of the 35 sq. arcmin field containing the quasar
BR 0951-04. Multislit spectroscopy of 35 candidate galaxies was performed and 8
of these candidates have been confirmed as z>3.5 Lyman-break galaxies. With
only BRI photometry, the photometric redshifts are quite accurate for the
spectroscopically confirmed galaxies but have a high rate of misclassification
due to color degeneracies between Lyman-break galaxies and low-redshift
ellipticals. Both of the z>3.5 galaxies found within 15'' of the quasar
line-of-sight appear to be causing absorption systems in the quasar spectrum.
We use a battery of statistical tests to look for clustering in the redshift
histogram of the z>3.5 galaxies but do not find measurable clustering of these
Lyman-break galaxies with the damped Lyman alpha absorbers. With a larger
sample of galaxies, our method should determine the cross-correlation between
these objects, which probes the bias and hence the mass of the damped Lyman
alpha absorbers.


title: Exact Lemaitre-Tolman-Bondi solutions for matter-radiation decoupling

abstract:
  A new class of solutions of Einstein's equations is derived providing a
physically plausible hydrodynamic description of cosmological matter in the
radiation era of the Universe expansion. The solutions are characterized by the
LTB metric with dissipative fluid sources, subjected to the following
conditions: (i) tthe nonequilibrium state variables satisfy the equations of
state of a mixture of relativistic and non-relativistic ideal gases, where the
internal energy of the latter has been neglected, (ii) the particle numbers of
the mixture components are independently conserved, (iii) the viscous stress is
consistent with the transport equation and entropy balance law of extended
irreversible thermodynamics with the coefficients of shear viscosity provided
by kinetic theory. The Jeans mass at decoupling is of the same order of
magnitude as that of baryons dominated models.


title: Stability of realistic strange stars (RSS)

abstract:
  A modified version of this paper is under process and with a new title and
abstract. Hence, this version of the article is completely withdrawn.


title: Observational Tests and Predictive Stellar Evolution

abstract:
  We compare eighteen binary systems with precisely determined radii and masses
from 23 to 1.1 M_sol, and stellar evolution models produced with our newly
revised code TYCHO. ``Overshooting'' and rotational mixing were suppressed in
order to establish a baseline for isolating these and other hydrodynamic
effects. Acceptable coeval fits are found for sixteen pairs without optimizing
for heavy element or helium abundance. The precision of these tests is limited
by the accuracies of the observed effective temperatures. High dispersion
spectra and detailed atmospheric modeling should give more accurate effective
temperatures and heavy element abundances. PV Cas, a peculiar early A system,
EK Cep B, a known post-T Tauri star, and RS Cha, a member of a young OB
association, are matched by pre-main sequence models. Predicted mass loss
agrees with upper limits from IUE for CW Cep A and B. Relatively poor fits are
obtained for binaries having at least one component in the mass range 1.7 <
M/M_sol <2.6, whose evolution is sensitive to mixing. These discrepancies are
robust and consistent with additional mixing in real stars. The predicted
apsidal motion implies that massive star models are systematically less
centrally condensed than the real stars. If these effects are due to
overshooting, then the overshooting parameter alpha_OV increases with stellar
mass. The apsidal motion constants are controlled by radiative opacity under
conditions close to those directly measured in laser experiments, making this
test more stringent than possible before.


title: Piecing together the big picture

abstract:
  This article (which appeared with illustrations) was the
astrophysics/cosmology contribution to the historical series ``Pathways of
Discovery'' which appeared in ``Science'' during last year.


title: The interaction of radio sources and cooling flows

abstract:
  The X-ray emission in many clusters of galaxies shows a central peak in
surface brightness coincident with a drop in temperature. These characterize a
cooling flow. There is often a radio source also at the centre of such regions.
Data from Chandra now enables us to map the interaction between the radio
source and the intracluster medium. Preliminary work shows no sign of heating
of the gas beyond the radio lobes, which are often devoid of cooler gas and so
appear as holes. In the case of the Perseus cluster around 3C84, the coolest
X-ray emitting gas occurs immediately around the inner radio lobes.


title: Status of MAGIC and Expected Capabilities

abstract:
  An overview of the status of the 17 m diameter MAGIC telescope project will
be given. During phase I, the telescope will reach a threshold of 30 GeV and a
sensitivity of 6.0 10$^{-11}$ cm$^{-2}$ s$^{-1}$. First light is foreseen in
mid 2001 and first observations in 2002. The expected capabilities of the
telescope for high energy astrophysics and fundamental physics will be
reviewed.


title: An X-ray selected sample of radio-loud quasars

abstract:
  We construct the first X-ray selected sample of broad line radio-loud AGN
from the EMSS survey. In order to test unifying schemes for radio-loud objects,
their spectral and statistical properties (both flat and steep spectrum
objects) are examined and compared with those of other samples of blazars. The
X-ray selection allows us to explore properties of radio-loud quasars 10-100
weaker in the radio band than classical samples. The most convincing
interpretation of our results is that there are no radio-loud quasars whose
synchrotron emission peak reaches the EUV-soft X-ray band at these (radio) flux
levels. Moreover, due to the comparatively weak non-thermal emission, a
quasi-thermal component appears to contribute at optical-UV energies. The
detection of sources at low radio fluxes also reveals a large population of
steep spectrum quasars and the lack of the predicted turnover in the quasar
(radio) counts. The evolution of X-ray selected radio-loud quasars does not
significantly differ from that of radio-selected ones, and flat spectrum and
steep spectrum sources appear to behave quite similarly.


title: Unified Model Photospheres for Ultracool Dwarfs of the Types L and T

abstract:
  We show that the two groups of ultracool dwarfs classified as L and T types
can be understood as a temperature effect in a unique sequence of the model
photospheres in which dust always exists but only in the restricted region deep
in the photosphere. Under the high density of the photospheres of cool dwarfs,
dust forms easily at the condensation temperature T_cond (about 2000 K), but
the dust will soon grow larger than the critical radius r_cr at the critical
temperature T_cr (about 1800 K). The large dust grains with r > r_cr will soon
segregate from the gas and precipitate below the photosphere. For this reason,
dust exists only in the limited region of T_cr < T < T_cond in the photo-
sphere which we referred to as an active dust zone, and this means that a cloud
layer is formed deep in the photosphere rather than in the cooler surface
region. With the simple assumption that only small dust grains (r < r_cr) can
be sustained in the photosphere, a self- consistent non-grey model photosphere
in radiative-convective equilibrium can be developed without any other ad-hoc
assumption. We show that the spectra and colours of L (the cloud layer appears
in the optically thin region) and T (the dust cloud is situated too deep to be
observable) dwarfs can consistently be reproduced by a single grid of our
unified model photospheres.


title:
A New Model for the Thermal X-ray Composites and the Neutral Pion Decay
  Gamma-Rays from Supernova Remnants

abstract:
  Recent nonthermal X-ray and gamma-ray observations, attributed to electron
emission processes, for the first time give an experimental confirmation that
electrons are accelerated on SNR shocks up to the energy 10^{14} eV. We have no
direct observational confirmations about proton acceleration by SNR. Different
models of gamma-emission from SNRs predict different emission mechanisms as
dominating. Only decays of neutral pion created in proton-nucleon interactions
allow us to look inside the CR nuclear component acceleration processes. A new
model for the thermal X-ray composites strongly suggest that thermal X-ray peak
inside the radio shell of SNR tells us about entering of one part of SNR shock
into a denser medium compared with other parts of the shell. This makes a TXCs
promising sites for gamma-ray generation via neutral pion decays. Detailed
consideration of SNR-cloud interaction allows to increase an expected proton
induced gamma-ray flux from SNR at least on an order of magnitude, that allows
to adjust the theoretical proton origin gamma-luminosities with observed fluxes
at least for a few SNRs even for low density (n=10-100 cm^{-3}) cloud.


title: A Measurement of the Flux of Cosmic Ray Iron at 5 x 10^13 eV

abstract:
  We present results from the initial flight of our Balloon Air CHerenkov
(BACH) payload. BACH detects air Cherenkov radiation from cosmic ray nuclei as
coincident flashes in two optical modules. The flight (dubbed PDQ BACH) took
place on April 22, 1998 from Ft. Sumner, New Mexico. During an exposure of 2.75
hours, with a typical threshold energy for iron nuclei of 2.2$\times10^{13}$
eV, we observed several events cleanly identifiable as iron group nuclei.
Analysis of the data yields a new flux measurement that is fully consistent
with that reported by other investigations.


title: AGN and Cooling Flows

abstract:
  For two decades the steady-state cooling-flow model has dominated the
literature of cluster and elliptical-galaxy X-ray sources. For ten years this
model has been in severe difficulty from a theoretical point of view, and it is
now coming under increasing pressure observationally. For two decades the
steady-state cooling-flow model has dominated the literature of cluster and
elliptical-galaxy X-ray sources. For ten years this model has been in severe
difficulty from a theoretical point of view, and it is now coming under
increasing pressure observationally. A small number of enthusiasts have argued
for a radically different interpretation of the data, but had little impact on
prevailing opinion because the unsteady heating picture that they advocate is
extremely hard to work out in detail. Here I explain why it is difficult to
extract robust observational predictions from the heating picture. Major
problems include the variability of the sources, the different ways in which a
bi-polar flow can impact on X-ray emission, the weakness of synchrotron
emission from sub-relativistic flows, and the sensitivity of synchrotron
emission to a magnetic field that is probably highly localized.


title: Three-dimensional MHD simulations of Radiatively cooling, Pulsed Jets

abstract:
  (Abridged) We here investigate, by means of fully 3-D Smoothed Particle
Magnetohydrodynamic numerical simulations, the effects of magnetic fields on
overdense, radiatively cooling, pulsed jets, using different initial magnetic
field topologies and strengths ($B \simeq 260 \mu$G-0). The relative
differences that have been previously detected in 2-D simulations involving
distinct magnetic field configurations are diminished in the 3-D flows. While
the presence of toroidal magnetic components can modify the morphology close to
the jet head inhibiting its fragmentation in the early jet evolution, as
previously reported in the literature, the impact of the pulsed-induced
internal knots causes the appearance of a complex morphology at the jet head
(as required by the observations of H-H jets) even in the MHD jet models with
toroidal components. The detailed structure and emission properties of the
internal working surfaces can be also significantly altered by the presence of
magnetic fields. The increase of the magnetic field strength improves the jet
collimation, and amplifies the density (by factors up to 1.4, and 4) and the
H\alpha$ intensity (by factors up to 4, and 5) behind the knots of jets with
helical field and $\beta \simeq 1-0.1$ (respectively), relative to a non
magnetic jet. As a consequence, the corresponding $I_{[SII]}}/I_{H}\alpha}$
ratio (which is frequently used to determine the excitation level of HH
objects) can be decreased in the MHD models with toroidal components relative
to non-magnetic calculations. We also find that the helical mode of the K-H
instability can be triggered in MHD models with helical magnetic fields,
causing some jet wiggling. No evidence for the formation of the nose cones is
found in the 3-D flows, nor even in the $\beta \simeq 0.1$ case.


title:
Infrared Space Observatory Spectra of R Coronae Borealis Stars. I.
  Emission Features in the Interval 3 - 25 microns

abstract:
  Infrared Space Observatory 3 - 25 $\mu$m spectra of the R Coronae Borealis
stars V854 Cen, R CrB, and RY Sgr are presented and discussed. Sharp emission
features coincident in wavelengths with the well known Unidentified Emission
Features are present in the spectrum of V854 Cen but not of R CrB or RY Sgr.
Since V854 Cen is not particularly H-poor and has a 1000 times more H than the
other stars, the emission features are probably from a carrier containing
hydrogen. There is a correspondence between the features and emission from
laboratory samples of hydrogenated amorphous carbon. A search for C$_{60}$ in
emission or absorption proved negative. Amorphous carbon particles account for
the broad emission features seen between 6 - 14 $\mu$m in the spectrum of each
star.


title: Discovery of X rays from Class 0 protostar candidates in OMC-3

abstract:
  We have observed the Orion Molecular Clouds 2 and 3 (OMC-2 and OMC-3) with
the Chandra X-ray Observatory (CXO). The northern part of OMC-3 is found to be
particularly rich in new X-ray features; four hard X-ray sources are located in
and along the filament of cloud cores. Two sources coincide positionally with
the sub$mm$-$mm$ dust condensations of MMS 2 and 3 or an outflow radio source
VLA 1, which are in a very early phase of star formation. The X-ray spectra of
these sources show an absorption column of (1-3) x 10^23 H cm-2. Assuming a
moderate temperature plasma, the X-ray luminosity in the 0.5-10 keV band is
estimated to be ~10^30 erg s^-1 at a distance of 450 pc. From the large
absorption, positional coincidence and moderate luminosity, we infer that the
hard X-rays are coming from very young stellar objects embedded in the
molecular cloud cores. We found another hard X-ray source near the edge of the
dust filament. The extremely high absorption of 3 x 10^23 H cm^-2 indicates
that the source must be surrounded by dense gas, suggesting that it is either a
YSO in an early accretion phase or a Type II AGN (e.g. a Seyfert 2), although
no counterpart is found at any other wavelength. In contrast to the hard X-ray
sources, soft X-ray sources are found spread around the dust filaments, most of
which are identified with IR sources in the T Tauri phase.


title: Analysis of Keck HIRES spectra of early L-type dwarfs

abstract:
  We present analyses of high resolution and medium resolution spectra of early
L dwarfs. We used our latest set of model atmospheres to reproduce and analyze
the observed features. We can model the optical flux and the atomic line
profiles with the best accuracy to date. The models used to reproduce the
observations include dust condensation and dust opacities. Compared to previous
studies using older models we find that our dust treatment is much improved.
The derived parameters for the objects are well in the expected range for old
very low mass objects. This is also supported by the absence of Li in most of
the objects. For the objects showing Li we can be almost certain that those are
brown dwarfs. However, a spectral analysis in general, and this one in
particular can only very roughly determine mass and age.


title: Two-colour photometric selection of high redshift galaxies

abstract:
  In this paper we describe a set of models to predict the colours of galaxies
over a wide range of redshifts. We present example output from the simulations,
and discuss their application to the selection of galaxies at high redshifts,
particularly through identification of the Lyman break. Additionally we
consider the optimal choices of filters for selection at a range of redshifts.
  An interface to a subset of the simulations has been made available on the
World Wide Web for the benefit of the community at the location
http://www-astro.physics.ox.ac.uk/~rejs/research/galcols.html.


title: Rapid Spindown of SGR 1900+14 Following Its Superburst

abstract:
  SGR 1900+14 had a brief episode of exceedingly rapid spindown immediately
following its 1998 Aug. 27 superburst. On a timescale of hours, it increased
its period by a part in 10^4. The corresponding Pdot ~ 10^{-8} is orders of
magnitude higher than the typical quiescent rate of Pdot ~ 6x10^{-11}.


title:
A Morphological-type dependence in the mu_0-log(h) plane of Spiral
  galaxy disks

abstract:
  We present observational evidence for a galaxy `Type' dependence to the
location of a spiral galaxy's disk parameters in the mu_0-log(h) (central disk
surface-brightness - disk scale-length) plane. With a sample of ~40 Low Surface
Brightness galaxies (both bulge- and disk-dominated) and ~80 High Surface
Brightness galaxies, the early-type disk galaxies (<=Sc) tend to define a
bright envelope in the mu_0-log(h) plane, while the late-type (>=Scd) spiral
galaxies have, in general, smaller and fainter disks. Below the defining
surface brightness threshold for a Low Surface Brightness galaxy (i.e. more
than 1 mag fainter than the 21.65 B-mag arcsec^(-2) Freeman value), the
early-type spiral galaxies have scale-lengths greater than 8-9 kpc, while the
late-type spiral galaxies have smaller scale-lengths. All galaxies have been
modelled with a seeing-convolved Sersic r^(1/n) bulge and exponential disk
model. We show that the trend of decreasing bulge shape parameter (n) with
increasing Hubble type and decreasing bulge-to-disk luminosity ratio, which has
been observed amongst the High Surface Brightness galaxies, extends to the Low
Surface Brightness galaxies, revealing a continuous range of structural
parameters.


title: The radio surroundings of the microquasar GRO J1655-40

abstract:
  We report the results of a study of the radio surroundings of the
superluminal microquasar GRO J1655-40. We have searched for extended continuum
structures that might be indicative of the presence of a supernova remnant
(SNR) associated with the formation of the compact object in the binary system.
We also carried out HI-line observations of the region looking for a local
minimum created by an explosive event. Our results indicate that there is, in
fact, a bubble in the large-scale HI distribution around GRO J1655-40. We
suggest that this structure might be created by the original supernova
explosion occurred a few hundred thousand years ago and whose signatures can be
traced by the overabundance of alpha-elements recently found by Israelian et
al. (1999) in the companion star.


title:
Detection of a new, low-brightness supernova remnant possibly associated
  with EGRET sources

abstract:
  We report on the discovery of a shell-type supernova remnant in the southern
sky. It is a large (8*8), low-brightness source with a nonthermal radio
spectrum, which requires background filtering to isolate it from the diffuse
background emission of the Galaxy. Three 3EG gamma-ray sources are spatially
correlated with the radio structure. We have made 21-cm line observations of
the region and found that two of these sources are coincident with HI clouds.
We propose that the gamma-ray emission is the result of hadronic interactions
between high-energy protons locally accelerated at the remnant shock front and
atomic nuclei in the ambient clouds.


title:
Line Forces in Keplerian Circumstellar Disks and Precession of Nearly
  Circular Orbits

abstract:
  We examine the effects of optically thick line forces on orbiting
circumstellar disks, such as occur around Be stars. For radially streaming
radiation, line forces are only effective if there is a strong radial velocity
gradient, as occurs, for example, in a line-driven stellar wind. However,
within an orbiting disk, the radial shear of the azimuthal velocity leads to
strong line-of-sight velocity velocity gradients along nonradial directions. As
such, in the proximity of a stellar surface extending over a substantial cone
angle, the nonradial stellar radiation can impart a significant line force,
even in the case of purely circular orbits. Given the highly supersonic nature
of orbital velocity variations, we use the Sobolev approximation, thereby
extending to the disk case the standard CAK formalism developed for line-driven
winds. We delineate the parameter regimes for which radiative forces might
alter disk properties; but even when radiative forces are small, we
analytically quantify higher-order effects in the linear limit, including the
precession of weakly elliptical orbits. We find that optically thick line
forces can have observable implications for the dynamics of disks around Be
stars, including the generation of either prograde or retrograde precession in
slightly eccentric orbits. However, our analysis suggests a net retrograde
effect, in apparent contradiction with observed long-term variations of
violet/red line profile asymmetries from Be stars, which are generally thought
to result from prograde propagation of a so-called ``one arm mode''. We also
conclude that radiative forces may alter the dynamical properties at the
surface of the disk where disk winds originate, and may even make low-density
disks vulnerable to being blown away.


title: Stability of accretion discs threaded by a strong magnetic field

abstract:
  We study the stability of poloidal magnetic fields anchored in a thin
accretion disc. The two-dimensional hydrodynamics in the disc plane is followed
by a grid-based numerical simulation including the vertically integrated
magnetic forces. The 3--dimensional magnetic field outside the disc is
calculated in a potential field approximation from the magnetic flux density
distribution in the disc. For uniformly rotating discs we confirm numerically
the existence of the interchange instability as predicted by Spruit, Stehle &
Papaloizou (1995). In agreement with predictions from the shearing sheet model,
discs with Keplerian rotation are found to be stabilized by the shear, as long
as the contribution of magnetic forces to support against gravity is small.
When this support becomes significant, we find a global instability which
transports angular momentum outward and allows mass to accrete inward. The
instability takes the form of a $m=1$ rotating `crescent', reminiscent of the
purely hydrodynamic nonlinear instability previously found in
pressure-supported discs. A model where the initial surface mass density
$\Sigma(r)$ and $B_{\mathrm{z}}(r)$ decrease with radius as power laws shows
transient mass accretion during about 6 orbital periods, and settles into a
state with surface density and field strength decreasing approximately
exponentially with radius. We argue that this instability is likely to be the
main angular momentum transport mechanism in discs with a poloidal magnetic
field sufficiently strong to suppress magnetic turbulence. It may be especially
relevant in jet-producing discs.


title: Collisional dynamics around binary black holes in galactic centers

abstract:
  We follow the sinking of two massive black holes in a spherical stellar
system where the black holes become bound under the influence of dynamical
friction. Once bound, the binary hardens by three-body encounters with
surrounding stars. We find that the binary wanders inside the core, providing
an enhanced supply of reaction partners for the hardening. The binary evolves
into a highly eccentric orbit leading to coalescence well beyond a Hubble time.
These are the first results from a hybrid ``self consistent field'' (SCF) and
direct Aarseth N-body integrator (NBODY6), which combines the advantages of the
direct force calculation with the efficiency of the field method. The code is
designed for use on parallel architectures and is therefore applicable to
collisional N-body integrations with extraordinarily large particle numbers (>
10^5). This creates the possibility of simulating the dynamics of both globular
clusters with realistic collisional relaxation and stellar systems surrounding
supermassive black holes in galactic nuclei.


title:
Primordial nucleosynthesis and hadronic decay of a massive particle with
  a relatively short lifetime

abstract:
  In this paper we consider the effects on big bang nucleosynthesis (BBN) of
the hadronic decay of a long-lived massive particle. If high-energy hadrons are
emitted near the BBN epoch ($t \sim 10^{-2}$ -- $10^2 \sec$), they
extraordinarily inter-convert the background nucleons each other even after the
freeze-out time of the neutron to proton ratio. Then, produced light element
abundances are changed, and that may result in a significant discrepancy
between standard BBN and observations. Especially on the theoretical side, now
we can obtain a lot of experimental data of hadrons and simulate the hadronic
decay process executing the numerical code of the hadron fragmentation even in
the high energy region where we have no experimental data. Using the light
element abundances computed in the hadron-injection scenario, we derive a
constraint on properties of such a particle by comparing our theoretical
results with observations.


title:
Thermal X-ray emission of the remnants of ashperical Supernova
  explosions

abstract:
  Evolution of adiabatic remnants of an aspherical supernova explosion in
uniform medium are considered. Thermal X-ray emission of such remnants are
investigated. It is shown that integral thermal X-ray characteristics (X-ray
luminosity and spectrum) of the objects do not allow us to reveal the assymetry
in the explosion because these characteristics are close to their Sedov
counterparts. Surface distribution of X-ray emission is sensitive to anisotropy
of the explosion and nonuniformity of the interstellar medium.


title: Weak Reprocessed Features in the Broad Line Radio Galaxy 3C382

abstract:
  We present a detailed X-ray study of the Broad Line Radio Galaxy 3C382,
observed with the BeppoSAX satellite in a very bright state. The continuum
emission is well modeled with a power law that steepens at high energies, with
an e-folding energy of about 120 keV. At soft energies a clear excess of
emission is detected, which can not be explained solely by the extended thermal
halo seen in a ROSAT HRI image. A second, more intense soft X-ray component,
possibly related to an accretion disk, is required by the data. Both a
reflection component (R=0.3) and an iron line (EW \sim 50) are detected, at
levels much weaker than in Seyfert galaxies, suggesting a common origin.
Combining our measurements with results from the literature we find that the
iron line has remained approximately constant over 9 years while the continuum
varied by a factor of 5. Thus the fluorescent gas does not respond promptly to
the variations of the X-ray primary source, suggesting that the reprocessing
site is located away, likely at parsec distances. While the continuum shape
indicates that X-rays derive from a thermal Comptonization process, the
weakness of other spectral features implies that either the upper layers of the
optically thick accretion disk are completely ionized or the corona above the
disk is outflowing with mildly relativistic velocity.


title: HST study of the LMC compact star forming region N83B

abstract:
  High resolution imaging with the HST uncovers the so far hidden stellar
content and the nebular features of the high excitation compact HII region N83B
in the Large Magellanic Cloud (LMC). We discover that the HII region is powered
by the most recent massive starburst in the OB association LH5 and the burst
has created about 20 blue stars spread over ~30" on the sky (7.5 pc). Globally
N83B displays a turbulent environment typical of newborn massive star formation
sites. It contains an impressive ridge, likely created by a shock and a cavity
with an estimated age of only ~30,000 yr, sculpted in the ionized gas by the
powerful winds of massive stars. The observations bring to light two compact
HII blobs, N83B-1 and N83B-2, and a small arc-nebula, N83B-3, lying inside the
larger HII region. N83B-1, only ~2.8" (0.7 pc) across, is the brightest and
most excited part of N83B. It harbors the presumably hottest star of the burst
and is also strongly affected by dust with an extinction of Av=2.5 mag. The
second blob, N83B-2, is even more compact, with a size of only ~1" (0.3 pc).
All three features are formed in the border zone between the molecular cloud
and the ionized gas possibly in a sequential process triggered by the
ionization front of an older HII region. Our HST imaging presents an
interesting and rare opportunity to observe details in the morphology of the
star formation in very small spatial scales in the LMC which are in agreement
with the concept of the fractal structure of molecular star forming clouds. A
scenario which supports hierarchical massive star formation in the LMC OB
association LH5 is presented.


title:
Thermal relaxation of very small solar magnetic structures in
  intergranules: a process that produces kG magnetic field strengths

abstract:
  The equilibrium configuration of very small magnetic fluxtubes in an
intergranular environment automatically produces kG magnetic field strengths.
We argue that such process takes place in the Sun and complements the
convective collapse (CC), which is traditionally invoked to explain the
formation of kG magnetic concentrations in the solar photosphere. In
particular, it can concentrate the very weak magnetic fluxes revealed by the
new IR spectro-polarimeters, for which the operation of the CC may have
difficulty. As part of the argument, we show the existence of solar magnetic
features of very weak fluxes yet concentrated magnetic fields (some 3 x 10^{16}
Mx and 1500 G).


title: A Molecular Tidal Tail in the Medusa Minor Merger

abstract:
  We have detected CO 1-0 emission along the tidal tail of the NGC 4194 (the
Medusa) merger. It is the first CO detection in the optical tail of a minor
merger. Emission is detected both in the centre of the tail and at its tip. The
molecular mass in the 33'' Onsala 20m beam is estimated to be >= 8.5 x 10^7
M_{sun} which is at least 4% of the total molecular mass measured so far in
this system. We suggest that the emission is a molecular tidal tail which is
part of the extended structure of the main body, and that the molecular gas was
thrown out by the collision instead of having formed in situ from condensing
atomic material. We find it unlikely that the emission is associated with a
tidal dwarf galaxy (even if the future formation of such an object is
possible), but high resolution HI, CO and optical observations are necessary to
resolve the issue. The Medusa is very likely the result of an elliptical+spiral
collison and our detection supports the notion that molecular gas in minor
mergers can be found at great distances from the merger centre.


title:
A hard X-ray constraint on the presence of an AGN in the ultra-luminous
  infrared galaxy Arp220

abstract:
  We present X-ray results on the ultraluminous infrared galaxy Arp220 obtained
with BeppoSAX. The X-ray emission up to 10 keV is detected. No significant
signal is detected with the PDS detector in the higher energy band. The 2-10
keV emission has a hard spectrum (photon-index of 1.7), similar to M82, and a
luminosity of 1e41 erg/s. A population of X-ray binaries may be a major source
of this X-ray emission. The upper limit of an iron K line equivalent width at
6.4 keV is ~600 eV. This observation imposes so far the tightest constraint on
an active nucleus if present in Arp220. We find that a column density of X-ray
absorption must exceed 10**25 cm-2 for an obscured active nucleus to be
significant in the energetics, and the covering factor of the absorption should
be almost unity. The underluminous soft X-ray starburst emission may need a
good explanation, if the bolometric luminosity is primarily powered by a
starburst.


title: Explosive nucleosynthesis

abstract:
  Many radioactive nuclei relevant for gamma-ray astrophysics are synthesized
during explosive events, such as classical novae and supernovae. A review of
recent results of explosive nucleosynthesis in these scenarios is presented,
with a special emphasis on the ensuing gamma-ray emission from individual nova
and supernova explosions. The influence of the dynamic properties of the ejecta
on the gamma-ray emission features, as well as the still remaining
uncertainties in nova and supernova models is also reviewed.


title:
The effect of stellar activity on the LiI 6708, NaI 5896 and KI 7699 \AA
  lines: A comparison with the Pleiades, field stars and the Sun

abstract:
  An analytical model has been developed to empirically study the effects of
stellar spots and faculae on the observed equivalent widths of Li {\sc i} 6708,
Na {\sc i} 5896 and K {\sc i} 7699 \AA lines (and abundances in the case of
lithium) in late-type stars, taking into account the changes in the observed
magnitudes and colors. Solar spectra corresponding to different active regions
are used as input data and a range of filling factors are applied to simulate
the surfaces of stars with different levels of activity. Detailed comparisons
between predicted and observed photometric colors and equivalent widths are
made for late-type stars of the Pleiades and the field. The observed
dispersions in K {\sc i} and Li {\sc i} equivalent widths for Pleiades stars
can be partially accounted by the simultaneous effects of activity on colors
and the line formation, indicating that the lithium-rotation connection
suggested for ~0.7-0.9 Msun Pleiades stars could be due in part to the stellar
activity. However, under realistic values for the filling factors, only a small
portion of the observed spread could be explained by these effects


title: Prospects for detectability of classical novae with INTEGRAL

abstract:
  Classical novae are potential gamma-ray emitters, both in lines and in a
continuum. Continuum emission (at energies between 20-30 and 511 keV) and line
emission at 511 keV are related to positron annihilation and its Comptonization
in the expanding shell; 18F is the main responsible of positron production. The
lines at 478 and 1275 keV have their origin in the decay of the radioactive
nuclei 7Be and 22Na. Updated models of nova explosions have been adopted for
the computation of the gamma-ray emission. New yields of some radioactive
isotopes directly translate into new detectability distances of classical novae
with INTEGRAL.


title:
The identification of the optical/IR counterpart of the 29.5-s transient
  X-ray pulsar GS 1843+009

abstract:
  We report on the identification of the optical/IR counterpart of the 29.5-s
transient X-ray pulsar GS 1843+009. We re-analysed an archival ROSAT HRI
observation of GS 1843+009, obtaining a new refined position. The optical and
IR follow-up observations carried out for the new error circle allowed us to
find a relatively faint (V=20.9) and variable early type reddened star
(V-R=2.1). The optical spectra show the Balmer and Paschen series in emission,
while the IR observations confirm the presence of a flux excess (H=13.2,
J-H=0.54), suggesting that the star is surrounded by a circumstellar envelope.
Spectroscopic and photometric data together indicate a B0-B2 IV-Ve
spectral-type star located at a distance of > 10kpc confirming the
Be-star/X-ray binary nature of GS 1843+009.


title: Deep Infrared Imaging of the Microquasars 1E1740-2942 and GRS 1758-258

abstract:
  We present deep infrared ($2.2 \mu$m) imaging of the Galactic microquasars
1E1740-2942 and GRS 1758-258 using the Keck-I 10-meter telescope in June 1998.
The observations were taken under excellent seeing conditions ($\sim 0.45
\arcsec$ full-width half-maximum), making them exceptionally deep for these
crowded fields. We used the USNO-A2.0 catalog to astrometrically calibrate the
infrared images (along with an optical CCD image in the case of GRS 1758-258),
providing independent frame ties to the known radio positions of the objects.
For 1E1740-2942, we confirm potential candidates for the microquasar previously
identified by Marti et al., and show that none of the objects near the
microquasar have varied significantly from 1998 to 1999. For GRS 1758-258, our
astrometry indicates a position shifted from previous reports of candidates for
the microquasar. We find no candidates inside our 90% confidence radius to a $2
\sigma$ limiting magnitude of $K_s = 20.3$ mag. We discuss the implications of
these results for the nature of the microquasar binary systems.


title: Precipitating Condensation Clouds in Substellar Atmospheres

abstract:
  We present a method to calculate vertical profiles of particle size
distributions in condensation clouds of giant planets and brown dwarfs. The
method assumes a balance between turbulent diffusion and sedimentation in
horizontally uniform cloud decks. Calculations for the Jovian ammonia cloud are
compared with results from previous methods. An adjustable parameter describing
the efficiency of sedimentation allows the new model to span the range of
predictions made by previous models. Calculations for the Jovian ammonia cloud
are consistent with observations. Example calculations are provided for water,
silicate, and iron clouds on brown dwarfs and on a cool extrasolar giant
planet. We find that precipitating cloud decks naturally account for the
characteristic trends seen in the spectra and colors of L- and T-type ultracool
dwarfs.


title: Internal shocks in the jets of radio-loud quasars

abstract:
  The central engine causing the production of jets in radio sources may work
intermittently, accelerating shells of plasma with different mass, energy and
velocity. Faster but later shells can then catch up slower earlier ones. In the
resulting collisions shocks develop, converting some of the ordered bulk
kinetic energy into magnetic field and random energy of the electrons which
then radiate. We propose that this internal shock scenario, which is the
scenario generally thought to explain the observed gamma-ray burst radiation,
can work also for radio sources in general, and for blazar in particular. We
investigate in detail this idea, simulating the birth, propagation and
collision of shells, calculating the spectrum produced in each collision, and
summing the locally produced spectra from those regions of the jet which are
simultaneously active in the observer's frame. We can thus construct snapshots
of the overall spectral energy distribution as well as time dependent spectra
and light curves. This allows us to characterize the predicted variability at
any frequency, study correlations among the emission at different frequencies,
specify the contribution of each region of the jet to the total emission, find
correlations between flares at high energies and the birth of superluminal
radio knots and/or radio flares. The model has been applied to qualitatively
reproduce the observed properties of 3C 279. Global agreement in terms of both
spectra and temporal evolution is found. In a forthcoming work, we explore the
constraints which this scenario sets on the initial conditions of the plasma
injected in the jet and the shock dissipation for different classes of blazars.


title: Age-Dating Ultraluminous Infrared Galaxies Along the Merger Sequence

abstract:
  Imaging spectroscopy using the new Palomar Integral Field Spectrograph is
presented for the Pa-alpha line in four ultraluminous infrared galaxies. In
most cases, we are able to establish the geometry of the merger, and thus
estimate the time in the merger process at which we are viewing the system. The
resulting range in estimated ages, some of which are very young encounters
(~5x10^7 yr), is not predicted by merger models, which produce high rates of
star formation either 1-2x10^8 years after the first encounter or very late
(~10^9 yr) in the merger process. Even in the very young mergers, the
ultraluminous activity appears to be centrally concentrated on the nucleus of
one of the progenitor galaxies---namely the galaxy with a prograde orbital
geometry. Very high star formation rates (2-5 M_sun/yr) are also observed in
structures believed to be newly formed tidal tails.
  The presence of a significant population of very young ultraluminous mergers,
together with the majority of ultraluminous infrared galaxies existing in the
final stages of merger activity, indicates that the ultraluminous galaxy phase
is at least bimodal in time. An evolutionary scenario is proposed for
ultraluminous infrared galaxies, wherein the far-infrared luminosity may
undergo multiple ultraluminous bursts during the course of the encounter. A
substantial fraction of the merger lifetime may be spent in a phase identified
with the less powerful luminous infrared galaxy class.


title: The Continuing Slow Decline of AG Pegasi

abstract:
  We analyze optical and ultraviolet observations of the symbiotic binary AG
Pegasi acquired during 1992-97. The bolometric luminosity of the hot component
declined by a factor of 2-3 from 1980-1985 to 1997. Since 1992, the effective
temperature of the hot component may have declined by 10%-20%, but this decline
is comparable to the measurement errors. Optical observations of H-beta and He
I emission show a clear illumination effect, where high energy photons from the
hot component ionize the outer atmosphere of the red giant. Simple illumination
models generally account for the magnitude of the optical and ultraviolet
emission line fluxes. High ionization emission lines - [Ne V], [Mg V], and [Fe
VII] - suggest mechanical heating in the outer portions of the photoionized red
giant wind. This emission probably originates in a low density region $\sim$
30-300 AU from the central binary.


title:
Relativistic corrections to astrometric shifts due to gravitational
  microlensing

abstract:
  Gravitational microlensing has proved to be a versatile astrophysical tool.
Recently, the question of whether higher order relativistic corrections can
influence the observable properties of microlensing has been addressed. This
letter extends earlier studies, investigating higher order relativistic terms
in astrometric shifts during microlensing events, revealing that they produce
no effects. The magnitude of the relativistic corrections is examined in an
astrophysical context and is found to be negligible.


title:
Use of Gas Electron Multiplier (GEM) Detectors for an Advanced X-ray
  Monitor

abstract:
  We describe a concept for a NASA SMEX Mission in which Gas Electron
Multiplier (GEM) detectors, developed at CERN, are adapted for use in X-ray
astronomy. These detectors can be used to obtain moderately large detector area
and two-dimensional photon positions with sub mm accuracy in the range of 1.5
to 15 keV. We describe an application of GEMs with xenon gas, coded mask
cameras, and simple circuits for measuring event positions and for
anticoincidence rejection of particle events. The cameras are arranged to cover
most of the celestial sphere, providing high sensitivity and throughput for a
wide variety of cosmic explosions. At longer timescales, persistent X-ray
sources would be monitored with unprecedented levels of coverage. The
sensitivity to faint X-ray sources on a one-day timescale would be improved by
a factor of 6 over the capability of the RXTE All Sky Monitor.


title: On the origin of the distribution of binary-star periods

abstract:
  Pre-main sequence and main-sequence binary systems are observed to have
periods, P, ranging from one day to 10^(10) days and eccentricities, e, ranging
from 0 to 1. We pose the problem if stellar-dynamical interactions in very
young and compact star clusters may broaden an initially narrow period
distribution to the observed width. N-body computations of extremely compact
clusters containing 100 and 1000 stars initially in equilibrium and in cold
collapse are preformed. In all cases the assumed initial period distribution is
uniform in the narrow range 4.5 < log10(P) < 5.5 (P in days) which straddles
the maximum in the observed period distribution of late-type Galactic-field
dwarf systems. None of the models lead to the necessary broadening of the
period distribution, despite our adopted extreme conditions that favour
binary--binary interactions. Stellar-dynamical interactions in embedded
clusters thus cannot, under any circumstances, widen the period distribution
sufficiently. The wide range of orbital periods of very young and old binary
systems is therefore a result of cloud fragmentation and immediate subsequent
magneto-hydrodynamical processes operating within the multiple proto-stellar
system.


title: The Metamorphosis of Tidally Stirred Dwarf Galaxies

abstract:
  We present results from high-resolution N-Body/SPH simulations of
rotationally supported dwarf irregular galaxies moving on bound orbits in the
massive dark matter halo of the Milky Way.The dwarf models span a range in disk
surface density and the masses and sizes of their dark halos are consistent
with the predictions of cold dark matter cosmogonies. We show that the strong
tidal field of the Milky Way determines severe mass loss in their halos and
disks and induces bar and bending instabilities that transform low surface
brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface
brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The
final central velocity dispersions of the remnants are in the range 8-30 km/s
and their final $v/\sigma$ falls to values $< 0.5$, matching well the
kinematics of early-type dwarfs. The transformation requires the orbital time
of the dwarf to be $\simlt 3-4$ Gyr, which implies a halo as massive and
extended as predicted by hierarchical models of galaxy formation to explain the
origin of even the farthest dSph satellites of the Milky Way, Leo I and Leo II.
Only dwarfs with central dark matter densities as high as those of Draco and
Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way: this is
naturally achieved within hierarchical models, where the densest objects should
have small orbital times due to their early formation epochs. Part of the gas
is stripped and part is funneled to the center due to the bar, generating one
strong burst of star formation in HSBs and smaller, multiple bursts in LSBs.
Extended low-surface brightness stellar and gaseous streams originate from LSBs
and, when projected along the line of sight, can lead to overestimate the
mass-to-light ratio of the bound remnant by a factor $\simlt 2$,


title: Properties of Black Holes in Stellar Binary Systems

abstract:
  Recent X-ray timing and spectral observations of black hole binaries in
outburst have redefined methods for investigating the properties of Galactic
black holes and the physics of accretion flows. High-frequency X-ray QPOs in 5
systems (60--300 Hz) continue to be investigated as a possible means to
constrain black hole mass and spin via interpretation as oscillations due to
strong-field effects in General Relativity. In principle, black hole mass and
spin may also be constrained via determination of the inner radius of the
accretion disk, using spectroscopic parallax of the X-ray thermal component.
However, an accurate application of this technique is fraught with
difficulties. Monitoring programs that track the evolution of the thermal and
power-law components in the X-ray spectrum provide new insights into modes of
energy flow and the formation of relativistic jets.


title: Star Formation in the Field and Clusters of NGC 5253

abstract:
  We investigate the star formation history of both the bright star clusters
and the diffuse `field star' population in the dwarf starburst galaxy NGC 5253
using STIS longslit ultraviolet spectroscopy. Our slit covers a physical area
of 370 x 1.6 pc and includes 8 apparent clusters and several inter-cluster
regions of diffuse light which we take to be the field. The diffuse light
spectrum lacks the strong O-star wind features which are clearly visible in
spectra of the brightest clusters. This discrepancy provides compelling
evidence that the diffuse light is not reflected light from nearby clusters,
but originates in a UV-bright field star population, and it raises the issue of
whether the star formation process may be operating differently in the field
than in clusters. We compare our spectra to STARBURST99 evolutionary synthesis
models which incorporate a new low metallicity atlas of O-star spectra. We
favor a scenario which accounts for the paucity of O-stars in the field without
requiring the field to have a different IMF than the clusters: stellar clusters
form continuously and then dissolve on ~10 Myr timescales and disperse their
remaining stars into the field. We consider the probable contribution of an
O-star deficient field population to the spatially unresolved spectra of high
redshift galaxies. (Abridged)


title:
FIRBACK: II. Data Reduction and Calibration of the 170 micron ISO Deep
  Cosmological Survey

abstract:
  We present the final reduction and calibration of the FIRBACK ISOPHOT data.
FIRBACK is a deep cosmological survey performed at 170 microns. This paper
deals with the ISOPHOT C200 camera with the C160 filter. We review the whole
data reduction process and compare our final calibration with DIRBE (for the
extended emission) and IRAS (for point sources). The FIRBACK source extraction
and galaxy counts is discussed in a companion paper (Dole et al., 2001).


title:
FIRBACK: III. Catalog, Source counts, and cosmological implications of
  the 170 micron ISO Deep survey

abstract:
  The FIRBACK (Far Infrared BACKground) survey is one of the deepest imaging
surveys carried out at 170 microns with ISOPHOT onboard ISO, and is aimed at
the study of the structure of the Cosmic Far Infrared Background. This paper
provides the analysis of resolved sources. After a validated process of data
reduction and calibration, we perform intensive simulations to optimize the
source extraction, measure the confusion noise (sigma_c = 45 mJy), and give the
photometric and astrometric accuracies. 196 galaxies with flux S > 3 sigma_c
are detected in the area of 3.89 square degrees. Counts of sources with flux S
> 4 sigma_c present a steep slope of 3.3 +/- 0.6 on a differential "logN-logS"
plot between 180 and 500 mJy. As a consequence, the confusion level is high and
will impact dramatically on future IR deep surveys. This strong evolution,
compared with a slope of 2.5 from Euclidian geometry, is in line with models
implying a strongly evolving Luminous Infrared Galaxy population. The resolved
sources account for less than 10% of the Cosmic Infrared Background at 170
microns, which is expected to be resolved into sources in the 1 to 10 mJy
range.


title: Evidence for an extended scattered disk

abstract:
  By telescopic tracking, we have established that the orbit of the
trans-neptunian object (2000 CR$_{105}$) has a perihelion of $\simeq$44 AU, and
is thus outside the domain controlled by strong gravitational close encounters
with Neptune. Because this object is on a very large, eccentric orbit (with
semimajor axis $a\simeq$216 AU and eccentricity $e\simeq$0.8) this object must
have been placed on this orbit by a gravitational perturbation which is {\it
not} direct gravitational scattering off of any of the giant planets (on their
current orbits). The existence of this object may thus have profound cosmogonic
implications for our understanding of the formation of the outer Solar System.
We discuss some viable scenarios which could have produced it, including
long-term diffusive chaos and scattering off of other massive bodies in the
outer Solar System. This discovery implies that there must be a large
population of trans-neptunian objects in an `extended scattered disk' with
perihelia above the previously-discussed 38 AU boundary.