title:
A Galactic Bar to Beyond the Solar Circle and its Relevance for
  Microlensing

abstract:
  The Galactic kinematics of Mira variables have been studied using infrared
photometry, radial velocities, and Hipparcos parallaxes and proper motions. For
Miras in the period range 145 to 200 days (probably corresponding to [Fe/H] in
the range -0.8 to -1.3) the major axes of the stellar orbits are concentrated
in the first quadrant of Galactic longitude. This is interpreted as a
continuation of the bar-like structure of the Galactic Bulge out to the solar
circle and beyond.


title:
Circumnuclear regions in barred spiral galaxies. I. Near-infrared
  imaging

abstract:
  We present sub-arcsecond resolution ground-based near-infrared images of the
central regions of a sample of twelve barred galaxies with circumnuclear star
formation activity, which is organized in ring-like regions typically one
kiloparsec in diameter. We also present Hubble Space Telescope near-infrared
images of ten of our sample galaxies, and compare them with our ground-based
data. Although our sample galaxies were selected for the presence of
circumnuclear star formation activity, our broad-band near-infrared images are
heterogeneous, showing a substantial amount of small-scale structure in some
galaxies, and practically none in others. We argue that, where it exists, this
structure is caused by young stars, which also cause the characteristic bumps
or changes in slope in the radial profiles of ellipticity, major axis position
angle, surface brightness and colour at the radius of the circumnuclear ring in
most of our sample galaxies. In 7 out of 10 HST images, star formation in the
nuclear ring is clearly visible as a large number of small emitting regions,
organised into spiral arm fragments, which are accompanied by dust lanes. NIR
colour index maps show much more clearly the location of dust lanes and, in
certain cases, regions of star formation than single broad-band images.
Circumnuclear spiral structure thus outlined appears to be common in barred
spiral galaxies with circumnuclear star formation.


title:
Early galaxy evolution from deep wide field star counts. I. The spheroid
  density law and mass function

abstract:
  As part of a global analysis of deep star counts to constrain scenarii of
galaxy formation and evolution, we investigate possible links between the
galactic spheroid and the dark matter halo. A wide set of deep star counts at
high and intermediate galactic latitudes is used to determine the large scale
density law of the spheroid. Assuming a power density law, the exponent,
flattening, local density and IMF slope of this population are estimated. The
estimation is checked for robustness against contamination of star counts by
the thick disc population. Contamination effects are derived from a model of
population synthesis under a broad variety of thick disc parameters. The
parameter fit is based on a maximum likelihood criterion. The best fit spheroid
density law has a flattening of 0.76, a power index of 2.44. There is a
significant degeneracy between these two parameters. The data are also
compatible with a slightly less flattened spheroid (c/a = 0.85), in combination
with a larger power index (2.75). A flatter spheroid (c/a = 0.6) with a power
index of 2 is not excluded either. We also constrain the spheroid IMF slope
\alpha to be 1.9 +/- 0.2, leading to a local density of 1.64 10$^{-4}$ stars
pc$^{-3}$ and a mass density of 4.15 10$^{-5}$ \Msun pc$^{-3}$. With this slope
the expected mass density of brown dwarfs in the halo makes a negligible part
of the dark matter halo, as already estimated from microlensing surveys.
  So, as star count data progresses in depth and extent, the picture of the
spheroid star population that comes out points to a shape quite compatible with
what we know about the distribution of baryonic dark matter if it is made of
stellar remnants, suggesting a common dynamical origin.


title: Cool Companions to Hot White Dwarfs

abstract:
  We describe a near-IR photometric search for cool red dwarf companions to hot
EUV-detected white dwarfs (WDs). While some composite systems have been found
optically among WDs detected in recent EUV All-Sky Surveys, we develop an IR
technique that probes farther down the main sequence, detecting yet more
companions. We use detailed DA model atmosphere fits to optical spectra to
predict K magnitudes and distances, against which we contrast our near-IR
observations. Our photometric survey reveals 10 DAs with a significant excess
in both J and K. Half are newly discovered, and are most likely previously
unrecognized binary systems. Neither the frequency of infrared excess nor the
mass estimate of the red dwarf companion correlate with white dwarf mass, as
might be expected if either the EUV detectability or mass of the white dwarfs
were significantly affected by a companion. Infrared spectra of these systems
should help to determine the mass and spectral type of the cool companions
presumably causing the IR excess, leading to better estimates of the mass ratio
distribution in binaries.
  Counting previously known binaries, and resolved pairs, we find the total
binary fraction of the sample is at least a third. Since most WD progenitors
had initial masses larger than about 2 solar masses, we thus provide a
photometric measure of the binary fraction of high mass stars that would be
difficult to perform in high mass main sequence stars. We estimate that 90% of
the companions are of type K or later.


title:
Testing Intermittency of the Galactic Star Formation History along with
  the Infall Model

abstract:
  We analyze the star formation history (SFH) of the Galactic disk by using an
infall model. Based on the observed SFH of the Galactic disk, we first
determine the timescales of the gas infall into the Galactic disk ($t_{\rm
in}$) and that of the gas consumption to form stars ($t_{\rm sf}$). Since each
of the two timescales does not prove to be determined independently from the
SFH, we first fix $t_{\rm sf}$. Then, $t_{\rm in}$ is determined so that we
minimize $\chi^2$. Consequently, we choose three parameter sets: ($t_{\rm sf}$
[Gyr], $t_{\rm in}$ [Gyr]) = (6.0, 23), (11, 12), and (15, 9.0), where we set
the Galactic age as 15 Gyr. All of the three cases predict almost identical
star formation history. Next, we test the intermittency (or variability) of the
star formation rate (SFR) along with the smooth SFH suggested from the infall
model. The large value of the $\chi^2$ statistic supports the violent
time-variation of the SFH. If we interpret the observed SFH with smooth and
variable components, the amplitude of the variable component is comparable to
the smooth component. Thus, intermittent SFH of the Galactic disk is strongly
suggested. We also examined the metallicity distribution of G-dwarfs. We found
that the true parameter set lies between ($t_{\rm sf}$ [Gyr], $t_{\rm in}$
[Gyr]) = (6, 23) and (11, 12), though we should need a more sophisticated model
including the process of metal enrichment within the Galactic halo.


title:
Microlensing of multiply-imaged compact radio sources: Evidence for
  compact halo objects in the disk galaxy of B1600+434

abstract:
  We present the first unambiguous case of external variability of a radio
gravitational lens, CLASS B1600+434. The VLA 8.5-GHz difference light curve of
the lensed images, taking the proper time-delay into account, shows the
presence of external variability with 14.6-sigma confidence. We investigate two
plausible causes of this external variability: scattering by the ionized
component of the Galactic interstellar medium and microlensing by massive
compact objects in the bulge/disk and halo of the lens galaxy. Based on the
tight relation between the modulation-index and variability time scale and the
quantitative difference between the light curves of both lensed images, we
conclude that the observed short-term variability characteristics of the lensed
images are incompatible with scintillation in our Galaxy. This conclusion is
strongly supported by multi-frequency WSRT observations at 1.4 and 5 GHz, which
are in strong disagreement with predictions based on the scintillation
hypothesis. ... On the other hand, a single superluminal jet-component, having
an apparent velocity 9<=(v_app/c)<=26, a radius of 2-5 micro-arcsec and
containing 5-11% of the observed 8.5-GHz source flux density, can reproduce the
observed modulation-indices and variability time scale at 8.5 GHz, when it is
microlensed by compact objects in the lens galaxy. It also reproduces the
frequency-dependence of the modulation-indices, determined from the independent
WSRT 1.4 and 5-GHz observations. ... The only conclusion fully consistent with
the data gathered thus far is that we have indeed detected radio microlensing.
The far reaching consequence of this statement is that a significant fraction
of the mass in the dark-matter halo at ~6 kpc (h=0.65) above the lens-galaxy
disk in B1600+434 consists of massive compact objects. [abridged]


title: Departures From Axisymmetric Morphology and Dynamics in Spiral Galaxies

abstract:
  New HI synthesis data have been obtained for six face-on galaxies with the
Very Large Array. These data and reanalyses of three additional data sets make
up a sample of nine face-on galaxies analyzed for deviations from axisymmetry
in morphology and dynamics. This sample represents a subsample of galaxies
already analyzed for morphological symmetry properties in the R-band. Four
quantitative measures of dynamical nonaxisymmetry are compared to one another
and to the quantitative measures of morphological asymmetry in HI and R-band to
investigate the relationships between nonaxisymmetric morphology and dynamics.
We find no significant relationship between asymmetric morphology and most of
the dynamical measures in our sample. A possible relationship is found,
however, between morphology and dynamical position angle differences between
approaching and receding sides of the galaxy.


title: Relativistic Diskoseismology. II. Analytical Results for C-modes

abstract:
  We first briefly review how we investigate the modes of oscillation trapped
within the inner region of accretion disks by the strong-field gravitational
properties of a black hole (or a compact, weakly-magnetized neutron star). Then
we focus on the `corrugation'(c)-modes, nearly incompressible perturbations of
the inner disk. The fundamental c-modes have eigenfrequencies (ordered by
radial mode number) which correspond to the Lense-Thirring frequency, evaluated
at the outer trapping radius of the mode, in the slow rotation limit. This
trapping radius is a decreasing function of the black hole angular momentum, so
a significant portion of the disk is modulated only for slowly rotating black
holes. The eigenfrequencies are thus strongly increasing functions of black
hole angular momentum. The dependence of the eigenfrequencies on the speed of
sound within (or the luminosity) within the disk is very weak, except for
slowly rotating black holes.


title: Substructure and halo density profiles in a Warm Dark Matter Cosmology

abstract:
  We performed a series of high-resolution simulations designed to study the
substructure of Milky Way-size galactic halos (host halos) and the density
profiles of halos in a warm dark matter (WDM) scenario with a non-vanishing
cosmological constant. The virial masses of the host halos range from 3.5 x
10^12 to 1.7 x 10^12 solar masses and they have more than 10^5 particles each.
A key feature of the WDM power spectrum is the free-streaming length R_f which
fixes an additional parameter for the model of structure formation. We analyze
the substructure of host halos using three R_f values: 0.2, 0.1, and 0.05 Mpc
and compare results to the predictions of the cold dark matter (CDM) model. We
find that guest halos (satellites) do form in the WDM scenario but are more
easily destroyed by dynamical friction and tidal disruption than their
counterparts in a CDM model. The small number of guest halos that we find
within the virial radii of host halos at z = 0 in the WDM models is the result
of a less efficient halo accretion and a higher satellite destruction rate.
Under the assumption that each guest halo hosts a luminous galaxy, we find that
the observed circular velocity function of satellites around the Milky Way and
Andromeda is well described by the R_f = 0.1 Mpc WDM model. In the R_f =
0.1-0.2 Mpc models, the surviving subhalos at z=0 have an average concentration
parameter c_1/5 which is approximately twice smaller than that of the
corresponding CDM subhalos. This difference, very likely, produces the higher
satellite destruction rate found in the WDM models. The density profile of host
halos is well described by the NFW fit whereas guest halos show a wide variety
of density profiles (abridged).


title:
The Globular Cluster System in the Inner Region of the Giant Elliptical
  Galaxy NGC 4472

abstract:
  We present a study of globular clusters in the inner region of the giant
elliptical galaxy NGC 4472, based on the HST WFPC2 archive data. We have found
about 1560 globular cluster candidates at the galactocentric radius r < 4
arcmin. V-(V-I) diagram of these objects shows a dominant vertical structure
which consists obviously of two components: blue globular clusters (BGCs) and
red globular clusters (RGCs). The luminosity function of the globular clusters
is derived to have a peak at V(max)=23.50+/-0.16 from Gaussian fitting. The
distance to NGC 4472 is estimated to be d=14.7+/-1.3 Mpc.The peak luminosity
for the RGCs is similar to that for the BGCs, which indicates that the RGCs may
be several Gyrs younger than the BGCs. The mean luminosity of the bright BGCs
decreases by 0.2 mag with increasing galactocentric radius over the range of 9
arcmin, while that of the RGCs does not. The observed color distribution of
these globular clusters is distinctively bimodal with peaks at (V-I) = 0.98 and
1.23. The mean observed color of all the globular clusters with V < 23.9 mag is
derived to be (V-I)=1.11. These colors are exactly the same as those for the
globular clusters in M87. It is found that the relative number of the BGCs to
the RGCs is increasing with the increasing galactocentric radius. Surface
number density profiles of both the BGCs and RGCs get flat in the central
region, and the core radii of the globular cluster systems are measured to be
r_c = 1.9 arcmin for the BGCs, r_c = 1.2 arcmin for the RGCs, and r_c = 1.3
arcmin for the total sample, which are much larger than the stellar core of the
galaxy. In general the properties of the globular clusters in the inner region
of NGC 4472 are consistent with those of the globular clusters in the outer
region of NGC 4472.


title: Population and Size Distribution of Small Jovian Trojan Asteroids

abstract:
  We present a study of Jovian Trojan objects detected serendipitously during
the course of a sky survey conducted at the University of Hawaii 2.2-meter
telescope. We used a 8192 x 8192 pixel charge-coupled device (CCD) mosaic to
observe 20 deg^2 at locations spread over the L4 Lagrangian swarm and reached a
limiting magnitude V = 22.5 mag (50% of maximum detection efficiency).
Ninety-three Jovian Trojans were detected with radii 2 - 20 km (assumed albedo
0.04). Their differential magnitude distribution has a slope of 0.40 +/- 0.05
corresponding to a power law size distribution index 3.0 +/- 0.3 (1-sigma). The
total number of L4 Trojans with radii > 1 km is of order 1.6 x 10^5 and their
combined mass (dominated by the largest objects) is ~ 10^{-4} M_{Earth}. The
bias-corrected mean inclination is 13.7 +/- 0.5 deg. We also discuss the size
and spatial distribution of the L4 swarm.


title:
Testing Comptonizing coronae on a long BeppoSAX observation of the
  Seyfert 1 galaxy NGC 5548

abstract:
  We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548, allowing for different geometries of the
scattering region, different temperatures of the input soft photon field and
different viewing angles. We find that the BeppoSAX data are well represented
by a plane parallel or hemispherical corona viewed at an inclination angle of
30$^{\circ}$. For both geometries the best fit temperature of the soft photons
is close to 15$^{+3}_{-9}$ eV. The corresponding best fit values of the hot
plasma temperature and optical depth are $kT_{\rm e}\simeq$ 250--260 keV and
$\tau\simeq$ 0.16--0.37 for the slab and hemisphere respectively. These values
are substantially different from those derived fitting the data with a
power-law + cut off approximation to the Comptonization component ($kT_{\rm
e}\lta$ 60 keV, $\tau\simeq$ 2.4). This is due to the fact that accurate
Comptonization spectra in anisotropic geometries show "intrinsic" curvature
which reduces the necessity of a high energy cut-off. The Comptonization
parameter derived for the slab model {is} larger than predicted for a two phase
plane parallel corona in energy balance, suggesting that a more
``photon-starved'' geometry is necessary. The spectral softening detected
during a flare which occurred in the central part of the observation
corresponds to a decrease of the Comptonization parameter, probably associated
with an increase of the soft photon luminosity, the {hard} photon luminosity
remaining constant.


title:
Studies of Binary Star Cluster Candidates in the Bar of the LMC. I: SL
  353 & SL 349

abstract:
  We present a detailed study of the cluster pair SL 353 & SL 349. This
candidate binary cluster is located at the northwestern rim of the LMC bar.
Based on photometric data we find that both clusters are coeval with an age of
550 +/- 100 Myr. We use the Ca II triplet in the spectra of individual red
giants to derive radial velocities. Both components of the binary cluster
candidate show very similar mean velocities (approx. 274 +/- 10 km/sec for SL
349 and approx. 279 +/- 4 km/sec for SL 353) while the field stars show lower
velocities (approx. 240 +/- 19 km/sec). These findings suggest a common origin
of the two clusters from the same GMC. In this sense the cluster pair may
constitute a true binary cluster. We furthermore investigate the stellar
densities in and around the star clusters and compare them with isopleths
created from artificial, interacting as well as non-interacting star clusters.
Gravitational interaction leads to a distortion which can also be found in the
observed pair.


title:
A self-consistent test of Comptonization models using a long BeppoSAX
  observation of NGC 5548

abstract:
  We test accurate models of Comptonization spectra over the high quality data
of the BeppoSAX long look at NGC 5548. The data are well represented by a plane
parallel corona with an inclination angle of 30$^{\circ}$, a soft photon
temperature of 5 eV and a hot plasma temperature and optical depth of $kT_{\rm
e}\simeq$ 360 keV and $\tau\simeq$ 0.1, respectively. If energy balance
applies, such values suggest that a more ``photon-starved'' geometry (e.g. a
hemispheric region) is necessary. The spectral softening detected during a
flare, appears to be associated to a decrease of the heating-to-cooling ratio,
indicating a geometric and/or energetic modification of the disk plus corona
system. The hot plasma temperature derived with the models above is
significantly higher than that obtained fitting the same data with a power law
plus high energy cut off model for the continuum. This is due to the fact that
in anisotropic geometries Comptonization spectra show "intrinsic" curvature
which moves the fitted high energy cut-off to higher energies.


title: Centrifugal buoyancy as a mechanism for neutron star glitches

abstract:
  The frequent glitches (sudden increases of the apparent angular velocity)
observed in certain pulsars are generally believed to be attributable to
discontinuous angular momentum transfer to the outer neutron star crust from a
differentially rotating superfluid layer, but the precise mechanism is not
quite elucidated. Most explanations invoke vortex pinning as the essential
mechanism responsible for the build up of strain in the crust that is relaxed,
either by fracture of the solid structure or by discontinous unpinning, during
the glitch. It is shown here that there is another mechanism that could give
rise to strain, and subsequent fracture, of the solid crust, even if vortex
pinning is ineffective: this is the effective force arising from the deficit of
centrifugal buoyancy that will be present whenever there is differential
rotation. This centrifugal buoyancy deficit force will be comparable in order
of magnitude, but opposite in direction, to the force that would arise from
vortex pinning if it were effective.


title: Pulsar optical observation with the Very Large Telescope

abstract:
  Optical data in the V-band gathered with the 8.2m ESO Very Large Telescope
(VLT) at the radio interferometric position of PSR 1706-44 are presented. The
pulsar is close to a bright star in projection and was not detected. The pulsar
magnitude limit must be fainter than V=24.5 for a distance of $\leq$ 2\farcs0
from the bright star. In the outer gap model for an aligned rotor the optical
flux should scale with the gamma-ray flux. For pulsars which emit pulsed
gamma-rays but are not detected in the optical bands, the synchrotron cutoff
frequency for the tertiary photons must be well below the optical frequencies
and the magnetic and spin axes may be misaligned.


title: Excess sub-millimetre emission from GRS 1915+105

abstract:
  We present the first detections of the black hole X-ray binary GRS 1915+105
at sub-millimetre wavelengths. We clearly detect the source at 350 GHz on two
epochs, with significant variability over the 24 hr between epochs.
Quasi-simultaneous radio monitoring indicates an approximately flat spectrum
from 2 - 350 GHz, although there is marginal evidence for a minimum in the
spectrum between 15 - 350 GHz. The flat spectrum and correlated variability
imply that the sub-mm emission arises from the same synchrotron source as the
radio emission. This source is likely to be a quasi-steady partially
self-absorbed jet, in which case these sub-mm observations probe significantly
closer to the base of the jet than do radio observations and may be used in
future as a valuable diagnostic of the disc:jet connection in this source.


title:
Physical properties of the pulsating hydrogen-deficient star LSS 3184
  (BX Cir)

abstract:
  We report new determinations of the radius and mass of the pulsating
helium-rich, hydrogen-deficient star LSS 3184 (BX Cir) using measurements of
radial velocity and angular radius throughout its pulsation cycle. Measurements
of radial velocity, and thus changes in stellar radius, were made using
Anglo-Australian Telescope echelle spectra. Hubble Space Telescope ultraviolet
spectra and ground-based BV photometry were used to find temperatures and
fluxes throughout the pulsation cycle. The temperatures and fluxes were used to
find the angular radius of the star (alpha). The alpha, Delta-alpha and
Delta-radius values thus found were used to calculate the mean stellar radius
<R_star> = 2.31 +/- 0.10 R_sun. If we use the previously determined logg = 3.35
+/- 0.1 for LSS 3184 and our radius estimate, we find its mass to be M_star =
0.42 +/- 0.12 M_sun.


title: Cosmological models from quintessence

abstract:
  A generalized quintessence model is presented which corresponds to a richer
vacuum structure that, besides a time-dependent, slowly varying scalar field,
contains a varying cosmological term. From first principles we determine a
number of scalar-field potentials that satisfy the constraints imposed by the
field equations and conservations laws, both in the conventional and
generalized quintessence models. Besides inverse-power law solutions, these
potentials are given in terms of hyperbolic functions or the twelve Jacobian
elliptic functions, and are all related to the luminosity distance by means of
an integral equation. Integration of this equation for the different solutions
leads to a large family of cosmological models characterized by luminosity
distance-redshift relations. Out of such models, only four appear to be able to
predict a required accelerating universe conforming to observations on
supernova Ia, at large or moderate redshifts.


title:
Galactic 1.275 MeV emission from ONe novae and its detectability by
  INTEGRAL/SPI

abstract:
  Models of Galactic 1.275 MeV emission produced by the decay of the
radionuclide $^{22}$Na have been computed. Several frequency-spatial
distributions of novae have been investigated using recent results of nova
rates and spatial distributions of novae in our Galaxy. These models allow us
to estimate the lower-limit of the $^{22}$Na mass ejected per ONe nova
detectable with the future spectrometer (SPI) of the INTEGRAL observatory as a
function of the frequency-spatial distribution of ONe novae in the Galaxy.
Calculations using recent estimations of the expected $^{22}$Na mass ejected
per ONe nova show that the detection of the Galactic emission of 1.275 MeV
photons will be difficult with the future spectrometer of the INTEGRAL
observatory, whereas the cumulative emission around the Galactic center has
some chances to be detected during the deep survey of the central radian of the
Galaxy.


title: Chandra X-Ray Observatory (CXO):Overview

abstract:
  The Chandra X-Ray Observatory (CXO), the x-ray component of NASA's Great
Observatories, was launched early in the morning of 1999, July 23 by the Space
Shuttle Columbia. The Shuttle launch was only the first step in placing the
observatory in orbit. After release from the cargo bay, the Inertial Upper
Stage performed two firings, and separated from the observatory as planned.
Finally, after five firings of Chandra's own Integral Propulsion System--- the
last of which took place 15 days after launch--- the observatory was placed in
its highly elliptical orbit of 140,000 km apogee and 10,000 km perigee. After
activation, the first x-rays focussed by the telescope were observed on 1999,
August 12. Beginning with these initial observations one could see that the
telescope had survived the launch environment and was operating as expected.
The month following the opening of the sunshade door was spent adjusting the
focus for each set of instrument configurations, determining the optical axis,
calibrating the star camera, establishing the relative response functions,
determining energy scales, and taking a series of "publicity" images. Each
observation proved to be far more revealing than was expected. Finally, and
despite an initial surprise and setback due to the discovery that the Chandra
x-ray telescope was far more efficient for concentrating low-energy protons
than had been anticipated, the observatory is performing well and is returning
superb scientific data. Together with other space observatories, most notably
the recently activated XMM-Newton, it is clear that we are entering a new era
of discovery in high-energy astrophysics.


title:
Identification of A-colored Stars and Structure in the Halo of the Milky
  Way from SDSS Commissioning Data

abstract:
  A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main
sequence A stars has been selected from 370 square degrees of Sloan Digital Sky
Survey (SDSS) commissioning observations. The data is from two long, narrow
stripes, each with an opening angle of greater than 60 deg, at Galactic
latitudes 36 < abs(b) < 63 on the celestial equator. An examination of the
sample's distribution shows that these stars trace considerable substructure in
the halo. Large overdensities of A-colored stars in the North at (l,b,R) =
(350, 50, 46 kpc) and in the South at (157, -58, 33 kpc) and extending over
tens of degrees are present in the halo of the Milky Way. Using photometry to
separate the stars by surface gravity, both structures are shown to contain a
sequence of low surface gravity stars consistent with identification as a blue
horizontal branch (BHB). Both structures also contain a population of high
surface gravity stars two magnitudes fainter than the BHB stars, consistent
with their identification as blue stragglers (BSs). From the numbers of
detected BHB stars, lower limits to the implied mass of the structures are
6x10^6 M_sun and 2x10^6 M_sun. The fact that two such large clumps have been
detected in a survey of only 1% of the sky indicates that such structures are
not uncommon in the halo. Simple spheroidal parameters are fit to a complete
sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to
a halo distribution with flattening (c/a = 0.65+/-0.2) and a density falloff
exponent of alpha = -3.2+/-0.3.


title: The Inner Boundary Condition for a Thin Disk Accreting Into a Black Hole

abstract:
  Contrary to some recent claims the `no torque inner boundary condition' as
applied at the marginally stable orbit is correct for geometrically thin disks
accreting into black holes.


title:
Candidate RR Lyrae Stars Found in Sloan Digital Sky Survey Commissioning
  Data

abstract:
  We present a sample of 148 candidate RR Lyrae stars selected from Sloan
Digital Sky Survey (SDSS) commissioning data for about 100 deg2 of sky surveyed
twice with Delta(t) = 1.9946 days. Although the faint magnitude limit of the
SDSS allows us to detect RR Lyrae stars to large galactocentric distances
(about 100 kpc, or r = 21), we find no candidates fainter than r = 20, i.e.
further than about 65 kpc from the Galactic center. On the assumption that all
148 candidates are indeed RR Lyrae stars (contamination by other species of
variable star is probably less than 10%), we find that their volume density has
roughly a power-law dependence on galactocentric radius, 1/R^(2.7+-0.2),
between 10 and 50 kpc, and drops abruptly at R = 50-60 kpc, possibly indicating
a sharp edge to the stellar halo as traced by RR Lyrae stars.
  The Galactic distribution of stars in this sample is very inhomogeneous and
shows a clump of over 70 stars at about 45 kpc from the Galactic center. This
clump is also detected in the distribution of nonvariable objects with RR Lyrae
star colors. When sources in the clump are excluded, the best power-law fit
becomes consistent with the 1/R^3 distribution found from surveys of bright RR
Lyrae stars. These results imply that the halo contains clumpy overdensities
inhomogeneously distributed within a smooth 1/R^3 background, with a possible
cutoff at about 50 kpc.


title:
Resonant Scattering of Emission Lines in Coronal Loops: Effects on Image
  Morphology and Line Ratios

abstract:
  We have investigated the effects of resonant scattering of emission lines on
the image morphology and intensity from coronal loop structures. It has
previously been shown that line of sight effects in optically thin line
emission can yield loop images that appear uniformly bright at one viewing
angle, but show ``looptop sources'' at other viewing angles. For optically
thick loops where multiple resonant scattering is important, we use a 3D Monte
Carlo radiation transfer code. Our simulations show that the intensity
variation across the image is more uniform than the optically thin simulation
and, depending on viewing angle, the intensity may be lower or higher than that
predicted from optically thin simulations due to scattering out of or into the
line of sight.


title: Merging history as a function of halo environment

abstract:
  According to the hierarchical scenario, galaxies form via merging and
accretion of small objects. Using N-body simulations, we study the frequency of
merging events in the history of the halos. We find that at z<~2 the merging
rate of the overall halo population can be described by a simple power law
(1+z)^3. The main emphasis of the paper is on the effects of environment of
halos at the present epoch (z=0). We find that the halos located inside
clusters have formed earlier (dz \approx 1) than isolated halos of the same
mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower
than that of isolated halos and 2 times lower than merger rate of halos that
end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster
and group halos have 3--5 times higher merger rates than isolated halos. We
briefly discuss implications of our results for galaxy evolution in different
environments.


title: Old White Dwarfs as a Microlensing Population

abstract:
  A popular interpretation of recent microlensing studies of the line of sight
towards the Large Magellanic Cloud invokes a population of old white dwarf
stars in the Galactic halo. Below I review the basic properties of old white
dwarf stars and the ongoing efforts to detect this population directly.


title:
A Dynamical Solution to the Problem of a Small Cosmological Constant and
  Late-time Cosmic Acceleration

abstract:
  Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.


title:
O VI and Multicomponent H I Absorption Associated with a Galaxy Group in
  the Direction of PG0953+415: Physical Conditions and Baryonic Content

abstract:
  We report the discovery of an O VI absorption system at z(abs) = 0.14232 in a
high resolution FUV spectrum of PG0953+415 obtained with the Space Telescope
Imaging Spectrograph (STIS). Both lines of the O VI 1032, 1038 doublet and
multicomponent H I Lya absorption are detected, but the N V doublet and the
strong lines of C II and Si III are not apparent. We examine the ionization
mechanism of the O VI absorber and find that while theoretical considerations
favor collisional ionization, it is difficult to observationally rule out
photoionization. If the absorber is collisionally ionized, it may not be in
equilibrium due to the rapid cooling of gas in the appropriate temperature
range. Non-equilibrium collisionally ionized models are shown to be consistent
with the observations. A WIYN survey of galaxy redshifts near the sight line
has revealed a galaxy at a projected distance of 395 kpc separated by ~130 km/s
from this absorber, and three additional galaxies are found within 130 km/s of
this redshift with projected separations ranging from 1.0 Mpc to 3.0 Mpc.
Combining the STIS observations of PG0953+415 with previous high S/N GHRS
observations of H1821+643, we derive a large number of O VI absorbers per unit
redshift, dN/dz ~20. We use this sample to obtain a first estimate of the
cosmological mass density of the O VI systems at z ~ 0. If further observations
confirm the large dN/dz derived for the O VI systems, then these absorbers
trace a significant reservoir of baryonic matter at low redshift.


title:
HI 21cm-line observations with the GMRT towards interstellar clouds
  previously seen in optical absorption

abstract:
  We have made HI 21cm-line absorption measurements using the GMRT towards 15
directions in the Galaxy which are known to have high random velocity clouds as
seen in the optical absorption lines of CaII and NaI. For the first time, in 6
out of these 15 directions we detect HI absorption features corresponding to
the high random velocity optical absorption lines. The mean optical depth of
these detections is ~ 0.08.


title:
Constraining the Location of Microlensing Objects by using the Finite
  Source Effect in EAGLE events

abstract:
  We propose a new method to constrain the location of microlensing objects
using EAGLE (Extremely Amplified Gravitational LEnsing) events. We have
estimated the rate of EAGLE events by taking the finite-source effect in to
account. We found that the EAGLE event rate for using a 1-m class telescope w
hose limiting magnitude is $V \sim 21$ is the same as or higher than that of
the ordinary microlensing events which have been found to date. We have also
found that the fraction of transit EAGLE events is large enough to detect:
between $4 \sim 80 % $ depending on the lens location. Since the lens proper
motion can be measured for a transit event, one can distinguish whether the
lens is a MACHO (MAssive Compact Halo Object) in our hal o or one of the known
stars in the Large Magellanic Cloud (LMC) from the proper motion measurement
for each transit EAGLE event. Moreover, we show that the fraction of transit
EAGLEs in all EAGLE events signif icantly depends on the lensing locations: the
transit EAGLE fraction for the sel f-lensing case is $2 \sim 15$ times larger
than that for halo MACHOs. Thus, one can constrain the location of lens objects
by the statistics of the tr ansit events fraction. We show that we can
reasonably expect $0 \sim 6$ transit events out of 21 EAGLE events in 3 years.
We can also constrain the lens population properties at a gre ater than 99%
confidence level depending on the number of transit events de tected. We also
present the duration of EAGLE events, and show how an hourly ob servational
mode is more suitable for an EAGLE event search program.


title: Properties of cosmologies with dynamical pseudo Nambu-Goldstone bosons

abstract:
  We study observational constraints on cosmological models with a quintessence
field in the form of a dynamical pseudo Nambu-Goldstone boson. After reviewing
the properties of the solutions, from a dynamical systems phase space analysis,
we consider the constraints on parameter values imposed by luminosity distances
from the 60 Type Ia supernovae published by Perlmutter et al., and also from
gravitational lensing statistics of distant quasars. In the case of the Type Ia
supernovae we explicitly allow for the possibility of evolution of the peak
luminosities of the supernovae sources, using simple empirical models which
have been recently discussed in the literature. We find weak evidence to
suggest that the models with supernovae evolution fit the data better in the
context of the quintessence models in question. If source evolution is a
reality then the greatest challenge facing these models is the tension between
current value of the expansion age, H_0 t_0, and the fraction of the critical
energy density, Omega_{phi0}, corresponding to the scalar field. Nonetheless
there are ranges of the free parameters which fit all available cosmological
data.


title: Detection of the VLBI-scale Counter Jet in NGC 6251

abstract:
  Mapping the central 5 pc region of the nearby radio galaxy NGC 6251 with a
0.2 pc resolution using VLBI at two radio frequencies, 5 GHz and 15 GHz, we
have found the sub-pc-scale counter jet for the first time in this radio
galaxy. This discovery allows us to investigate the jet acceleration based on
the relativistic beaming model (Ghisellini et al. 1993).


title:
The sign of temperature inhomogeneities deduced from time-distance
  helioseismology

abstract:
  Inhomogeneities in wave propagation conditions near and below the solar
surface have been detected by means of time-distance helioseismology. Here we
calculate the effect of temperature inhomogeneities on the travel times of
sound waves. A temperature increase, e.g. in active regions, not only increases
the sound speed but also lengthens the path along which the wave travels
because the expansion of the heated layers shifts the upper turning of the
waves upward. Using a ray tracing approximation we find that in many cases the
net effect of a temperature enhancement is an {\it increase} of the travel
times. We argue that the reduced travel times that are observed are caused by a
combination of magnetic fields in the active region and {\it reduced}
subsurface temperatures. Such a reduction may be related to the increased
radiative energy loss from small magnetic flux tubes.


title: BeppoSAX observations of Mrk 509 and MCG+8-11-11

abstract:
  BeppoSAX observations of the Seyfert galaxies Mrk 509 and MCG+8-11-11 are
presented. Earlier evidence of a soft excess in Mrk 509 is confirmed. This
excess is found to be better represented by a power law than by a black body:
with a photon slope of 2.5, its extrapolation matches the flux recorded in the
far UV. An ASCA observation, which appeared to exclude the presence of the
excess while showing instead evidence of a warm absorber, turns out to be
compatible with the coexistence of the excess seen with BeppoSAX and of the
warm absorber. The hard power law of Mrk 509 is seen for the first time to be
affected by a cut-off at high energies, with an e-folding energy of about 70
keV. In MCG+8-11-11 the cut-off is found at about 170 keV, consistent within
the combined errors with a previous estimate from a ASCA+OSSE/CGRO observation.
In both objects the reflection component is clearly detected. In Mrk 509 its
strength, together with that of the iron K line, indicates a solid angle Omega,
subtended by the reprocessing gas in the accretion disk, much less than 2pi,
but lacking a valid constraint on the inclination angle this evidence is not as
convincing as that found with BeppoSAX in IC 4329A. In MCG+8-11-11 the same
parameters are instead consistent with Omega=2pi, but the comparison with an
ASCA observation, when the flux level was about 2.5 times weaker, suggests that
a substantial fraction of the angle might be associated with gas farther out
than the accretion disk.


title:
A Finding List for Delta_mu Binaries derived from a Comparison of
  HIPPARCOS Proper Motions with Long-term Averaged Data

abstract:
  The comparison of quasi-instantaneously measured HIPPARCOS proper motions
with long-term averaged proper motions, derived from ground-based data, allows
the identification of many stellar objects as Delta_mu binaries (Wielen et al.,
1999, A&A 346, 675). We have used this method to find Delta_mu binaries among
the fundamental stars, among the GC stars, and among the Tycho-2 stars. A
finding list for about 4000 Delta_mu binaries is given under the URL
http://www.ari.uni-heidelberg.de/dmubin in a machine-readable format. This
information on the probable duplicity of the listed objects can be used for
planning specific observing programs, for improving our knowledge of binary
statistics, or for avoiding the use of these binaries in some astrophysical
calibrations or as astrometric reference stars.


title:
Neutrino-antineutrino transition under CP-transformation and solar
  neutrino puzzle

abstract:
  This paper has been withdrawn by the author due to the presented idea is
wrong.


title:
Homogeneous Photometry for Star Clusters and Resolved Galaxies. II.
  Photometric Standard Stars

abstract:
  Stars appearing in CCD images obtained over 224 nights during the course of
69 observing runs have been calibrated to the Johnson/Kron-Cousins BVRI
photometric system defined by the equatorial standards of Landolt (1992, AJ,
104, 340). More than 15,000 stars suitable for use as photometric standards
have been identified, where "suitable" means that the star has been observed
five or more times during photometric conditions and has a standard error of
the mean magnitude less than 0.02 mag in at least two of the four bandpasses,
and shows no significant evidence of intrinsic variability. Many of these stars
are in the same fields as Landolt's equatorial standards or Graham's (1982,
PASP, 94, 244) southern E-region standards, but are considerably fainter. This
enhances the value of those fields for the calibration of photometry obtained
with large telescopes. Other standards have been defined in fields containing
popular objects of astrophysical interest, such as star clusters and famous
galaxies, extending Landolt-system calibrators to declinations far from the
equator and to stars of sub-Solar chemical abundances. I intend to continue to
improve and enlarge this set of photometric standard stars as more observing
runs are reduced. The full current database of photometric indices is being
made freely available via a site on the World-Wide Web, or by direct request to
the author. Although the contents of the database will evolve in detail, at any
given time it should represent the largest sample of precise BVRI broad-band
photometric standards available anywhere.


title:
Strong Balmer lines in old stellar populations: No need for young ages
  in ellipticals?

abstract:
  Comparing models of Simple Stellar Populations (SSP) with observed line
strengths generally provides a tool to break the age-metallicity degeneracy in
elliptical galaxies. Due to the wide range of Balmer line strengths observed,
ellipticals have been interpreted to exhibit an appreciable scatter in age. In
this paper, we analyze Composite Stellar Population models with a simple mix of
an old metal-rich and an old metal-poor component. We show that these models
simultaneously produce strong Balmer lines and strong metallic lines without
invoking a young population. The key to this result is that our models are
based on SSPs that better match the steep increase of Hbeta in metal-poor
globular clusters than models in the literature. Hence, the scatter of Hbeta
observed in cluster and luminous field elliptical galaxies can be explained by
a spread in the metallicity of old stellar populations. We check our model with
respect to the so-called G-dwarf problem in ellipticals. For a galaxy subsample
covering a large range in UV-V colors we demonstrate that the addition of an
old metal-poor subcomponent does not invalidate other observational constraints
like colors and the flux in the mid-UV.


title: Population synthesis of HII galaxies

abstract:
  We study the stellar population of galaxies with active star formation,
determining ages of the stellar components by means of spectral population
synthesis of their absorption spectra. The data consist of optical spectra of
185 nearby ($z \leq 0.075$) emission line galaxies. They are mostly HII
galaxies, but we also include some Starbursts and Seyfert 2s, for comparison
purposes. They were grouped into 19 high signal-to-noise ratio template
spectra, according to their continuum distribution, absorption and emission
line characteristics. The templates were then synthesized with a star cluster
spectral base. The synthesis results indicate that HII galaxies are typically
age-composite stellar systems, presenting important contribution from
generations up to as old as 500 Myr. We detect a significant contribution of
populations with ages older than 1 Gyr in two groups of HII galaxies. The age
distributions of stellar populations among Starbursts can vary considerably
despite similarities in the emission line spectra. In the case of Seyfert 2
groups we obtain important contributions of old population, consistent with a
bulge. From the diversity of star formation histories, we conclude that typical
HII galaxies in the local universe are not systems presently forming their
first stellar generation.


title:
Measurements of the Diffuse Ultraviolet Background and the Terrestrial
  Airglow with the Space Telescope Imaging Spectrograph

abstract:
  Far-UV observations in and near the Hubble Deep Fields demonstrate that the
Space Telescope Imaging Spectrograph (STIS) can potentially obtain unique and
precise measurements of the diffuse far-ultraviolet background. Although STIS
is not the ideal instrument for such measurements, high-resolution images allow
Galactic and extragalactic objects to be masked to very faint magnitudes, thus
ensuring a measurement of the truly diffuse UV signal. The programs we have
analyzed were not designed for this scientific purpose, but would be sufficient
to obtain a very sensitive measurement if it were not for a weak but
larger-than-expected signal from airglow in the STIS 1450-1900 A bandpass. Our
analysis shows that STIS far-UV crystal quartz observations taken near the limb
during orbital day can detect a faint airglow signal, most likely from NI\1493,
that is comparable to the dark rate and inseparable from the far-UV background.
Discarding all but the night data from these datasets gives a diffuse
far-ultraviolet background measurement of 501 +/- 103 ph/cm2/sec/ster/A, along
a line of sight with very low Galactic neutral hydrogen column (N_HI = 1.5E20
cm-2) and extinction (E(B-V)=0.01 mag). This result is in good agreement with
earlier measurements of the far-UV background, and should not include any
significant contribution from airglow. We present our findings as a warning to
other groups who may use the STIS far-UV camera to observe faint extended
targets, and to demonstrate how this measurement may be properly obtained with
STIS.


title:
Signatures of Interstellar-Intracluster Medium Interactions: Spiral
  Galaxy Rotation Curves in Abell 2029

abstract:
  We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and
long-slit spectral observations of 52 disk galaxies distributed throughout the
cluster field. No strong emission-line galaxies are present within ~400 kpc of
the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit
line emission. H-alpha rotation curves of 14 cluster members are used in
conjunction with a deep I band image to study the environmental dependence of
the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches
that of clusters at lower redshifts, although we do observe a relatively larger
scatter about the Tully-Fisher relation. We do not observe any systematic
variation in the data with projected distance to the cluster center: we see no
environmental dependence of Tully-Fisher residuals, R-I color, H-alpha
equivalent width, and the shape and extent of the rotation curves.


title: The Observational Mass Function of Loose Galaxy Groups

abstract:
  We analyze the three catalogs of nearby loose groups identified by Garcia
(1993). She used a percolation and a hierarchical method, and proposed a third
group catalog defined as a combination of the two. Each catalog contains almost
500 groups. In agreement with previous works on earlier catalogs, we find that
groups can be described as collapsing systems. Their sampled size is much
larger than their expected virialized region. We compute the virial masses and
correct them by taking into account the young dynamical status. We estimate
group masses, M, for two cosmological models, a flat one with Omega_0=1 and an
open one with Omega_0=0.2. For each of the three catalogs we calculate the mass
function, MF. The number density of groups with M>9x10^{12}\msun, which is the
adopted limit of sample completeness, ranges within 1.3-1.9x10^{-3}h^3/Mpc^3
for Omega_0=1, and it is about a factor of 15% lower for Omega_0=0.2. The MFs
of the hierarchical and combined catalogs have essentially the same shape,
while the MF of the percolation catalog shows a flattening towards large
masses. However, the difference decreases if we do not consider the most
massive groups, for which reliable results come from galaxy cluster studies.
After having estimated the mass contained within the central, presumably
virialized, regions of groups by adopting a reduction in mass of 30-40%, we do
a comparison with the results coming from the virial analysis of nearby rich
clusters (Girardi et al. 1998). All three group MFs turn out to be a smooth
extrapolation of the cluster MF at M<4x10^{14}\msun, which is the completeness
limit of the cluster sample. The resulting optical virial MF of galaxy systems,
which extends over two orders of magnitude, is fitted to a Schechter expression
with a slope of about -1.5 and a characteristic mass of about 3x10^{14}\msun.


title: Orbits in a Neighboring Dwarf Galaxy According to MOND

abstract:
  We study the orbits in the MOND theory within a dwarf galaxy of mass Md at a
distance of ~100kpc from a neighboring galaxy of mass Mg, such as ours. It is
assumed that a second mass m<<Md is gravitationally bound to Md by a previously
calculated potential for the MOND theory. This potential is obtained for a free
falling mass Md in a constant external gravitational acceleration field. The
numerical technique of surfaces of section is used to study the stability of
the phase-space orbits in the dwarf galaxy. Equatorial orbits with sufficiently
small eccentricities e<0.65 are found to be stable with respect to small chang
es in the initial conditions. (The equatorial plane is perpendicular to the
direction of the external gravitational acceleration, which is along the line
joining Md and Mg.) For decreasing values of the conserved component of the
angular momentum, in the direction of the external gravitational acceleration,
equatorial stability is lost.


title: Weak Gravitational Lensing Bispectrum

abstract:
  Weak gravitational lensing observations probe the spectrum and evolution of
density fluctuations and the cosmological parameters which govern them. The
non-linear evolution of large scale structure produces a non-Gaussian signal
which is potentially observable in galaxy shear data. We study the three-point
statistics of the convergence, specifically the bispectrum, using the dark
matter halo approach which describes the density field in terms of correlations
between and within dark matter halos. Our approach allows us to study the
effect of the mass distribution in observed fields, in particular the bias
induced by the lack of rare massive halos (clusters) in observed fields. We
show the convergence skewness is primarily due to rare and massive dark matter
halos with skewness converging to its mean value only if halos of mass M >
10^15 M_\sun are present. This calculational method can in principle be used to
correct for such a bias as well as to search for more robust statistics related
to the two and three point correlations.


title:
Measurements with the Chandra X-Ray Observatory's flight contamination
  monitor

abstract:
  NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.


title: Optimal Masks for Low-Degree Solar Acoustic Modes

abstract:
  We suggest a solution to an important problem of observational
helioseismology of the separation of lines of solar acoustic (p) modes of low
angular degree in oscillation power spectra by constructing optimal masks for
Doppler images of the Sun. Accurate measurements of oscillation frequencies of
low-degree modes are essential for the determination of the structure and
rotation of the solar core. However, these measurements for a particular mode
are often affected by leakage of other p modes arising when the Doppler images
are projected on to spherical-harmonics masks. The leakage results in
overlaping peaks corresponding to different oscillation modes in the power
spectra. In this paper we present a method for calculating optimal masks for a
given (target) mode by minimizing the signals of other modes appearing in its
vicinity. We apply this method to time series of 2 years obtained from
Michelson Doppler Imager (MDI) instrument on board SOHO space mission and
demonstrate its ability to reduce efficiently the mode leakage.


title:
A Uniform Analysis of the Ly-alpha forest at z = 0 - 5: I. The sample
  and distribution of clouds at z > 1.7

abstract:
  We present moderate resolution data for 39 QSOs at z $\approx$ 2 obtained at
the Multiple Mirror Telescope. These data are combined with spectra of
comparable resolution of 60 QSOs with redshifts greater than 1.7 found in the
literature to investigate the distribution of Ly-alpha forest lines in redshift
and equivalent width. We find a value for $\gamma$, the parameter describing
the number distribution of Ly-alpha forest lines in redshift, of $1.88\pm0.22$
for lines stronger than a rest equivalent width of 0.32 $\AA$, in good
agreement with some previous studies. The Kolmogorov-Smirnov test was applied
to the data and it is found that this single power law is a good fit over the
relevant redshift ranges. Simulations of the Lyman alpha forest were performed
to determine the completeness of the line lists and to test how well the
analysis the underlying line statistics, given this level of completeness.


title:
A Uniform Analysis of the Ly-alpha forest at z = 0 - 5: II. Measuring
  the mean intensity of the extragalactic ionizing background using the
  proximity effect

abstract:
  A homogeneous sample of 99 moderate resolution QSO spectra at z > 1.7 were
presented in Paper I, including 39 previously unpublished spectra from the
Multiple Mirror Telescope. The statistics of the Lyman alpha forest were
discussed. In this analysis, we demonstrate that a proximity effect is present
in the data, ie. there exists a significant (5.5$\sigma$) deficit of lines at
$z_{abs} \approx z_{em}$. Within 1.5 $h^{-1}$ Mpc of the QSO emission redshift,
the significance does depend on QSO luminosity, in accordance with the theory
that this effect is caused by enhanced ionization of hydrogen in the vicinity
of the QSO from UV photons from the QSO itself. The photoionization model of
Bajtlik, Duncan, and Ostriker (1988) permits an estimate of the mean intensity
of the extragalactic background radiation at the Lyman limit. We compare the
results of this standard analysis with those obtained using a maximum
likelihood technique. The best fit value for $J(\nu_{0})$ is
7.0$^{+3.4}_{-4.4}$ x 10$^{-22}$ ergs/s/cm$^{2}$/Hz/sr, over the redshift range
1.7 < z < 3.8, using QSO redshifts based on narrow emission lines. The best fit
value for the HI ionization rate is 1.9$^{+1.2}_{-1.0}$ x 10$^{-12}$ s$^{-1}$,
in good agreement with models of the background which incorporate QSOs only.
This large absorption line sample and these techniques for measuring the
background and understanding the systematics involved allow us to place what we
believe are are the firmest limits on the background at these redshifts.


title: Quintessence and Gravitational Waves

abstract:
  We investigate some aspects of quintessence models with a non-minimally
coupled scalar field and in particular we show that it can behave as a
component of matter with $-3 \lesssim P/\rho \lesssim 0$. We study the
properties of gravitational waves in this class of models and discuss their
energy spectrum and the cosmic microwave background anisotropies they induce.
We also show that gravitational waves are damped by the anisotropic stress of
the radiation and that their energy spectrum may help to distinguish between
inverse power law potential and supergravity motivated potential. We finish by
a discussion on the constraints arising from their density parameter
$\Omega_\GW$.


title:
Abundances of light elements in metal-poor stars. IV. [Fe/O] and [Fe/Mg]
  and the history of star formation in the solar neighborhood

abstract:
  The accurate O, Mg and Fe abundances derived in previous papers of this
series from a homogenous reanalysis of high quality data for a large sample of
stars are combined with stellar kinematics in order to discuss the history of
star formation in the solar neighborhood. We found that the Fe/O and Fe/Mg
abundance ratios are roughly constant in the (inner) halo and the thick disk;
this means that the timescale of halo collapse was shorter than or of the same
order of typical lifetime of progenitors of type Ia SNe (~ 1 Gyr), this
conclusion being somewhat relaxed (referring to star formation in the
individual fragments) in an accretion model for the Galaxy formation. Both Fe/O
and Fe/Mg ratios raised by ~ 0.2 dex while the O/H and Mg/H ratios hold
constant during the transition from the thick to thin disk phases, indicating a
sudden decrease in star formation in the solar neighbourhood at that epoch.
These results are discussed in the framework of current views of Galaxy
formation; they fit in a scenario where both dissipational collapse and
accretions were active on a quite similar timescale.


title:
Vertical structure of the accreting two-temperature corona and the
  transition to an ADAF

abstract:
  We investigate the model of the disc/corona accretion flow around the black
hole. The model, parameterized by the total (i.e. disk plus corona) accretion
rate, $\dot m$, mass of the black hole, $M$ and the viscosity parameter,
$\alpha$, uniquely determines the fraction of energy released in the corona as
a function of radius and, in particular, the transition radius to the
single-phase ADAF flow. Self-consistent solutions with the mass exchange
between phases display radial dependence of the parameters. This model
reproduces all characteristic luminosity states of accretion black hole without
any additional ad hoc assumptions. In particular. the mechanism of the disk
evaporation leads to a new, almost horizontal branch on the accretion flow's
stability curve (i.e. the dependence of accretion rate on surface density) at
the critical accretion rate.


title:
Smoothed Particle Hydrodynamics Simulations of Apsidal and Nodal
  Superhumps

abstract:
  In recent years a handful of systems have been observed to show "negative"
(nodal) superhumps, with periods slightly shorter than the orbital period. It
has been suggested that these modes are a consequence of the slow retrograde
precession of the line of nodes in a disk tilted with respect to the orbital
plane. Our simulations confirm and refine this model: they suggest a roughly
axisymmetric, retrogradely-precessing, tilted disk that is driven at a period
slightly less than half the orbital period as the tidal field of the orbiting
secondary encounters in turn the two halves of the disk above and below the
midplane. Each of these passings leads to viscous dissipation on one face of an
optically-thick disk -- observers on opposite sides of the disk would each
observe one brightening per orbit, but 180 degrees out of phase with each
other.


title:
Gas properties of HII and Starburst galaxies: relation with the stellar
  population

abstract:
  We study the gas emission of galaxies with active star formation, consisting
mostly of HII galaxies and Starbursts, as well as some Seyfert 2s, and
determine chemical and physical parameters. The data consist of 19 high
signal-to-noise ratio optical templates, a result of grouping 185 emission line
galaxy spectra. Underlying stellar population models were subtracted from the
templates in order to isolate the pure emission component. We analyse the
distribution of these improved signal-to-noise ratio emission spectra in
diagnostic diagrams and find that the HII templates show a smaller spread in
log([OIII]/H\beta) values than the individual galaxies, apparently due to the
population subtraction and better signal-to-noise ratio. We thus suggest the
template sequence as a fiducial observational locus for HII galaxies which can
be used as reference for models. The sequence of line ratios presented by the
HII galaxies in the diagram log([OIII}]\lambda5007/H\beta) X
log([NII]\lambda6584/H\alpha) is primarily due to the gas metallicity, of which
the log([NII]/H\alpha) ratio is a direct estimator. We also study the
properties of the Starburst galaxies and those intermediate between HII and
Starburst galaxies, which are more metal rich and sit on more massive galaxies.
We discuss the present results in the frame of a recently proposed equivalent
width diagnostic diagram for emission line galaxies and conclude that the
observed ranges in W([OII])/W(H\beta) and W(H\beta) are mostly due to the
non-ionizing stellar population contribution. W(H\beta) can be used as an
estimator of this contribution to the continuum, and briefly discuss
implications to the cosmological use of HII galaxies.


title: Infrared observations of gravitational lensing in Abell 2219 with CIRSI

abstract:
  We present the first detection of a gravitational depletion signal at
near-infrared wavelengths, based on deep panoramic images of the cluster Abell
2219 (z=0.22) taken with the Cambridge Infrared Survey Instrument (CIRSI) at
the prime focus of the 4.2m William Herschel Telescope. Infrared studies of
gravitational depletion offer a number of advantages over similar techniques
applied at optical wavelengths, and can provide reliable total masses for
intermediate redshift clusters. Using the maximum likelihood technique
developed by Schneider, King & Erben (1999), we detect the gravitational
depletion at the 3 sigma confidence level. By modeling the mass distribution as
a singular isothermal sphere and ignoring uncertainty in the unlensed number
counts, we find an Einstein radius of 13.7 +3.9/-4.2 arcsec (66% confidence
limit). This corresponds to a projected velocity dispersion of approximately
800 km/s, in agreement with constraints from strongly-lensed features. For a
Navarro, Frenk and White mass model, the radial dependence observed indicates a
best-fitting halo scale length of 125/h kpc}. We investigate the uncertainties
arising from the observed fluctuations in the unlensed number counts, and show
that clustering is the dominant source of error. We extend the maximum
likelihood method to include the effect of incompleteness, and discuss the
prospects of further systematic studies of lensing in the near-infrared band.


title:
The Hamburg/ESO survey for bright QSOs. III. A large flux-limited sample
  of QSOs

abstract:
  We present a new sample of 415 bright QSOs and Seyfert~1 nuclei drawn from
the Hamburg/ESO survey (HES). The sample is spectroscopically 99 % complete and
well-defined in terms of flux and redshift limits. Optical magnitudes are in
the interval 13 < B_J < 17.5, redshifts range within 0< z < 3.2. More than 50 %
of the objects in the sample are new discoveries. We describe the selection
techniques and discuss sample completeness and potential selection effects.
There is no evidence for redshift-dependent variations of completeness; in
particular, low-redshift QSOs - notoriously missed by other optical surveys -
are abundant in this sample, since no discrimination against extended sources
is imposed. For the same reason, the HES is not biased against multiply imaged
QSOs due to gravitational lensing. The sample forms the largest homogeneous set
of bright QSOs currently in existence, useful for a variety of statistical
studies. We have redetermined the bright part of the optical quasar
number-magnitude relation. We confirm that the Palomar-Green survey is
significantly incomplete, but that its degree of incompleteness has recently
been overestimated.


title: Fine Structure in the Energy Spectrum of Cosmic Ray Protons at 50 GeV ?

abstract:
  The recently published precise spectrum of cosmic ray protons from the Alpha
Magnetic Spectrometer has been examined in some detail from the standpoint of a
search for deviations from a smooth, simple, power law. We find a significant
excess ($\simeq$ 10% over $\sim$ 0.3 interval in logE) centered on 50 GeV in
the published data. It is possible that the 'unfolding technique' adopted by
the experimenters causes an overestimate of the excess but it is difficult to
reduce it much below about (5 $\pm$ 3)%.
  We have examined other recent data, too. There is also evidence here, for an
excess in the same energy region, although of only (1.6 $\pm$ 0.9)%. A value of
(3 $\pm$ 1.5)% would be consistent with all the data. There are hints of
similar excesses for heavier nuclei and electrons.
  Possible explanations are put forward for an excess, should it prove to be
genuine.


title:
Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from
  Novae. I. Limits on the Positron Annihilation Line in Five Individual Novae

abstract:
  The Transient Gamma Ray Spectrometer (TGRS) on board the WIND spacecraft has
spent most of the interval 1995-1997 in a high-altitude orbit where gamma-ray
backgrounds are low. Its high-resolution Ge spectrometer is thus able to detect
weak lines which are slightly offset from stronger background features. One
such line is predicted from nucleosynthesis in classical novae, where
beta-decays on a time-scale of a few hours in an expanding envelope produce
positrons that annihilate to generate a line which is blueshifted by a few keV
away from the background annihilation line at 511 keV. The broad TGRS field of
view contained five known Galactic novae during 1995 January - 1997 June, and
we have searched the spectra taken around the times of these events for the
blueshifted nova annihilation line. Although no definite detections were made,
the method is shown to be sensitive enough to detect novae occurring on
ONeMg-rich white dwarfs out to about 2.5 kpc.


title:
On the Role of Irradiation and Evaporation in Strongly Irradiated
  Accretion Disks in the Black Hole X-ray Binaries: Toward an Understanding of
  FREDs and Secondary Maxima

abstract:
  We examine a new paradigm to account for the exponential decay seen in the
light curves of some of the bright X-ray novae. These systems show an
exponential decay in soft X-rays with an e-folding time constant of ~30 d. We
investigate a scenario in which evaporation of matter into a corona is the
dominant mass removal mechanism from the accretion disk. We utilize the thermal
evaporative instability discovered by Shaviv and Wehrse. First we parametrize
local mass loss rates from the disk (fitted to vertical structure computations
of the optically thin structure using the photoionization code CLOUDY), and
then we utilize the scalings in our numerical time dependent model for the
decay. Both the ~30 d e-folding time scale for the decay and the secondary
maximum with its rapid rise time ~1-3 d which is seen in the X-ray nova light
curves can be produced by adjusting the strength of the evaporation.


title:
Cosmic Ray Acceleration in Superbubbles and the Composition of Cosmic
  Rays

abstract:
  We review the evidence for cosmic ray acceleration in the superbubble/hot
phase of the interstellar medium, and discuss the implications for the
composition of cosmic rays and the structure and evolution of the interstellar
medium. We show that the bulk of the galactic supernovae, their expanding
remnants, together with their metal-rich grain and gas ejecta, and their cosmic
ray accelerating shocks, are all confined within the interiors of superbubbles,
generated by the multiple supernova explosions of massive stars formed in giant
OB associations. This superbubble/hot phase of the ISM provides throughout the
age of the Galaxy a cosmic ray source of essentially constant metallicity for
acceleration by the shocks of many supernovae over time scales of a few Myr,
consistent with both the Be/Fe evolution and ACE observations of Ni-59/Co-59.
We also show that if the refractory cosmic ray metals come from the sputtering
of fast refractory grains then the accompanying scattering of ambient gas by
these fast grains can also account for the relative abundance of cosmic ray
volatiles.


title:
Transient Gamma Ray Spectrometer Measurements of Gamma-Ray Lines from
  Novae. II. Constraining the Galactic Nova Rate from a Survey of the Southern
  Sky during 1995-1997

abstract:
  The good energy resolution (3--4 keV FWHM) of the Transient Gamma Ray
Spectrometer (TGRS) on board the WIND spacecraft makes it sensitive to
Doppler-shifted outbursts of 511 keV electron-positron annihilation radiation,
the reason being that the Doppler shift causes the cosmic line to be slightly
offset from a strong instrumental background 511 keV line at rest, which is
ubiquitous in space environments. Such a cosmic line (blueshifted) is predicted
to arise in classical novae due to the annihilation of positrons from
$\beta$-decay on a timescale of a few hours in an expanding envelope. A further
advantage of TGRS - its broad field of view, containing the entire southern
ecliptic hemisphere - has enabled us to make a virtually complete and unbiased
3-year search for classical novae at distances up to ~1 kpc. We present
negative results of this search, and estimate its implications for the
highly-uncertain Galactic classical nova rate and for future space missions.


title:
Temperature Profiles of Accretion Disks around Rapidly Rotating Neutron
  Stars in General Relativity and Implications for Cygnus X-2

abstract:
  We calculate the temperature profiles of (thin) accretion disks around
rapidly rotating neutron stars (with low surface magnetic fields), taking into
account the full effects of general relativity. We then consider a model for
the spectrum of the X-ray emission from the disk, parameterized by the mass
accretion rate, the color temperature and the rotation rate of the neutron
star. We derive constraints on these parameters for the X-ray source Cygnus X-2
using the estimates of the maximum temperature in the disk along with the disk
and boundary layer luminosities, using the spectrum inferred from the EXOSAT
data. Our calculations suggest that the neutron star in Cygnus X-2 rotates
close to the centrifugal mass-shed limit. Possible constraints on the neutron
star equation of state are also discussed.


title: TGRS Observations of Positron Annihilation in Classical Novae

abstract:
  The TGRS experiment on board the Wind spacecraft has many advantages as a sky
monitor --- broad field of view (~2 pi) centered on the south ecliptic pole),
long life (1994-present), and stable low background and continuous coverage due
to Wind's high altitude high eccentricity orbit. The Ge detector has sufficient
energy resolution (3-4 keV at 511 keV) to resolve a cosmic positron
annihilation line from the strong background annihilation line from beta-decays
induced by cosmic ray impacts on the instrument, if the cosmic line is
Doppler-shifted by this amount. Such lines (blueshifted) are predicted from
nucleosynthesis in classical novae. We have searched the entire TGRS database
for 1995-1997 for this line, with negative results. In principle such a search
could yield an unbiased upper limit on the highly-uncertain Galactic nova rate.
We carefully examined the times around the known nova events during this
period, also with negative results. The upper limit on the nova line flux in a
6-hr interval is typically <3.8 E-3 photon/(cm2 s) at 4.6 sigma. We performed
the same analysis for times around the outburst of Nova Vel 1999, obtaining a
worse limit due to recent degradation of the detector response caused by cosmic
ray induced damage.


title: Extremely Red Objects from the NICMOS/HST Parallel Imaging Survey

abstract:
  We present a catalog of extremely red objects discovered using the NICMOS/HST
parallel imaging database and ground-based optical follow-up observations.
Within an area of 16 square arc-minutes, we detect 15 objects with $\rm R -
F160W > 5$ and $\rm F160W < 21.5$. We have also obtained K-band photometry for
a subset of the 15 EROs. All of the $\rm R - F160W$ selected EROs imaged at
K-band have $\rm R - K > 6$. Our objects have $\rm F110W - F160W$ colors in the
range of 1.3 - 2.1, redder than the cluster ellipticals at $z \sim 0.8$ and
nearly 1 magnitude redder than the average population selected from the F160W
images at the same depth. In addition, among only 22 NICMOS pointings, we
detected two groups or clusters in two fields, each contains 3 or more EROs,
suggesting that extremely red galaxies may be strongly clustered. At bright
magnitudes with $\rm F160W < 19.5$, the ERO surface density is similar to what
has been measured by other surveys. At the limit of our sample, F160W = 21.5,
our measured surface density is 0.94$\pm 0.24$ arcmin^{-2}. Excluding the two
possible groups/clusters and the one apparently stellar object, reduces the
surface density to 0.38$\pm 0.15$ arcmin^{-2}.


title: Resolving SNR 0540-6944 from LMC X-1 with Chandra

abstract:
  We examine the supernova remnant (SNR) 0540-697 in the Large Magellanic Cloud
(LMC) using data from the Chandra ACIS. The X-ray emission from this SNR had
previously been hidden in the bright emission of nearby X-ray binary LMC X-1;
however, new observations with Chandra can finally reveal the SNR's structure
and spectrum. We find the SNR to be a thick-shelled structure about 19 pc in
diameter, with a brightened northeast region. Spectral results suggest a
temperature of 0.31 keV and an X-ray luminosity (0.3-3.0 keV) of 8.4 x 10^33
erg/s. We estimate an age of 12,000-20,000 yr for this SNR, but note that this
estimate does not take into account the possibility of cavity expansion or
other environmental effects.


title:
Multifrequency studies of Gamma-Ray bursts: towards the understanding of
  the mystery

abstract:
  GRBs have remained a puzzle for many high-energy astrophysicists since their
discovery in 1967. With the advent of the X-ray satellites BeppoSAX and
RossiXTE, it has been possible to carry out deep multi-wavelength observations
of the counterparts associated with the GRBs just within a few hours of
occurence, thanks to the observation of the fading X-ray emission that follows
the more energetic gamma-ray photons once the GRB event has ended. The fact
that this emission (the afterglow) extends at longer wavelengths, has led to
the discovery of the first optical/IR/radio counterparts in 1997-99, greatly
improving our understanding of these sources. Now it is widely accepted that
GRBs originate at cosmological distances. The observed afterglow satisfies the
predictions of the "standard" relativistic fireball model, and the central
engines that power these extraordinary events are thought to be the collapse of
massive stars or the merging of compact objects.


title:
The Multiply Scattering Effect on the Energy Measurement of UHE Cosmic
  Rays using Atmospheric Fluorescence Technique

abstract:
  Point sources in the atmosphere are surrounded by aureole because of
atmospheric scattering. The properties of the time-dependent aureole radiance
are calculated by use of a Monte Carlo approach and an iterative method. Since
the aureole is particularly important in the ultraviolet, which is the region
the Ultra-High-Energy (UHE) cosmic ray experiment using the air fluorescence
technique like Fly's Eye or High-Resolution-Fly's-Eye(HiRes) are set in. The
effect of the multiply scatteing on the energy measurement is studied.


title:
Membership and Multiplicity among Very Low-Mass Stars and Brown Dwarfs
  in the Pleiades Cluster

abstract:
  We present near-infrared photometry and optical spectroscopy of very low-mass
stars and brown dwarf candidates in the Pleiades open cluster. The membership
status of these objects is assessed. Eight objects out of 45 appear to be
non-members. A search for companions among 34 very low-mass Pleiades members
(M$\le$0.09 M$_\odot$) in high-spatial resolution images obtained with the
Hubble Space Telescope and the adaptive optics system of the
Canada-France-Hawaii telescope produced no resolved binaries with separations
larger than 0.2 arcsec (a ~ 27 AU; P ~ 444 years). Nevertheless, we find
evidence for a binary sequence in the color-magnitude diagrams, in agreement
with the results of Steele & Jameson (1995) for higher mass stars. We compare
the multiplicity statistics of the Pleiades very low-mass stars and brown
dwarfs with that of G and K-type main sequence stars in the solar neighborhood
(Duquennoy & Mayor 1991). We find that there is some evidence for a deficiency
of wide binary systems (separation >27 AU) among the Pleiades very low-mass
members. We briefly discuss how this result can fit with current scenarios of
brown dwarf formation. We correct the Pleiades substellar mass function for the
contamination of cluster non-members found in this work. We find a
contamination level of 33% among the brown dwarf candidates identified by
Bouvier et al. (1998). Assuming a power law IMF across the substellar boundary,
we find a slope dN/dM ~ M^{-0.53}, implying that the number of objects per mass
bin is still rising but the contribution to the total mass of the cluster is
declining in the brown dwarf regime.


title: Detection of a Far IR Excess with DIRBE at 60 and 100 Microns

abstract:
  From analysis of the DIRBE weekly averaged sky maps, we have detected
substantial flux in the 60 and 100 micron channels in excess of expected
zodiacal and Galactic emission. Two methods are used to separate zodiacal light
from more distant emission. Method I makes use of the time-dependence of the
North-South annual variation observed at the ecliptic poles. This method is
robust against errors in the inter-planetary dust (IPD) model, but does not
demonstrate isotropy of the background. Method II measures the ecliptic
latitude dependence of the dust over a range of ecliptic latitudes at solar
elongation e=90 degrees. This allows the excess to be determined in each week
of the DIRBE mission for high redundancy, but the results depend weakly on the
IPD model. Both methods give consistent results at 60 and 100 microns. The
observed signal is consistent with an isotropic background at the level \nu
I_\nu = 28.1 +- 1.8 +- 7(syst) nW/m^2/sr at 60 microns and 24.6 +- 2.5 +- 8
nW/m^2/sr at 100 microns.
  The IR excess detected at 140 and 240 microns by these methods agrees with
previous measurements, which are thought to be the cosmic infra-red background
(CIB). The detections at 60 and 100 microns are new. While this new excess is
not necessarily the CIB, we have ruled out all known sources of emission in the
solar system and Galaxy. We therefore tentatively interpret this signal as the
CIB and consider the implications of such energy production from the viewpoint
of star formation efficiency and black hole accretion efficiency. However, the
IR excess exceeds limits on the CIB derived from the inferred opacity of the
IGM to observed TeV photons, thus casting doubt on this interpretation. There
is currently no satisfactory explanation for the 60-100 micron excess.


title:
Redshifts For 220 BATSE Gamma-Ray Bursts Determined by Variability and
  the Cosmological Consequences

abstract:
  We show that the time variability of a gamma-ray burst (GRB) appears to be
correlated with the absolute luminosity of the burst: smooth bursts are
intrinsically less luminous. This Cepheid-like relationship can be used to
determine the redshift of a GRB from parameters measured solely at gamma-ray
energies. The relationship is based on only seven events at present and needs
to be further confirmed with more events. We present the details of converting
GRB observables to luminosities and redshifts for 220 bright, long GRBs from
the Burst and Transient Source Experiment (BATSE) and explore the cosmological
consequences. In particular, we derive the GRB rate as a function of $z$
without assuming either a luminosity function or that the GRB rate follows the
star formation rate (SFR). We find that the GRB formation rate scales as
(1+z)^{3.3 \pm 0.3}. The observations used to derive the SFR can be strongly
affected by dust for z > 2 whereas GRB observations are not. If GRBs trace star
formation, then our results indicate that the SFR does not peak at z\sim 2 but
instead continues to increase until z \sim 10. We have used the burst formation
rate to correct the observed GRB luminosity function for the incompleteness due
to the detection threshold, resulting in a luminosity function with a power law
index of \sim -2.3 that slightly rolls over at low luminosities. The reality of
our variability--luminosity relationship requires confirmation but, if valid,
will provide a powerful tool for studying both GRBs and the early universe.


title:
The Chandra X-Ray Observatory's Radiation Environment and the AP-8/AE-8
  Model

abstract:
  The Chandra X-ray Observatory (CXO) was launched on July 23, 1999 and reached
its final orbit on August 7, 1999. The CXO is in a highly elliptical orbit,
approximately 140,000 km x 10,000 km, and has a period of approximately 63.5
hours (~ 2.65 days). It transits the Earth's Van Allen belts once per orbit
during which no science observations can be performed due to the high radiation
environment. The Chandra X-ray Observatory Center (CXC) currently uses the
National Space Science Data Center's ``near Earth'' AP-8/AE-8 radiation belt
model to predict the start and end times of passage through the radiation
belts. However, our scheduling software uses only a simple dipole model of the
Earth's magnetic field. The resulting B, L magnetic coordinates, do not always
give sufficiently accurate predictions of the start and end times of transit of
the Van Allen belts. We show this by comparing to the data from Chandra's
on-board radiation monitor, the EPHIN (Electron, Proton, Helium Instrument
particle detector) instrument. We present evidence that demonstrates this
mis-timing of the outer electron radiation belt as well as data that also
demonstrate the significant variablity of one radiation belt transit to the
next as experienced by the CXO. We also present an explanation for why the
dipole implementation of the AP-8/AE-8 model is not ideally suited for the CXO.
Lastly, we provide a brief discussion of our on-going efforts to identify a
model that accounts for radiation belt variability, geometry, and one that can
be used for observation scheduling purposes.


title: Multiparameter estimation with the Pseudo-C_l method

abstract:
  We apply the Pseudo-C_l formalism to obtain an unbiased, approximate method
for efficient simultaneous estimation of several cosmological parameters from
large, almost full-sky cosmic microwave background data sets.


title: Fluctuations in Isothermal Spheres

abstract:
  Isolated isothermal spheres of N gravitationally interacting points with
equal mass are believed to be stable when density contrasts do not exceed 709.
That stability limit does, however, not take into consideration fluctuations of
temperature near the onset of instability. These are important when N is
finite. Here we correlate {\it global mean quadratic temperature fluctuations}
with onset of instability. We show that such fluctuations trigger instability
when the density contrast reaches a value near $709\cdot\exp(-3.3N^{-1/3})$.
These lower values of limiting density contrasts are significantly smaller than
709 when N is not very big and this suggests (i) that numerical calculations
with small N may not reflect correctly the onset of core collapse in clusters
with big N and (ii) that a greater number of globular clusters than is normally
believed may already be in an advanced stage of core collapse because most of
observed globular clusters whose parameters fit quasi-isothermal configurations
are close to marginal stability.


title: BeppoSAX observation of Cen X-4 in quiescence

abstract:
  We report on a 61 ks BeppoSAX observation of the soft X-ray transient Cen X-4
during quiescence which allowed to study the source X-ray spectrum from ~ 0.3
keV to ~ 8 keV. A two-component spectral model was required, consisting of a
black body with temperature of ~ 0.1 keV and a power law with photon index ~ 2.
These values are compatible with earlier ASCA results indicating that Cen X-4
may be stable, within a factor of a few, over a 5 year baseline.


title: On the Poynting-Robertson Effect and Analytical Solutions

abstract:
  Solutions of the two-body problem with the simultaneous action of the solar
electromagnetic radiation in the form of the Poynting-Robertson effect are
discussed. Special attention is devoted to pseudo-circular orbits and terminal
values of osculating elements. The obtained results complete those of Klacka
and Kaufmannova (1992) and Breiter and Jackson (1998). Terminal values of
osculating elements presented in Breiter and Jackson (1998) are of no physical
sense due to the fact that relativistic equation of motion containing only
first order of $\vec{v}/c$ was used in the paper.


title: Cold dust in the starburst galaxy M82

abstract:
  The starburst galaxy M82 has been mapped at 1.2 mm using the IRAM 30-m
telescope and the MPIfR 19-channel bolometer. The influence of the sidelobe
pattern and the 12CO(2-1) line has been carefully analyzed. Based on this
analysis, we conclude that the cold dust does not extend far into the halo as
has been previously claimed. The total mass of dust in the inner 3 kpc of the
galaxy is 7.5*10^6 M(sun). Assuming a solar gas-to-dust ratio, the inferred
total mass of gas is 7.5*10^8 M(sun).
  Unlike in any other galaxy we find the CO(2-1) line emission to be clearly
more extended than the 1.2 mm dust emission. A likely explanation for this
exceptional behaviour, seen for the first time in galaxies, is heating of
molecular gas by low-energy cosmic rays and soft X-rays in the halo of M82.


title:
Bulk viscosity in superfluid neutron star cores. I. Direct Urca
  processes in npe\mu matter

abstract:
  The bulk viscosity of the neutron star matter due to the direct Urca
processes involving nucleons, electrons and muons is studied taking into
account possible superfluidity of nucleons in the neutron star cores. The cases
of singlet-state pairing or triplet-state pairing (without and with nodes of
the superfluid gap at the Fermi surface) of nucleons are considered. It is
shown that the superfluidity may strongly reduce the bulk viscosity. The
practical expressions for the superfluid reduction factors are obtained. For
illustration, the bulk viscosity is calculated for two models of dense matter
composed of neutrons, protons,electrons and muons. The presence of muons
affects the bulk viscosity due to the direct Urca reactions involving electrons
and produces additional comparable contribution due to the direct Urca
reactions involving muons. The results can be useful for studying damping of
vibrations of neutron stars with superfluid cores.


title: GI2T/REGAIN spectro-interferometry with a new infrared beam combiner

abstract:
  We have built an infrared beam combiner for the GI2T/REGAIN interferometer of
the Observatoire de la Cote d'Azur. The beam combiner allows us to record
spectrally dispersed Michelson interference fringes in the near-infrared J-, H-
or K-bands. The beam combiner has the advantage that Michelson interferograms
can simultaneously be recorded in about 128 different spectral channels. The
tilt of the spectrally dispersed fringes is a measure of the instantaneous
optical path difference. We present the optical design of the beam combiner and
GI2T/REGAIN observations of the Mira star R Cas with this beam combiner in the
spectral range of 2.00 micron - 2.18 micron (observations on 22 and 25 August
1999; variability phase 0.08; V-magnitude approx. 6; seven baselines between
12m and 24m; reference stars Vega and Beta Peg). The spectrograph of the beam
combiner consists of an anamorphotic cylindrical lens system, an image plane
slit, and a grism. A system of digital signal processors calculates the
ensemble average power spectrum of the spectrally dispersed Michelson
interferograms and the instantaneous optical path difference error in real
time. From the observed R Cas visibilities at baselines 12.0m, 13.8m and 13.9m,
a 2.1 micron uniform-disk diameter of 25.3mas +/-3.3mas was derived. The
unusually high visibility values at baselines >16m show that the stellar
surface of R Cas is more complex than previously assumed. The visibility values
at baselines >16m can be explained by high-contrast surface structure on the
stellar surface of R Cas or other types of unexpected center-to-limb
variations. The R Cas observations were compared with theoretical Mira star
models yielding a linear Rosseland radius of 276Rsun +/-66Rsun and an effective
temperature of 2685K+/-238K for R Cas at phase 0.08.


title: Cooling neutron stars with localized protons

abstract:
  We analyze cooling of neutron stars, assuming the presence of localized
protons in the densest region of their cores. Choosing a single threshold
density for proton localization and adjusting neutron star mass, we reproduce
the observational data on effective surface temperatures of Vela and PSR
0656+14, with or without an accreted hydrogen envelope. However, the presence
of a tiny hydrogen envelope is mandatory, in this model, for reproducing the
Geminga data.


title: Unusually rapid variability of the GRB000301C optical afterglow

abstract:
  We present BVRI light curves of the afterglow of GRB000301C, one of the
brightest ever detected at a day time scale interval after GRB trigger. The
monitoring started 1.5 days after the GRB and ended one month later. Inspection
of the extremely well sampled R band light curve and comparison with BVI data
has revealed a complex behavior, with a long term flux decrease and various
short time scale features superimposed. These are uncommon among other observed
afterglows, and might trace either intrinsic variability within the
relativistic shock (re-acceleration and re-energization) or inhomogeneities in
the medium in which the shock propagates.


title: The Mass of the Andromeda Galaxy

abstract:
  This paper argues that the Milky Way galaxy is probably the largest member of
the Local Group. The evidence comes from estimates of the total mass of the
Andromeda galaxy (M31) derived from the three dimensional positions and radial
velocities of its satellite galaxies, as well as the projected positions and
radial velocities of its distant globular clusters and planetary nebulae. The
available dataset comprises ten satellite galaxies, seventeen distant globular
clusters and nine halo planetary nebulae with radial velocities. We find the
halo of Andromeda has a mass of about 12 x 10^11 solar masses, together with a
scalelength of roughly 90 kpc and a predominantly isotropic velocity
distribution. For comparison, our earlier estimate for the Milky Way halo is 19
x 10^11 solar masses. Though the error bars are admittedly large, this suggests
that the total mass of M31 is probably less than that of the Milky Way. We
verify the robustness of our results to changes in the modelling assumptions
and to errors caused by the small size and incompleteness of the dataset.
Future prospects using radial velocity surveys of globular clusters and
planetary nebulae, as well as microarcsecond astrometry from the upcoming
satellite missions, are discussed.


title: X-ray Spectral Diagnostics of Gamma-Ray Burst Environments

abstract:
  Recently, the detection of discrete features in the X-ray afterglow spectra
of GRB970508 and GRB970828 was reported. The most natural interpretation of
these features is that they are redshifted Fe K emission complexes. The
identification of the line emission mechanism has drastic implications for the
inferred mass of radiating material, end hence the nature of the burst site.
X-ray spectroscopy provides a direct observational constraint on these
properties of gamma-ray bursters. We briefly discuss how these constraints
arise, in the context of an application to the spectrum of GRB970508.


title: Local Group Dwarf Galaxies and the Star Formation Law at High Redshift

abstract:
  I show how the existing observational data on Local Group dwarf galaxies can
be used to estimate the average star formation law during the first 3 Gyr of
the history of the universe. I find that the observational data are consistent
with the orthodox Schmidt law with a star formation efficiency of about 4
percent if the star formation is continuous (during the first 3 Gyr). The
efficiency is proportionally higher if most of the gas in the dwarfs was
consumed (and never replenished) in a short time interval well before the
universe turned 3 Gyr.


title:
Interferometric measurement of resonance transition wavelengths in C IV,
  Si IV, Al III, Al II, and Si II

abstract:
  We have made the first interferomeric measurements of the wavelengths of the
important ultraviolet diagnostic lines in the spectra \ion{C}{4} near 155 nm
and \ion{Si}{4} near 139 nm with a vacuum ultraviolet Fourier transform
spectrometer and high-current discharge sources. The wavelength uncertainties
were reduced by one order of magnitude for the \ion{C}{4} lines and by two
orders of magnitude for the \ion{Si}{4} lines. Our measurements also provide
accurate wavelengths for resonance transitions in \ion{Al}{3}, \ion{Al}{2}, and
\ion{Si}{2}.


title: On the Emergence and Discovery of Hot Spots in Supernova Remnant 1987A

abstract:
  We present the discovery of several new regions of interaction between the
ejecta and equatorial circumstellar ring of SN1987A, an interaction leading to
a much expanded development of the supernova remnant. We also trace the
development of the first such "hot spot," discovered in 1997, back to 1995.
Later hot spots seem to have emerged by early 1999. We discuss mechanisms for
the long delay between the first and later spots.


title:
DIRBE Minus 2MASS: Confirming the Cosmic Infrared Background at 2.2
  Microns

abstract:
  Stellar fluxes from the 2MASS catalog are used to remove the contribution due
to Galactic stars from the intensity measured by DIRBE in four regions in the
North and South Galactic polar caps. After subtracting the interplanetary and
galactic foregrounds, a consistent residual intensity of 14.8 +/- 4.6 kJy/sr or
20.2 +/- 6.3 nW/m^2/sr at 2.2 microns is found. At 1.25 microns the residuals
show more scatter and are a much smaller fraction of the foreground, leading to
a weak limit on the CIRB of 12.0 +/- 6.8 kJy/sr or 28.9 +/- 16.3 nW/m^2/sr (1
sigma).


title: Apse Alignment of Narrow Eccentric Planetary Rings

abstract:
  The boundaries of the Uranian epsilon, alpha, and beta rings can be fitted by
Keplerian ellipses. The pair of ellipses that outline a given ring share a
common line of apsides. Apse alignment is surprising because the quadrupole
moment of Uranus induces differential precession. We propose that rigid
precession is maintained by a balance of forces due to ring self-gravity,
planetary oblateness, and interparticle collisions. Collisional impulses play
an especially dramatic role near ring edges. Pressure-induced accelerations are
maximal near edges because there (1) velocity dispersions are enhanced by
resonant satellite perturbations, and (2) the surface density declines steeply.
Remarkably, collisional forces felt by material in the last 100 m of a 10 km
wide ring can increase equilibrium masses up to a factor of 100. New ring
surface densities are derived which accord with Voyager radio measurements. In
contrast to previous models, collisionally modified self-gravity appears to
allow for both negative and positive eccentricity gradients; why all narrow
planetary rings exhibit positive eccentricity gradients remains an open
question.