title: Quantum Astronomy. Part II

abstract:
  A new method of fundamental quantum data sampling on basis of an optimum
measuring scale has been designed. The method was applied for minimization of
redundant experimental data in different fields of physics. Basing of this
method there has been found the laws of binding electromagnetic, strong and
gravitational interaction. It is shown that the laws of quantizing of physical
quantities are the effect of space fibering. Moreover the said laws have a
general electromagnetic nature as well. They are related to dimensionless
electromagnetic constants - Na =861 and a-1=137,0360547255. These constants
cover both the atomic and the space scales.


title: On the Interpretation of the Optical Spectra of L-type Dwarfs

abstract:
  We present synthetic optical spectra in the range 640-930 nm of a sample of
field L dwarfs and the brown dwarf GL229B. Our computations show that Li, Na,
K, Rb, and Cs govern the spectra of these objects, with Na and K contributing
significantly to block the optical emergent radiation. Molecular absorptions of
TiO, VO, CrH, FeH and CaH also dominate in the early L-types showing a strength
that progressively decreases for later types. We find that the densities of
these molecules in the atmospheres are considerably smaller by larger factors
than those predicted by the chemical equilibrium. In order to reproduce the
overall shape of the optical spectra an additional opacity (AdO) is required to
be implemented in the computations. We have modelled it with a simple law [a0
(nu / nu0)^N, N=4], and found that this provides a good fit to the data. This
AdO could be due to molecular/dust absorption or to dust scattering. The EWs
and intensities of the alkali lines are highly affected by this opacity. The Li
line at 670.8 nm, widely used as a substellarity discriminator, is more
affected by the AdO than by the natural depletion of neutral Li. Our
computations display a rather strong feature even at Teffs around 1000 K;
depending on the Teff and on the amount of dust in the atmospheres of very cool
dwarfs, it might be possible to detect Li even at temperatures this cool.
Changes in the physical conditions of dust formation in L-type dwarfs will
cause variability of the alkali lines.


title: Observational Tests for the Cosmological Principle and World Models

abstract:
  We review observational tests for the homogeneity of the Universe on large
scales. Redshift and peculiar velocity surveys, radio sources, the X-Ray
Background, the Lyman-$\alpha$ forest and the Cosmic Microwave Background are
used to set constraints on inhomogeneous models and in particular on
fractal-like models.
  Assuming the Cosmological Principle and the FRW metric, we estimate
cosmological parameters by joint analysis of peculiar velocities, the CMB,
cluster abundance, IRAS and Supernovae.
  Under certain assumptions the best fit density parameter is Omega_m = 1 -
lambda \approx 0.4 .
  We present a new method for joint estimation by combining different data sets
in a Bayesian way, and utilising `Hyper-Parameters'.


title:
The Discovery of a Second Field Methane Brown Dwarf from Sloan Digital
  Sky Survey Commissioning Data

abstract:
  We report the discovery of a second field methane brown dwarf from the
commissioning data of the Sloan Digital Sky Survey (SDSS). The object, SDSS
J134646.45-003150.4 (SDSS 1346-00), was selected because of its very red color
and stellar appearance. Its spectrum between 0.8-2.5 mic is dominated by strong
absorption bands of H_2O and CH_4 and closely mimics those of Gliese 229B and
SDSS 162414.37+002915.6 (SDSS 1624+00), two other known methane brown dwarfs.
SDSS 1346-00 is approximately 1.5 mag fainter than Gliese 229B, suggesting that
it lies about 11 pc from the sun. The ratio of flux at 2.1 mic to that at 1.27
mic is larger for SDSS 1346-00 than for Gliese 229B and SDSS 1624+00, which
suggests that SDSS 1346-00 has a slightly higher effective temperature than the
others. Based on a search area of 130 sq. deg. and a detection limit of z* =
19.8, we estimate a space density of 0.05 pc^-3 for methane brown dwarfs with
T_eff ~ 1000 K in the 40 pc^3 volume of our search. This estimate is based on
small-sample statistics and should be treated with appropriate caution.


title: The nature of RXJ0052.1-7319

abstract:
  The nature of the X-ray source RXJ0052.1-7319 is discussed from observational
data obtained from ROSAT observations performed in 1995 and 1996. An accurate
position is derived from ROSAT HRI observations performed in 1995. The 6" error
circle contains two OGLE microlensing optical variables of which one has
previously been identified with a 14.5 mag Be-type star in the Small Magellanic
Cloud. During the October 1996 observation RXJ0052.1-7319 was found to be
extremely bright (with a count rate of ~1.1+/-0.1 s^-1) and 15.3 second X-ray
pulsations have been discovered during this observation. This indicates for a
high-mass X-ray binary nature of the source. During the 1995 observation the
X-ray source detected at the position of RXJ0052.1-7319 was a factor 200
fainter. The corresponding luminosity has changed from 5.2x10^37 erg s^-1 to
2.6x10^35 erg s^-1 assuming SMC membership of the source. It is unclear whether
the so-far unidentified second optical variable contributes to the X-ray flux
of the source.


title: Photometric modal discrimination in Delta Scuti and Gamma Doradus stars

abstract:
  The potential of photometric methods for the identification of l, the degree
of spherical surface harmonic of a pulsating star, is investigated with special
emphasis on Stromgren photometry applied to delta Scuti and gamma Dor
variables. Limitations of actual model atmospheres when fine precision is
required for the calculations of partial derivatives and integrals, which
depend on limb darkening coefficients, are discussed. Two methods are discussed
to calculate the phase lags, the angle between maximum temperature and minimum
radius, and R, a parameter which describes departure from adiabaticity of the
atmospheres of these pulsating stars. These quantities appear to be very
dependent on the convection as parametrized by the mixing length theory. When
one of the methods is applied to the gamma Dor stars gives phase lags close to
0 degrees, which are 90-180 degrees out of phase from typical delta Scuti
stars. Examples are given for some High Amplitude Delta Scuti Stars (HADS)
where the method can be easily applied and gives results consistent to
interpret them as radial (l=0) pulsating stars. Other low amplitude delta Scuti
stars could be oscillating in a non-radial (l=1, 2) mode. Multi-band photometry
is concluded to be a very powerful tool for mode identification of delta Scuti
and gamma Dor stars, specially with the more accurate photometry that will be
achieved in the near future with the asteroseismological space missions now in
progress.


title: The Angular Momentum Evolution of Very Low Mass Stars

abstract:
  We present theoretical models of the angular momentum evolution of very low
mass stars (0.1 - 0.5 M_sun) and solar analogues (0.6 - 1.1 M_sun). We
investigate the effect of rotation on the effective temperature and luminosity
of these stars. We find that the decrease in T_eff and L can be significant at
the higher end of our mass range, but becomes negligible below 0.4 M_sun.
Formulae for relating T_eff to mass and v_rot are presented.
  We compare our models to rotational data from young open clusters of
different ages to infer the rotational history of low mass stars, and the
dependence of initial conditions and rotational evolution on mass. We find that
the qualitative conclusions for stars below 0.6 M_sun do not depend on the
assumptions about internal angular momentum transport, which makes these low
mass stars ideal candidates for the study of the angular momentum loss law and
distribution of initial conditions. We find that neither models with solid body
nor differential rotation can simultaneously reproduce the observed stellar
spin down in the 0.6 to 1.1 M_sun mass range and for stars between 0.1 and 0.6
M_sun. The most likely explanation is that the saturation threshold drops more
steeply at low masses than would be predicted with a simple Rossby scaling. In
young clusters there is a systematic increase in the mean rotation rate with
decreased temperature below 3500 K (0.4 M_sun). This suggests either
inefficient angular momentum loss or mass-dependent initial conditions for
stars near the fully convective boundary. (abridged)


title:
Generation of the Primordial Magnetic Fields during Cosmological
  Reionization

abstract:
  We investigate the generation of magnetic field by the Biermann battery in
cosmological ionization fronts, using new simulations of the reionization of
the universe by stars in protogalaxies. Two mechanisms are primarily
responsible for magnetogenesis: i) the breakout of I-fronts from protogalaxies,
and ii) the propagation of I-fronts through the high density neutral filaments
which are part of the cosmic web. The first mechanism is dominant prior to
overlapping of ionized regions (z ~ 7), whereas the second continues to operate
even after that epoch. However, after overlap the field strength increase is
largely due to the gas compression occurring as cosmic structures form. As a
consequence, the magnetic field at z ~ 5 closely traces the gas density, and it
is highly ordered on megaparsec scales. The mean mass-weighted field strength
is B_0 ~ 10^{-19} G in the simulation box. There is a relatively well-defined,
nearly linear correlation between B_0 and the baryonic mass of virialized
objects, with B_0 ~ 10^{-18} G in the most massive objects (M ~ 10^9 M_sun) in
our simulations. This is a lower limit, as lack of numerical resolution
prevents us from following small scale dynamical processes which could amplify
the field in protogalaxies. Although the field strengths we compute are
probably adequate as seed fields for a galactic dynamo, the field is too small
to have had significant effects on galaxy formation, on thermal conduction, or
on cosmic ray transport in the intergalactic medium. It could, however, be
observed in the intergalactic medium through innovative methods based on
analysis of gamma-ray burst photon arrival times.


title: An asymmetric relativistic model for classical double radio sources

abstract:
  There is substantial observational evidence against the symmetric
relativistic model of FRII radio sources. An asymmetric relativistic model is
proposed which takes account of both relativistic effects and
intrinsic/environmental asymmetries to explain the structural asymmetries of
their radio lobes. A key parameter of the model is the jet-side of the double
sources, which is estimated for 80% of the FRII sources in the 3CRR complete
sample. Statistical analyses of the properties of these sources show that the
asymmetric model is in agreement with a wide range of observational data, and
that the relativistic and intrinsic asymmetry effects are of comparable
importance. Intrinsic/environmental asymmetry effects are more important at
high radio luminosities and small physical scales. The mean translational speed
of the lobes is found to be 0.11c, consistent with the speeds found from
spectral ageing arguments. According to a Gaussian model, the standard
deviation of lobe speeds is 0.04c. The results are in agreement with an
orientation-based unification scheme in which the critical angle separating the
radio galaxies from quasars is about 45 degrees.


title:
On the peculiar red clump morphology in the open clusters NGC 752 and
  NGC 7789

abstract:
  The red clump stars in the open cluster NGC 752 present a peculiar
distribution in the colour-magnitude diagran (CMD): the clump is observed to
present a faint extension, slightly to the blue of the main concentration of
clump stars. We point out that a similar structure is present in the CMD of NGC
7789, and discuss their possible origins. This feature may be understood as the
result of having, at the same time, stars of low-mass which undergo the
helium-flash, and those just massive enough for avoiding it. The ages of both
clusters are compatible with this interpretation. Similar features can be
produced in theoretical models which assume a non-negligible mass spread for
clump stars, of about 0.2 Mo. However, one can probably exclude that the
observed effect is due to the natural mass range of core helium burning stars
found in single isochrones, although present models do not present the level of
detail necessary to completely explore this possibility. Also the possibility
of a large age spread among cluster stars can be refuted on observational
grounds. We then suggest a few alternatives. This spread may be resulting
either from star-to-star variations in the mass-loss rates during the RGB
phase. Alternatively, effects sush as stellar rotation or convective core
overshooting, could be causing a significant spread in the core mass at
He-ignition for star of similar mass. Finally, we point out that similar
effects could also help to understand the distribution of clump stars in the
CMDs of the clusters NGC 2660 and NGC 2204.


title:
2-D Radiation Transfer Model of Non-Spherically Symmetric Dust Shell in
  P roto-Planetary Nebulae

abstract:
  We have fitted the HST images and the spectral energy distributions (SEDs) of
three proto-planetary nebulae (PPN) with a 2-D radiation transfer model. The
geometric and mass loss properties of these PPN are also derived.


title: Cold gas, the HI 21cm line and evolving galactic potentials

abstract:
  Neutral hydrogen traces gravitational potentials. In the nearby universe,
21cm emission-line surveys show that the bulk of the HI resides in well-formed,
optically-luminous galaxies. At high redshift, 21cm line absorption against
background radio quasars occurs in gas-rich systems identified with the highest
HI column densities -- the ``damped Lyman alpha'' quasar absorption-line
systems. High spatial-resolution observations of the redshifted 21cm line
absorbers measure sizes and kinematics of the neutral absorbers.


title: Discovery of a Low Surface Brightness Object near Seyfert's Sextet

abstract:
  We report the discovery of a low surface brightness (LSB) object
serendipitously found during deep CCD imaging of a compact group of galaxies,
Seyfert's Sextet, in VR and I bands. The LSB object is located 2.3 arcmin
southwest from the group center. Its surface brightness within the angular
effective radii of r_e(VR)=4.8 arcsec and r_e(I)=4.8 arcsec is very low ---
mu_e(VR)=25.28 mag arcsec-2 and mu_e(I)=24.47 mag arcsec-2, respectively. The
apparent magnitudes are m_AB(VR)=19.87 mag and m(I)=19.06 mag. The object is
most likely a LSB dwarf galaxy, but other possibilities are also discussed.


title:
The Stellar Population Histories of Local Early-Type Galaxies. I.
  Population Parameters

abstract:
  We present single stellar population (SSP) equivalent ages, metallicities,
and abundance ratios for local elliptical galaxies derived from Hbeta, Mgb, and
<Fe> absorption line strengths. We use an extension of the Worthey (1994)
stellar population models that incorporates non-solar line-strength "response
functions" by Tripicco & Bell (1995), allowing us to correct the models for the
enhancements of Mg and other alpha-like elements relative to the Fe-peak
elements. SSP-equivalent ages of local ellipticals from Gonzalez (1993) are
found to vary widely, 1.5 < t < 18 Gyr, while metallicities [Z/H] and
enhancement ratios [E/Fe] are strongly peaked around <[Z/H]>=+0.26 and
<[E/Fe]>=+0.20 (in an aperture of radius Re/8). The enhancement ratios are
milder than previous estimates, owing to the application of non-solar abundance
corrections to both Mgb and <Fe> for the first time. Gradients in stellar
populations within galaxies are found to be mild, with SSP-equivalent age
decreasing by 25%, metallicity decreasing by <[Z/H]>=0.20 dex, and [E/Fe]
remaining nearly constant out to an aperture of radius Re/2 for nearly all
systems. Our ages have an overall zeropoint uncertainty of at least 25% due to
uncertainties in the stellar evolution prescription, the oxygen abundance, the
effect of non-solar abundances on the isochrones, and other unknowns. However,
the relative age rankings of stellar populations should be largely unaffected
by these errors. In particular, the large spread in ages appears to be real and
cannot be explained by contamination of Hbeta by blue stragglers or hot
horizontal branch stars, or by fill-in of Hbeta by emission. Correlations
between these derived SSP-equivalent parameters and other galaxy observables
will be discussed in future papers. (Abridged)


title: Distribution functions for evolved stars in the inner galactic Plane

abstract:
  We present dynamical distribution functions for evolved stars in the inner
galactic plane. We use an axisymmetric, two-component Stackel potential that
satisfies recent constraints on the galactic potential, amongst others a
slightly declining local rotation curve. We show that this potential is
adequate to model stellar-kinematic samples with radial extent ranging from
\~100 pc to ~5 kpc in the Galaxy. The two-integral model that gives the best
fit to the first three projected moments provides a very good global
representation of the data but fails to reproduce the central dispersion, the
central apparent scaleheight and the cylindrical rotation at intermediate
longitudes. All these features are fitted well by a three--integral model. We
discuss various properties of the 2I- and 3I models and the implications for
galactic structure. A somewhat thicker disk component is needed to explain the
distribution of older AGB stars in the plane; this component also fits the
kinematics at higher latitudes better. We find that the Disk and the Bulge, as
traced by AGB stars, are very similar dynamically and could well be one and the
same component. There is a dynamically distinct component in the inner 100 pc
of the Bulge, however.


title:
Supernova Remnants, Pulsars and the Interstellar Medium - Summary of a
  Workshop Held at U Sydney, March 1999

abstract:
  We summarise the proceedings of the SRCfTA workshop on ``Supernova Remnants,
Pulsars and the Interstellar Medium'' that was held at the University of Sydney
on Mar 18 and 19, 1999.


title: The First Galaxies: Clues from Element Abundances

abstract:
  It has recently become possible to measure directly the abundances of several
chemical elements in a variety of environments at redshifts up to z = 5. In
this review I summarise the latest observations of Lyman break galaxies, damped
Lyman alpha systems and the Lyman alpha forest with a view to uncovering any
clues which these data may offer to the first episodes of star formation. The
picture which is emerging is one where the universe at z = 3 already included
many of the components of today's galaxies--even at these early times we see
evidence for Populations I and II stars, while the `smoking gun' for Population
III objects may be hidden in the chemical composition of the lowest density
regions of the IGM, yet to be deciphered.


title:
Cosmological SPH simulations with four million particles: statistical
  properties of X-ray clusters in a low-density universe

abstract:
  We present results from a series of cosmological SPH (smoothed particle
hydrodynamics) simulations coupled with the P3M
(Particle-Particle-Particle-Mesh) solver for the gravitational force. The
simulations are designed to predict the statistical properties of X-ray
clusters of galaxies as well as to study the formation of galaxies. We have
seven simulation runs with different assumptions on the thermal state of the
intracluster gas. Following the recent work by Pearce et al., we modify our SPH
algorithm so as to phenomenologically incorporate the galaxy formation by
decoupling the cooled gas particles from the hot gas particles. All the
simulations employ 128^3 particles both for dark matter and for gas components,
and thus constitute the largest systematic catalogues of simulated clusters in
the SPH method performed so far. These enable us to compare the analytical
predictions on statistical properties of X-ray clusters against our direct
simulation results in an unbiased manner. We find that the luminosities of the
simulated clusters are quite sensitive to the thermal history and also to the
numerical resolution of the simulations, and thus are not reliable. On the
other hand, the mass-temperature relation for the simulated clusters is fairly
insensitive to the assumptions of the thermal state of the intracluster gas,
robust against the numerical resolution, and in fact agrees well with the
analytic prediction. Therefore the prediction for the X-ray temperature
function of clusters on the basis of the Press-Schechter mass function and the
virial equilibrium is fairly reliable.


title: Neutron Star Mass Determinations

abstract:
  I review attempts made to determine the properties of neutron stars. The
focus is on the maximum mass that a neutron star can have, or, conversely, the
minimum mass required for the formation of a black hole. There appears to be
only one neutron star for which there is strong evidence that its mass is above
the canonical 1.4 solar masses, viz., Vela X-1, for which a mass close to 1.9
solar masses is found. Prospects for progress appear brightest for studies of
systems in which the neutron star should have accreted substantial amounts of
matter.


title:
Radiative Processes and Geometry of Spectral States of Black-Hole
  Binaries

abstract:
  I review radiative processes responsible for X-ray emission in the hard (low)
and soft (high) spectral states of black-hole binaries. The main process in the
hard state appears to be thermal Comptonization (in a hot plasma) of blackbody
photons emitted by a cold disk. This is supported by correlations between the
spectral index, the strength of Compton reflection, and the peak frequencies in
the power density spectrum, as well as by the frequency-dependence of
Fourier-resolved spectra. Spectral variability may then be driven by the
variable truncation radius of the disk. The soft state appears to correspond to
the smallest truncation radii. However, the lack of high-energy cutoffs
observed in the soft state implies that its main radiative process is Compton
scattering of disk photons by non-thermal electrons. The bulk-motion
Componization model for the soft state is shown to be ruled out by the data.


title: Can DNA bases be produced during molecular cloud collapse?

abstract:
  Using a reasonably large chemical reaction network consisting of 421 species,
we show that along with normal chemical evolution of molecular cloud during
collapse and star formation, significant amount of adenine, a DNA base, may be
produced after an evolution of 10^{6-7} years. This findings may shed light on
whether life on earth had to begin from scratch or these molecules could have
contaminated the earth from the beginning.


title:
X-ray and optical observations of three clusters of galaxies: Abell 901,
  Abell 1437, and Abell 3570

abstract:
  We analyse three clusters of galaxies, Abell 901 (z=0.17), Abell 1437
(z=0.13) and Abell 3570 (z=0.037). They have low to intermediate X-ray fluxes
and an irregular morphology in the ROSAT All-Sky Survey (RASS). These clusters
are chosen to test the abilities and limitations of the RASS in terms of
cluster fluxes and cluster morphologies. Therefore some ``worst'' cases are
used here. X-ray observations with the ROSAT/HRI and optical spectroscopic
observations are carried out. The ROSAT/HRI observations, which have a much
better spatial resolution than the RASS, reveal in two of the three cases a
significantly different morphology than seen in the RASS. The reasons are point
sources which could not be resolved in the RASS and were therefore confused
with the cluster emission. For A3570 we could confirm the relaxed state of the
cluster by the optical determination of a small velocity dispersion. In the
cluster with the lowest flux (Abell 901) the countrate measurement is strongly
affected by point sources, in the two other cases the countrate measurements of
the RASS are reliable, i.e. they are reproduced by the ROSAT/HRI measurement.
We conclude that for clusters with a flux of a few times 10^{-12} erg/cm^2/s or
smaller, which show at the same time a non-relaxed morphology, the flux
measurement of the RASS can be seriously affected by fore- or background
sources. We point out that an all-sky survey of a second ABRIXAS mission would
provide a much clearer source distinction for low-flux clusters and thus a much
improved countrate determination.


title:
A limit-cycle model for the binary supersoft X-ray source RX
  J0513.9-6951

abstract:
  We present new results of our X-ray monitoring of the transient binary
supersoft X-ray source RX J0513.9-6951 in the LMC and of our re-analysis of
optical light curves obtained during the MACHO project. We have covered a
complete X-ray outburst cycle with the ROSAT HRI detector. From the amplitude
and timescale of the soft X-ray variability, tight limits are derived for the
temporal behaviour of the white-dwarf radius and the effective temperature of
its envelope. A limit-cycle model is proposed to explain the observed optical
and X-ray variability, the characteristic timescales of the durations of the
X-ray on and off states, and those of the transitions between both states. Our
observations confirm that the radius changes of the white-dwarf envelope occur
on the Kelvin-Helmholtz timescale. The duration of the X-ray on and off states
is compatible with the viscous timescales of the inner and outer accretion
disk, respectively.


title: The Dark Matter Distribution in Disk Galaxies

abstract:
  We use high-quality optical rotation curves of 9 low-luminosity disk galaxies
to obtain the velocity profile of the surrounding dark matter halos. We find
that they increase linearly with radius at least out to the stellar disk edge,
implying that, over the entire region where the stars reside, the density of
the dark halo is constant. The properties of the halo mass structure found are
similar to that claimed for a number of dwarf and low surface brightness
galaxies, but provide a more substantial evidence of the discrepancy between
the halo mass distribution predicted in standard cold dark matter scenario and
those actually detected around galaxies. We find that the density profile
proposed by Burkert (1995) reproduces the halo rotation curves, with halo
central densities and core radii scaling as $\rho_0 \propto r_0^{-2/3}$.


title:
Observation of Microlensing towards the Galactic Spiral Arms. EROS II 3
  year survey

abstract:
  We present an analysis of the light curves of 9.1 million stars observed
during three seasons by EROS (Experience de Recherche d'Objets Sombres), in the
Galactic plane away from the bulge. Seven stars exhibit luminosity variations
compatible with gravitational microlensing effects due to unseen objects. The
corresponding optical depth, averaged over four directions, is tau = 0.45 +0.24
-0.11 x 10^-6. While this value is compatible with expectations from simple
galactic models under reasonable assumptions on the target distances, we find
an excess of events with short timescale towards the direction closest to the
Galactic Centre.


title:
Signatures of galactic magnetic lensing upon ultra high energy cosmic
  rays

abstract:
  We analyse several implications of lensing by the regular component of the
galactic magnetic field upon the observed properties of ultra high energy
cosmic rays. Magnetic fields deflect cosmic ray trajectories, causing flux
(de)magnification, formation of multiple images of a single source, and time
delays. We derive the energy dependence of these effects near the caustics at
which the flux amplification of a point source diverges. We show that the large
magnification of images around caustics leads to an amplification bias, which
can make them dominate the flux in some energy ranges. We argue that clustering
in the arrival directions of UHECRs of comparable energy may be due to magnetic
lensing around caustics. We show that magnetic lensing can also significantly
alter the observed composition of cosmic rays at the highest energies. We also
show that the time delay between events from a single image may monotonically
decrease with decreasing energy in the neighborhood of a caustic, opposite to
its behaviour in normal regions.


title:
The discovery of the optical/IR counterpart of the 12s transient X-ray
  pulsar GS 0834-43

abstract:
  We report the discovery of the optical/infra-red counterpart of the 12.3s
transient X-ray pulsar GS0834-43. We re-analysed archival ROSAT PSPC
observations of GS0834-43, obtaining two new refined positions, about 14" and
18" away from the previously published one, and a new spin period measurement.
Within the new error circles we found a relatively faint (V=20.1) early type
reddened star (V-R=2.24). The optical spectrum shows a strong Halpha emission
line. The IR observations of the field confirm the presence of an IR excess for
the Halpha-emitting star (K'=11.4, J-K'=1.94) which is likely surrounded by a
conspicuous circumstellar envelope. Spectroscopic and photometric data indicate
a B0-2 V-IIIe spectral-type star located at a distance of 3-5kpc and confirm
the Be-star/X-ray binary nature of GS0834-43.


title: Fluorescent Excitation of Spectral Lines in Planetary Nebulae

abstract:
  Fluorescent excitation of spectral lines is demonstrated as a function of
temperature-luminosity and the distance of the emitting region from the central
stars of planetary nebulae. The electron densities and temperatures are
determined, and the method is exemplified through a detailed analysis of
spectral observations of a high excitation PN, NGC 6741, observed by Hyung and
Aller(1997). Fluorescence should also be important in the determination of
element abundances. It is suggested that the method could be generally applied
to determine or constrain the luminosity and the region of spectral emission in
other intensively radiative sources such as novae, supernovae, and active
galactic nuclei.


title: Testing population synthesis models with globular cluster colors

abstract:
  We have measured an extensive set of UBVRIJHK colors for M31 globular
clusters [Barmby et al. 2000]. We compare the predicted simple stellar
population colors of three population synthesis models to the intrinsic colors
of Galactic and M31 globular clusters. The best-fitting models fit the cluster
colors very well -- the weighted mean color offsets are all < 0.05 mag. The
most significant offsets between model and data are in the U and B passbands;
these are not unexpected and are likely due to problems with the spectral
libraries used by the models. The metal-rich clusters ([Fe/H] > -0.8) are best
fit by young (8 Gyr) models, while the metal-poor clusters are best fit by
older (12--16 Gyr) models. If this range of globular cluster ages is correct,
it implies that conditions for cluster formation must have existed for a
substantial fraction of the galaxies' lifetimes.


title: Evolution of Cluster Galaxies in Hierarchical Clustering Universes

abstract:
  Using cosmological N-body simulations of critical (SCDM) and open ($\Omega =
0.3$, OCDM) cold dark matter models, we investigate evolution of cluster
galaxies. From our numerical simulation, we construct merging history trees of
the galaxies. By following their merging history, we show major merger
fractions of the galaxies in cluster forming regions is roughly proportional to
(1+z)^{4.5} at low redshifts (z < 2) and has a steep peak at $z \simeq 2.5$ and
$z \simeq 3$ in SCDM and OCDM, respectively. We also show that the cluster
galaxies are affected by tidal interaction after the clusters formed. Because
the formation redshift of the cluster in SCDM, z_{form} = 0.15, is much more
recent than that of the cluster in OCDM, z_{form} = 1.6, the cluster galaxies
in SCDM show more rapid evolution by tidal interactions from z = 0.5 than those
in OCDM.


title:
Constraints on Cosmological Parameters from the Lyman-alpha Forest Power
  Spectrum and COBE-DMR

abstract:
  We combine COBE-DMR measurements of cosmic microwave background anisotropy
with a recent measurement of the mass power spectrum at redshift z=2.5 from Lya
forest data to derive constraints on cosmological parameters and test the
inflation+CDM scenario of structure formation. By treating the inflationary
spectral index n as a free parameter, we can find successful fits to the COBE
and Lya forest constraints in Omega_m=1 models with and without massive
neutrinos and in low-Omega_m models with and without a cosmological constant.
Within each class of model, the combination of COBE and the Lya forest P(k)
constrains a parameter combination of the form (Omega_m h^a n^b Omega_b^c),
with different indices for each case. This new constraint breaks some of the
degeneracies in cosmological parameter determinations from other measurements.
The Lya forest P(k) provides the first measurement of the slope of the linear
mass power spectrum on ~Mpc scales, and it confirms a basic prediction of the
inflationary CDM scenario: a nearly scale-invariant spectrum of primeval
fluctuations (n~1) that bends towards k^{n-4} on small scales. Considering
additional observational data, we find that COBE-normalized, Omega_m=1 models
that match the Lya forest P(k) do not match the observed masses of rich galaxy
clusters and that a low-Omega_m model with a cosmological constant provides the
best overall fit, even without the direct evidence for cosmic acceleration from
supernovae. Modest improvements in the Lya forest P(k) measurement could
greatly restrict the allowable region of parameter space for CDM models,
constrain the contribution of tensor fluctuations to CMB anisotropy, and
achieve a more stringent test of the current consensus model of structure
formation.


title: Emission Lines in the Spectrum of the 3He Star 3 Cen A

abstract:
  Emission in the 4d - 4f transitions of MnII (multiplet 13, 6122-6132 Ang), in
the 4f - 6g transitions of PII, and in 6149.5 Ang of HgII has been detected in
the spectrum of the helium weak star 3 Centauri A (B5 III-IVp). Weaker emission
from the same MnII multiplet is also seen in the hot, mild HgMn star 46 Aquila
(B9 III).It is suggested that the emission is of photospheric origin and may be
evidence for the stratification of manganese, phosphorus and mercury in the
photosphere of 3 CenA, and of manganese in 46Aql.


title:
X-ray observations of the high magnetic field radio pulsar PSR
  J1814-1744

abstract:
  PSR J1814-1744 is a 4 s radio pulsar with surface dipole magnetic field
strength 5.5*10^13 G, inferred assuming simple magnetic dipole braking. This
pulsar's spin parameters are very similar to those of anomalous X-ray pulsars
(AXPs), suggesting that this may be a transition object between the radio
pulsar and AXP population, if AXPs are isolated, high magnetic field neutron
stars as has recently been hypothesized. We present archival X-ray observations
of PSR J1814-1744 made with ROSAT and ASCA. X-ray emission is not detected from
the position of the radio pulsar. The derived upper flux limit implies an X-ray
luminosity significantly smaller than those of all known AXPs. This conclusion
is insensitive to the possibility that X-ray emission from PSR J1814-1744 is
beamed or that it undergoes modest variability. When interpreted in the context
of the magnetar mechanism, these results argue that X-ray emission from AXPs
must depend on more than merely the inferred surface magnetic field strength.
This suggests distinct evolutionary paths for radio pulsars and AXP, despite
their proximity in period--period derivative phase space.


title:
Hubble Space Telescope imaging and ground-based spectroscopy of old nova
  shells - I. FH Ser, V533 Her, BT Mon, DK Lac, V476 Cyg

abstract:
  In this paper we report on the first five out of eleven observations in our
programme of Hubble Space Telescope (HST) imaging of old nova shells. We
present new WFPC2 images of the shells around FH Ser and V533 Her taken in the
F656N (Halpha+[NII]) filter. We also show long-slit spectra taken using the
William Herschel Telescope (WHT) in La Palma for these objects in the same
spectral range. The shell around FH Ser is found to be a prolate ellipsoid of
ellipticity 1.3 +/- 0.1 inclined at 62 +/- 4 deg to the line of sight. The
shell has an equatorial ring which is found to be due to increased emission in
the two [NII] lines rather than Halpha. The expansion velocity, systemic
velocity and distance are found to be 490 +/- 20 km/s, -45 km/s and 950 +/- 50
pc. The origin of the [NII] equatorial ring is discussed in the context of a
photoionization feature resulting from aspherical illumination by the central
source rather than a simple density enhancement. It is possible however that
the ring is also in part due to an extremely localised increase in the nitrogen
abundance. Similar imaging and spectroscopy of the nova V533 Her reveal a shell
of radius 5 +/- 0.7 arcsec with an axial ratio of 1.2 +/- 0.2, expansion
velocity of 850 +/- 150 km/s and distance 1250 +/- 300 pc. The shells around BT
Mon, DK Lac and V476 Cyg were not detected with HST.


title: Chemo - Photometric evolution of star forming disk galaxy

abstract:
  The chemical and photometric evolution of star forming disk galaxies is
investigated. Numerical simulations of the complex gasdynamical flows are based
on our own coding of the Chemo - Dynamical Smoothed Particle Hydrodynamical (CD
- SPH) approach, which incorporates the effects of star formation. As a first
application, the model is used to describe the chemical and photometric
evolution of a disk galaxy like the Milky Way.


title: Photoionization of Galactic Halo Gas by Old Supernova Remnants

abstract:
  We present new calculations on the contribution from cooling hot gas to the
photoionization of warm ionized gas in the Galaxy. We show that hot gas in
cooling supernova remnants (SNRs) is an important source of photoionization,
particularly for gas in the halo. We find that in many regions at high latitude
this source is adequate to account for the observed ionization so there is no
need to find ways to transport stellar photons from the disk. The flux from
cooling SNRs sets a floor on the ionization along any line of sight. Our model
flux is also shown to be consistent with the diffuse soft X-ray background and
with soft X-ray observations of external galaxies.
  We consider the ionization of the clouds observed towards the halo star HD
93521, for which there are no O stars close to the line of sight. We show that
the observed ionization can be explained successfully by our model EUV/soft
X-ray flux from cooling hot gas. In particular, we can match the H alpha
intensity, the S++/S+ ratio, and the C+* column. From observations of the
ratios of columns of C+* and either S+ or H0, we are able to estimate the
thermal pressure in the clouds. The slow clouds require high (~10^4 cm^-3 K)
thermal pressures to match the N(C+*)/N(S+) ratio. Additional heating sources
are required for the slow clouds to maintain their ~7000 K temperatures at
these pressures, as found by Reynolds, Hausen & Tufte (1999).


title:
Phase Lag and Coherence Function of X-ray emission from Black Hole
  Candidate XTE J1550-564

abstract:
  We report the results from measuring the phase lag and coherence function of
X-ray emission from black hole candidate (BHC) XTE J1550-564. These X-ray
temporal properties have been recognized to be increasingly important in
providing important diagnostics of the dynamics of accretion flows around black
holes. For XTE J1550-564, we found significant hard lag --- the X-ray
variability in high energy bands {\em lags} behind that in low energy bands ---
associated both with broad-band variability and quasi-periodic oscillation
(QPO). However, the situation is more complicated for the QPO: while hard lag
was measured for the first harmonic of the signal, the fundamental component
showed significant {\em soft} lag. Such behavior is remarkably similar to what
was observed of microquasar GRS 1915+105. The phase lag evolved during the
initial rising phase of the 1998 outburst. The magnitude of both the soft and
hard lags of the QPO increases with X-ray flux, while the Fourier spectrum of
the broad-band lag varies significantly in shape. The coherence function is
relatively high and roughly constant at low frequencies, and begins to drop
almost right after the first harmonic of the QPO. It is near unity at the
beginning and decreases rapidly during the rising phase. Also observed is that
the more widely separated the two energy bands are the less the coherence
function between the two. It is interesting that the coherence function
increases significantly at the frequencies of the QPO and its harmonics. We
discuss the implications of the results on the models proposed for BHCs.


title:
Mapping the Evolution of High Redshift Dusty Galaxies with Submillimeter
  Observations of a Radio-Selected Sample

abstract:
  Direct submm imaging has recently revealed the 850-micron background to be
mostly composed of a population of distant ultraluminous infrared galaxies, but
identifying the optical/NIR counterparts to these sources has proved difficult
due to the poor submm spatial resolution. However, the proportionality of both
cm and submm data to the star formation rate suggests that high resolution
radio continuum maps with subarcsecond positional accuracy can be exploited to
locate submm sources. In this paper we present results from a targeted SCUBA
survey of micro-Jansky radio sources in the flanking fields of the Hubble Deep
Field. Even with relatively shallow 850-micron SCUBA observations (>6 mJy at
3-sigma), we were successful at making submm detections of optical/NIR-faint
(I>24 and K~21-22) radio sources, and our counts closely match the bright
counts from submm surveys. Redshift estimates can be made from the ratio of the
submm flux to the radio flux across the 100 GHz break in the spectral energy
distribution. This millimetric redshift estimation places the bright submm
population at z=1-3 where it forms the high redshift tail of the faint radio
population. The star formation rate density (SFRD) due to ultraluminous
infrared galaxies increases by more than two orders of magnitude from z~0 to
z~1-3. The SFRD at high redshift inferred from our >6 mJy submm observations is
comparable to that observed in the UV/optical. (Abridged)


title: HST NICMOS Observation of Proto-Planetary Nebulae

abstract:
  We report NICMOS wide-band and polarimetric observations of four
proto-planetary nebulae. Molecular hydrogen emission is detected near the ends
of the bipolar lobes of IRAS 17150-3224, which is evidence for the interaction
of a fast, collimated outflow with the remnant of the asymptotic giant branch
wind.


title: The X-ray/submillimetre link

abstract:
  It is widely believed that most of the cosmic X-ray background (XRB) is
produced by a vast, hitherto undetected population of obscured AGN. Deep X-ray
surveys with Chandra and XMM will soon test this hypothesis. Similarly, recent
sub-mm surveys with SCUBA have revealed an analogous population of
exceptionally luminous, dust-enshrouded {\em star-forming} galaxies at high
redshift. There is now growing evidence for an intimate link between these
obscured populations. There are currently large uncertainties in the models,
but several independent arguments lead to the conclusion that a significant
fraction of the SCUBA sources ($10-30% $) will contain quasars. Recent
observational studies of SCUBA survey sources appear to confirm these
predictions, although the relative roles of AGN and star-forming activity in
heating the dust are unclear. Forthcoming surveys combining X-ray and sub-mm
observations will provide a very powerful tool for disentangling these
processes.


title:
Biased Estimates of Omega from Comparing Smoothed Predicted Velocity
  Fields to Unsmoothed Peculiar Velocity Measurements

abstract:
  We show that a regression of unsmoothed peculiar velocity measurements
against peculiar velocities predicted from a smoothed galaxy density field
leads to a biased estimate of the cosmological density parameter Omega, even
when galaxies trace the underlying mass distribution and galaxy positions and
velocities are known perfectly. The bias arises because the errors in the
predicted velocities are correlated with the predicted velocities themselves.
We investigate this bias using cosmological N-body simulations and analytic
arguments. In linear perturbation theory, for cold dark matter power spectra
and Gaussian or top hat smoothing filters, the bias in Omega is always
positive, and its magnitude increases with increasing smoothing scale. This
linear calculation reproduces the N-body results for Gaussian smoothing radii
R_s > 10 Mpc/h, while non-linear effects lower the bias on smaller smoothing
scales, and for R_s < 3 Mpc/h Omega is underestimated rather than
overestimated. The net bias in Omega for a given smoothing filter depends on
the underlying cosmological model. The effect on current estimates of Omega
from velocity-velocity comparisons is probably small relative to other
uncertainties, but taking full advantage of the statistical precision of future
peculiar velocity data sets will require either equal smoothing of the
predicted and measured velocity fields or careful accounting for the biases
discussed here.


title: Spectral Energy Distributions for Disk and Halo M--Dwarfs

abstract:
  We have obtained infrared (1 to 2.5 micron) spectroscopy for 42 halo and disk
dwarfs with spectral type M1 to M6.5. These data are compared to synthetic
spectra generated by the latest model atmospheres of Allard & Hauschildt.
Photospheric parameters metallicity, effective temperature and radius are
determined for the sample. We find good agreement between observation and
theory except for known problems due to incomplete molecular data for metal
hydrides and water. The metal-poor M subdwarfs are well matched by the models
as oxide opacity sources are less important in this case. The derived effective
temperatures for the sample range from 3600K to 2600K; at these temperatures
grain formation and extinction are not significant in the photosphere. The
derived metallicities range from solar to one-tenth solar. The radii and
effective temperatures derived agree well with recent models of low mass stars.


title:
CCD Photometry of the Globular Cluster Omega Centauri. I. Metallicity of
  RR Lyrae Stars from Caby Photometry

abstract:
  We present new measurements of the metallicity of 131 RR Lyrae stars in the
globular cluster Omega Centauri, using the hk index of the Caby photometric
system. The hk method has distinct advantages over Delta-S and other techniques
in determining the metallicity of RR Lyrae stars, and has allowed us to obtain
the most complete and homogeneous metallicity data to date for the RR Lyrae
stars in this cluster. For RR Lyrae stars in common with the Delta-S
observations of Butler et al. (1978) and Gratton et al. (1986), we have found
that our metallicities, [Fe/H](hk), deviate systematically from their Delta-S
metallicity, while our [Fe/H](hk) for well observed field RRab stars are
consistent with previous spectroscopic measurements. We conclude that this is
due to the larger errors associated with the previous Delta-S observations for
this cluster. The Mv(RR) - [Fe/H] and period-shift - [Fe/H] relations obtained
from our new data are consistent with the evolutionary models predicted by Lee
(1991), confirming that the luminosity of RR Lyrae stars depends on
evolutionary status as well as metallicity. Using the period - amplitude
diagram, we have also identified highly evolved RRab stars in the range of -1.9
< [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.


title: RR Lyrae Variables in the Globular Cluster M5

abstract:
  We present V-band CCD photometry of 65 RR Lyr variables from the globular
cluster M5. We have estimated the basic physical parameters for 16 RRc stars
and 26 RRab stars using a Fourier decomposition of the light curves of the
variables. The mean values of mass, luminosity, effective temperature and
relative helium abundance for the RRc stars are measured to be ${\cal
M}=0.54{\cal M}_\odot$, log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively.
For the RRab variables the derived mean values of absolute magnitude,
metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and
T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period
is a function of metal abundance rather than Oosterhoff type. We find
significant problems with the calibration of both the zero point and the scale
of the luminosities measured with the Fourier technique. The apparent distance
modulus derived from RRc stars is equal to $14.47\pm0.11$ and it is in good
agreement with recent determinations. On the other hand distance modulus
obtained from the sample of RRab stars (calibrated by the Baade-Wesselink
observations of field RR Lyr variables) is significantly smaller and equal to
$14.27\pm0.04$


title: A model-independent analysis of the variability of GRS 1915+105

abstract:
  We analyzed 163 observations of the microquasar GRS 1915+105 made with the
Rossi X-ray Timing Explorer (RXTE) in the period 1996-1997. For each
observation, we produced light curves and color-color diagrams. We classified
the observations in 12 separate classes, based on their count rate and color
characteristics. From the analysis of these classes, we reduced the variability
of the source to transitions between three basic states: a hard state
corresponding to the non-observability of the innermost parts of the accretion
disk, and two softer states with a fully observable disk. These two soft states
represent different temperatures of the accretion disk, related to different
local values of the accretion rate. The transitions between these states can be
extremely fast. The source moves between these three states following certain
patterns and avoiding others, giving rise to a relatively large but limited
number of variability classes. These results are the first step towards a
linking of the properties of this exceptional source with standard black-hole
systems and with accretion disk models.


title: Young and Intermediate Stellar population in Seyfert 2 galaxies

abstract:
  We present and discuss near-ultraviolet and optical ground-based spectra of
the 20 brightest Seyfert 2 nuclei, and HST ultraviolet images and ultraviolet
spectroscopy for a few of them. The goal is to study the starburst-AGN
connection and the origin of the featureless continuum. The results indicate
that half of the nuclei in the sample harbor a young and/or intermediate age
population. These stars are formed in powerful and dusty starbursts of short
duration, that have bolometric luminosities similar to the estimated bolometric
luminosities of their obscured Seyfert 1 nuclei.


title: Clustering of galaxies at faint magnitudes

abstract:
  Significant uncertainties exist in the measured amplitude of the angular
two-point correlation function of galaxies at magnitudes $I\approx26$ and
fainter. Published results from HST and ground-based galaxy catalogs seem to
differ by as much as a factor of 3, and it is not clear whether the correlation
amplitude as a function of magnitude increases or decreases in the faintest
magnitude bins. In order to clarify the situation, we present new results from
both ground-based and HST galaxy catalogs. The angular two-point correlation
function as a function of limiting R and I magnitudes was computed from a
galaxy catalog created from the Hubble Deep Field - South (HDF-S) WFPC2 image.
The measured amplitudes of the correlation at an angular separation of 1 arcsec
are consistent with those measured in the Northern counter part of the field.
The flanking fields (FF fields) of the Hubble deep fields were used to extend
the magnitude range for which we compute correlation amplitudes towards
brighter magnitude bins. This allows easier comparison of the amplitudes to
ground based data. The newly measured correlation amplitudes as a function of
magnitude limit were compared to previously published measurements at larger
separations. For this comparison, the correlation function was approximated by
a power law with an index of 0.8. The scatter in the correlation amplitudes is
too large to be explained by random errors. We argue that the most likely cause
is the assumption that the shape of the correlation function does not depend on
the magnitude limit.


title: Cosmology with SZ and X-ray cluster surveys

abstract:
  Hydrodynamical simulations are used in combination with the Press-Schechter
expression to simulate Sunyaev-Zel'dovich (SZ) galaxy cluster sky maps. These
are used to gauge the ability of future SZ observations to provide information
about the cosmological parameters H_0, Omega_M, and the gas fraction f_g in
clusters. This work concentrates on prospects for AMI, the Arcminute
MicroKelvin Imager, a new type of compact interferometric array currently
proposed in Cambridge. The expectations are contrasted with those for X-ray
missions, such as XMM, and the benefits of combining SZ and X-ray data are
highlighted.


title:
The variability of the Seyfert galaxy NGC 2992: the case for a revived
  AGN

abstract:
  We report the transition to an active state of the nucleus in the Seyfert 1.9
galaxy NGC 2992, discovered by means of new hard X-ray data. While the 2-10 keV
flux declined by a factor of ~ 20 from 1978 to 1994, two recent BeppoSAX
observations in 1997 and in 1998 caught the nuclear emission raising back to
the same level of activity observed in 1978.
  In both BeppoSAX observations the X-ray spectrum of the source is well
represented by a power law with spectral index Gamma ~ 1.7, absorbed by a
column density of N_H ~ 10^{22} cm^{-2} and characterized by a prominent iron
K\alpha line. While in the second BeppoSAX data set the line properties appear
to be consistent with those expected from accretion disc models, in the first
BeppoSAX data set the iron feature is rather peculiar. The broadening is not
significant and the line energy is E_{K\alpha}=6.62 +/- 0.07 keV, indicating
emission from highly ionized iron. The line has too high equivalent width (~
700 eV) to be produced by a hot scattering medium. By comparing these data with
data previously in the literature, we interpret the spectral and flux changes
in terms of different phases of rebuilding an accretion disc. The timescale for
the disc rebuilding is estimated to range between 1 and 5 years.
  The X-ray data are complemented with optical and near-infrared followup
spectra taken 1.5 months after the discovery of the X-ray burst. The spectra
are characterized by prominent broad emission lines. There is also evidence for
hot dust emission in the H and K bands that, however, is probably still in the
process of increasing.


title:
Micro & strong lensing with the Square Kilometer Array: The
  mass--function of compact objects in high--redshift galaxies

abstract:
  We present the results from recent VLA 8.5-GHz and WSRT 1.4 and 4.9-GHz
monitoring campaigns of the CLASS gravitational lens B1600+434 and show how the
observed variations argue strongly in favor of microlensing by MACHOs in the
halo of a dark-matter dominated edge-on disk galaxy at z=0.4. The population of
flat-spectrum radio sources with micro-Jy flux-densities detected with the
Square-Kilometer-Array is expected to have dimensions of micro-arcsec. They
will therefore vary rapidly as a result of Galactic scintillation (diffractive
and refractive). However, when positioned behind distant galaxies they will
also show variations due to microlensing, even more strongly than in the case
of B1600+434. Relativistic or superluminal motion in these background sources
typically leads to temporal variations on time scales of days to weeks.
Scintillation and microlensing can be distinguished, and separated, by their
different characteristic time scales and the frequency dependence of their
modulations. Monitoring studies with Square-Kilometer-Array at GHz frequencies
will thus probe both microscopic and macroscopic properties of dark matter and
its mass-function as a function of redshift, information very hard to obtain by
any other method.


title:
Chemical Evolution of Damped Ly alpha galaxies: The [S/Zn] abundance
  ratio at redshift z > 2

abstract:
  Relative elemental abundances, and in particular the alpha/Fe ratio, are an
important diagnostic tool of the chemical evolution of damped Ly alpha systems
(DLAs). The S/Zn ratio is not affected by differential dust depletion and is an
excellent estimator of the alpha/Fe ratio. We report 6 new determinations of
sulphur abundance in DLAs at zabs greater than or equal to 2 with already known
zinc abundances. The combination with extant data from the literature provides
a measure of the S/Zn abundance ratio for a total of 11 high redshift DLA
systems. The observed [S/Zn] ratios do not show the characteristic [alpha/Fe]
enhancement observed in metal-poor stars of the Milky Way at comparable level
of metallicity ([Zn/H] ~ -1). The behaviour of DLAs data is consistent with a
general trend of decreasing [S/Zn] ratio with increasing metallicity [Zn/H].
This would be the first evidence of the expected decrease of the alpha/Fe ratio
in the course of chemical evolution of DLA systems. However, in contrast to
what observed in our Galaxy, the alpha/iron-peak ratio seems to attain solar
values when the metallicity is still low ([Zn/H] < -1) and to decrease below
solar values at higher metallicities. The behaviour of the alpha/Fe ratio
challenges the frequently adopted hypothesis that high redshift DLAs are
progenitors of spiral galaxies and favours instead an origin in galaxies
characterized by low star formation rates, in agreement with the results from
imaging studies of low redshift DLAs, where the candidate DLA galaxies show a
variety of morphological types including dwarfs and LSBs and only a minority of
spirals.


title:
BeppoSAX observations of the nearby low-mass X-ray binary and fast
  transient SAX J1819.3-2525

abstract:
  SAX J1819.3-2525 is a nearby X-ray transient which exhibited a fast and large
X-ray outburst on Sep. 15, 1999 (Smith et al. 1999). The Wide Field Cameras and
the Narrow Field Instruments (NFI) on board BeppoSAX observed SAX J1819.3-2525
at various stages of its activity before that, in the spring and fall of 1999.
The fluxes range between 0.012 and 0.3 Crab units (2-10 keV). The NFI
observation is unique because it is the longest semi-continuous observation of
SAX J1819.3-2525 so far, and it offers a study of the spectrum at a relatively
high resolution of 8% full width at half maximum at 6 keV. We discuss the
observations with emphasis on the X-ray spectrum. A strong Fe-K emission line
was detected in SAX J1819.3-2525 with an equivalent width between 0.3 and 1
keV. The line energy is up to 6.85 +/- 0.02 keV and suggests the presence of
highly ionized iron. We identify this as fluorescent emission from a
photo-ionized plasma. The continuum spectrum is typical for a low-mass X-ray
binary in which emission from an accretion disk corona plays an important role.
There is no sign of an eclipse or periodic signal due to the binary orbit in
this exposure, despite the fact that the twin jets seen at radio wavelengths
suggest a high inclination angle.


title: Evidence for TeV Emission from GRB 970417a

abstract:
  Milagrito, a detector sensitive to very high energy gamma rays, monitored the
northern sky from February 1997 through May 1998. With a large field of view
and a high duty cycle, this instrument was well suited to perform a search for
TeV gamma-ray bursts (GRBs). We report on a search made for TeV counterparts to
GRBs observed by BATSE. BATSE detected 54 GRBs within the field of view of
Milagrito during this period. An excess of events coincident in time and space
with one of these bursts, GRB 970417a, was observed by Milagrito. The excess
has a chance probability of $2.8 \times 10^{-5}$ of being a fluctuation of the
background. The probability for observing an excess at least this large from
any of the 54 bursts is $1.5 \times 10^{-3}$. No significant correlations were
detected from the other bursts.


title: A Sociological Study of the Optically Emitting Isolated Neutron Stars

abstract:
  Although less than 1% of all radio pulsars are detected at optical
wavelengths, their optical emission can yield a wealth of information that is
either very difficult or plainly impossible to obtain at other wavelengths.


title:
The APM Survey for Cool Carbon Stars in the Galactic Halo - II The
  Search for Dwarf Carbon Stars

abstract:
  We present proper motion measurements for carbon stars found during the APM
Survey for Cool Carbon Stars in the Galactic Halo (Totten & Irwin, 1998).
Measurements are obtained using a combination of POSSI, POSSII and UKST survey
plates supplemented where necessary by CCD frames taken at the Isaac Newton
Telescope. We find no significant proper motion for any of the new APM
colour-selected carbon stars and so conclude that there are no dwarf carbon
stars present within this sample. We also present proper motion measurements
for three previously known dwarf carbon stars and demonstrate that these
measurements agree favourably with those previously quoted in the literature,
verifying our method of determining proper motions. Results from a
complimentary program of JHK photometry obtained at the South African
Astronomical Observatory are also presented. Dwarf carbon stars are believed to
have anomalous near-infrared colours, and this feature is used for further
investigation of the nature of the APM carbon stars. Our results support the
use of JHK photomtery as a dwarf/giant discriminator and also reinforce the
conclusion that none of the new APM-selected carbon stars are dwarfs. Finally,
proper motion measurements combined with extant JHK photometry are presented
for a sample of previously known Halo carbon stars, suggesting that one of
these stars, CLS29, is likely to be a previously unrecognised dwarf carbon
star.


title: Advection-dominated Inflow/Outflows from Evaporating Accretion Disks

abstract:
  In this Letter we investigate the properties of advection-dominated accretion
flows (ADAFs) fed by the evaporation of a Shakura-Sunyaev accretion disk (SSD).
In our picture the ADAF fills the central cavity evacuated by the SSD and
extends beyond the transition radius into a coronal region. We find that,
because of global angular momentum conservation, a significant fraction of the
hot gas flows away from the black hole forming a transsonic wind, unless the
injection rate depends only weakly on radius (if $r^2\dot\sigma\propto
r^{-\xi}$, $\xi< 1/2$). The Bernoulli number of the inflowing gas is negative
if the transition radius is $\lesssim 100$ Schwarzschild radii, so matter
falling into the hole is gravitationally bound. The ratio of inflowing to
outflowing mass is $\approx 1/2$, so in these solutions the accretion rate is
of the same order as in standard ADAFs and much larger than in
advection-dominated inflow/outflow models (ADIOS). The possible relevance of
evaporation-fed solutions to accretion flows in black hole X-ray binaries is
briefly discussed.


title: The Circumstellar Matter Around OH44.8-2.3

abstract:
  I-band images of the OH maser source OH44.8-2.3 are presented. Having a
well-determined distance, this object is shown to be very young and physically
small, with an asymmetric compact bipolar nebula already present. In the
morphological terminology of Meixner et al. (2000) and Ueta et al. (2000), this
object would be classified as a DUPLEX nebula because of its high optical depth
and pole-to-equator density contrast.


title: Magnetic ionization fronts II: Jump conditions for oblique magnetization

abstract:
  We present the jump conditions for ionization fronts with oblique magnetic
fields. The standard nomenclature of R- and D-type fronts can still be applied,
but in the case of oblique magnetization there are fronts of each type about
each of the fast- and slow-mode speeds. As an ionization front slows, it will
drive first a fast- and then a slow-mode shock into the surrounding medium.
Even for rather weak upstream magnetic fields, the effect of magnetization on
ionization front evolution can be important. [Includes numerical MHD models and
an application to observations of S106.]


title: The PSCz Catalogue

abstract:
  We present the catalogue, mask, redshift data and selection function for the
PSCz survey of 15411 IRAS galaxies across 84% of the sky. Most of the IRAS data
is taken from the Point Source Catalog, but this has been supplemented and
corrected in various ways to improve the completeness and uniformity. We
quantify the known imperfections in the catalogue, and we assess the overall
uniformity, completeness and data quality. We find that overall the catalogue
is complete and uniform to within a few percent at high latitudes and 10% at
low latitudes. Ancillary information, access details, guidelines and caveats
for using the catalogue are given.


title: Evolution of the cosmological density distribution function

abstract:
  We present a new calculation for the evolution of the 1-point Probability
Distribution Function (PDF) of the cosmological density field based on an exact
statistical treatment. Using the Chapman-Kolmogorov equation and second-order
Eulerian perturbation theory we propagate the initial density distribution into
the nonlinear regime. Our calculations yield the moment generating function,
allowing a straightforward derivation of the skewness of the PDF to second
order. We find a new dependency on the initial perturbation spectrum. We
compare our results with other approximations to the 1-pt PDF, and with N-body
simulations. We find that our distribution accurately models the evolution of
the 1-pt PDF of dark matter.


title: The Most Distant X-ray Clusters and the Evolution of their Space Density

abstract:
  We briefly review our current knowledge of the space density of distant X-ray
clusters as measured by several ROSAT serendipitous surveys. We compare old and
new determinations of the cluster X-ray Luminosity Function (XLF) at increasing
redshifts, addressing the controversial issue of the evolution of its high end.
We use complete subsamples, drawn from the ROSAT Deep Cluster Survey (RDCS), to
quantify the statistical significance of the XLF evolution out to z ~1. A
consistent observational picture emerges in which the bulk of the cluster
population shows no significant evolution out to z ~1, whereas the most
luminous systems (L_x >~ L* [0.5-2 keV] =~ 5x10^44 erg/s) were indeed rarer, at
least at z >0.5, in keeping with the original findings of the EMSS. We also
report on the recent spectroscopic identification of four clusters in the RDCS
lying beyond z =1, the most distant X-ray clusters known to date, which set an
interesting lower limit on the space density of clusters at z >1.


title:
Sunyaev-Zeldovich effect from hydrodynamical simulations: maps and low
  order statistics

abstract:
  We use moving mesh hydrodynamical simulations to make maps of
Sunyaev-Zeldovich effect. We present these maps for several cosmological models
and explore their lowest moments. We find that the first moment, the mean
Compton $y$ parameter, is typically between $1-2\times 10^{-6}$ for cluster
abundance normalized models, the lower value corresponding to the high density
models, and scales as $\sigma_8^{3-4}$. Rms fluctuations at 10' scale have an
amplitude $ \Delta T/T \sim 1-3 \times 10^{-6}$ in the Rayleigh-Jeans regime.
The amplitude of the power spectrum strongly depends on the power spectrum
normalization and scales as $\sigma_8^{6-9}$. On smaller scales ($l>1000$) the
spectrum is dominated by halos below $10^{13}M_{\sun}$ and so is sensitive to
the thermal history of galactic halo gas. On larger scales ($l<1000$) the power
spectrum is less sensitive to nongravitational energy injection, but becomes
very noisy, with large field to field variations in the power spectrum caused
by rare bright sources in the maps, which dominate in the spectrum over the
large scale structure correlations. Cross-correlation power spectrum with weak
lensing or projected galaxy distribution is significant and the
cross-correlation coefficient is around 0.5 over a wide range of scales.
Comparison with Press-Schechter predictions gives very good agreement for all
the statistics, indicating that there is no significant contribution to SZ from
non-virialized structures. The one point distribution function shows clear
deviations from gaussianity and can be well-approximated as log-normal on small
scales.


title:
Luminosities, Space Densities and Redshift Distributions of Gamma-Ray
  Bursts

abstract:
  We use the BD2 sample of gamma-ray bursts (GRBs) based on 5.9 years of BATSE
DISCLA data with a variety of models of the luminosity function to derive
characteristic GRB luminosities, space densities and redshift distributions.
Previously published results for an open universe and modest density evolution
of the GRBs showed characteristic peak luminosities around 5x10**51 erg/s in
the 50-300 keV band if the emission is isotropic, and local space densities
around 0.2 Gpc**-3 yr**-1. In this paper, we illustrate for several luminosity
function models the predicted distributions of peak flux, luminosity and
redshifts. We use the luminosity function models also to address the connection
between supernovae and GRB. If all supernovae of Type Ib/c harbor a GRB, the
beaming fraction would have to be 10**-5 - 10**-3.5. We find that GRB 980425,
if correctly identified with SN 1998bw, has to be part of a population
different from that of the bulk of GRBs.


title: Derivation of a Sample of Gamma-Ray Bursts from BATSE DISCLA Data

abstract:
  We have searched for gamma-ray bursts (GRBs) in the BATSE DISCLA data over a
time period of 5.9 years. We employ a trigger requiring an excess of at least 5
sigma over background for at least two modules in the 50-300 keV range. After
excluding certain geographic locations of the satellite, we are left with 4485
triggers. Based on sky positions, we exclude triggers close to the sun, to Cyg
X-1, to Nova Persei 1992 and the repeater SGR 1806-20, while these sources were
active. We accept 1013 triggers that correspond to GRBs in the BATSE catalog,
and after visual inspection of the time profiles classify 378 triggers as
cosmic GRBs. We denote the 1391 GRBs so selected as the "BD2 sample". The BD2
sample effectively represents 2.003 years of full sky coverage for a rate of
694 GRBs per year. Euclidean V/Vmax values have been derived through
simulations in which each GRB is removed in distance until the detection
algorithm does not produce a trigger. The BD2 sample produces a mean value
<V/Vmax> = 0.334 +- 0.008.


title: Impeding and Unimpeding Gamma: The Principle of Impedance

abstract:
  We demonstrate that gamma (from the Lorentz transformation of the Special
Theory of Relativity) acts as an impediment to velocity changes only when it
occurs inside motion energy transfer relationships. We also examine whether or
not gamma unimpeded velocity changes violate either the constancy of light or
elementary causality. We find that no violations occur. We conclude the paper
with the procedure for a simple experiment, and further empirical evidence from
the Hubble Space Telescope and W.M. Keck Observatory, all of which help to
establish the veracity of our "principle of impedance."


title: On the Internal Absorption of Galaxy Clusters

abstract:
  A study of the cores of galaxy clusters with the Einstein SSS indicated the
presence of absorbing material corresponding to 1E+12 Msun of cold cluster gas,
possibly resulting from cooling flows. Since this amount of cold gas is not
confirmed by observations at other wavelengths, we examined whether this excess
absorption is present in the ROSAT PSPC observations of 20 bright galaxy
clusters. For 3/4 of the clusters, successful spectral fits were obtained with
absorption due only to the Galaxy, and therefore no extra absorption is needed
within the clusters, in disagreement with the results from the Einstein SSS
data for some of the same clusters. For 1/4 of the clusters, none of our
spectral fits was acceptable, suggesting a more complicated cluster medium than
the two-temperature and cooling flow models considered here. However, even for
these clusters, substantial excess absorption is not indicated.


title: Star Formation: Lessons from Taurus

abstract:
  The Taurus molecular cloud complex is the paradigm for quiescent,
low-density, isolated star formation. Yet the age distribution of its stellar
population indicates that star formation is a rapid and dynamic process,
inconsistent with the old picture of magnetically-controlled protostellar cloud
collapse. Instead, Taurus seems to have formed stars in a manner qualitatively
consistent with the rapid cloud formation and dispersal inferred for other,
higher-density star-forming regions. I suggest that the Taurus clouds were
formed rapidly by the collision of atomic gas streams in the interstellar
medium, and that star formation ensued immediately afterward.


title:
Multi-wavelength Observations of Dusty Star Formation at Low and High
  Redshift

abstract:
  This paper examines what can be learned about high-redshift star formation
from the small fraction of high-redshift galaxies' luminosities that is emitted
at accessible wavelengths. We review and quantify empirical correlations
between bolometric luminosities produced by star formation and the UV, mid-IR,
sub-mm, and radio luminosities of galaxies in the local universe. These
correlations suggest that observations of high-redshift galaxies at any of
these wavelengths should constrain their star-formation rates to within
0.2--0.3 dex. We assemble the limited evidence that high-redshift galaxies obey
these locally calibrated correlations. The characteristic luminosities and dust
obscurations of galaxies at z ~ 0, z ~ 1, and z ~ 3 are reviewed. After
discussing the relationship between the high-redshift populations selected in
surveys at different wavelengths, we calculate the contribution to the 850um
background from each. The available data show that a correlation between
star-formation rate and dust obscuration L_dust/L_UV exists at low and high
redshift. This correlation plays a central role in the major conclusion of this
paper: most star formation at high redshift occurred in galaxies with 1 <
L_dust/L_UV < 100 similar to those that host the majority of star formation in
the local universe and to those that are detected in UV-selected surveys.
(abridged)


title:
EUV and X-ray observation of Abell 2199: a three-phase intracluster
  medium with a massive warm component

abstract:
  Various independent ways of constraining the Hubble constant and the baryonic
content of the universe finally converged at a consensus range of values which
indicates that at the present epoch the bulk of the universe's baryons is in
the form of a warm $\sim$ 10$^6$ K gas - a temperature regime which renders
them difficult to detect. The discovery of EUV and soft X-ray excess emission
from clusters of galaxies was originally interpreted as the first direct
evidence for the large scale presence of such a warm component. We present
results from an EUVE Deep Survey (DS) observation of the rich cluster Abell
2199 in the Lex/B (69 - 190 eV) filter passband. The soft excess radial trend
(SERT), shown by a plot against cluster radius $r$ of the percentage EUV
emission $\eta$ observed above the level expected from the hot intracluster
medium (ICM), reveals that $\eta$ is a simple function of $r$ which decreases
monotonically towards $r = 0$; it smoothly turns negative at $r \sim$ 6 arcmin,
inwards of this radius the EUV is absorbed by cold matter with a line-of-sight
column density of $\geq$ 2.7 $\times$ 10$^{19}$ cm$^{-2}$. The area of
absorption is much larger than that of the cooling flow. These facts together
provide strong evidence for a centrally concentrated but cluster-wide
distribution of clumps of cold gas which co-exist with warm gas of similar
spatial properties. Further, the simultaneous modeling of EUV and X-ray data
requires a warm component even within the region of absorption. The phenomenon
demonstrates a three phase ICM, with the warm phase estimated to be $\sim$ 5-10
times more massive than the hot.


title: The multi-phase nature of three intracluster media

abstract:
  Among the models proposed to account for the new component of diffuse EUV and
soft X-ray emission from clusters of galaxies (first discovered in Virgo) are
two key contestants: the non-thermal scenario which postulates a population of
relativistic electrons undergoing inverse-Compton (IC) interaction with the
cosmic microwave background, and the original conjecture that the radiation is
from a thermal warm gas at a temperature of $\sim$ 10$^{5-6}$ K. Currently a
consensus set of limiting values on cosmological parameters favor the thermal
gas interpretation. We also argued, based on pressure balance within the
intracluster medium (ICM), that the non-thermal approach has formidable
difficulties. Here we describe a spatial analysis of the soft X-ray excess
emission of three clusters (Virgo, A2199, and Coma), using archival ROSAT/PSPC
data, which reveals resolved features of cold intracluster clouds in absorption
spreading over vast distances. Within the sample there is good indication that
the soft excess radial trend (SERT, which qualitatively means a rising
importance of the soft component with cluster radius) is due to a centrally
peaked distribution of cold matter, with Coma having the least effect and no
direct evidence for absorption. The data strongly suggest an intermixed ICM
which contains gas masses at a wide range of temperatures, and the soft excess
is due to a warm intermediate phase.


title: Gravitational Waves from Low-Mass X-ray Binaries: a Status Report

abstract:
  We summarize the observations of the spin periods of rapidly accreting
neutron stars. If gravitational radiation is responsible for balancing the
accretion torque at the observed spin frequencies of ~300 Hz, then the
brightest of these systems make excellent gravitational wave sources for
LIGO-II and beyond. We review the recent theoretical progress on two mechanisms
for gravitational wave emission: mass quadrupole radiation from deformed
neutron star crusts and current quadrupole radiation from r-mode pulsations in
neutron star cores.


title: Nature of eclipsing pulsars

abstract:
  We present a model for pulsar radio eclipses in some binary systems, and test
this model for PSRs B1957+20 and J2051-0827. We suggest that in these binaries
the companion stars are degenerate dwarfs with strong surface magnetic fields.
The magnetospheres of these stars are permanently infused by the relativistic
particles of the pulsar wind. We argue that the radio waves emitted by the
pulsar split into the eigenmodes of the electron-positron plasma as they enter
the companion's magnetosphere and are then strongly damped due to cyclotron
resonance with the ambient plasma particles. Our model explains in a natural
way the anomalous duration and behavior of radio eclipses observed in such
systems. In particular, it provides stable, continuous, and frequency-dependent
eclipses, in agreement with the observations. We predict a significant
variation of linear polarization both at eclipse ingress and egress. In this
paper we also suggest several possible mechanisms of generation of the optical
and $X$-ray emission observed from these binary systems.


title: A Search for X-ray emission from Saturn, Uranus and Neptune

abstract:
  We present an analysis of X-ray observations of the trans-Jovian planets
Saturn, Uranus and Neptune with the ROSAT PSPC in comparison with X-ray
observations of Jupiter. For the first time a marginal X-ray detection of
Saturn was found and 95% confidence upper limits for Uranus and Neptune were
obtained. These upper limits show that Jupiter-like X-ray luminosities can be
excluded for all three planets, while they are consistent assuming intrinsic
Saturn-like X-ray luminosities. Similar X-ray production mechanisms on all
trans-Jovian planets can therefore not be ruled out, and spectral shape and
total luminosity observed from Saturn are consistent with thick-target
bremsstrahlung caused by electron precipitation as occurring in auroral
emission from the Earth.


title: Nature of microstructure in pulsar radio emission

abstract:
  We present a model for microstructure in pulsar radio emission. We propose
that micropulses result from the alteration of the radio wave generation region
by nearly transverse drift waves propagating across the pulsar magnetic field
and encircling the bundle of the open magnetic field lines. It is demonstrated
that such waves can modify significantly curvature of these dipolar field
lines. This in turn affects strongly fulfillment of the resonance conditions
necessary for the excitation of radio waves. The time-scale of micropulses is
therefore determined by the wavelength of drift waves. Main features of the
microstructure are naturally explained in the frame of this model.


title: Giant protostellar outflows revealed by infrared imaging

abstract:
  We present new infrared data from a survey for embedded protostellar jets in
the Orion A cloud. This survey makes use of the S(1) v = 1-0 line of molecular
hydrogen at lambda = 2.12 mum to search for infrared jets deep inside the cloud
and thus hidden from view at optical wavelengths. We present data on the three
flows associated with the Herbig-Haro objects HH 43/38 & HH 64, HH 65, and the
L1641-N giant flow. HH 43, HH 38 and HH 64 are part of one H_2 flow extending
over at least 1.4 pc. We identify a deeply embedded 1.3 mm and IRAS source (HH
43 MMS1 = IRAS 05355-0709C) as the likely driving source, while the infrared
source previously assumed to drive HH 43/38 (HH 43-IRS1 = IRAS 05357-0710) is
seen to drive a smaller jet. The morphology of HH 43-IRS1 suggests that it is a
star+disk system seen close to edge-on. We identify another large H_2 flow
apparently comprising the L1641-S3 CO outflow and the redshifted lobe of the
L1641-S CO outflow containing HH 65. This flow extends over at least 2.6 pc and
appears strongly curved. It is driven by L1641-S3 IRS, a deeply embedded 1.3 mm
and IRAS source (L1641-S3 MMS1 = IRAS 05375-0731). Finally, we have found some
additional large H_2 features to the east of V 380 Ori and the HH 1/2 system,
which probably outline another part of the L1641-N outflow. The molecular flow
MB 20/21, which extends to the south from V 380 Ori, also appears to be a part
of the L1641-N outflow.


title: Phase lag variability associated with the 0.5-10 Hz QPO in GRS 1915+105

abstract:
  We have used Rossi X-ray Timing Explorer data to measure the lags between
soft (2-5 keV) and hard (5-13 keV) photons and to study the aperiodic
variability of the superluminal black hole candidate GRS 1915+105 during
low-flux states. The power density spectra exhibit quasi-periodic oscillations
(QPO) whose frequency increases with increasing count rate and varies in the
frequency range 0.6-8 Hz. A correlation between the QPO frequency and the phase
lag spectra is reported for the first time. This correlation is found for both
the phase lag continuum and the phase lag at the QPO frequency. We find that as
the QPO frequency moves to higher values the phase lags reverse sign from
positive to negative. The absolute value of the lag always increases with
photon energy. The negative (soft) lags are associated with a softer energy
spectrum, whereas the positive (hard) lags are seen when the source is harder.
We describe a possible scenario that may account for the change in the sign of
the lags.


title: Theory and Observations of Type I X-Ray Bursts from Neutron Stars

abstract:
  I review our understanding of the thermonuclear instabilities on accreting
neutron stars that produce Type I X-Ray bursts. I emphasize those observational
and theoretical aspects that should interest the broad audience of this
meeting. The easily accessible timescales of the bursts (durations of tens of
seconds and recurrence times of hours to days) allow for a very stringent
comparison to theory. The largest discrepancy (which was found with EXOSAT
observations) is the accretion rate dependence of the Type I burst properties.
Bursts become less frequent and energetic as the global accretion rate
increases, just the opposite of what the spherical theory predicts. I present a
resolution of this issue by taking seriously the observed dependence of the
burning area on the global accretion rate, which implies that as the accretion
rate increases, the accretion rate per unit area decreases. This resurrects the
unsolved problem of knowing where the freshly accreted material accumulates on
the star, equally relevant to the likely signs of rotation during the bursts
summarized by Swank at this meeting. I close by highlighting the Type I bursts
from GS 1826-238 that were found with BeppoSAX and RXTE. Their energetics,
recurrence times and temporal profiles clearly indicate that hydrogen is being
burned during these bursts, most likely by the rapid-proton (rp) process.


title:
Deformations of Accreting Neutron Star Crusts and Gravitational Wave
  Emission

abstract:
  Motivated by the narrow range of spin frequencies of nearly 20 accreting
neutron stars, Bildsten (1998) conjectured that their spin-up had been halted
by the emission of gravitational waves. He also pointed out that small
nonaxisymmetric temperature variations in the accreted crust will lead to
"wavy" electron capture layers, whose horizontal density variations naturally
create a mass quadrupole moment.
  We present a full calculation of the crust's elastic adjustment to these
density perturbations and find that the elastic response of the crust reduces
Bildsten's original estimate of the quadrupole moment in the thin outer crust
by a factor of 20-50. However, this basic picture, when applied to capture
layers in the deep inner crust, can generate quadrupoles in the necessary range
as long as there are ~5% lateral temperature variations in the inner crust. By
calculating the thermal flow throughout the core and the crust, we find that
temperature gradients this large are easily maintained by asymmetric heat
sources or lateral composition gradients in the crust.
  We also derive a general relation between the stresses and strains in the
crust and the maximum quadrupole moment they can generate. We show under quite
general conditions that maintaining a quadrupole of the magnitude necessary to
balance the accretion torque requires dimensionless strains close to 0.01 at
near-Eddington accretion rates, of order the breaking strain of conventional
materials.


title: Searching the Footprint of WIMPZILLAs

abstract:
  We constrain mass, lifetime and contribution of a very slowly decaying Ultra
Heavy Dark Matter (UHDM) by simulating the cosmological evolution of its
remnants. Most of interactions which participate in energy dissipation are
included in the numerical solution of the Boltzmann equation. Cross-sections
are calculated either analytically or by using PYTHIA Monte Carlo program. This
paper describes in detail our simulation. To show the importance of the
distribution of matter in constraining WIMPZILLA characteristics, we consider
two extreme cases: a homogeneous universe, and a local halo with uniform
distribution. We show that in a homogeneous universe, the decay of UHDM with a
mass \sim 10^15 GeV and a lifetime \sim a few times \tau_0 the age of the
Universe, can not explain the flux of observed Ultra High Energy Cosmic Rays
(UHECRs). This shows the importance of nearby sources, notably galactic halo.
In a uniform clump with an over-density of \sim 200 extended to 100 kpc or
more, the lifetime must be \sim 10 - 100 \tau_0 or the contribution in the DM
must be proportionally smaller. We also compare our calculation with observed
gamma-rays at E \sim 10^11 eV by EGRET and CASA-MIA limit at E \sim 10^15 eV.
They are compatible with a UHDM with relatively short lifetime.


title: A Preliminary Visual Orbit of BY Draconis

abstract:
  We report on the preliminary determination of the visual orbit of the
double-lined spectroscopic binary system BY Draconis with data obtained by the
Palomar Testbed Interferometer in 1999. BY Dra is a nearly equal-mass
double-lined binary system whose spectroscopic orbit is well known. We have
estimated the visual orbit of BY Dra from our interferometric visibility data
fit both separately and in conjunction with archival radial velocity data. Our
BY Dra orbit is in good agreement with the spectroscopic results. Due to the
orbit's face-on orientation, the physical parameters implied by a combined fit
to our visibility data and radial velocity data do not yet result in precise
component masses and a system distance, but with continued interferometric
monitoring we hope to improve the mass estimates to better than 10%
determinations.


title:
Analysis and interpretation of the X-ray properties of black hole
  candidates

abstract:
  The thesis is mainly devoted to the study of spectral and timing
characteristics of the X-ray emission from Galactic black hole candidates GRS
1915+105, GX 339-4, 4U 1630-47, XTE J1748-288, GRS 1739-278, KS/GRS 1730-312
and GRS 1737-31. The main results include: observation of the correlated
evolution of spectral and timing parameters of these sources and its
interpretation in the framework of two-phase model of the accretion flow near
the compact objects; development of the quantitative model for the evolution of
GRS 1915+105 during flaring state providing direct estimate of the disk
accretion rate in the system; restrictions on the parameters of the spatial
distribution and luminosity function of the Galactic hard X-ray transient
sources; application of the the bulk motion comptonization model to the
analytic approximation of the broad--band energy spectra of Galactic black hole
candidates in the high/very high state obtained with RXTE.


title:
Nearby Optical Galaxies: Selection of the Sample and Identification of
  Groups

abstract:
  In this paper we describe the Nearby Optical Galaxy (NOG) sample, which is a
complete, distance-limited ($cz\leq$6000 km/s) and magnitude-limited
(B$\leq$14) sample of $\sim$7000 optical galaxies. The sample covers 2/3 (8.27
sr) of the sky ($|b|>20^{\circ}$) and appears to have a good completeness in
redshift (98%). We select the sample on the basis of homogenized corrected
total blue magnitudes in order to minimize systematic effects in galaxy
sampling.
  We identify the groups in this sample by means of both the hierarchical and
the percolation {\it friends of friends} methods. The resulting catalogs of
loose groups appear to be similar and are among the largest catalogs of groups
presently available. Most of the NOG galaxies ($\sim$60%) are found to be
members of galaxy pairs ($\sim$580 pairs for a total of $\sim$15% of objects)
or groups with at least three members ($\sim$500 groups for a total of
$\sim$45% of objects). About 40% of galaxies are left ungrouped (field
galaxies). We illustrate the main features of the NOG galaxy distribution.
Compared to previous optical and IRAS galaxy samples, the NOG provides a denser
sampling of the galaxy distribution in the nearby universe. Given its large sky
coverage, the identification of groups, and its high-density sampling, the NOG
is suited for the analysis of the galaxy density field of the nearby universe,
especially on small scales.


title: CTQ 839: Candidate for the Smallest Projected Separation Binary Quasar

abstract:
  We report the discovery of the new double quasar CTQ 839. This B = 18.3,
radio quiet quasar pair is separated by 2.1" in BRIH filters with magnitude
differences of delta m_B = 2.5, delta m_R = delta m_I = 1.9, and delta m_H =
2.3. Spectral observations reveal both components to be z = 2.24 quasars, with
relative redshifts that agree at the 100 km/s level, but exhibit pronounced
differences in the equivalent widths of related emission features, as well as
an enhancement of blue continuum flux in the brighter component longward of the
Ly alpha emission feature. In general, similar redshift double quasars can be
the result of a physical binary pair, or a single quasar multiply imaged by
gravitational lensing. Empirical PSF subtraction of R and H band images of CTQ
839 reveal no indication of a lensing galaxy, and place a detection limit of R
= 22.5 and H = 17.4 for a third component in the system. For an Einstein-de
Sitter cosmology and SIS model, the R band detection limit constrains the
characteristics of any lensing galaxy to z_lens >= 1 with a corresponding
luminosity of L >~ 5 L_*, while an analysis based on the redshift probability
distribution for the lensing galaxy argues against the existence of a z_lens >~
1 lens at the 2 sigma level. A similar analysis for a Lambda dominated
cosmology, however, does not significantly constrain the existence of any
lensing galaxy. The broadband flux differences, spectral dissimilarities, and
failure to detect a lensing galaxy make the lensing hypothesis for CTQ 839
unlikely. The similar redshifts of the two components would then argue for a
physical quasar binary. At a projected separation of 8.3/h kpc (Omega_matter =
1), CTQ 839 would be the smallest projected separation binary quasar currently
known.


title: Gas Rich Dwarf Spheroidals

abstract:
  We present evidence that nearly half of the dwarf spheroidal galaxies (dSph
and dSph/dIrr) in the Local Group are associated with large reservoirs of
atomic gas, in some cases larger than the stellar mass. The gas is sometimes
found at large distance (~10 kpc) from the center of a galaxy and is not
necessarily centered on it. Similarly large quantities of ionized gas could be
hidden in these systems as well. The properties of some of the gas reservoirs
are similar to the median properties of the High-Velocity Clouds (HVCs); two of
the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf
spheroidals might thus provide a link between the HVCs and stars. We show that
the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if
the galaxy is within 250 kpc of either the Milky Way or M31. This can be
explained if both galaxies have a sufficiently massive x-ray emitting halo that
produces ram-pressure stripping if a dwarf ventures too close to either giant
spiral. We also investigate tidal stripping of the dwarf galaxies and find that
although it may play a role, it cannot explain the apparent total absence of
neutral gas in most dSph galaxies at distances less than 250 kpc. For the
derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping
is found to be more than an order of magnitude more effective in removing the
gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar
masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs
or true dwarf galaxies similar to those we observe now.


title:
X-ray Measurements of the Gravitational Potential Profile in the Central
  Region of the Abell 1060 Cluster of Galaxies

abstract:
  X-ray spectral and imaging data from ASCA and ROSAT were used to measure the
total mass profile in the central region of Abell 1060, a nearby and relatively
poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the
spatial response of the X-ray telescope, show an isothermal distribution of the
intra-cluster medium (ICM) within at least $\sim$ 12' (or $160h_{70}^{-1}$ kpc;
$H_0 = 70 h_{70}$ km s$^{-1}$Mpc$^{-1}$) in radius of the cluster center. The
azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a
central excess above an isothermal $\beta$ model. The ring-sorted ASCA GIS
spectra and the radial surface brightness distribution from the ROSAT PSPC were
simultaneously utilized to constrain the gravitational potential profile. Some
analytic models of the total mass density profile were examined. The ICM
density profile was also specified by analytic forms. The ICM temperature
distribution was constrained to satisfy the hydrostatic equilibrium, and to be
consistent with the data. Then, the total mass distribution was found to be
described better by the universal dark halo profile proposed by Navarro, Frenk,
and White (1996;1997) than by a King-type model with a flat density core. A
profile with a central cusp together with a logarithmic radial slope of $\sim
1.5$ was also consistent with the data. Discussions are made concerning the
estimated dark matter distribution around the cluster center.


title: The Chemical Composition of Carbon Stars II: The J-type stars

abstract:
  Abundances of Li, heavy elements and carbon isotope ratios have been measured
in 12 J-type galactic carbon stars. The abundance analysis shows that in these
stars the abundances of s-process elements with respect to the metallicity are
nearly normal. Tc is not present in most of them, although upper limits have
been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type
carbon stars. The Rb abundances, obtained from the resonance 7800 {\AA} Rb I
line, are surprisingly low, probably due to strong non-LTE effects in the
formation of this line in cool carbon-rich stars. Lithium and $^{13}$C are
found to be enhanced in all the stars. These results together with the nitrogen
abundances and oxygen isotope ratios measured by Lambert et al. (1986) and
Harris et al. (1987) are used to discuss the origin of J-stars. The luminosity
and variability class of the stars studied would indicate that they are low
mass (M$\leq 2-3$ M$_\odot$), less evolved objects than the normal carbon
stars, although the presence of some luminous (M$_{bol}<-5.5$) J-stars in our
galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence
of at least two types of J-stars, with different formation scenarios depending
upon the initial mass of the parent star. Standard evolutionary AGB models are
difficult to reconcile with all the observed chemical characteristics. In fact,
they suggest the existence of an extra-mixing mechanism which transports
material from the convective envelope down to hotter regions where some nuclear
burning occurs. This mechanism would act preferably on the early-AGB phase in
low-mass stars. Mixing at the He-core flash and the binary system hypothesis
are also discussed as alternatives to the above scenario.


title: The Lake Baikal neutrino experiment

abstract:
  We rewiew the present status of the Baikal Neutrino Project and present the
results of a search for high energy neutrinos with the detector intermediate
stage NT-96.