title: Abundances and Evolution of Lithium in the Galactic Halo and Disk

abstract:
  We have measured the Li abundance of 18 stars with -2 <~ [Fe/H] ~< -1 and
6000 K <~ Teff <~ 6400 K, a parameter range that was poorly represented in
previous studies. We examine the Galactic chemical evolution (GCE) of this
element, combining these data with previous samples of turnoff stars over the
full range of halo metallicities. We find that A(Li) increases from a level of
\~2.10 at [Fe/H] = -3.5, to ~2.40 at [Fe/H] = -1.0, where A(Li) = log_10
(n(Li)/n(H)) + 12.00.
  We compare the observations with several GCE calculations, including existing
one-zone models, and a new model developed in the framework of inhomogeneous
evolution of the Galactic halo. We show that Li evolved at a constant rate
relative to iron throughout the halo and old-disk epochs, but that during the
formation of young-disk stars, the production of Li relative to iron increased
significantly. These observations can be understood in the context of models in
which post-primordial Li evolution during the halo and old-disk epochs is
dominated by Galactic cosmic ray fusion and spallation reactions, with some
contribution from the $\nu$-process in supernovae. The onset of more efficient
Li production (relative to iron) in the young disk coincides with the
appearance of Li from novae and AGB stars. The major challenge facing the
models is to reconcile the mild evolution of Li during the halo and old-disk
phases with the more efficient production (relative to iron) at [Fe/H] > -0.5.
We speculate that cool-bottom processing (production) of Li in low-mass stars
may provide an important late-appearing source of Li, without attendant Fe
production, that might explain the Li production in the young disk.


title:
Spectral distortion of cosmic background radiation by scattering on hot
  electrons. Exact calculations

abstract:
  The spectral distortion of the cosmic background radiation produced by the
inverse Compton scattering on hot electrons in clusters of galaxies (thermal
Sunyaev-Zeldovich effect) is calculated for arbitrary optical depth and
electron temperature. The distortion is found by a numerical solution of the
exact Boltzmann equation for the photon distribution function. In the limit of
small optical depth and low electron temperature our results confirm the
previous analyses. In the opposite limits, our method is the only one that
permits to make accurate calculations.


title: Ultra-Lithium-Deficient Halo Stars and Blue Stragglers: A Common Origin?

abstract:
  We present data for four ultra-Li-deficient, warm, halo stars. The Li
deficiency of two of these is a new discovery. Three of the four stars have
effective temperatures Teff ~ 6300 K, in contrast to previously known
Li-deficient halo stars which spanned the temperature range of the Spite
Plateau. In this paper we propose that these, and previously known
ultra-Li-deficient halo stars, may have had their surface lithium abundances
reduced by the same mechanism as produces halo field blue stragglers. Even
though these stars have yet to reveal themselves as blue stragglers, they might
be regarded as "blue-stragglers-to-be." In our proposed scenario, the surface
abundance of Li in these stars could be destroyed (a) during the normal
pre-main-sequence single star evolution of their low mass precursors, (b)
during the post-main-sequence evolution of a evolved mass donor, and/or (c) via
mixing during a mass-transfer event or stellar merger. The warmest Li-deficient
stars at the turnoff would be regarded as emerging "canonical" blue stragglers,
whereas cooler ones represent sub-turnoff-mass "blue-stragglers-to-be." The
latter are presently hidden on the main sequence, Li depletion being possibly
the clearest signature of their past history and future significance.
Eventually, the main sequence turnoff will reach down to their mass, exposing
those Li-depleted stars as canonical blue stragglers when normal stars of that
mass evolve away. Arguing against this unified view is the observation that the
three Li-depleted stars at Teff ~ 6300 K are all binaries, whereas very few of
the cooler systems show evidence for binarity; it is thus possible that two
separate mechanisms are responsible for the production of Li-deficient
main-sequence halo stars.


title: The autocorrelation function of the soft X-ray background

abstract:
  The first positive detection of the X-ray background fluctuations at small
angular scales is reported. ROSAT PSPC archive pointed observations are used to
measure fluctuations at scales of 0.03 - 0.4 deg. The pointings have been
selected from an area free from galactic contamination. At separations below
0.1 deg clusters of galaxies become a substantial source of the background
fluctuations. The autocorrelation function of the fluctuations in the power law
approximation has a slope of ~1 for all the data but is substantially flatter
(with slope of ~0.7) when pointings containing bright clusters are removed. At
separations 0.3 - 0.4 deg where the ACF estimates based on the ROSAT pointings
and All-Sky Survey are available, both data sets give consistent results.


title: Constraining non standard recombination: A worked example

abstract:
  We fit the BOOMERANG, MAXIMA and COBE/DMR measurements of the cosmic
microwave background anisotropy in spatially flat cosmological models where
departures from standard recombination of the primeval plasma are parametrized
through a change in the fine structure constant $\alpha$ compared to its
present value. In addition to $\alpha$ we vary the baryon and dark matter
densities, the spectral index of scalar fluctuations, and the Hubble constant.
Within the class of models considered, the lack of a prominent second acoustic
peak in the measured spectrum can be accomodated either by a relatively large
baryon density, by a tilt towards the red in the spectrum of density
fluctuations, or by a delay in the time at which neutral hydrogen formed. The
ratio between the second and first peak decreases by around 25% either if the
baryon density $\Omega_bh^2$ is increased or the spectral index $n$ decreased
by a comparable amount, or if neutral hydrogen formed at a redshift $z_*$ about
15% smaller than its standard value. We find that the present data is best
fitted by a delay in recombination, with a lower baryon density than the best
fit if recombination is standard. Our best fit model has $z_*= 900$,
$\Omega_bh^2=0.024$, $\Omega_mh^2=0.14$, $H_0=49$ and $n=1.02$. Compatible with
present data at 95% confidence level $780< z_*<1150$,
$0.018<\Omega_bh^2<0.036$, $0.07< \Omega_m h^2 < 0.3$ and $0.9<n<1.2$.


title:
First XMM-Newton observations of a cataclysmic variable II: the X-ray
  spectrum of OY Car

abstract:
  We present XMM-Newton X-ray spectra of the disc accreting cataclysmic
variable OY Car, which were obtained during the performance verification phase
of the mission. These data were taken 4 days after a short outburst. In the
EPIC spectra we find strong iron Kalpha emission with weaker iron Kbeta
emission together with silicon and sulphur lines. The spectra are best fitted
with a three temperature plasma model with a partial covering absorber.
Multiple temperature emission is confirmed by the emission lines seen in the
RGS spectrum and the H/He like intensity ratio for iron and sulphur which imply
temperatures of ~7keV and ~3keV respectively.


title: Cepheid Mass-Luminosity Relations from the Magellanic Clouds

abstract:
  The OGLE data base is used in conjunction with Kurucz atmosphere models to
generate sets of period, effective temperature and luminosity for fundamental
and overtone Magellanic Cloud Cepheids. The Florida pulsation code (with linear
turbulent convection) is then used to compute masses for these stars, assuming
an average composition of ($X$=0.716, $Z$=0.010) for the LMC and of ($X$=0.726,
$Z$=0.004) for the SMC. The average $M$--$L$ relation for the fundamental
Cepheids matches closely that for the first overtone Cepheids for each
Magellanic Cloud. Neither the SMC nor the LMC average $\Log M$--$\Log L$
relations are straight, but have a noticeable curvature.
  In view of the uncertainties in distance and reddening we have adopted three
different choices for these quantities. The results based on the 'long'
distance scale to the clouds give a better agreement between theory and and
observations than the 'short' one.
  All the current evolutionary tracks predict systematically larger masses for
given luminosities than our observationally derived ones, especially at the
high end. Moreover, our study confirms that the evolutionary tracks of the low
mass stars in SMC are not in agreement with the observations as they do not
extend sufficiently blueward and do not penetrate deep enough into the
instability strip, or not at all.
  The inference of masses directly from the observational database yields a
novel and strong constraint on evolutionary calculations.


title: INT-WFS + 2dFGRS: The Local Space & Luminosity Density

abstract:
  We discuss the quantification of the local galaxy population and the impact
of the ``New Era of Wide-Field Astronomy'' on this field, and, in particular,
systematic errors in the measurement of the Luminosity Function. New results
from the 2dFGRS are shown in which some of these selection effects have been
removed. We introduce an INT-WFS project which will further reduce the
selection biases. We show that there is a correlation between the surface
brightness and the luminosity of galaxies and that new technologies are having
a big impact on this field. Finally selection criteria from different surveys
are modelled and it is shown that some of the major selection effects are
surface brightness selection effects.


title:
The radio luminosity function from the low-frequency 3CRR, 6CE & 7CRS
  complete samples

abstract:
  We measure the radio luminosity function (RLF) of steep-spectrum radio
sources using three redshift surveys of flux-limited samples selected at low
(151 & 178 MHz) radio frequency, low-frequency source counts and the local RLF.
The redshift surveys used are the new 7C Redshift Survey (7CRS) and the
brighter 3CRR and 6CE surveys totalling 356 sources with virtually complete
redshift information. This yields unprecedented coverage of the radio
luminosity versus z plane for steep-spectrum sources, and hence the most
accurate measurements of the steep-spectrum RLF yet made. We find that a simple
dual-population model for the RLF fits the data well, requiring differential
density evolution (with z) for the two populations. The low-luminosity
population can be associated with radio galaxies with weak emission lines, and
includes sources with both FRI and FRII radio structures; its comoving space
density $\rho$ rises by about one dex between z~0 and z~1 but cannot yet be
meaningfully constrained at higher redshifts. The high-luminosity population
can be associated with FRII radio galaxies and quasars with strong emission
lines; its $\rho$ rises by nearly three dex between z~0 and z~2. These results
mirror the situation seen in X-ray and optically-selected AGN. The integrated
radio luminosity density of the combination of the two populations is
controlled by the value of $\rho$ at the low-luminosity end of the RLF of the
high-luminosity population, a quantity which has been directly measured at z~1
by the 7CRS. We argue that robust determination of this quantity at higher
redshifts requires a new redshift survey based on a large (~1000 source) sample
about five times fainter than the 7CRS.


title: A Search for Wolf-Rayet Stars in the Small Magellanic Cloud

abstract:
  We conducted an extensive search for Wolf-Rayet stars (W-Rs) in the SMC,
using the same interference filter imaging techniques that have proved
successful in finding W-Rs in more distant members of the Local Group.
Photometry of some 1.6 million stellar images resulted in some 20 good
candidates, which we then examined spectroscopically. Two of these indeed
proved to be newly found W-Rs, bringing the total known in the SMC from 9 to
11. Other finds included previously unknown Of-type stars (one as early as
O5f?p)),the recovery of the Luminous Blue Variable S18, and the discovery of a
previously unknown SMC symbiotic star. More important, however, is the fact
that there does not exist a significant number of W-Rs waiting to be discovered
in the SMC. The number of W-Rs in the SMC is a factor of 3 lower than in the
LMC (per unit luminosity), and we argue this is the result of the SMC's low
metallicity on the evolution of the most massive stars.


title: The impact of spanish astrophysics in the 1990's

abstract:
  Spanish astronomy has grown rapidly during the 1990s. The number of papers
published per year has doubled, to about 4% of the world total. The number of
high-impact papers, however, remains constant at about 1% of the world total.
Papers from France, Germany and Italy also have lower impact than their numbers
suggest, while for the US, UK, Japan, Netherlands and Australia, the fraction
of high-impact papers is approximately the same as the fraction of all papers.
We suggest a few possible explanations for this difference.


title: Ho from Type Ia Supernovae

abstract:
  The Hubble diagrams in B, V, and I of a complete sample of 35 SNeIa with
(B-V) < 0.06 and 1200 < v < 30000 kms^{-1} have a scatter of only 0.1 mag,
after small corrections are applied for differences in decline rate Delta
m_{15} and color (B-V). The tightness of the Hubble diagrams proves blue SNeIa
to be the best ``standard candles'' known. Their absolute magnitudes M_{B,V,I}
are calibrated by eight SNeIa with Cepheid distances from HST. Combining this
calibration with the appropriate Hubble diagrams yields a large-scale value of
H_0 = 58.5+/-6.3 at the 90-percent confidence level.
  The Hubble diagram of SNeIa has so small scatter that it seems feasable to
determine Lambda ``locally'', i.e. within z<0.12, once 100-200 SNeIa with good
photometry will be available. Such a local determination would minimize
evolutionary effects and K-term corrections.


title: XMM-Newton observations of the cluster of galaxies Sersic 159-03

abstract:
  The cluster of galaxies Sersic 159-03 was observed with the XMM-Newton X-ray
observatory as part of the Guaranteed Time program. X-ray spectra taken with
the EPIC and RGS instruments show no evidence for the strong cooling flow
derived from previous X-ray observations. There is a significant lack of cool
gas below 1.5 keV as compared to standard isobaric cooling flow models. While
the oxygen is distributed more or less uniformly over the cluster, iron shows a
strong concentration in the center of the cluster, slightly offset from the
brightness center but within the central cD galaxy. This points to enhanced
type Ia supernova activity in the center of the cluster. There is also an
elongated iron-rich structure extending to the east of the cluster, showing the
inhomogeneity of the iron distribution. Finally, the temperature drops rapidly
beyond 4 arcmin from the cluster center.


title:
Separating Thermal and Nonthermal X-Rays in Supernova Remnants I: Total
  Fits to SN 1006 AD

abstract:
  The remnant of SN 1006 has an X-ray spectrum dominated by nonthermal
emission, and pre-ASCA observations were well described by a synchrotron
calculation with electron energies limited by escape. We describe the results
of a much more stringent test: fitting spatially integrated ASCA GIS (0.6-8
keV) and RXTE PCA (3-10 keV) data with a combination of the synchrotron model
SRESC newly ported to XSPEC and a new thermal shock model VPSHOCK. The new
model can describe the continuum emission above 2 keV well, in both spatial
distribution and spectrum. We find that the emission is dominantly nonthermal,
with a small but noticeable thermal component: Mg and Si are clearly visible in
the integrated spectrum. The synchrotron component rolls off smoothly from the
extrapolated radio spectrum, with a characteristic rolloff frequency of 3.1E17
Hz, at which the spectrum has dropped about a factor of 6 below a powerlaw
extrapolation from the radio spectrum. Comparison of TeV observations with new
TeV model images and spectra based on the X-ray model fits gives a mean
post-shock magnetic field strength of about 9 microGauss, implying (for a
compression ratio of 4) an upstream magnetic field of 3 microGauss, and fixing
the current energy content in relativistic electrons at about 7E48 ergs,
resulting in a current electron-acceleration efficiency of about 5%. This total
energy is about 100 times the energy in the magnetic field. Our results
indicate that joint thermal and nonthermal fitting, using sophisticated models,
will be required for analysis of most supernova-remnant X-ray data in the
future.


title:
The Nature of the Radiative Hydrodynamic Instabilities in Radiatively
  Supported Thomson Atmospheres

abstract:
  Atmospheres having a significant radiative support are shown to be
intrinsically unstable at luminosities above a critical fraction Gamma_crit ~
0.5-0.85 of the Eddington limit, with the exact value depending on the boundary
conditions. Two different types of absolute radiation-hydrodynamic
instabilities of acoustic waves are found to take place even in the electron
scattering dominated limit. Both instabilities grow over dynamical time scales
and both operate on non radial modes. One is stationary and arises only after
the effects of the boundary conditions are taken into account, while the second
is a propagating wave and is insensitive to the boundary conditions. Although a
significant wind can be generated by these instabilities even below the
classical Eddington luminosity limit, quasi-stable configurations can exist
beyond the Eddington limit due to the generally reduced effective opacity.
  The study is done using a rigorous numerical linear analysis of a gray plane
parallel atmosphere under the Eddington approximation. We also present more
simplified analytical explanations.


title: Super Eddington Atmospheres and their Winds

abstract:
  We present a model for the steady state winds of super-Eddington systems.
These radiatively driven winds are expected to be optically thick and clumpy as
they arise from a porous atmosphere. The model is then used to derive the mass
loss of bright classical novae. The long duration super-Eddington outflows that
are clearly observed in at least two cases (Nova LMC 1988 #1 & Nova FH Ser) are
naturally explained. Moreover, the predicted mass loss and temperature
evolution agree nicely with the observations, as do additional features.
eta-Car is then used to double check the theory which predicts the observed
mass shed in the great eruption.


title:
An Imaging and Spectroscopic Study of the z=3.38639 Damped Lyman Alpha
  System in Q0201+1120: Clues to Star Formation Rate at High Redshift

abstract:
  We present the results of a series of imaging and spectroscopic observations
aimed at identifying and studying the galaxy responsible for the z = 3.38639
damped lya system in the z = 3.61 QSO Q0201+1120. We find that the DLA is part
of a concentration of matter which includes at least four galaxies (probably
many more) over linear comoving dimensions, greater than 5h^-1Mpc. The absorber
may be a 0.7 L* galaxy at an impact parameter of 15 h^-1 kpc, but follow-up
spectroscopy is still required for positive identification. The gas is
turbulent, with many absorption components distributed over approximately 270
km/s and a large spin temperature, T_s greater than 4000K. The metallicity is
relatively high for this redshift, Z(DLA) approximately 1/20 Z(solar). From
consideration of the relative ratios of elements which have different
nucleosynthetic timescales, it would appear that the last major episode of star
formation in this DLA occurred at z greater than 4.3, more than approximately
  500 Myr prior to the time when we observe it.


title: Irradiance or luminosity changes?

abstract:
  Whereas a variation of the solar luminosity, L, will inevitably cause a
similar change of the total solar irradiance, S, the opposite is not true. In
fact, the bulk of the days to months variations of S can be explained entirely
in terms of the passage of active regions across the solar disk. In this case,
L remains essentially unchanged.
  For the total irradiance variation observed over the solar cycle, the issue
is more uncertain. One view explains this modulation primarily as a combined
action of active regions and magnetic network. These components would be
superposed to an otherwise unchanging photosphere. the other view suggests that
the activity cycle modulation of S is primarily produced by a variation of L
(both in terms of R and T_{eff}) caused by structural reajustments of the
interior of the Sun induced by a changing magnetic field. We will present
evidence in support of this second interpretation, and a model for it. We will
also present the S variations over the last 5 centuries implied by our model.


title: BeppoSAX Observations of the Maser Sy2 Galaxy: ESO103-G35

abstract:
  We have made BeppoSAX observations of the Seyfert 2/1.9 galaxy ESO103-G35,
which contains a nuclear maser source and is known to be heavily absorbed in
the X-rays. Analysis of the X-ray spectra observed by SAX in October 1996 and
1997 yields a spectral index 0.74+/-0.07, typical of Seyfert galaxies and
consistent with earlier observations of this source. The strong, soft X-ray
absorption has column density 1.79E(23)/cm^2, again consistent with earlier
results. The best fitting spectrum is that of a power law with a high energy
cutoff at 29+/-10 keV, a cold, marginally resolved Fe Kalpha line with EW 290
eV (1996) and a mildly ionized Fe K-edge at 7.37 keV. The Kalpha line and cold
absorption are consistent with origin in a accretion disk/torus through which
our line-of-sight passes at a radial distance of $\sim 50$ pc. The Fe K-edge is
mildly ionized suggesting the presence of ionized gas probably in the inner
accretion disk, close to the central source or in a separate warm absorber. The
data quality is too low to distinguish between these possibilities but the
edge-on geometry implied by the water maser emission favors the former.
Comparison with earlier observations of ESO103-G35 shows little/no change in
spectral parameters while the flux changes by factors of a few on timescales of
a few months. The 2--10 keV flux decreased by a factor of 2.7 between Oct 1996
and Oct 1997 with no detectable change in the count rate >20 keV suggesting a
constant or delayed response reflection component. The high energy cutoff is
lower than the typical 300keV values seen in Seyfert galaxies. A significant
subset of similar sources would affect current models of the AGN contribution
to the cosmic X-ray background which generally assume a high energy cutoff of
300 keV.


title:
The Quiescent Emission Spectrum of Cen X-4 and other X-ray Transients
  containing Neutron Stars

abstract:
  We use the observed optical-UV and X-ray emission spectrum of Cen X-4 during
quiescence to constrain models for the accretion flow in this system. We argue
that the optical-UV emission is not due to an optically-thick quiescent
accretion disk, nor due to synchrotron emission from an Advection-Dominated
Accretion Flow (ADAF). Emission from the bright spot could account for the
observed optical-UV component if the mass transfer rate in Cen X-4 is >~
2.10^16 g/s. Although the presence of an ADAF around the neutron star leads to
Compton upscattering of the soft X-ray photons radiated from the stellar
surface, we find that this process alone cannot account for the power law
component seen in the quiescent X-ray spectrum of Cen X-4 and other X-ray
transients containing neutron stars; this result is independent of whether the
source of soft photons is incandescent thermal emission or accretion-powered
emission. We conclude that, in models which invoke the presence of an ADAF and
a propeller effect for the quiescence of X-ray transients containing neutron
stars, the intrinsic emission from the ADAF must contribute very little to the
optical-UV and X-ray emission observed. If these ADAF+propeller models are
correct, the X-ray power law component observed must arise from regions where
the gas impacts the neutron star surface. Variability studies could greatly
help clarify the role of the various emission mechanisms involved.


title: Status of VHE Astronomy c.2000

abstract: The status of VHE Astronomy is reviewed


title: Semi-Analytical Models for Lensing by Dark Halos: I. Splitting Angles

abstract:
  We use the semi-analytical approach to analyze gravitational lensing of
quasars by dark halos in various cold dark matter (CDM) cosmologies, in order
to determine the sensitivity of the prediction probabilities of images
separations to the input assumptions regarding halos and cosmologies. The mass
function of dark halos is assumed to be given by the Press-Schechter function.
The mass density profile of dark halos is alternatively taken to be the
singular isothermal sphere (SIS), the Navarro-Frenk-White (NFW) profile, or the
generalized NFW profile. The cosmologies include: the Einstein-de Sitter model
(SCDM), the open model (OCDM), and the flat \Lambda-model (LCDM). As expected,
we find that the lensing probability is extremely sensitive to the mass density
profile of dark halos, and somewhat less so to the mean mass density in the
universe, and the amplitude of primordial fluctuations. NFW halos are very much
less effective in producing multiple images than SIS halos. However, none of
these models can completely explain the current observations: the SIS models
predict too many large splitting lenses, while the NFW models predict too few
small splitting lenses. This indicates that there must be at least two
populations of halos in the universe. A combination of SIS and NFW halos can
reasonably reproduce the current observations if we choose the mass for the
transition from SIS to NFW to be ~ 10^{13} solar masses. Additionally, there is
a tendency for CDM models to have too much power on small scales, i.e. too much
mass concentration; and it appears that the cures proposed for other apparent
difficulties of CDM would help here as well, an example being the warm dark
matter (WDM) variant which is shown to produce large splitting lenses fewer
than the corresponding CDM model by one order of magnitude.


title: On the Local Separation of E and B Polarization Patterns in CMBR

abstract:
  The work examines the small-scale E and B patterns of the CMBR polarization
anisotropy. Particularly we address the topological natures and spatial
symmetries of the E and B modes, and how one may make use of the local
measurements of Stokes parameters to separate these two modes. The analysis of
a local map in the Fourier space for separating the E and B modes can be an
ill-defined problem, due to the non-periodic boundary condition in the map for
each mode. A strategy for the E-B separation in a local map can be through an
appropriate projection of the polarization tensor field into a unique vector
field, which naturally contains a curl-free E component and a divergence-free B
component. An integral method, equivalent to the real-space, top-hat filtering,
is proposed for extracting the smoothed "charge" and "current" of the E and B
modes separately. The top-hat filter function exhibits, in its power spectrum,
an oscillation of a well-defined frequency comparable to the filter size. It is
pointed out that when the inverse filter size is chosen properly, matching the
oscillation period in the E-mode power spectrum, the sensitivity for the E-mode
detection can be made to be significantly improved.


title:
UVES observations of QSO 0000-2620: Argon and Phosphorus abundances in
  the dust-free damped Ly-alpha system at zabs = 3.3901

abstract:
  The UV resonance transitions of neutral argon ArI 1066 A, and of singly
ionized phosphorus PII 963 A, originated in the damped Ly_alpha system (DLA) at
zabs = 3.3901 towards QSO 0000--2620 have been detected by means of the UVES
spectrograph at the 8.2m ESO KUEYEN telescope. So far, this is the first
measurement of ArI, and the second of PII, ever performed in damped galaxies
and in high redshift objects. This DLA is well known for having one of the
lowest metal abundances and dust content, and the lowest fractional abundance
of molecular hydrogen H_2. The measured Ar abundance is [Ar/H] = - 1.91 (+/-
0.09) which is equal to the abundances of the other alpha-chain elements (O, S
and Si). The similarity of the Ar abundance with the other alpha-chain elements
implies the absence of significant photoionization by either UV background or
stellar sources along the sightline throughout the damped Ly_alpha system. Both
log(Ar/O) and log(Ar/S) ratios are found close to those measured in the
extragalactic HII regions and in blue compact galaxies where O is more abundant
by at least one order of magnitude. This strengthens the universality of the
Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The
abundance of the non-refractory element phosphorus [P/H] = - 2.31 (+/- 0.10)
confirms the low amount of chemical evolution in the DLA. This is the
measurement of P in the most metal-poor material and shows a subsolar [P/Fe] =
--0.27 value. The measured ratios [P/Si] = - 0.40 (+/- 0.13) and [P/S] = - 0.33
(+/- 0.13) provide evidence for a mild odd-even effect. Finally, a stringent
upper limit to the population of the 3P_1 level in the ground state of OI is
derived, which provides a lower limit to the physical dimensions of the zabs =
3.3901 system of L > 7 pc.


title: The NGC4839 group falling into the Coma cluster observed by XMM-Newton

abstract:
  We present here the first analysis of the XMM Newton EPIC-MOS data of the
galaxy group around NGC4839, which lies at a projected distance to the Coma
cluster center of 1.6Mpc. In our analysis, which includes imaging,
spectro-imaging and spectroscopy we find compelling evidence for the sub group
being on its first infall onto the Coma cluster. The complex temperature
structure around NGC 4839 is consistent with simulations of galaxies falling
into a cluster environment. We see indications of a bow shock and of ram
pressure stripping around NGC4839. Furthermore our data reveal a displacement
between NGC4839 and the center of the hot gas in the group of about 300kpc.
With a simple approximation we can explain this displacement by the pressure
force originating from the infall, which acts much stronger on the group gas
than on the galaxies.


title: Limits on Collisional Dark Matter from Elliptical Galaxies in Clusters

abstract:
  The dynamical evolution of galaxies in clusters is modified if dark matter is
self-interacting. Heat conduction from the hot cluster halo leads to
evaporation of the relatively cooler galactic halos. The stellar distribution
would adiabatically expand as it readjusts to the loss of dark matter, reducing
the velocity dispersion and increasing the half-light radius. If the dark
matter content within that radius was f_dm = 25-50% of the total, as indicated
by current observations, the ellipticals in clusters would be offset from the
fundamental plane relation beyond the observational scatter. The requirement
that their halos survive for a Hubble time appears to exclude just that range
of the dark matter cross-section, 0.3 < sigma/m < 10^4 cm^2/g, thought to be
optimal for reducing central halo cusps, unless f_dm < 15%. If the
cross-section is allowed to vary with the relative velocity of dark matter
particles, sigma \propto v^{-2 delta}, a new problem of evaporation of dark
matter arises in the dwarf galaxies with low velocity dispersion. The halos of
large galaxies in clusters and dwarf galaxies in the Local Group can both
survive only if delta < 1.1 or delta > 1.8. In either case the problem of
central density cusps remains.


title: Spatial Structure and Periodicity in the Universe

abstract:
  We analyze the possibility that the spatial periodicity of 128 h^{-1}Mpc in
the galaxy count number observed recently in deep pencil-beam surveys could be
explained by the mere existence of structure with the appropriate scale in the
galaxy distribution in the Universe. We simulate a universe where the
distribution of galaxies has an intrinsic length scale and then investigate the
probability of observing in a given direction a spatial periodicity similar to
that observed in the pencil-beam surveys of the galactic polar regions. A
statistical analysis shows that this probability is of the order of 10^{-8}, a
value which excludes the afore mentioned explanation. This result contrasts
with the estimates based on Voronoi foam models which yield probabilities of
the order 10^{-1} - 10^{-2}. The conclusion that emerges from these studies put
together with the present one, is that although they can be obtained in some
models, the presence of an intrinsic scale in the structure is not sufficient
to achieve reasonable probabilities for observation of the detected
periodicities.


title:
The Red Stellar Contents of Three Selected Fields of The Local Group
  Dwarf Irregular Galaxy IC1613

abstract:
  We present a moderately-deep JK photometry for three selected areas of the
dwarf irregular galaxy IC1613. The color-magnitude diagrams contain a mixture
of red supergiants, asymptotic giant branch stars and the brightest red giant
stars. The red supergiants are massive (20 - 25 M_\odot) and young -with ages
between 8 and 25 Myr. The most important result is the evidence of the
decreasing density of the intermediate age AGB population in the vicinity of
the HII regions in the galaxy. We also find age differences between AGB stars
in the main body of the galaxy and those near the HII regions in the
North-East. The former span a range in ages between 1 and 10 Gyr, while the
latter are younger than 1 Gyr. Using the period-luminosity relation derived by
Madore & Freedman (1991) and JK magnitudes of the Cepheid variable V20, we
calculated (m-M)_K = 24.37+-0.2. The recently discovered Nova (King et al.
1999) was identified in Field III. Its presence of our images and its
brightness questioned its classification as a nova.


title: Cygnus X-3 with ISO: investigating the wind

abstract:
  We observed the energetic binary Cygnus X-3 in both quiescent and flaring
states between 4 and 16 microns using the ISO satellite. We find that the
quiescent source shows the thermal free-free spectrum typical of a hot, fast
stellar wind, such as from a massive helium star. The quiescent mass-loss rate
due to a spherically symmetric, non-accelerating wind is found to be in the
range 0.4-2.9 x 10E-4 solar masses per year, consistent with other infrared and
radio observations, but considerably larger than the 10E-5 solar masses per
year deduced from both the orbital change and the X-ray column density. There
is rapid, large amplitude flaring at 4.5 and 11.5 microns at the same time as
enhanced radio and X-ray activity, with the infrared spectrum apparently
becoming flatter in the flaring state. We believe non-thermal processes are
operating, perhaps along with enhanced thermal emission.


title:
Hyper- and suspended-accretion states of rotating black holes and the
  durations of gamma-ray bursts

abstract:
  We analyze the temporal evolution of accretion onto rotating black holes
subject to large-scale magnetic torques. Wind torques alone drive a disk
towards collapse in a finite time $\sim t_{ff} E_k/E_B$, where $t_{ff}$ is the
initial free-fall time and $E_k/E_B$ is the ratio of kinetic-to-poloidal
magnetic energy. Additional spin-up torques from a rapidly rotating black hole
can arrest the disk's inflow. We associate short/long gamma-ray bursts with
hyperaccretion/suspended-accretion onto slowly/rapidly spinning black holes.
This model predicts afterglow emission from short bursts, and may be tested by
HETE-II.


title: Testing the origin of the gamma-ray background

abstract:
  Fluctuations of the gamma-ray background (GRB) are investigated. It is
assumed that the GRB is produced by discrete sources. Two basic classes of
objects as dominant contributors to the GRB are considered: a) blazars and b)
starburst galaxies. Predicted logN-logS counts are calculated for these two
models. To produce substantial fraction of the XRB, both classes of sources are
subject to strong evolution. Since blazars are several orders of magnitude more
luminous than starburst galaxies, their number required to produce the GRB is
much smaller than the number of starburst galaxies. In effect, fluctuations of
the GRB in the blazar model are relatively high. It is shown that moderate
improvement in the measurements of the GRB flux will allow us to discriminate
beteween these models.


title: J1343+3758: The third largest FRII-type radio galaxy

abstract:
  A radio source of the Fanaroff--Riley type II with the angular size of 11.3
arc min is identified with an optical galaxy at z=0.229. Thus, the projected
linear extent of the radio structure is 3.14 Mpc which makes it the third
largest classical double radio source known after 3C236 and WNB2147+816. The
high-frequency VLA observations, a galaxy identification and its optical
spectroscopy are reported. The equipartition magnetic field and energy density
in the source is calculated and compared with corresponding parameters of other
giants known, indicating that either parameter of the source investigated is
extremely low. On the other hand, the age estimate of relativistic electrons
and the advance speed of the lobe material are comparable to the respective
parameters characterizing other low-luminosity giant sources, as well as much
smaller and brighter 3CR sources. The environment analysis suggests that
J1343+3758 lies in a significantly poor region of intergalactic medium.


title: Supernovae Ia, evolution and quintessence

abstract:
  Quintessence models with a dark energy generated by pseudo Nambu-Goldstone
bosons provide a natural framework in which to test the possibility that type
Ia supernovae luminosity distance measurements are at least partially due to an
evolution of the sources, since these models can have parameter values for
which the expansion of the Universe is decelerating as well as values for which
it is accelerating, while being spatially flat in all cases and allowing for a
low density of clumped matter. The results of a recent investigation
(astro-ph/0004138) of current observational bounds which allow for SNe Ia
source evolution are discussed. It is found that models with source evolution
still favour cosmologies with an appreciable amount of acceleration in the
recent past, but that the region of parameter space which is most favoured
shifts significantly.


title: The ATESP radio survey III. Source counts

abstract:
  This paper is part of a series reporting the results of the ATESP radio
survey obtained at 1.4 GHz with the Australia Telescope Compact Array. The
survey consists of 16 radio mosaics with ~8"x14" resolution and uniform
sensitivity (1sigma noise level ~79 microJy) over the whole area of the ESO
Slice Project redshift survey (~26 sq. degrees at decl. = -40 degrees). The
ATESP survey has produced a catalogue of 2960 radio sources down to a flux
limit (6sigma) of ~0.5 mJy. In this paper we present the 1.4 GHz log N - log S
relation derived from the ATESP radio source catalogue. The possible causes of
incompleteness at the faint end of the source counts are extensively discussed
and their effects are quantified and corrected for. The ATESP counts are
consistent with others reported in the literature, even though some significant
discrepancies are present at low fluxes (below a few mJy). We investigate
whether such discrepancies may be explained in terms of field-to-field
anisotropies, considering the fact that all the existing sub-mJy surveys cover
small areas of sky (from a fraction of square degree to a few square degrees).
We stress that the ATESP survey, covering 26 sq. degrees, provides the best
determination of source counts available today in the flux range 0.7 <
S(1.4GHz) < 2 mJy.


title:
A sample of 6C radio sources with virtually complete redshifts. II -
  optical spectroscopy

abstract:
  This is the second of two papers presenting basic observational data on the
6CE sample of extragalactic radio sources. It presents the results of optical
spectroscopy which has yielded virtually complete redshift information for the
6CE sample: 56 of the 59 sample members have spectroscopic redshifts which,
with the exception of seven cases, are secure. The redshift distribution N(z)
is fairly flat over the redshift range 0 < z < 2, with a median redshift of 1.1
and a high-redshift tail reaching to z = 3.4. The highest-redshift (z > 1.75)
members of the 6CE sample have similar optical spectra and a tight (less than
one dex) spread in narrow-Lyalpha emission line luminosity.


title:
Spectrophotometry and period analysis of the sdB eclipsing binary HW
  Virginis

abstract:
  New CCD observations of the pre-cataclysmic binary HW Virginis are presented
and discussed. The R-filtered CCD photometry was supplemented with medium-band
(Cousins VRI) spectrophotometry based on low-resolution objective-prism
spectra. The period variation is reanalysed by means of the standard O-C
technique. The new data support the conclusions of Kilkenny et al. (2000) on
the strong and continuous period change. The long-term period variation can be
described approximately with two linear branches in the O-C diagram
corresponding to a sudden period jump between two constant periods around JD
2448500 (1991). Additional smooth small-scale changes in the period distort the
linearity. The present data do not support the hypothetical light-time effect
of Cakirli & Devlen (1999). The eclipse depths in V, R, and I do not show
colour dependence, which suggests negligible continuum variations due to the
cool secondary component in the far-red region (up to 8800 AA). The magnitude
of the reflection effect was used to estimate the mean effective temperature of
the illuminated hemisphere of the secondary. The result is 13300\pm200 K or
11000\pm200 K, depending on the primary temperature (35000 K or 26000 K). An
albedo near unity is implied for the cool component.


title: FUSE Observations of the hottest DA white dwarfs

abstract:
  We report early FUSE observations of the very hot DA white dwarfs PG1342+444
and REJ0558-371. Detection of photospheric absorption lines allows us to
estimate the abundances of C, O, Si, P, S and Fe, the first measurements
reported for DA stars at such high temperatures. Values of Teff and log g
determined for PG1342+444 from the Lyman line series disagree with the results
of the standard Balmer line analysis, an issue that requires further
investigation.


title: Heavy element abundances in DA white dwarfs

abstract:
  We present a series of systematic abundance measurements for a group of hot
DA white dwarfs in the temperature range 20,000-75,000K, based on far-UV
spectroscopy with STIS on HST, IUE and FUSE. Using our latest heavy element
blanketed non-LTE stellar atmosphere calculations we have addressed the heavy
element abundance patterns for the hottest stars for the first time, showing
that they are similar to objects like G191-B2B. The abundances observed in the
cooler (<50,000K) white dwarfs are something of a mystery. Some of the patterns
(e.g. REJ1032) can be explained by self-consistent levitation-diffusion
calculations but there is then a serious difficulty in understanding the
appearance of the apparently pure H atmospheres. We also report the detection
of photospheric HeII in the atmosphere of WD2218+706.


title:
Detection of arcs in Saturn's F ring during the 1995 Sun ring-plane
  crossing

abstract:
  Observations of the November 1995 Sun crossing of the Saturn's ring-plane
made with the 3.6m CFH telescope, using the UHAO adaptive optics system, are
presented here. We report the detection of four arcs located in the vicinity of
the F ring. They can be seen one day later in HST images. The combination of
both data sets gives accurate determinations of their orbits. Semi-major axes
range from 140020 km to 140080 km, with a mean of 140060 +- 60 km. This is
about 150 km smaller than previous estimates of the F ring radius from Voyager
1 and 2 data, but close to the orbit of another arc observed at the same epoch
in HST images.


title: Chandra Observation of M84, Radio Lobe Elliptical in Virgo cluster

abstract:
  We analyzed a deep Chandra observation of M84, a bright elliptical galaxy in
the core of the Virgo cluster. We find that the spatial distribution of the
soft X-ray emission is defined by the radio structure of the galaxy. In
particular we find two low density regions associated with the radio lobes and
surrounded by higher density X-ray filaments. In addition to a central AGN and
a population of galactic sources, we find a diffuse hard source filling the
central 10 kpc region. Since the morphology of the hard source appears round
and is different from that seen in the radio or in soft X-rays, we propose that
it is hot gas heated by the central AGN. Finally, we find that the central
elemental abundance in the X-ray gas is comparable to that measured optically.


title:
Fitting BOOMERANG and MAXIMA-1 data with a Einstein-Yang-Mills
  Cosmological Model

abstract:
  We analyse the implications of recent Cosmic Microwave Background (CMB) data
for a specific cosmological model, based on the higher-dimensional
Einstein-Yang-Mills System compactified on a $R\times S^3\times S^d$ topology
and conclude that the model parameters are tightly constrained by CMB spectra.
Moreover, the model predicts a relationship between the deceleration parameter
at present, $q_0$, and some characteristic features of CMB spectra, namely the
height of the first peak and the location of the second peak, that is
consistent with the observations and which can be further tested by future CMB
and other experiments.


title: On the use of a non-extensive distribution in the solar plasma

abstract:
  We describe the physical motivations why we are urged to look for an ionic
equilibrium distribution function slightly different from the Maxwellian to
calculate nuclear reaction rates in the solar core plasma answering, at the
same time, to a recent paper by Bahcall et al. (astro-ph/0010055).


title: Precision of inflationary predictions and recent CMB anisotropy data

abstract:
  Inflationary predictions of the cosmic microwave background anisotropy are
often based on the slow-roll approximation. We study the precision of these
predictions and compare them with the recent data from BOOMERanG and MAXIMA-1.


title: IRAM observations of JVAS/CLASS gravitational lenses

abstract:
  We have searched for molecular absorption lines at millimeter wavelengths in
eleven gravitational lens systems discovered in the JVAS/CLASS surveys of flat
spectrum radio sources. Spectra of only one source 1030+074 were obtained in
the 3-, 2- and 1.3-millimeter band at the frequencies corresponding to common
molecular transitions of CO and HCO+ as continuum emission was not found in any
of the other sources. We calculated upper limits to the column density in
molecular absorption for 1030+074, using an excitation temperature of 15 K, to
be N_{CO} < 6.3 x 10^{13} cm^{-2} and N_{HCO+} < 1.3 x 10^{11} cm^{-2},
equivalent to hydrogen column density of the order N_H < 10^{18} cm^{-2},
assuming standard molecular abundances. We also present the best upper limits
of the continuum at the lower frequency for the other 10 gravitational lenses.


title: Mining the Digital Hamburg/ESO Objective Prism Survey

abstract:
  We report on the exploitation of the stellar content of the Hamburg/ESO
objective prism survey by quantitative selection methods, such as automatic
spectral classification, and first results obtained.


title:
Measurement of the magnetic field direction in the NGC2024-FIR5
  protostellar outflow

abstract:
  Molecular outflows from young protostars are widely believed to be collimated
by magnetic fields, but there has been little observational evidence to support
this hypothesis. Using the new technique of millimetre-wavelength
spectro-polarimetry, we demonstrate the existence of a magnetic field in the
NGC2024-FIR5 outflow lobe. The 1.3mm J=2-1 transition of carbon monoxide (CO)
is polarized at a level of approximately 1%, in a direction within 10-15
degrees of the outflow axis. This agrees with theoretical models where the
magnetic field channels the outflowing gas, and shows that the process can be
effective as far as 0.1pc from the protostar.


title: Population synthesis for double white dwarfs I.Close detached systems

abstract:
  We model the population of double white dwarfs in the Galaxy and find a
better agreement with observations compared to earlier studies, due to two
modifications. The first is the treatment of the first phase of unstable mass
transfer and the second the modelling of the cooling of the white dwarfs. A
satisfactory agreement with observations of the local sample of white dwarfs is
achieved if we assume that the initial binary fraction is ~ 50% and that the
lowest mass white dwarfs (M < 0.3 Msun) cool faster than the most recently
published cooling models predict. With this model we find a Galactic birth rate
of close double white dwarfs of 0.05 yr^{-1}, a birth rate of AM CVn systems of
0.005 yr^{-1}, a merger rate of pairs with a combined mass exceeding the
Chandrasekhar limit (which may be progenitors of SNe Ia) of 0.003 yr^{-1} and a
formation rate of planetary nebulae of 1 yr^{-1}. We estimate the total number
of double white dwarfs in the Galaxy as 2.5 10^8. In an observable sample with
a limiting magnitude V_lim = 15 we predict the presence of ~855 white dwarfs of
which ~220 are close pairs. Of these 10 are double CO white dwarfs of which one
has a combined mass exceeding the Chandrasekhar limit and will merge within a
Hubble time.


title: Evolution of O Abundance Relative to Fe

abstract:
  We present a three-component mixing model for the evolution of O abundance
relative to Fe, taking into account the contributions of the first very massive
(> 100 solar masses) stars formed from Big Bang debris. We show that the
observations of O and Fe abundances in metal-poor stars in the Galaxy by
Israelian et al. and Boesgaard et al. can be well represented both
qualitatively and quantitatively by this model. Under the assumption of an
initial Fe ([Fe/H] = -3) and O inventory due to the prompt production by the
first very massive stars, the data at -3 < [Fe/H] < -1 are interpreted to
result from the addition of O and Fe only from type II supernovae (SNII) to the
prompt inventory. At [Fe/H] = -1, SNII still contribute O while both SNII and
type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops
off to an asymptotic value of 0.8(O/Fe)_sun. The value of (O/Fe) for the prompt
inventory at [Fe/H] = -3 is found to be (O/Fe) = 20(O/Fe)_sun. This result
suggests that protogalaxies with low ``metallicities'' should exhibit high
values of (O/Fe). The C/O ratio produced by the first very massive stars is
expected to be much less than 1 so that all the C should be tied up as CO and
that C dust and hydrocarbon compounds should be quite rare at epochs
corresponding to [Fe/H] < -3.


title:
Can dusty Lyman break galaxies produce the submillimeter counts and
  background? Lessons from lensed Lyman break galaxies

abstract:
  Can the submillimeter counts and background be produced by applying a locally
derived extinction correction to the population of Lyman break galaxies? We
investigate the submillimeter emission of two strongly lensed Lyman break
galaxies (MS1512+36-cB58 and MS1358+62-G1) and find that the procedure that is
used to predict the submillimeter emission of the Lyman break galaxy population
overpredicts the observed 850micron fluxes by up to a factor of 14. This result
calls for caution in applying local correlations to distant galaxies. It also
shows that large extinction corrections on Lyman break galaxies should be
viewed with skepticism. It is concluded that the Lyman break galaxies may
contribute to the submillimeter background at the 25 to 50% level. The brighter
submillimeter galaxies making up the rest of the background are either not
detected in optical surveys, or if they are detected, their submillimeter
emission cannot be reliably estimated from their rest-frame ultraviolet
properties.


title: The Impact of Galaxy Formation on the Diffuse Background Radiation

abstract:
  The far infrared background is a sink for the hidden aspects of galaxy
formation. At optical wavelengths, ellipticals and spheroids are old, even at
$z \sim 1.$ Neither the luminous formation phase nor their early evolution is
seen in the visible. We infer that ellipticals and, more generally, most
spheroids must have formed in dust-shrouded starbursts. In this article, we
show how separate tracking of disk and spheroid star formation enables us to
infer that disks dominate near the peak in the cosmic star formation rate at $z
\lapproxeq 2$ and in the diffuse ultraviolet/optical/infrared background,
whereas spheroid formation dominates the submillimetre background.


title: The Two Micron All-Sky Survey: Removing the Infrared Foreground

abstract:
  We introduce the properties of the Two Micron All-Sky Survey (2MASS) survey
for IAU Symposium 204. 2MASS is a near-infrared survey of the entire sky
characterized by high reliability and completeness. Catalogs and images for 47%
of the sky are now available online. This data release has been used by Wright
(2000) and Cambresy et al. (2000) to subtract the stellar foreground at 1.25
and 2.2 microns from COBE DIRBE data, revealing the cosmological near-infrared
background.


title:
Chemical Evolution of the Stellar and Gaseous Components of Galaxies in
  Hydrodynamical Cosmological Simulations

abstract:
  We present preliminary results on the effects of mergers on the chemical
properties of galactic objects in hierarchical clustering scenarios. We adopt a
hydrodynamical chemical code that allows to describe the coupled evolution of
dark matter and baryons within a cosmological context. We found that disk-like
and spheroid-like objects have distinctive metallicity patterns that may be the
result of different evolution.


title: Determination of the orbital parameters of binary pulsars

abstract:
  We present a simple novel method for determining the orbital parameters of
binary pulsars. This method works with any sort of orbital sampling, no matter
how sparse, provided that information on the period derivatives is available
with each measurement of the rotational period of the pulsar, and it is
applicable to binary systems with nearly circular orbits. We use the technique
to precisely estimate the hitherto unknown orbital parameters of two binary
millisecond pulsars in the globular cluster 47 Tucanae, 47 Tuc S and T. The
method can also be used more generally to make first-order estimates of the
orbital parameters of binary systems using a minimal amount of data.


title: On the Initial Mass Function of Population III Stars

abstract:
  The collapse and fragmentation of filamentary primordial gas clouds are
explored using 1D and 2D hydrodynamical simulations coupled with the
nonequilibrium processes of H2 formation. The simulations show that depending
upon the initial density,there are two occasions for the fragmentation of
primordial filaments. If a filament has relatively low initial density, the
radial contraction is slow due to less effective H2 cooling. This filament
tends to fragment into dense clumps before the central density reaches
$10^{8-9}$ cm$^{-3}$, where H2 cooling by three-body reactions is effective and
the fragment mass is more massive than some tens $M_\odot$. In contrast, if a
filament is initially dense, the more effective H2 cooling with the help of
three-body reactions allows the filament to contract up to $n\sim 10^{12}$
cm$^{-3}$. After the density reaches $n\sim 10^{12}$ cm$^{-3}$, the filament
becomes optically thick to H2 lines and the radial contraction subsequently
almost stops. At this final hydrostatic stage, the fragment mass is lowered
down to $\approx 1M_\odot$ because of the high density of the filament. The
dependence of the fragment mass upon the initial density could be translated
into the dependence on the local amplitude of random Gaussian density fields or
the epoch of the collapse of a parent cloud. Hence, it is predicted that the
initial mass function of Population III stars is likely to be bimodal with
peaks of $\approx 10^2 M_\odot$ and $\approx 1M_\odot$, where the relative
heights could be a function of the collapse epoch.


title:
Timing Analysis of the Light Curve of the Dipping-Bursting X-ray Binary
  X1916-053

abstract:
  We present the timing analysis results for our observations of the x-ray dip
source X1916-053 conducted with RXTE between February and October of 1996. Our
goal was to finally measure the binary period - as either the x-ray dip period
or ~1% longer optical modulation period, thereby establishing if the binary has
a precessing disk (SU UMa model) or a third star (triple model). Combined with
historical data (1979-96), the x-ray dip period is measured to be 3000.6508
$\pm$ 0.0009 sec with a 2$\sigma$ upper limit $|\dot P| \leq 2.06 \times
10^{-11}$. From our quasi-simultaneous optical observations (May 14-23, 1996)
and historical data (1987-96), we measure the optical modulation period to be
3027.5510 $\pm$ 0.0052 sec with a 2$\sigma$ upper limit $|\dot P| \leq 2.28
\times 10^{-10}$. The two periods are therefore each stable (over all recorded
data) and require a $3.9087 \pm 0.0008$d beat period. This beat period, and
several of its harmonics is also observed as variations in the dip shape. Phase
modulation of x-ray dips, observed in a 10 consecutive day observation, is
highly correlated with the $\sim$3.9d dip shape modulation. The 1987-1996
optical observations show that the optical phase fluctuations are a factor of 3
larger than those in the x-ray. We discuss SU UMa vs. triple models to describe
the X1916-053 light curve behavior and conclude that the x-ray dip period, with
smaller phase jitter, is probably the binary period so that the required
precession is most likely similar to that observed in SU UMa and x-ray nova
systems. However the ``precession'' period stability and especially the fact
that the times of x-ray bursts may partially cluster to occur just after x-ray
dips, continue to suggest that this system may be a hierarchical triple.


title: Global VLBI Observations of Compact Radio Sources in M82

abstract:
  Observations of the starburst galaxy, M82, have been made with a 20-station
global VLBI array at $\lambda$18cm. Maps are presented of the brightest young
supernova remnants (SNR) in M82 and the wide-field mapping techniques used in
making images over a field of view of $\sim$1 arcminute with 3 milliarcsecond
resolution are discussed. A limit has been placed on the power law deceleration
of the young SNR, 43.31+592 with an index greater than 0.73 $\pm$ 0.11 from
observations with the European VLBI Network. Using the global array we have
resolved compact knots of radio emission in the source which, with future
global observations, will enable better constraints to be placed on the
expansion parameters of this SNR.
  The latest global observations have also provided high resolution images of
the most compact radio source in M82, 41.95+575. We determine an upper limit to
the radial expansion rate along the major axis of 2000 km s$^{-1}$. However,
the new images also show structure resembling that of collimated ejection which
brings into question the previous explanation of the source as being due to the
confinement of a supernova by a high density circumstellar medium.
  It is apparent that we are now able to image the brightest supernova remnants
in M82 with a linear scale which allows direct comparison with galactic SNR
such as Cassiopeia A.


title: The Reionization of the Universe by the First Stars and Quasars

abstract:
  The first light from stars and quasars ended the ``dark ages'' of the
universe and led to the reionization of hydrogen by redshift 7. Current
observations are at the threshold of probing this epoch. The study of
high-redshift sources is likely to attract major attention in observational and
theoretical cosmology over the next decade.


title:
In the Beginning: The First Sources of Light and the Reionization of the
  Universe

abstract:
  The formation of the first stars and quasars marks the transformation of the
universe from its smooth initial state to its clumpy current state. In popular
cosmological models, the first sources of light began to form at redshift 30
and reionized most of the hydrogen in the universe by redshift 7. Current
observations are at the threshold of probing the hydrogen reionization epoch.
The study of high-redshift sources is likely to attract major attention in
observational and theoretical cosmology over the next decade.


title: A Search for Jovian Planets around Hot White Dwarfs

abstract:
  Current searches for extrasolar planets have concentrated on observing the
reflex Doppler shift of solar-type stars. Little is known, however, about
planetary systems around non-solar-type stars. We suggest a new method to
extend planetary searches to hot white dwarfs. Near a hot white dwarf, the
atmosphere of a Jovian planet will be photoionized and emit hydrogen
recombination lines, which may be detected by high- dispersion spectroscopic
observations. Multi-epoch monitoring can be used to distinguish between non-LTE
stellar emission and planetary emission, and to establish the orbital
parameters of the detected planets. In the future, high-precision astrometric
measurements of the hot white dwarf will allow the masses of the detected
planets to be determined. Searches for Jovian planets around hot white dwarfs
will provide invaluable new insight on the development of planetary systems
around stars more massive than the Sun and on how stellar evolution affects
these systems. We present high-dispersion spectroscopic observations of the
white dwarf Feige 34 to demonstrate the complexity and feasibility of the
search method.


title:
The First Hour of Extra-galactic Data of the Sloan Digital Sky Survey
  Spectroscopic Commissioning: The Coma Cluster

abstract:
  On 26 May 1999, one of the Sloan Digital Sky Survey (SDSS) fiber-fed
spectrographs saw astronomical first light. This was followed by the first
spectroscopic commissioning run during the dark period of June 1999. We present
here the first hour of extra-galactic spectroscopy taken during these early
commissioning stages: an observation of the Coma cluster of galaxies. Our data
samples the Southern part of this cluster, out to a radius of 1.5degrees and
thus fully covers the NGC 4839 group. We outline in this paper the main
characteristics of the SDSS spectroscopic systems and provide redshifts and
spectral classifications for 196 Coma galaxies, of which 45 redshifts are new.
For the 151 galaxies in common with the literature, we find excellent agreement
between our redshift determinations and the published values. As part of our
analysis, we have investigated four different spectral classification
algorithms: spectral line strengths, a principal component decomposition, a
wavelet analysis and the fitting of spectral synthesis models to the data. We
find that a significant fraction (25%) of our observed Coma galaxies show signs
of recent star-formation activity and that the velocity dispersion of these
active galaxies (emission-line and post-starburst galaxies) is 30% larger than
the absorption-line galaxies. We also find no active galaxies within the
central (projected) 200 h-1 Kpc of the cluster. The spatial distribution of our
Coma active galaxies is consistent with that found at higher redshift for the
CNOC1 cluster survey. Beyond the core region, the fraction of bright active
galaxies appears to rise slowly out to the virial radius and are randomly
distributed within the cluster with no apparent correlation with the potential
merger of the NGC 4839 group. [ABRIDGED]


title: Periodic Photometric Variability in the Becklin-Neugebauer Object

abstract:
  The Becklin-Neugebauer (BN) object in the Orion Nebula Cluster (ONC) is a
well-studied optically invisible, infrared-bright young stellar object, thought
to be an intermediate-mass protostar. We report here that BN exhibited
nearly-sinusoidal periodic variability at the near-infrared H- and Ks-bands
during a one month observing campaign in 2000 March/April. The period was 8.28
days and the peak-to-peak amplitude ~0.2 mag. Plausible mechanisms for
producing the observed variability characteristics are explored.


title:
Winds and Shocks in Galaxy Clusters: Shock Acceleration on an
  Intergalactic Scale

abstract:
  We review the possible roles of large scale shocks as particle accelerators
in clusters of galaxies. Recent observational and theoretical work has
suggested that high energy charged particles may constitute a substantial
pressure component in clusters. If true that would alter the expected dynamical
evolution of clusters and increase the dynamical masses consistent with
hydrostatic equilibrium. Moderately strong shocks are probably common in
clusters, through the actions of several agents. The most obvious of these
agents include winds from galaxies undergoing intense episodes of starbursts,
active galaxies and cosmic inflows, such as accretion and cluster mergers. We
describe our own work derived from simulations of large scale structure
formation, in which we have, for the first time, explicitly included passive
components of high energy particles. We find, indeed that shocks associated
with these large scale flows can lead to nonthermal particle pressures big
enough to influence cluster dynamics. These same simulations allow us also to
compute nonthermal emissions from the clusters. Here we present resulting
predictions of gamma-ray fluxes.


title: Determining gravitational lensing effects on supernovae observations

abstract:
  In this paper we use a generalized version of a method originally proposed by
Holz and Wald to investigate the effects from gravitational lensing on Type Ia
supernovae measurements. We find that results for different mass distributions
in smooth dark matter halos are very similar, making lensing effects
predictable for a broad range of density profiles. Also, a sample of 100
supernovae at z = 1, should be sufficient to discriminate between the case of
all dark matter in smooth halos and the extreme case of all dark matter in
point-like objects.


title: Testing Fundamental Physics with High-Energy Cosmic Rays

abstract:
  Cosmic rays may provide opportunities for probing fundamental physics. For
example, ultra-high-energy cosmic rays might originate from the decays of
metastable heavy particles, and astrophysical gamma rays can be used to test
models of quantum gravity. Both scenarios offer ways to avoid the GZK cut-off.


title: The rotation curve of spiral galaxies and its cosmological implications

abstract:
  We review the topic of rotation curves of spiral galaxies emphasizing the
standard interpretation as evidence for the existence of dark matter halos.
Galaxies other than spirals and late-type dwarfs may also possess great amounts
of dark matter, and therefore ellipticals, dwarf spirals, lenticulars and polar
ring galaxies are also considered. Furthermore, other methods for determining
galactic dark matter, such as those provided by binaries, satellites or
globular clusters, have to be included. Cold dark matter hierarchical models
constitute the standard way to explain rotation curves, and thus the problem
becomes just one aspect of a more general theory explaining structure and
galaxy formation. Alternative theories also are included. In the magnetic
model, rotation curves could also be a particular aspect of the whole history
of cosmic magnetism during different epochs of the Universe. Modifications of
Newtonian Dynamics provide another interesting possibility which is discussed
here.


title:
BeppoSAX observations of Narrow-Line Seyfert 1 galaxies: II. Ionized
  iron features in Arakelian 564

abstract:
  The BeppoSAX observations of the bright Narrow-Line Seyfert 1 galaxy Ark 564
are presented along with a high quality optical spectrum taken at the 1.5m
telescope at La Silla. The 0.1-10 keV X-ray spectrum is characterized by a
strong soft component which is best described by blackbody-like emission with a
temperature of about 160 eV. At higher energies a steep (Gamma = 2.4) power-law
tail is present. There is evidence of an ionized reflector in the form of an
iron line and edge. We do not find significant evidence of soft X-ray features
if the spectrum is modelled with a two component continuum. The optical and
X-ray spectral properties support the hypothesis of a high accretion rate onto
a low mass black hole.


title:
The Quest for the Dominant Stellar Population in the Giant Elliptical
  NGC 5018

abstract:
  Newly obtained HST/WFPC2 images of the disturbed elliptical galaxy NGC 5018
show that the average amount of internal reddening due to the its complex
``dust web'' is as low as E(B-V)~0.02 within the IUE aperture, thus implying
that its observed and intrinsic energy distributions do not differ
significantly down to UV wavelengths. This, in turn, is quite relevant to the
current debate on the age of its dominant stellar population.


title:
Maximum Likelihood Estimates of the Two- and Three-Dimensional Power
  Spectra of the APM Galaxy Survey

abstract:
  We estimate the two- and three-dimensional power spectra, P_2(K) and P_3(k),
of the galaxy distribution by applying a maximum likelihood estimator to pixel
maps of the APM Galaxy Survey. The analysis provides optimal estimates of the
power spectra and of their covariance matrices if the fluctuations are assumed
to be Gaussian. Our estimates of P_2(K) and P_3(k) are in good agreement with
previous work but we find that the errors at low wavenumbers have been
underestimated in some earlier studies. If the galaxy power spectrum is assumed
to have the same shape as the mass power spectrum, then the APM maximum
likelihood P_3(k) estimates at k < 0.19 h Mpc^-1 constrain the amplitude and
shape parameter of a scale-invariant CDM model to lie within the 2 sigma ranges
0.78 < sigma_8_g < 1.18 and 0.05 < Gamma < 0.38. Using the Galactic extinction
estimates of Schlegel, Finkbeiner and Davis, we show that Galactic obscuration
has a negligible effect on galaxy clustering over most of the area of the APM
Galaxy Survey.


title: Collimating, relativistic, magnetic jets from rotating disks

abstract:
  The magnetic flux distribution is determined by the solution of the
Grad-Shafranov equation. With differential rotation, i.e. the variation of the
iso-rotation parameter, the shape of the light surface must be calculated in an
iterative way. For the first time, we have calculated the force-free magnetic
structure of truly two-dimensional, relativistic jets, anchored in a
differentially rotating disk. Such an approach allows for a direct connection
between parameters of the central source (mass, rotation) and the extension of
the radio jet. We present an analytical estimate for the jet opening angle
along the asymptotic branches of the light surface. In general, differential
rotation of the iso-rotation parameter leads to an increase of the jet opening
angle. Comparison to the M87 jet shows agreement in the collimation distance.
We derive a light cylinder radius of the M87 jet of 50 Schwarzschild radii.


title: Radial and Transverse Velocities of Nearby Galaxies

abstract:
  Analysis of the peculiar velocities of galaxies should take account of the
uncertainties in both redshifts and distances. We show how this can be done by
a numerical application of the action principle. The method is applied to an
improved catalog of the galaxies and tight systems of galaxies within
4$h_{75}^{-1}$ Mpc, supplemented with a coarser sample of the major
concentrations at 4$h_{75}^{-1}$ Mpc to 20$h_{75}^{-1}$ Mpc distance. Inclusion
of this outer zone improves the fit of the mass tracers in the inner zone to
their measured redshifts and distances, yielding best fits with reduced $\chi
^2$ in redshift and distance in the range 1.5 to 2. These solutions are based
on the assumption that the galaxies in and near the Local Group trace the mass,
and a powerful test would be provided by observations of proper motions of the
nearby galaxies. Predicted transverse galactocentric velocities of some of the
nearby galaxies are confined to rather narrow ranges of values, and are on the
order of 100 km~s$^{-1}$, large enough to be detected and tested by the
proposed SIM and GAIA satellite missions.


title:
Optical photometry of the UCM Lists I and II. II-B band surface
  photometry and morphological discussion

abstract:
  We present Johnson B surface photometry for the UCM Survey galaxies.
One-dimensional bulge-disk decomposition is attempted, discussing on fitting
functions and computational procedures. The results from this decomposition,
jointly with concentration indices and an asymmetry coefficient, are employed
to study the morphological properties of these galaxies. We also compare our
results with the previous morphological classification established using Gunn r
imaging data and with other samples of galaxies. No major statistical
differences in morphology are found between red and blue data, although some
characteristics such as size and luminosity concentration vary. We find a
correlation between luminosity and size. Several parameters are used to
segregate the objects according to their morphological type.


title: HI absorption and the ISM around radio galaxies

abstract:
  We present a study, done with the Australian LBA, of HI absorption for two
compact radio galaxies (PKS 1549-79 and PKS 1814-63). In both the radio
galaxies, the HI appears to give us information about the environment in which
the radio sources are embedded, the effect that the ISM can have on the
observed characteristics and the possible presence of interaction between the
ISM and the radio plasma.


title: Starburst activity in a ROSAT Narrow Emission-Line Galaxy

abstract:
  We present multiwaveband photometric and optical spectropolarimetric
observations of the R=15.9 narrow emission line galaxy R117_A which lies on the
edge of the error circle of the ROSAT X-ray source R117 (from McHardy et al
1998). The overall spectral energy distribution of the galaxy is well modelled
by a combination of a normal spiral galaxy and a moderate-strength burst of
star formation. The far infra-red and radio emission is extended along the
major axis of the galaxy, indicating an extended starburst.
  On positional grounds, the galaxy is a good candidate for the identification
of R117 and the observed X-ray flux is very close to what would be expected
from a starburst of the observed far infra-red and radio fluxes. Although an
obscured high redshift QSO cannot be entirely ruled out as contributing some
fraction of the X-ray flux, we find no candidates to K=20.8 within the X-ray
errorbox and so conclude that R117_A is responsible for a large fraction, if
not all, of the X-ray emission from R117.
  Searches for indicators of an obscured AGN in R117_A have so far proven
negative; deep spectropolarimetric observations show no signs of broad lines to
a limit of one per cent and, for the observed far infra-red and radio emission,
we would expect a ten times greater X-ray flux if the overall emission were
powered by an AGN. We therefore conclude that the X-ray emission from R117 is
dominated by starburst emission from the galaxy R117_A.


title:
Quasar luminosity and twin effects induced by filamentary and planar
  structures

abstract:
  We consider filamentary and planar large scale structures as possible
refraction channels for electromagnetic radiation coming from cosmological
structures. By this hypothesis, it is possible to explain the quasar luminosity
distribution and, in particular, the presence of "twin" and "brother" objects.
Method and details of simulation are given.


title: The circumnuclear X-ray reflectors in NGC 1068 and the Circinus Galaxy

abstract:
  The ASCA and BeppoSAX spectra of the Circinus Galaxy and NGC 1068 are
analysed and compared with photoionization models based on cloudy. In the case
of Circinus, a single, mildly ionized reflector can account for the line
spectrum, while in NGC 1068 at least three different reflectors (with different
ionization states) are needed. We suggest that the reflector in Circinus, and
the low ionized one in NGC1068 are the inner and visible part of the material
responsible for the X-ray absorption. With this assumption, we estimate for the
inner radius of the absorber a value of 0.2 pc for Circinus and of a few
parsecs for NGC 1068.


title:
HD172481: a super lithium-rich metal-deficient post-AGB binary with a
  red AGB companion

abstract:
  We present in this paper a study on the peculiar supergiant HD172481. Its
spectral type (F2Ia), high galactic latitude (b=-10.37), circumstellar dust,
high radial velocity and moderate metal deficiency ([Fe/H]=-0.55) confirm the
post-AGB character of this object. A detailed chemical analysis shows slight
but real s-process overabundances, however no CNO-enhancement was detected.
Furthermore, the spectral energy distribution and the TiO bands in the red part
of the spectrum reveal a red luminous companion. The luminosity ratio of the
hot F type component and this cool M type companion L(F)/L(M) is derived for a
reddening of E(B-V)=0.44 (L(F)/L(M)=1.8) and indicates that the companion must
also be strongly evolved and probably evolving along the AGB. Neither our
photometric data-set, nor our radial velocity monitoring show evidence for
orbital variability which may indicate that the period is too large for direct
binary interaction. Most interestingly, a strong lithium resonance line is
detected, which yields an abundance of log(Li)=3.6. Several explanations for
this large lithium content are explored.


title:
The Best Brown Dwarf Yet?: A Companion to the Hyades Eclipsing Binary
  V471 Tau

abstract:
  We have carried out an analysis of about 160 eclipse timings spanning over 30
years of the Hyades eclipsing binary V471 Tauri that shows a long-term
quasi-sinusoidal modulation of its observed eclipse arrival times. The O-Cs
have been analyzed for the ``light-time'' effect that arises from the
gravitational influence of a tertiary companion. The presence of a third body
causes the relative distance of the eclipsing pair to the Earth to change as it
orbits the barycenter of the triple system. The result of the analysis of the
eclipse times yields a light-time semi-amplitude of 137.2+/-12.0 s, an orbital
period of P_3 = 30.5+/-1.6 yr and an eccentricity of e_3 = 0.31+/-0.04. The
mass of the tertiary component is M_3 sin i_3 = 0.0393+/-0.0038 Mo when a total
mass of 1.61+/-0.06 Mo for V471 Tau is adopted. For orbital inclinations i_3 >
35 deg, the mass of the third body would be below the stable hydrogen burning
limit of M = 0.07 Mo and it thus would be a brown dwarf. In the next several
years (near maximum elongation), it should be feasible to obtain IR images and
spectra of V471 Tau C that, when combined with the known mass, age, distance,
and [Fe/H], will serve as a benchmark for understanding the physical properties
and evolution of brown dwarfs.


title: Extended Air Showers and Muon Interactions

abstract:
  The objective of this work is to report on the influence of muon interactions
on the development of air showers initiated by astroparticles. We make a
comparative study of the different theoretical approaches to muon
bremsstrahlung and muonic pair production interactions. A detailed algorithm
that includes all the relevant characteristics of such processes has been
implemented in the AIRES air shower simulation system. We have simulated ultra
high energy showers in different conditions in order to measure the influence
of these muonic electromagnetic interactions. We have found that during the
late stages of the shower development (well beyond the shower maximum) many
global observables are significantly modified in relative terms when the
mentioned interactions are taken into account. This is most evident in the case
of the electromagnetic component of very inclined showers. On the other hand,
our simulations indicate that the studied processes do not induce significant
changes either in the position of the shower maximum or the structure of the
shower front surface.


title: On the Mass and Inclination of the PSR J2019+2425 Binary System

abstract:
  We report on nine years of timing observations of PSR J2019+2425, a
millisecond pulsar in a wide 76.5 day orbit with a white dwarf. We measure a
significant change over time of the projected semi-major axis of the orbit,
x-dot/x=(1.3+-0.2)x10^-15 s^-1, where x=(a sin i)/c. We attribute this to the
proper motion of the binary. This constrains the inclination angle to i<72
degrees, with a median likelihood value of 63 degrees. A similar limit on
inclination angle arises from the lack of a detectable Shapiro delay signal.
These limits on inclination angle, combined with a model of the evolution of
the system, imply that the neutron star mass is at most 1.51 solar masses; the
median likelihood value is 1.33 solar masses. In addition to these timing
results, we present a polarization profile of this source. Fits of the linear
polarization position angle to the rotating vector model indicate the magnetic
axis is close to alignment with the rotation axis, alpha<30 degrees.


title:
XMM-Newton observation of an unusual iron line in the quasar Markarian
  205

abstract:
  XMM-Newton observations of the low luminosity, radio-quiet quasar Markarian
205 have revealed a unique iron K emission line profile. In marked contrast to
the broad and redshifted iron K line commonly seen in ASCA observations of
Seyfert 1 galaxies, we find that a substantial amount of the line flux in Mrk
205 occurs above the neutral line energy of 6.4 keV. Furthermore, we find that
the iron line emission has two distinct components, a narrow, unresolved
neutral line at 6.4 keV and a broadened line centred at 6.7 keV. We suggest
that the most likely origin of the 6.7 keV line is from X-ray reflection off
the surface of a highly ionised accretion disk, whilst the 6.4 keV component
may arise from neutral matter distant from the black hole, quite possibly in
the putative molecular torus. Overall this observation underlines the potential
of XMM-Newton for using the iron K line as a diagnostic of matter in the
innermost regions of AGN.


title:
Spatiotemporal fragmentation as a mechanism for different dynamical
  modes of behaviour in the solar convection zone

abstract:
  Recent analyses of the helioseismic observations indicate that the previously
observed surface torsional oscillations with periods of about 11 years extend
significantly downwards into the solar convective zone. Furthermore, there are
indications that the dynamical regimes at the base of the convection zone are
different from those observed at the top, having either significantly shorter
periods or non-periodic behaviour.
  We propose that this behaviour can be explained by the occurrence of {\it
spatiotemporal fragmentation}, a crucial feature of which is that such
behaviour can be explained solely through nonlinear spatiotemporal dynamics,
without requiring separate mechanisms with different time scales at different
depths.
  We find evidence for this mechanism in the context of a two dimensional
axisymmetric mean field dynamo model operating in a spherical shell, with a
semi-open outer boundary condition, in which the only nonlinearity is the
action of the azimuthal component of the Lorentz force of the dynamo generated
magnetic field on the solar angular velocity.


title: Anatomy of giant spiral galaxies

abstract:
  We present B and I band photometry, gas and star kinematics and 3D modelling
of 7 giant spiral galaxies. The stellar systems studied have morphological
types spanning from S0/a to Sc and absolute magnitudes from -20.6 to -22.5.
  The spectra have been collected with the spectrographs Boller & Chivens and
EFOSC2 of the 2.2m ESO-MPI telescope. Images have been taken with the same
telescope.
  The models fit simultaneously the photometric and kinematics data using a
disk+bulge tridimensional model. The distribution of luminous matter, coming
from the fit of the photometric data is compared with the distribution of total
matter derived from the velocity dispersion and velocity curves.
  The intrinsic properties of these galaxies, such as the disk/bulge mass
ratio, the total mass and the scale length of the galaxy components are
presented and discussed.


title:
Differential Rotation in Neutron Stars: Magnetic Braking and Viscous
  Damping

abstract:
  Diffferentially rotating stars can support significantly more mass in
equilibrium than nonrotating or uniformly rotating stars, according to general
relativity. The remnant of a binary neutron star merger may give rise to such a
``hypermassive'' object. While such a star may be dynamically stable against
gravitational collapse and bar formation, the radial stabilization due to
differential rotation is likely to be temporary. Magnetic braking and viscosity
combine to drive the star to uniform rotation, even if the seed magnetic field
and the viscosity are small. This process inevitably leads to delayed collapse,
which will be accompanied by a delayed gravitational wave burst and, possibly,
a gamma-ray burst. We provide a simple, Newtonian, MHD calculation of the
braking of differential rotation by magnetic fields and viscosity. The star is
idealized as a differentially rotating, infinite cylinder consisting of a
homogeneous, incompressible conducting gas. We solve analytically the simplest
case in which the gas has no viscosity and the star resides in an exterior
vacuum. We treat numerically cases in which the gas has internal viscosity and
the star is embedded in an exterior, low-density, conducting medium. Our
evolution calculations are presented to stimulate more realistic MHD
simulations in full 3+1 general relativity. They serve to identify some of the
key physical and numerical parameters, scaling behavior and competing
timescales that characterize this important process.


title:
The Giant Flare of 1998 August 27 from SGR 1900+14: I. An Interpretive
  Study of BeppoSAX and Ulysses Observations

abstract:
  The giant flare of 1998 August 27 from SGR 1900+14 was extraordinary in many
ways: it was the most intense flux of gamma rays ever detected from a source
outside our solar system; it was longer than any previously detected burst from
a soft gamma repeater (SGR) in our Galaxy by more than an order of magnitude;
and it showed a remarkable four-peaked, periodic pattern in hard X-rays with
the same rotation period that was found modulating soft X-rays from the star in
quiescence. The event was detected by several gamma-ray experiments in space,
including the Ulysses gamma-ray burst detector and the BeppoSAX Gamma Ray Burst
Monitor. These instruments operate in different energy ranges, and comparisons
of their measurements reveal complex patterns of spectral evolution as the
intensity varies. In this paper, we present a joint analysis of the BeppoSAX
and Ulysses data and discuss some implications of these results for the SGRs.
We also present newly-analyzed Venera/SIGNE and ISEE-3 data on the 1979 March 5
giant flare from an SGR in the Large Magellanic Cloud (SGR 0526-66), and
compare them with the August 27 event. Our results are consistent with the
hypothesis that giant flares are due to catastrophic magnetic instabilities in
highly magnetized neutron stars, or "magnetars".


title: Numerical simulations of turbulent dynamos

abstract:
  Using a periodic box calculation it is shown that, owing to helicity
conservation, a large scale field can only develop on a resistive timescale.
This behaviour can be reproduced by a mean-field dynamo with alpha and eta_t
quenchings that are equally strong and `catastrophic'.


title:
Balmer-Dominated Spectra of Nonradiative Shocks in the Cygnus Loop, RCW
  86 and Tycho Supernova Remnants

abstract:
  We present an observational and theoretical study of the optical emission
from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86
and Tycho. The spectra of these shocks are dominated by collisionally excited
hydrogen Balmer lines which have both a broad component caused by
proton-neutral charge exchange and a narrow component caused by excitation of
cold neutrals entering the shock. In each remnant we have obtained the broad to
narrow flux ratios of the Halpha and Hbeta lines and measured the Halpha broad
component width.
  A new numerical shock code computes the broad and narrow Balmer line emission
from nonradiative shocks in partially neutral gas. The Balmer line fluxes are
sensitive to Lyman line trapping and the degree of electron-proton temperature
equilibration. The code calculates the density, temperature and size of the
postshock ionization layer and uses a Monte Carlo simulation to compute narrow
Balmer line enhancement from Lyman line trapping. The initial fraction of the
shock energy allocated to the electrons and protons (the equilibration) is a
free parameter. Our models show that variations in electron-proton temperature
equilibration and Lyman line trapping can reproduce the observed range of broad
to narrow ratios. The results give 80%-100% equilibration in nonradiative
portions of the NE Cygnus Loop (v_shock ~ 300 km/s), 40%-50% equilibration in
nonradiative portions of RCW 86 (v_shock ~ 600 km/s) and <~ 20% equilibration
in Tycho (v_shock ~ 2000 km/s). Our results suggst an inverse correlation
between magnetosonic Mach number and equilibration in the observed remnants.


title: Modelling Self-Interacting CDM Haloes with a Cosmological Boltzmann Code

abstract:
  We investigate the density profiles and evolution of weakly self-interacting
cold dark matter haloes using a numerical code based on the collisional
Boltzmann equation. This approach is alternative to N-body techniques in
following the dynamical evolution of haloes in the cosmological context and
taking into account particle self-interaction. The physical case with a cross
section inversely proportional to the relative velocity of the colliding
particles is modelled with an unprecedented resolution, spanning five orders of
magnitude on the radius for each halo. The modelled haloes cover a mass range
from dwarf galaxies to galaxy clusters. We find that for \sigma v_{100} \approx
10^{-24} cm^2/GeV, where \sigma is the cross section per unit mass and v_{100}
is the collision velocity in units of 100 km/s, soft cores in good agreement
with observations on galactic as well as on galaxy cluster scales are obtained.
Remarkably, the observed nearly invariance of the halo central density with
mass is reproduced.


title: Clusters of Galaxies in the 2dF Galaxy Redshift Survey

abstract:
  The 2dF Galaxy Redshift Survey has obtained 135,000 redshifts for galaxies in
two broad strips. Here we present the first results of a 3-dimensional search
for galaxy clusters based on known 2-dimensional compilations. We derive new
redshifts and velocity dispersions for clusters, assess the level of
contamination in the sample, analyze the accuracy of photometric redshift
estimates and study the space distribution of clusters.


title: Magnetic Fields in Young Galaxies

abstract:
  We have studied the fate of initial magnetic fields in the hot halo gas out
of which the visible parts of galaxies form, using three-dimensional numerical
MHD-experiments. The halo gas undergoes compression by several orders of
magnitude in the subsonic cooling flow that forms the cold disk. The magnetic
field is carried along and is amplified considerably in the process, reaching
muG levels for reasonable values of the initial ratio of magnetic to thermal
energy density.


title: Formation of Proto-Globular Cluster Clouds by Thermal Instability

abstract:
  Many models of globular cluster formation assume the presence of cold dense
clouds in early universe. Here we re-examine the Fall & Rees (1985) model for
formation of proto-globular cluster clouds (PGCCs) via thermal instabilities in
a protogalactic halo. We first argue, based on the previous study by others,
that under the protogalactic environments only nonlinear density
inhomogeneities can condense into PGCCs. We then carry out numerical
simulations of the collapse of overdense clouds in one-dimensional spherical
geometry, including self-gravity and radiative cooling down to T=10^4 K. Since
imprinting of Jeans mass at 10^4 K is essential to this model, here we focus on
the cases where external UV background radiation prevents the formation of H2
molecules and so prevent the cloud from cooling below 10^4 K. The quantitative
results from these simulations can be summarized as follows: 1) Perturbations
smaller than M_min ~ (10^{5.6} M_sun) (n_h/0.05 cm3)^{-2} cool isobarically,
while perturbations larger than M_max ~ (10^8 M_sun) (n_h/0.05 cm3)^{-2} cool
isochorically. On the other hand, intermediate size perturbations (M_min<
M_pgcc < M_max) are compressed supersonically. 2) For supersonically collapsing
clouds, the density compression factor after they cool to T_c=10^4 K range
10^{2.5}-10^6. 3) For supersonically collapsing clouds the Jeans mass can be
reduced to as small as 10^{5.5} M_sun (n_h/0.05 cm3)^{-1/2} at the maximum
compression. 4) The density profile of simulated PGCCs can be approximated by a
constant core with a halo of rho ~ r^{-2} rather than a singular isothermal
sphere.


title: Faint Quasar Surveys

abstract:
  Faint quasar surveys are necessary complements to bright quasar surveys to
remove the degeneracy between redshift and luminosity inherent in any single
magnitude-limited sample. I discuss two ongoing surveys for faint quasars at
3.3<z<5 and 5<z<6 using imaging data from the Big Throughput Camera on the CTIO
4m. I also discuss a sample of faint spectroscopically selected AGN with z<4.7
found serendipitously in the CNOC2 Field Galaxy Redshift Survey. Faint quasars
at 2<z<3.5 from this sample, when combined with literature data on more
luminous quasars at the same redshift, show evidence for a much weaker Baldwin
Effect in CIV 1549 and Ly-alpha than previously seen at z<1.5. This may imply
that the slopes of the Baldwin Effect for these transitions evolve with
redshift, steepening with cosmic time. Finally I discuss the prospects for
extending faint quasar surveys to z~7 using near-IR followup of very red
objects in the Red-Sequence Cluster Survey.


title: Do "isolated" planetary mass objects orbit mirror stars?

abstract:
  We propose that the ``isolated'' planetary mass objects observed by Zapatero
Osorio et al in the $\sigma$ Orionis cluster might actually be in orbit around
invisible stellar mass companions such as mirror stars. Mirror matter is
expected to exist if parity is an unbroken symmetry of nature. Future
observations can test this idea by looking for a periodic Doppler shift in the
radiation emitted by the planets. The fact that the observations show an
inverse dependence between the abundance of the these objects and their mass
may argue in favour of the mirror matter hypothesis.


title:
X-ray measurements of the field and particle energy distributions in the
  west lobe of the radio galaxy NGC 1316 (Fornax A)

abstract:
  A follow-up X-ray study was made of the west lobe of the radio galaxy Fornax
A, (NGC 1316) based on new ASCA observations made in 1997 for 98 ks, and
incorporating the previous observation in 1994 for 39 ks. The 0.7--10 keV
spectrum of the emission can be described by a power-law of energy index 0.74
+/- 0.10, which agrees with the synchrotron radio index of 0.9 +/- 0.2.
Therefore, the X-rays are reconfirmed to arise via inverse-Compton scattering
of the cosmic microwave photons, as Kaneda et al. (1995) and Feigelson et al.
(1995) concluded. The surface brightness of the inverse-Compton X-rays exhibits
a relatively flat distribution over the west lobe, indicative of an
approximately spherical emissivity distribution with a radius of ~11' (75 kpc).
n contrast, the 1.4 GHz radio image by Ekers et al. (1983) exhibits a rim-
brightened surface brightness, consistentwith a shell-like emissivity
distribution whose inner and outer boundaries are 4' and 11', respectively.
These morphological differences between radio and X-rays suggest that the
relativistic electrons are distributed homogeneously over the lobe volume,
whereas the magnetic field is amplified toward the lobe rim region.


title: Pressure- and magnetic shear- driven instabilities in rotating MHD jets

abstract:
  We derive new stability criteria for purely MHD instabilities in rotating
jets, in the framework of the ballooning ordering expansion. Quite
unexpectedly, they involve a term which is linear in the magnetic shear. This
implies that cylindrical configurations can be destabilized by a negative
magnetic shear as well as by a favorable equilibrium pressure gradient, in
distinction with the predictions of Suydam's stability criterion, which
suggests on the contrary that the shear is always stabilizing.
  We have used these criteria to establish sufficient conditions for
instability. In particular, the magnetic shear can always destabilize jets with
vanishing current density on the axis, a feature which is generically found in
jets which are launched from an accretion disk. We also show that standard
nonrotating jet models (where the toroidal field dominates the poloidal one),
which are known to be unstable, are not stabilized by rotation, unless the
plasma $\beta$ parameter and the strength of the rotation forces are both close
to the limit allowed by the condition of radial equilibrium.
  The new magnetic shear-driven instability found in this paper, as well as the
more conventional pressure-driven instability, might provide us with a
potential energy source for the particle acceleration mechanisms underlying the
high energy emission which takes place in the interior of AGN jets.