title: The Frequency of Binary Stars in the Core of 47 Tucanae

abstract:
  Differential time series photometry has been derived for 46422 main-sequence
stars in the core of 47 Tucanae. The observations consisted of near-continuous
160-s exposures alternating between the F555W and F814W filters for 8.3 days in
1999 July with WFPC2 on the Hubble Space Telescope. Using Fourier and other
search methods, eleven detached eclipsing binaries and fifteen W UMa stars have
been discovered, plus an additional ten contact or near-contact non-eclipsing
systems. After correction for non-uniform area coverage of the survey, the
observed frequencies of detached eclipsing binaries and W UMa's within 90
arcseconds of the cluster center are 0.022% and 0.031% respectively. The
observed detached eclipsing binary frequency, the assumptions of a flat binary
distribution with log period and that the eclipsing binaries with periods
longer than about 4 days have essentially their primordial periods, imply an
overall binary frequency of 13 +/- 6 %. The observed W UMa frequency and the
additional assumptions that W UMa's have been brought to contact according to
tidal circularization and angular momentum loss theory and that the contact
binary lifetime is 10^{9} years, imply an overall binary frequency of 14 +/- 4
%. An additional 71 variables with periods from 0.4 - 10 days have been found
which are likely to be BY Draconis stars in binary systems. The radial
distribution of these stars is the same as that of the eclipsing binaries and W
UMa stars and is more centrally concentrated than average stars, but less so
than the blue straggler stars. A distinct subset of six of these stars fall in
an unexpected domain of the CMD, comprising what we propose to call red
stragglers.


title:
LANNDD---A Massive Liquid Argon Detector for Proton Decay, Supernova and
  Solar Neutrino Studies, and a Neutrino Factory Detector

abstract:
  We describe a possible Liquid Argon Neutrino and Nuclear Decay Detector
(LANNDD) that consists of a 70kt magnetized liquid argon tracking detector. The
detector is being designed for the Carlsbad Underground Laboratory.


title: Is Cold Dark Matter Still a Strong Buy? The Lesson from Galaxy Clusters

abstract:
  For the last few years the Cold Dark Matter model (ticker: CDM), has been the
dominant theory of structure formation. We briefly review the recent
advancements and predictions of the model in the field of galaxy clusters. A
new set of very high resolution simulations of galaxy clusters show that they
have (1) density profiles with central slopes very close to -1.6 and (2)
abundance of subhalos similar to the ones observed in real clusters. These
results show a remarkably small cluster to cluster variation and a weak
dependence from the particular CDM cosmology chosen (LCDM having ~40% less
substructure than SCDM). While still a speculative theory with a high
prediction/evidence ratio, subject to strong challenges from observational data
and competition from other hierarchical theories, we give CDM a rating of
``market outperform'' and of ``long term BUY''.


title: Termination of planetary accretion due to gap formation

abstract:
  The process of gap formation by a growing planetary embryo embedded in a
planetesimal disk is considered. It is shown that there exists a single
parameter characterizing this process, which represents the competition between
the gravitational influence of the embryo and planetesimal-planetesimal
scattering. For realistic assumptions about the properties of the planetesimal
disk and the planetary embryo, a gap is opened long before the embryo can
accrete all the bodies within its region of influence. The implication of this
result is that the embryo stops growing and, thus, large bodies formed during
the coagulation stage should be less massive than is usually assumed. For
conditions expected at 1 AU in the solar protoplanetary disk, gap formation is
expected to occur around bodies of mass < 10^24 g. The effect of protoplanetary
radial migration is also discussed.


title:
Solar Models with Helioseismic Constraints and the Solar Neutrino
  Problem

abstract:
  Imposing a constraint of the sound-speed profile determined from
helioseismology and updating the microphysics, we have revised our seismic
solar model, constructed with the assumption of a homogeneous metal abundance
distribution, and have shown that the theoretically expected neutrino fluxes
are still significantly more than the observations. With the same sound-speed
profile constraint, we also constructed solar models with low metal abundance
in the core, and evaluated the neutrino fluxes of these models to see if
nonstandard solar models with a low metal core can solve the solar neutrino
problem. Some of these models are in agreement with the Homestake data, the
Super-Kamiokande data, and the sound-speed profile simultaneously, but none of
these satisfy both the neutrino flux data, including GALLEX and SAGE, and the
helioseismically determined density profile.


title:
The erratic luminosity behavior of SAX J1808.4-3658 during its 2000
  outburst

abstract:
  We report on the highly variable and erratic long-term X-ray luminosity
behavior of the only known accretion-driven millisecond X-ray pulsar SAX
J1808.4-3658 during its 2000 outburst, as observed with RXTE. The maximum
observed luminosity is ~2.5 x 10^35 erg s^-1 (3-25 keV; for a distance of 2.5
kpc), which is a factor of ~10 lower than that observed during the 1996 and
1998 outbursts. Due to solar constraints, the source could not be observed for
several months with RXTE before 21 Jan. 2000. Therefore, the exact moment of
the outburst onset is unknown and the peak luminosity could have been
significantly higher. On some occasions SAX J1808.4-3658 was observed with
luminosities of ~10^35 erg s^-1, but on other occasions it could not be
detected with RXTE resulting in upper limits of a few times 10^33 erg s^-1
(3-25 keV). The non-detections of the source during its 2000 outburst obtained
with BeppoSAX demonstrate that its luminosity was at times <10^32 erg s^-1
(0.5-10 keV). However, only a few days after these BeppoSAX observations, we
detected the source again with RXTE at high luminosities, giving a factor of
>1000 of luminosity swings in this system on time scales of days. The last
detection of SAX J1808.4-3658 with RXTE was on 2000 May 13, almost 4 months
after the first detection. Due to the lack of sensitivity and observations
during the 1996 and 1998 outbursts, it cannot be excluded that after those
outbursts the source remained active for months and that the source behavior
during the 2000 outburst is not unique. Long duration activity at low
luminosities has been observed in other transients (both neutron stars and
black holes), although not with such extreme variability which might point to a
different origin for this behavior for the pulsar.


title:
COSMOCR: A Numerical Code for Cosmic Ray Studies in Computational
  Cosmology

abstract:
  We present COSMOCR, a numerical code for the investigation of cosmic ray
related studies in computational cosmology. The code follows the diffusive
shock acceleration, the mechanical and radiative energy losses and the spatial
transport of the supra-thermal particles in cosmic environment. Primary cosmic
ray electrons and ions are injected at shocks according to the thermal leakage
prescription. Secondary electrons are continuously injected as a results of p-p
inelastic collisions of primary cosmic ray ions and thermal background nuclei.
The code consists of a conservative, finite volume method with a power-law
sub-grid model in momentum space. Two slightly different schemes are
implemented depending on the stiffness of the cooling terms. Comparisons of
numerical results with analytical solution for a number of tests of direct
interest show remarkable performance of the present code.


title: Hydrogen-poor planetary nebulae

abstract:
  Five planetary nebulae are known to show hydrogen-poor material near the
central star. In the case of A58, this gas was ejected following a late thermal
pulse similar to Sakurai's Object. In this paper I will review these five
objects. One of them, IRAS 18333$-$2357, may not be a true PN. I will show that
there is a strong case for a relation to the [WC] stars and their relatives,
the weak emission-line stars. The surface abundances of the [WC] stars are
explained via diffuse overshoot into the helium layer. The hydrogen-poor PNe do
not support this: their abundances indicate a change of abundance with depth in
the helium layer. A short-lived phase of very high mass loss, the r-AGB, is
indicated. Sakurai's Object may be at the start of such a phase, and may evolve
to very low stellar temperatures.


title: Synthetic Doppler maps of gaseous flows in IP Peg

abstract:
  We present synthetic Doppler maps of gaseous flows in binary IP Peg based on
the results of 3D gasdynamical simulations. Using of gasdynamical calculations
alongside with the Doppler tomography technique permits us to identify the main
features of the flow on the Doppler maps without solution of an ill-posed
inverse problem. Comparison of synthetic tomograms with observations shows that
in quiescence there are two zones of high emission: a shock wave on the edge of
the stream from L_1 caused by the interaction of the gas of circumbinary
envelope and the stream, and the dense region in a apoastron of
quasi-elliptical accretion disk. A single arm of the spiral shock and the
stream itself give a minor input to the total brightness. During outburst the
accretion disk dominates, and the most emitive regions are the two arms of the
spiral shock.


title: Measuring large-scale structure with the 2dF Galaxy Redshift Survey

abstract:
  The 2dF Galaxy Redshift Survey is the first to measure more than 100,000
redshifts. This allows precise measurements of many of the key statistical
measures of galaxy clustering, in particular redshift-space distortions and the
large-scale power spectrum. This paper presents the current 2dFGRS results in
these areas. Redshift-space distortions are detected with a high degree of
significance, confirming the detailed Kaiser distortion from large-scale infall
velocities, and measuring the distortion parameter beta = 0.43 +/- 0.07. The
power spectrum is measured to < 10% accuracy for k > 0.02 h Mpc^-1, and is well
fitted by a CDM model with Omega_m h = 0.20 +/- 0.03 and a baryon fraction of
0.15 +/- 0.07.


title:
Spatially-resolved Thermal Continuum Absorption against the Supernova
  Remnant W49B

abstract:
  We present sub-arcminute resolution imaging of the Galactic supernova remnant
W49B at 74 MHz (25") and 327 MHz (6"), the former being the lowest frequency at
which the source has been resolved. While the 327 MHz image shows a shell-like
morphology similar to that seen at higher frequencies, the 74 MHz image is
considerably different, with the southwest region of the remnant almost
completely attenuated. The implied 74 MHz optical depth (~ 1.6) is much higher
than the intrinsic absorption levels seen inside two other relatively young
remnants, Cas A and the Crab Nebula, nor are natural variations in the
relativistic electron energy spectra expected at such levels. The geometry of
the absorption is also inconsistent with intrinsic absorption. We attribute the
absorption to extrinsic free-free absorption by a intervening cloud of thermal
electrons. Its presence has already been inferred from the low-frequency
turnover in the integrated continuum spectrum and from the detection of radio
recombination lines toward the remnant. Our observations confirm the basic
conclusions of those measurements, and our observations have resolved the
absorber into a complex of classical HII regions surrounded either partially or
fully by low-density HII gas. We identify this low-density gas as an extended
HII region envelope (EHE), whose statistical properties were inferred from low
resolution meter- and centimeter-wavelength recombination line observations.
Comparison of our radio images with HI and H_2CO observations show that the
intervening thermal gas is likely associated with neutral and molecular
material as well.


title: Phase-resolved spectra of the millisecond pulsar SAX J1808.4-3658

abstract:
  We analyze the RXTE observations of the April 1998 outburst of SAX
J1808.4-3658. We show that the soft blackbody component of the X-ray spectrum
is from the hotspot on the neutron star surface, while the hard component is
from Comptonization of these blackbody photons in the plasma heated by the
accretion shock. The disc cannot provide enough seed photons for
Comptonization. It is truncated into an optically thin flow at ~20-40 R_g by
some generic mechanisms common for black holes and neutron stars. Only below
the truncation radius the flow is disrupted by the magnetic field and
collimated onto the surface. The pulse profiles of both spectral components
imply several constraints on the geometry of the system: the inclination angle,
location and size of the hotspot and the shock.


title:
An upper mass limit for the progenitor of the TypeII-P supernova
  SN1999gi

abstract:
  Masses and progenitor evolutionary states of TypeII supernovae remain almost
unconstrained by direct observations. Only one robust observation of a
progenitor (SN1987A) and one plausible observation (SN1993J) are available.
Neither matched theoretical predictions and in this Letter we report limits on
a third progenitor (SN1999gi). The Hubble Space Telescope has imaged the site
of the TypeII-P supernova SN1999gi with the WFPC2 in two filters (F606W and
F300W) prior to explosion. The distance to the host galaxy (NGC3184) of 7.9Mpc
means that the most luminous, massive stars are resolved as single objects in
the archive images. The supernova occurred in a resolved, young OB association
2.3kpc from the centre of NGC3184 with an association age of about 4Myrs.
Follow-up images of SN1999gi with WFPC2 taken 14 months after discovery
determine the precise position of the SN on the pre-explosion frames. An upper
limit of the absolute magnitude of the progenitor is estimated (M_v >= -5.1).
By comparison with stellar evolutionary tracks this can be interpreted as a
stellar mass, and we determine an upper mass limit of 9(+3/-2)M_solar. We
discuss the possibility of determining the masses or mass limits for numerous
nearby core-collapse supernovae using the HST archive enhanced by our current
SNAP programme.


title:
A Study of Nine High-Redshift Clusters of Galaxies: IV. Photometry and
  Sp ectra of Clusters 1324+3011 and 1604+4321

abstract:
  New photometric and spectroscopic observations of galaxies in the directions
of three distant clusters are presented as part of our on-going high-redshift
cluster survey. The clusters are CL1324+3011 at z = 0.76, CL1604+4304 at z =
0.90, and CL1604+4321 at z = 0.92. The observed x-ray luminosities in these
clusters are at least a factor of 3 smaller than those observed in clusters
with similar velocity dispersions at z <= 0.4. These clusters contain a
significant population of elliptical-like galaxies, although these galaxies are
not nearly as dominant as in massive clusters at z <= 0.5. We also find a large
population of blue cluster members. Defining an active galaxy as one in which
the rest equivalent width of [OII] is greater than 15 Angstroms, the fraction
of active cluster galaxies, within the central 1.0 Mpc, is 45%. In the field
population, we find that 65% of the galaxies with redshifts between z = 0.40
and z = 0.85 are active, while the fraction is 79% for field galaxies at z >
0.85. The star formation rate normalized by the rest AB B-band magnitude, SFRN,
increases as the redshift increases at a given evolving luminosity. At a given
redshift, however, SFRN decreases linearly with increasing luminosity
indicating a remarkable insensitivity of the star formation rate to the
intrinsic luminosity of the galaxy over the range -18 >= ABB >= -22. Cluster
galaxies in the central 1 Mpc regions exhibit depressed star formation rates.
We are able to measure significant evolution in the B-band luminosity function
over the range 0.1 <= z <= 1. The characteristic luminosity increases by a
factor of 3 with increasing redshift over this range.


title:
Spatially Resolved Near-Infrared Spectroscopy of Seyfert 2 Galaxies Mk
  1066, NGC 2110, NGC 4388, and Mk 3

abstract:
  We present near-infrared spectra with resolutions of lambda/dlambda~1200 in
the emission lines of Pa-beta, [FeII] (1.2567um), Br-gamma, and H2 v=1-0S(1) of
the nuclei and circumnuclear regions of the four Seyfert 2 galaxies Mk 1066,
NGC 2110, NGC 4388, and Mk 3. All of these galaxies show strong near-infrared
line emission that is detected at radii several times the spatial resolution,
corresponding to projected physical scales of 0.07 to 0.7 kpc. Velocity
gradients are detected in these nuclei, as are spatial variations in line
profiles and flux ratios. We compare the spatial and velocity distribution of
the line emission to previously observed optical line and radio emission. The
evidence indicates that the [FeII] emission is associated with the Seyfert
activity in the galaxies. Our data are consistent with X-ray heating being
responsible for most of the [FeII] emission, although differences in [FeII] and
Pa-beta line profiles associated with radio emission suggests that the [FeII]
emission is enhanced by fast shocks associated with radio outflows. The H2
emission is not as strongly associated with outflows or ionization cones as is
the emission in other lines, but rather appears to be primarily associated with
the disk of the galaxy.


title:
The Stellar Population and Star Clusters in the Unusual Local Group
  Galaxy IC 10

abstract:
  We present analysis of HST U,V,I, and Halpha images of the peculiar Local
Group irregular galaxy IC 10. The images are used to determine the nature of
the stellar products in a portion of the recent starburst in this galaxy. We
identified 13 stellar associations and clusters, two of which are probably old
(>350 Myr) and the rest of which are young (4-30 Myr) and presumably formed in
the starburst. We found the following: 1) The slope of the stellar initial mass
function (IMF) for 6.3--18 Msolar stars formed in the starburst lies between
two limiting cases: a value of -1.9+/-0.4 under the assumption of coevality
over the past 13 Myr and of -0.9+/-0.3 under the assumption of constant star
formation over the past 40 Myr. Thus, most likely, the IMF of the intermediate
mass stars is not very unusual. The slope of the IMF for the underlying galaxy
population under the assumption of constant star formation is -2.6+/-0.3 for
4.8-18 Msolar stars. 2) The lower stellar mass limit in the starburst is <6.3
Msolar. This constraint is less than some predictions of what lower stellar
mass limits might be in starbursts, but higher than others. 3) There are two
modest-sized Halpha shells that could easily have been produced in the past few
Myr by the clusters they encircle. 4) The dominant mode of star formation in
the starburst has been that of a scaled-up OB association. This mode, with a
few compact clusters sprinkled in, is similar to the star formation that took
place in Constellation III in the LMC, as well as that in the Blue Compact
Dwarfs IZw18 and VIIZw403. The starburst in this part of IC10 has not produced
a super star cluster. We suggest that the high WC/WN ratio could be reproduced
if there were small, well-synchronized (<1 Myr), but widely scattered, pockets
of secondary star formation 3-4 Myr ago.


title: Constraining Quintessence with the New CMB Data

abstract:
  The CMB data recently released by BOOMERANG and MAXIMA suggest that the
anisotropy spectrum has a third peak in the range 800<l_3<900. A combination of
this result with constraints from large-scale structure permit us to
differentiate between different quintessence models. In particular, we find
that inverse power law models with power \alpha >1 are disfavoured. Models with
more than 5% quintessence before last scattering require a spectral index
greater than 1. These constraints are compared with supernovae observations. We
also show that the CMB alone now provides strong evidence for an accelerating
universe.


title: Merger Shocks and Nonthermal Processes in Clusters of Galaxies

abstract:
  Clusters of galaxies generally form by the gravitational merger of smaller
clusters and groups. Major cluster mergers are the most energetic events in the
Universe since the Big Bang. Mergers drive shocks into the intracluster gas,
and these shocks heat the intracluster gas, and should also accelerate
nonthermal relativistic particles. The thermal effects of merger shocks will be
briefly discussed. Mergers can increase the temperature and X-ray luminosities
of clusters, and this may affect statistical conclusions about cosmological
parameters. As a result of particle acceleration in shocks, clusters of
galaxies should contain very large populations of relativistic electrons and
ions. Electrons with Lorentz factors gamma~300 (energies E = gamma m_e c^2 ~
150 MeV) are expected to be particularly common. Observations and models for
the radio, extreme ultraviolet, hard X-ray, and gamma-ray emission from
nonthermal particles accelerated in these shocks will also be described. The
predicted gamma-ray fluxes of clusters should make them easily observable with
GLAST. Chandra X-ray observations of the interaction between the radio lobes
and cooling flow gas in Abell 2052 are also discussed briefly.


title:
Relativistic fine structure and resonance effects in electron-ion
  recombination and excitation of (e + C IV)

abstract:
  Relativistic close coupling calculations are reported for unified electronic
recombination of (e + C IV) including non-resonant and resonant recombination
processes, radiative and dielectronic recombination (RR and DR). Detailed
comparison of the theoretical unified results with two recent experiments on
ion storage rings (Mannervik etal. [1] and Schippers etal. [2]) shows very good
agreement in the entire measured energy region 2s -- 2p with 2pnl resonances.
The results benchmark theory and experiments to uncertainties of ~15%, and show
that the resonant and the background cross sections are not an incoherent sum
of separate RR and DR contributions. The limiting values of the DR cross
sections, as n ---> infinity, are shown to correspond to those due to electron
impact excitation (EIE) at the 2Po_(1/2,3/2) fine structure thresholds,
delineated for the first time. The near-threshold 2s 2S_(1/2) - 2p
2Po_(1/2,3/2) EIE cross sections are also compared with recent experimental
measurements. The demonstrated threshold fine structure and resonance effects
should be of general importance in excitation and recombination of positive
ions.


title: Search for Supernova Neutrino-Bursts with the AMANDA Detector

abstract:
  The core collapse of a massive star in the Milky Way will produce a neutrino
burst, intense enough to be detected by existing underground detectors. The
AMANDA neutrino telescope located deep in the South Pole ice can detect MeV
neutrinos by a collective rate increase in all photo-multipliers on top of dark
noise. The main source of light comes from positrons produced in the
CC-reaction of anti-electron neutrinos on free protons $\antinue + p \to e^+ +
n$. This paper describes the first supernova search performed on the full sets
of data taken during 1997 and 1998 (215 days of live time) with 302 of the
detector's optical modules. No candidate events resulted from this search. The
performance of the detector is calculated, yielding a 70% coverage of the
Galaxy with one background fake per year with 90% efficiency for the detector
configuration under study. An upper limit at the 90% c.l. on the rate of
stellar collapses in the Milky Way is derived, yielding 4.3 events per year. A
trigger algorithm is presented and its performance estimated. Possible
improvements of the detector hardware are reviewed.


title: Simulations of Glitches in Isolated Pulsars

abstract:
  Many radio pulsars exhibit glitches wherein the star's spin rate increases
fractionally by $\sim 10^{-10} - 10^{-6}$. Glitches are ascribed to variable
coupling between the neutron star crust and its superfluid interior. With the
aim of distinguishing among different theoretical explanations for the glitch
phenomenon, we study the response of a neutron star to two types of
perturbations to the vortex array that exists in the superfluid interior: 1)
thermal motion of vortices pinned to inner crust nuclei, initiated by sudden
heating of the crust, (e.g., a starquake), and 2) mechanical motion of
vortices, (e.g., from crust cracking by superfluid stresses). Both mechanisms
produce acceptable fits to glitch observations in four pulsars, with the
exception of the 1989 glitch in the Crab pulsar, which is best fit by the
thermal excitation model. The two models make different predictions for the
generation of internal heat and subsequent enhancement of surface emission. The
mechanical glitch model predicts a negligible temperature increase. For a pure
and highly-conductive crust, the thermal glitch model predicts a surface
temperature increase of as much as $\sim$ 2%, occurring several weeks after the
glitch. If the thermal conductivity of the crust is lowered by a high
concentration of impurities, however, the surface temperature increases by
$\sim$ 10% about a decade after a thermal glitch. A thermal glitch in an impure
crust is consistent with the surface emission limits following the January 2000
glitch in the Vela pulsar. Future surface emission measurements coordinated
with radio observations will constrain glitch mechanisms and the conductivity
of the crust.


title: Hidden Broad Line Seyfert 2 Galaxies in the CfA and 12micron Samples

abstract:
  We report the results of a spectropolarimetric survey of the CfA and 12micron
samples of Seyfert 2 galaxies (S2s). Polarized (hidden) broad line regions
(HBLRs) are confirmed in a number of galaxies, and several new cases
(F02581-1136, MCG -3-58-7, NGC 5995, NGC 6552, NGC 7682) are reported. The
12micron S2 sample shows a significantly higher incidence of HBLR (50%) than
its CfA counterpart (30%), suggesting that the latter may be incomplete in
hidden AGNs. Compared to the non-HBLR S2s, the HBLR S2s display distinctly
higher radio power relative to their far-infrared output and hotter dust
temperature as indicated by the f25/f60 color. However, the level of
obscuration is indistinguishable between the two types of S2. These results
strongly support the existence of two intrinsically different populations of
S2: one harboring an energetic, hidden S1 nucleus with BLR, and the other, a
``pure S2'', with weak or absent S1 nucleus and a strong, perhaps dominating
starburst component. Thus, the simple purely orientation-based unification
model is not applicable to all Seyfert galaxies.


title: Self-similarity of clusters of galaxies and the L_X-T relation

abstract:
  In this paper based on ROSAT/PSPC data we investigate the emission measure
profiles of a sample of hot clusters of galaxies (kT>3.5keV) in order to
explain the differences between observed and theoretically predicted L_X-T
relation. Looking at the form of the emission measure profiles as well as their
normalizations we find clear indication that indeed the profiles have similar
shapes once scaled to the virial radius, however, the normalization of the
profiles shows a strong temperature dependence. We introduce a M_gas-T relation
with the dependence M_gas propto T^1.94. This relationship explains the
observed L_X-T relation and reduces the scatter in the scaled profiles by a
factor of 2 when compared to the classical scaling. We interpret this finding
as strong indication that the M_gas-T relation in clusters deviates from
classical scaling.


title:
The Blandford-Znajek mechanism with Vacuum Breakdown as a Gamma-ray
  Burst Progenitor

abstract:
  The energetics of the long duration GRB phenomenum is compared with the BZ
mechanism. A rough estimate of the energy extracted from a rotating Black Hole
with the Blandford-Znajek mechanism is evaluated with a very simple assumption:
an inelastic collision between the rotating BH and an accreting torus. The GRB
energetics requires an high magnetic field that breaks down the vacuum around
the BH and gives origin to a e$^\pm$ fireball.


title:
Cosmic Ray Protons Accelerated at Cosmological Shocks and Their Impact
  on Groups and Clusters of Galaxies

abstract:
  We investigate the production of cosmic ray (CR) protons at cosmological
shocks by performing, for the first time, numerical simulations of large scale
structure formation that include directly the acceleration, transport and
energy losses of the high energy particles. CRs are injected at shocks
according to the thermal leakage model and, thereafter, accelerated to a
power-law distribution as indicated by the test particle limit of the diffusive
shock acceleration theory. The evolution of the CR protons accounts for losses
due to adiabatic expansion/compression, Coulomb collisions and inelastic p-p
scattering. Our results suggest that CR protons produced at shocks formed in
association with the process of large scale structure formation could amount to
a substantial fraction of the total pressure in the intra-cluster medium. Their
presence should be easily revealed by GLAST through detection of gamma-ray flux
from the decay of neutral pions produced in inelastic p-p collisions of such CR
protons with nuclei of the intra-cluster gas. This measurement will allow a
direct determination of the CR pressure contribution in the intra-cluster
medium. We also find that the spatial distribution of CR is typically more
irregular than that of the thermal gas because it is more influenced by the
underlying distribution of shocks. This feature is reflected in the appearance
of our gamma-ray synthetic images. Finally, the average CR pressure
distribution appears statistically slightly more extended than the thermal
pressure.


title:
The Diversity of High- and Intermediate-Velocity Clouds: Complex C
  versus IV Arch

abstract:
  We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption
lines in the Galactic high-velocity cloud Complex C and the Intermediate
Velocity Arch (IV Arch) in direction of the quasar PG 1259+593 (l=120,b=+58
deg). Absorption lines from CII, NI, NII, OI, AlII, SiII, PII, SII, ArI, FeII,
and FeIII are used to study the atomic abundances in these two halo clouds at
V_LSR=-130 km/s (Complex C) and V_LSR=-55 km/s (IV Arch). The OI/HI ratio
provides the best measure of the overall metallicity in the diffuse
interstellar medium, because ionization effects do not alter the ratio, and
oxygen is at most only lightly depleted from the gas into dust grains. For
Complex C, we find an oxygen abundance of 0.093 (+0.125, -0.047) solar,
consistent with the idea that Complex C represents the infall of low
metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV
Arch is 0.98 (+1.21,-0.46) solar, which indicates a Galactic origin. We report
the detection of an intermediate- velocity absorption component at +60 km/s
that is not seen in HI 21cm emission. The clouds along the PG 1259+593 sight
line have a variety of properties, proving that multiple processes are
responsible for the creation and circulation of intermediate- and high-velocity
gas in the Milky Way halo.


title: Identification of the donor in the X-ray binary GRS 1915+105

abstract:
  We report on the results of medium-resolution spectroscopy of GRS 1915+105 in
the near-infrared H and K band using the 8m VLT at ESO. We clearly identify
absorption bandheads from 12CO and 13CO. Together with other features this
results in a classification of the mass-donating star in this binary as a K-M
III star, clearly indicating that GRS 1915+105 belongs to the class of low-mass
X-ray binaries (LMXB).


title: Gamma-ray burst: probe of a black hole

abstract:
  There is strong evidence for the existence of black holes(BHs) in some X-ray
binaries and most galatic nuclei, based on different measuremental approaches,
but black holes aren't finally identified for the lack of very firm
observational evidence up to now. Because the direct evidence for BHs should
come from determination of strong gravitational redshift, we hope an object can
fall into the region near the BH horizon where radiation can be detected.
Therefore the object must be compact stars as neutron stars(NSs), then the
intense astrophysical processes will release very high energy radiation which
is transient, fast-variant. These phenomena may point to gamma-ray bursts(GRBs)
observed. And recent observations of iron lines suggest that afterglows of GRBs
show the similar property in active galatic nuclei(AGNs), implying GRBs may
originate from intense events related to black holes. In this letter, a model
for GRBs and afterglows is proposed to obtain the range of gravitational
redshifts($z_g$) of GRBs with known cosmological redshifts. Hence, we provide a
new method that with a search for high-energy emission lines(X- or
$\gamma$-rays) in GRBs, one can determine the gravitational redshift. We expect
$z_g>0.5$ or even larger, so that we can rule out the possibility of other
compact objects such as NSs, suggesting that the central progenitors of GRBs
should be black holes.


title:
Photometric Solutions for Detached Eclipsing Binaries: selection of
  ideal distance indicators in the SMC

abstract:
  Detached eclipsing binary stars provide a robust one-step distance
determination to nearby galaxies. As a by-product of Galactic microlensing
searches, catalogs of thousands of variable stars including eclipsing binaries
have been produced by the OGLE, MACHO and EROS collaborations. We present
photometric solutions for detached eclipsing binaries in the Small Magellanic
Cloud (SMC) discovered by the OGLE collaboration. The solutions were obtained
with an automated version of the Wilson-Devinney program. By fitting mock
catalogs of eclipsing binaries we find that the normalized stellar radii
(particularly their sum) and the surface brightness ratio are accurately
described by the fitted parameters and estimated standard errors, despite
various systematic uncertainties. In many cases these parameters are well
constrained. In addition we find that systems exhibiting complete eclipses can
be reliably identified where the fractional standard errors in the radii are
small. We present two quantitatively selected sub-samples of eclipsing binaries
that will be excellent distance indicators. These can be used both for
computation of the distance to the SMC and to probe its structure. One
particularly interesting binary has a very well determined solution, exhibits
complete eclipses, and is comprised of well detached G-type, class $II$ giants.


title: Cosmic Flows: A Status Report

abstract:
  We give a brief review of recent developments in the study of the large-scale
velocity field of galaxies since the international workshop on Cosmic Flows
held in July 1999 in Victoria, B.C. Peculiar velocities (PVs) yield a tight and
unique constraint on cosmological characteristics, independent of Lambda and
biasing, such as the cosmological matter density parameter Omega_m and the
convergence of bulk flows on large scales. Significant progress towards
incorporating non-linear dynamics and improvements of velocity field
reconstruction techniques have led to a rigorous control of errors and much
refined cosmic flow analyses. Current investigations favor low-amplitude (< 250
km/s) bulk flows on the largest scales (< ~100 Mpc/h) probed reliably by
existing redshift-distance surveys, consistent with favored LambdaCDM
cosmogonies. Tidal field analyses also suggest that the Shapley Concentration,
located behind the Great Attractor (GA), might play an important dynamical
role, even at the Local Group. Low-amplitude density fluctuations on very large
scales generate the overall large-scale streaming motions while massive
attractors like the GA, and Perseus-Pisces account for smaller scale motions
which are superposed on the large-scale flow. Likelihood analyses of galaxy
PVs, in the framework of flat CDM cosmology, now provide tight constraints of
Omega_m = 0.35 +/- 0.05. A four-fold size increase of our data base is expected
in the next 4-5 years with the completion of next generation FP/TF surveys and
automated supernovae searches within 20,000 km/s.


title:
No Evidence for Coherent Diffractive Radiation Patterns in the
  Millisecond Pulsar PSR J0437-4715

abstract:
  An independent analysis of the 326.5 MHz data obtained by Ooty Radio
Telescope reveals no evidence for coherent diffractive radiation patterns in
the millisecond pulsar PSR J0437-4715.


title: RXTE observations of single pulses of PSR B0531+21 I: Flux variations

abstract:
  This article is the first in the series that analyze about 1.87 million
periods of PSR B0531+21 (Crab pulsar), observed by the PCA detector aboard the
RXTE x-ray observatory. The Crab pulsar's x-ray light curve shows little
variation over time scales ranging from days to a period (33.46 milli seconds).
The standard deviation of its x-ray flux variation is $\approx$ 0.7% of its
mean value, which is negligible compared to its radio flux variations. The
phase resolved power spectrum of pulse to pulse x-ray flux variation shows no
spectral feature; an upper limit to the peak of any possible broad spectral
feature is 0.06% of the mean power. The x-ray fluxes in the two components of
its integrated profile are unrelated to each other; their linear correlation
coefficient is 0.0004$\pm$0.0010. ``Giant pulses'' that are routinely seen at
radio wavelengths are absent here. This work sets very strong constraints on
the connection (if any) between the flux variations at radio and x-ray
energies, for example due to variation in the degree of coherence of the basic
emitters. Its phase resolved x-ray flux variation shows a weak correlation with
the integrated profile. If confirmed, this might be an important clue to
understanding the x-ray emission mechanism of Crab pulsar.


title:
Stable Operation of a 300-m Laser Interferometer with Sufficient
  Sensitivity to Detect Gravitational-Wave Events within our Galaxy

abstract:
  TAMA300, an interferometric gravitational-wave detector with 300-m baseline
length, has been developed and operated with sufficient sensitivity to detect
gravitational-wave events within our galaxy and sufficient stability for
observations; the interferometer was operated for over 10 hours stably and
continuously. With a strain-equivalent noise level of $h\sim 5 \times 10^{-21}
/\sqrt{\rm Hz}$, a signal-to-noise ratio (SNR) of 30 is expected for
gravitational waves generated by a coalescence of 1.4 $M_\odot$-1.4 $M_\odot$
binary neutron stars at 10 kpc distance. %In addition, almost all noise sources
which limit the sensitivity and which %disturb the stable operation have been
identified. We evaluated the stability of the detector sensitivity with a
2-week data-taking run, collecting 160 hours of data to be analyzed in the
search for gravitational waves.


title:
Formation of Obscuring Walls by Radiation Force from Circumnuclear
  Starbursts and Implications for Starburst-Active Galactic Nucleus Connection

abstract:
  We explore the formation of dusty gas walls induced by a circumnuclear
starburst around an AGN. We concentrate our attention on the role of the
radiation force by a starburst as well as an AGN, where the effects of optical
depth of dusty gas are taken into consideration. In two-dimensional
axisymmetric space, we analyze the configuration and the stability of
geometrically thin walls which are in balance between radiation pressure and
gravity. As a result, it is shown that the radiation force by the circumnuclear
starburst works to stabilize optically thick walls surrounding the nucleus. In
the case of a brighter starburst with a fainter AGN, there form double walls,
an inner one of which is located between the nucleus and the circumnuclear
starburst, and an outer one of which enshrouds both the starburst regions and
the nucleus. The total extinction (Av) of both walls turns out to be ~10 for a
brighter starburst. As a consequence, double walls could heavily obscure the
nucleus to make the AGN type 2. The outer wall may provide an explanation for
the recent indications for large-scale obscuring materials in Seyfert 2's.
Also, it is predicted that the AGN type is shifts from type 2 to type 1 in
several times 10^7 yr according to the stellar evolution in the starburst. In
contrast, if the AGN itself is much brighter than the starburst as a quasar is,
then neither wall forms regardless of the starburst activity and the nucleus is
likely to be identified as type 1. To conclude, the radiatively-supported gas
walls could be responsible for the putative correlation between AGN type and
the starbursts, whereby Seyfert 2 galaxies are more frequently associated with
circumnuclear starbursts than type 1, whereas quasars are mostly observed as
type 1 regardless of star-forming activity in the host galaxies.


title: The coronal FeXXI 1354.094 line in AB Doradus

abstract:
  The active late-type star AB Doradus was observed in February 1996 with the
Goddard High Resolution Spectrograph of the Hubble Space Telescope using the
low resolution G140L grating. The observations covered one half of the star's
rotation cycle (P = 0.514 d) with 11.5 min time resolution. The strong coronal
Fe XXI 1354.094 line formed at 10^7 K was analysed and its emission measure
(EM) derived. This EM is much higher than that derived from recent XMM-Newton
observations (Guedel et al. 2001), and earlier EXOSAT (Collier Cameron et al.
1988) and ASCA/EUVE (Mewe et al. 1996) data, as well, requiring a variability
by a factor of 5. The physical reason for the variability remains unknown,
since (outside flares) the observed broad band variability of AB Dor is much
smaller.


title: Energy Loss of Particles in Pulsar Radio Emission Region

abstract:
  Extremely relativistic particles play an important role in various radiative
mechanisms of pulsar radio as well as high-energy emission. It is thus very
essential to estimate the Lorentz factor of these particles in the radiation
regions both observationally and theoretically. In this paper, according to the
frequency dependence of component separation and considering some radiation
processes, we obtain the Lorentz factor and its variation in the radiation
region observationally for seven multi-frequency observed pulsars. It is found
that the Lorentz factor decays substantially in the emission region, and the
decay can not be well understood by all known energy loss processes. This
result hints that there are some unknown processes which are responsible to
loosing energy effectively.


title:
The Influence of N14(e-,nu)C14(alpha,gamma)O18 reaction on the
  He-Ignition in Degenerate Physical Conditions

abstract:
  The importance of NCO chain on the onset of the He-flash in degenerate
physical conditions has been reevaluated. We find that low-mass, metal-rich (Z
$\ge$ 0.001) structures climbing the Red Giant Branch do never attain the
physical conditions suitable for the onset of this chain, while at lower
metallicities the energy contribution provided by NCO reaction is too low to
affect the onset of the central He-flash. At the same time, our evolutionary
models suggest that for a Carbon-Oxygen White Dwarf of mass M_{WD}=0.6 M_sun
accreting He-rich matter, directly or as a by-product of an overlying H-burning
shell, at rates suitable for a dynamical He-flash, the NCO energy contribution
is not able to keep hot enough the He-shell and in turn to avoid the occurrence
of a strong electron degeneracy and the ensuing final explosion.


title: X-Ray Bursts from the Galactic X-Ray Transient Source GRS 1915+105

abstract:
  We have analyzed publicly available RXTE/PCAs archival data of GRS 1915+105
during its burst/flaring state. The burst cycle ranges from 30 to 1300 s. These
bursts are different from the type I and type II classical bursts seen in Low
Mass X-ray Binaries (LMXBs) in terms of their temporal and spectral properties.
We have classified these bursts on the basis of properties observed during the
quiescent (low flux) phase. The 2 - 10 Hz QPOs are present during the quiescent
phase and disappear during the burst phase of all types of these X-ray bursts.
The duration of the quiescent phase can be explained assuming an outflow from
the post-shock regions and the catastrophic Compton cooling.


title: Interacting Quintessence

abstract:
  We demonstrate that a suitable coupling between a quintessence scalar field
and a pressureless cold dark matter (CDM) fluid leads to a constant ratio of
the energy densities of both components which is compatible with an accelerated
expansion of the Universe.


title: Old and new tools for understanding the evolution of stars in clusters

abstract:
  The uncertainties in the modelling of some important inputs of stellar
evolution must be taken into account for a correct interpretation, both of the
HR diagrams of individual stars, and of the integrated colors, of stellar
clusters. After a short discussion of Globular Cluster ages, we focus on the
problem of convection and discuss the parametrization of convection efficiency
and overshooting. Convective efficiency affects the reliability of the giants
colors, especially for high metallicity: age-metallicity relations found from
integrated colors of clusters must be regarded with caution. Non-instantaneous
mixing, both in the formal convective region and in the overshooting region
must be taken into account: this is necessary to compute nucleosynthesis in
`hot bottom burning' envelopes, but it may also affect the color distribution
in clump stars as shown here for the case of the LMC cluster NGC 1866.


title: Theoretical insights into the RR Lyrae K-band Period-Luminosity relation

abstract:
  Based on updated nonlinear, convective pulsation models computed for several
values of stellar mass, luminosity and metallicity, theoretical constraints on
the K-band Period-Luminosity (PLK) relation of RR Lyrae stars are presented. We
show that for each given metal content the predicted PLK is marginally
dependent on uncertainties of the stellar mass and/or luminosity. Then, by
considering the RR Lyrae masses suggested by evolutionary computations for the
various metallicities, we obtain that the predicted infrared magnitude M_K over
the range 0.0001< Z <0.02 is given by the relation
MK=0.568-2.071logP+0.087logZ-0.778logL/Lo, with a rms scatter of 0.032 mag.
Therefore, by allowing the luminosities of RR Lyrae stars to vary within the
range covered by current evolutionary predictions for metal-deficient (0.0001<
Z <0.006) horizontal branch models, we eventually find that the infrared
Period-Luminosity- Metallicity (PLZK) relation is
MK=0.139-2.071(logP+0.30)+0.167logZ, with a total intrinsic dispersion of 0.037
mag. As a consequence, the use of such a PLZK relation should constrain within
+-0.04 mag the infrared distance modulus of field and cluster RR Lyrae
variables, provided that accurate observations and reliable estimates of the
metal content are available. Moreover, we show that the combination of K and V
measurements can supply independent information on the average luminosity of RR
Lyrae stars, thus yielding tight constraints on the input physics of stellar
evolution computations. Finally, for globular clusters with a sizable sample of
first overtone variables, the reddening can be estimated by using the PLZK
relation together with the predicted MV-logP relation at the blue edge of the
instability strip (Caputo et al. 2000).


title:
The spectral energy distribution of Centaurus A (NGC 5128) - A summary
  of all observations including all GCRO results -

abstract:
  Due to its proximity and its importance for the understanding of active
galaxies and their active nuclei (AGN), Centaurus A has been observed
frequently within the last 150 years in all accessible wavelength bands. Thus a
wealth of data exists which has been compiled into the "NASA Extragalactic
Database" (NED). Missing from this compilation are to date almost completely
the results of recent high energy observations as e.g. obtained by the Compton
Gamma Ray Observatory (CGRO). A combination of those recent high energy results
with all other observations in the NED enables us for the first time to
establish the important spectral energy distributions (SEDs) of this closest
AGN in different emission states. The combined data have been analyzed to
produce SEDs which are from contemporaneous data and in addition an attempt was
made to derive SEDs which are spatially resolved, i.e. SEDs from observations
which can resolve nucleus and jet in Cen A are treated separately.


title:
The stellar origin of 7Li - Do AGB stars contribute a substantial
  fraction of the local Galactic lithium abundance?

abstract:
  We adopt up-to-date 7Li yields from asymptotic giant branch stars in order to
study the temporal evolution of 7Li in the solar neighbourhood in the context
of a revised version of the two-infall model for the chemical evolution of our
galaxy.
  We consider several lithium stellar sources besides the asymptotic giant
branch stars such as Type II supernovae, novae, low-mass giants as well as
Galactic cosmic rays and low-mass X-ray binaries.
  We conclude that asymptotic giant branch stars cannot be considered as
important 7Li producers as believed in so far and that the contribution of
low-mass giants and novae is necessary to reproduce the steep rise of the 7Li
abundance in disk stars as well as the meteoritic 7Li abundance. Lithium
production in low-mass X-ray binaries hardly affects the temporal evolution of
7Li in the solar neighbourhood.


title: Nonlinear States of the Screw Dynamo

abstract:
  The self-excitation of magnetic field by a spiral Couette flow between two
coaxial cylinders is considered. We solve numerically the fully nonlinear,
three-dimensional MHD equations for magnetic Prandtl numbers Pm (ratio of
kinematic viscosity to magnetic diffusivity) between 0.14 and 10 and kinematic
and magnetic Reynolds numbers up to about 2000. In the initial stage of
exponential field growth (kinematic dynamo regime), we find that the dynamo
switches from one distinct regime to another as the radial width \delta r_B$ of
the magnetic field distribution becomes smaller than the separation of the
field maximum from the flow boundary. The saturation of magnetic field growth
is due to a reduction in the velocity shear resulting mainly from the
azimuthally and axially averaged part of the Lorentz force, which agrees with
an asymptotic result for the limit of Pm<<1. In the parameter regime
considered, the magnetic energy decreases with kinematic Reynolds number as
Re^{-0.84}, which is approximately as predicted by the nonlinear asymptotic
theory (\sim Re^{-1}). However, when the velocity field is maintained by a
volume force (rather than by viscous stress) the dependence of magnetic energy
on the kinematic Reynolds number is much weaker.


title: Apparent radii of neutron stars and equation of state of dense matter

abstract:
  Apparent (radiation) radius of neutron star,R_\infty, depends on the star
gravitational mass in quite a different way than the standard coordinate radius
in the Schwarzschild metric, R. We show that, for a broad set of equations of
state of dense matter,
  R_\infty(M_max) for the configurations with maximum allowable masses is very
close to the absolute lower bound on R_\infty at fixed M, resulting from the
very definition of R_\infty.
  Also, the value of R_infty at given M, corresponding to the maximum
compactness (minimum R) of neutron star consistent with general relativity and
condition v_sound<c, is only 0.6% higher than this absolute lower bound.
Theoretical predictions for R_\infty are compared with existing observational
estimates of the apparent radii of neutron stars.


title: The Kr85 s-process Branching and the Mass of Carbon Stars

abstract:
  We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.


title: Constraints on a Local Group X-ray Halo

abstract:
  A simple model for a hot Local Group halo is constructed, using the standard
beta-model for the halo density and by choosing model parameters based on all
available observations of X-ray emission in other groups of galaxies and on
optical data on Local Group morphology. From the predicted X-ray intensities,
total Local Group mass, and central cooling time of the halo, we derive very
conservative upper limits on the central halo density N_0 and global
temperature T of N_0 = 5e-4 cm-3 and kT = 0.5 keV, irrespective of realistic
values of the density profile parameters r_c and beta. A typical poor group
value of beta = 0.5 requires kT < 0.15 keV and N_0 < 1e-4 cm-3, from which it
is concluded that the Local Group is very unlikely to possess a significant
X-ray halo. The prospects for further constraining of halo parameters from UV
absorption line observations are considered. We explicitly calculate the
ability of the halo to distort the cosmic microwave background (CMB) in terms
of the resulting CMB temperature variations and multipole anisotropies.


title: Galactic Magnetic Fields as a consequence of Inflation

abstract:
  The generation of a magnetic field in the Early Universe is considered, due
to the gravitational production of the Z-boson field during inflation. Scaled
to the epoch of galaxy formation this magnetic field suffices to trigger the
galactic dynamo and explain the observed galactic magnetic fields. The
mechanism is independent of the inflationary model.


title: Open, Massive and Globular Clusters - Part of the same family?

abstract:
  Populations of young star clusters show significant differences even among
"normal" disk galaxies. In this contribution I discuss how properties of young
cluster systems are related to those of their host galaxies, based on a recent
study of clusters in a sample of 22 nearby spiral galaxies. Luminous young
clusters similar to the "super" star clusters observed in starbursts and
mergers exist in several of these galaxies, and it is found that the luminosity
of the brightest star cluster as well as the specific luminosity of the cluster
systems both correlate well with the host galaxy star formation rate. When
considering star clusters in different environments the traditional distinction
between "open", "massive" and "globular" clusters breaks down, underscoring the
need for a universal physical description of cluster formation.


title:
The Luminosity of SN 1999by in NGC 2841 and the Nature of `Peculiar'
  Type Ia Supernovae

abstract:
  We present UBVRIJHK photometry and optical spectroscopy of the so-called
'peculiar' Type Ia supernova 1999by in NGC 2841. The observations began one
week before visual maximum light which is well-defined by daily observations.
The light curves and spectra are similar to those of the prototypical
subluminous event SN 1991bg. We find that maximum light in B occurred on 1999
May 10.3 UT (JD 2,451,308.8 +/- 0.3) with B=13.66 +/- 0.02 mag and a color of
B_max-V_max=0.51 +/- 0.03 mag. The late-time color implies minimal dust
extinction from the host galaxy. Our photometry, when combined with the recent
Cepheid distance to NGC 2841 (Macri et al. 2001), gives a peak absolute
magnitude of M_B=-17.15 +/- 0.23 mag, making SN 1999by one of the least
luminous Type Ia events ever observed. We estimate a decline rate parameter of
dm15(B)=1.90 mag, versus 1.93 for SN 1991bg, where 1.10 is typical for
so-called 'normal' events. We compare SN 1999by with other subluminous events
and find that the B_max-V_max color correlates strongly with the decline rate
and may be a more sensitive indicator of luminosity than the fading rate for
these objects. We find a good correlation between luminosity and the depth of
the spectral feature at 580 nm, which had been attributed solely to Si II. We
show that in cooler photospheres the 580 nm feature is dominated by Ti II,
which provides a simple physical explanation for the correlation. Using only
subluminous Type Ia supernovae we derive a Hubble parameter of H_0=75 +12 -11
km/s Mpc, consistent with values found from brighter events.


title:
The Discovery of Cepheids and a New Distance to NGC 2841 Using the
  Hubble Space Telescope

abstract:
  We report on the discovery of Cepheids in the spiral galaxy NGC 2841, based
on observations made with the Wide Field and Planetary Camera 2 on board the
Hubble Space Telescope. NGC 2841 was observed over 12 epochs using the F555W
filter, and over 5 epochs using the F814W filter. Photometry was performed
using the DAOPHOT/ALLFRAME package.
  We discovered a total of 29 variables, including 18 high-quality Cepheids
with periods ranging from 15 to 40 days. Period-luminosity relations in the V
and I bands, based on the high-quality Cepheids, yield an extinction-corrected
distance modulus of 30.74 +/- 0.23 mag, which corresponds to a distance of 14.1
+/- 1.5 Mpc. Our distance is based on an assumed LMC distance modulus of 18.50
+/- 0.10 mag (D = 50+/- 2.5 kpc) and a metallicity dependence of the Cepheid
P-L relation of gamma (VI) = -0.2 +/- 0.2 mag/dex.


title: Dynamics and the Origin of Low-Mass Galaxies in the Virgo Cluster

abstract:
  Early-type dwarfs are the most common galaxy in the local universe, yet their
origin and evolution remain a mystery. Various cosmological scenarios predict
that dwarf-like galaxies in dense areas are the first to form and hence should
be the oldest stellar systems in clusters. By using radial velocities of
early-type dwarfs in the Virgo cluster we demonstrate that these galaxies are
not an old cluster population but have signatures of production from the infall
of field galaxies. Evidence of this includes the combined large dispersions and
sub-structure in spatial and kinematic distributions for Virgo early-type
dwarfs, and a velocity dispersion ratio with giant ellipticals expected for
virialized and accreted populations. We also argue that these galaxies cannot
originate from accreted field dwarfs, but must have physically evolved from a
precursor population, of different morphology, that fell into Virgo some time
in the past.


title:
Constraining the Cosmological Constant from Large-Scale Redshift-Space
  Clustering

abstract:
  We show how the cosmological constant can be estimated from redshift surveys
at different redshifts, using maximum-likelihood techniques. The apparent
redshift-space clustering on large scales (\simgt 20 \himpc) are affected in
the radial direction by infall, and curvature influences the apparent
correlations in the transverse direction. The relative strengths of the two
effects will strongly vary with redshift. Using a simple idealized survey
geometry, we compute the smoothed correlation matrix of the redshift-space
correlation function, and the Fisher matrix for $\omm$ and $\oml$. These
represent the best possible measurement of these parameters given the geometry.
We find that the likelihood contours are turning, according to the behavior of
the angular-diameter distance relation. The clustering measures from redshift
surveys at intermediate-to-high redshifts can provide a surprisingly tight
constraint on $\oml$. We also estimate confidence contours for real survey
geometries, using the SDSS LRG and QSO surveys as specific examples. We believe
that this method will become a practical tool to constrain the nature of the
dark energy.


title:
X-ray flares/bursts from GRS 1915+105 and the two component accretion
  flow

abstract:
  We present results of our analysis of a set of RXTE/PCA observations of X-ray
flares/bursts with burst cycle ranging from 30 to 1300 s from the Galactic
X-ray transient source GRS 1915+105 during last four years. These flares/bursts
can be classified into four types: 1) regular bursts with short burst phase of
about 20 s (R1), 2) regular bursts with short quiescent phase of about 20 s
(R2), 3) long regular bursts (R3) with burst cycle > 1000 s, and 4) irregular
bursts (IR). For all the observed bursts, the duration of the quiescent phase
is inversely proportional to the square of the QPO frequency (2-10 Hz).


title: Dust and Nebular Emission in Star Forming Galaxies

abstract:
  Star forming galaxies exhibit a variety of physical conditions, from
quiescent normal spirals to the most powerful dusty starbursts. In order to
study these complex systems, we need a suitable tool to analyze the information
coming from observations at all wavelengths. We present a new
spectro-photometric model which considers in a consistent way starlight as
reprocessed by gas and dust. We discuss preliminary results to interpret some
observed properties of VLIRGs.


title: Clusters of Galaxies as Self-Gravitating Systems

abstract:
  Self-gravitating systems such as elliptical galaxies appear to have a
constant specific entropy and obey a scaling law relating their potential
energy to their mass. These properties can be interpreted as due to the
physical processes involved in the formation of these structures. Dark matter
haloes obtained through numerical simulations have also been found to obey a
scaling law relating their potential energy to their mass with the same slope
as ellipticals. Since the X-ray gas in clusters is weakly dissipative, we have
checked the hypothesis that it might verify similar properties.
  We have analyzed ROSAT-PSPC images of 24 clusters, and also found that: 1)
the S\'ersic law parameters (intensity, shape and scale) describing the X-ray
gas emission are correlated two by two; 2) the hot gas in all these clusters
roughly has the same specific entropy; 3) a scaling law linking the cluster
potential energy to the mass of the X-ray gas is observed, with the same slope
as for elliptical galaxies and dark matter haloes.


title:
The Helium content and age of the Hyades: Constraints from five binary
  systems and Hipparcos parallaxes

abstract:
  We compare the accurate empirical mass-luminosity (M-L) relation based on
five Hyades binary systems to predictions of stellar models calculated with
various input parameters (helium, metallicity, age) or physics (mixing-length
ratio, model atmosphere, equation of state, microscopic diffusion). Models
based on a helium content Ysim0.28 inferred from the dydz enrichment law are
more than 3sigma beyond the observations, suggesting that the initial helium
abundance is lower than expected from its supersolar metallicity. With the
photometric metallicity (FeH=0.144pm0.013 dex, Grenon (2000) we derive
Y=0.255\pm0.009. Because of the (Y,FeH) degeneracy in the M-L plane, the
uncertainty grows to Delta Y=0.013 if the metallicity from spectroscopy is
adopted (FeH=0.14pm0.05 dex, Cayrel de Strobel et al 1997). We use these
results to discuss the Hertzsprung-Russell (HR) diagram of the Hyades, in the
(Mv,B-V) plane, based on the very precise Hipparcos dynamical parallaxes.
Present models fit the tight observed sequence very well except at low
temperatures. In the low mass region of the HR diagram sensitive to the
mixing-length parameter (aMLT), the slope of the main sequence (MS) suggests
that aMLT could decrease from a solar (or even supersolar) value at higher mass
to subsolar values at low mass, which is also supported by the modeling of the
vB22 M-L relation. We find that the discrepancy at low temperatures (B-V\gtrsim
1.2) remains, even if an improved equation of state or better model atmospheres
are used. Finally, we discuss the positions of the stars at turn-off in the
light of their observed rotation rates and we deduce that the maximum age of
the Hyades predicted by the present models is sim650 Myr.


title:
A Comparison of the EUV and X-Ray Emission Profiles in Six Clusters of
  Galaxies

abstract:
  We have performed the wavelet analysis and image reconstructions for six
clusters of galaxies observed both at extreme ultraviolet and X-ray energies.
The comparison of the profiles of the EUV and X-ray reconstructed images shows
that they differ at a very large confidence level in five of the six clusters,
and are similar only for the Coma cluster, which is by far the hottest one in
the sample.


title: Spectroscopy and orbital periods of four cataclysmic variable stars

abstract:
  We present spectroscopy and orbital periods Porb of four relatively
little-studied cataclysmic variable stars. The stars and their periods are: AF
Cam, Porb = 0.324(1) d (the daily cycle count is slightly ambiguous); V2069 Cyg
(= RX J2123.7+4217), 0.311683(2) d; PG 0935+075, 0.1868(3) d; and KUV
03580+0614, 0.1495(6) d. V2069 Cyg and KUV 03580+0614 both show HeII lambda
4686 emission comparable in strength to H beta. V2069 Cyg appears to be a
luminous novalike variable, and the strong HeII suggests it may be an
intermediate polar. The period of KUV 03580+0614 is similar to members of the
SW Sex-type novalike variables, and it shows the phase-dependent absorption in
the Balmer and He I lines typical of this subclass. AF Cam shows absorption
features from a K-type secondary, as expected given its rather long orbital
period. The secondary spectrum and the outburst magnitude both suggest that AF
Cam is about 1 kpc distant. The spectrum of PG 0935+075 resembles that of a
dwarf nova at minimum light, with a noticeable contribution from an M-dwarf
secondary star. The secondary spectrum and a tentative outburst magnitude both
suggest a distance near 500 pc.


title:
The 2dF Galaxy Redshift Survey: Luminosity dependence of galaxy
  clustering

abstract:
  We investigate the dependence of the strength of galaxy clustering on
intrinsic luminosity using the Anglo-Australian two degree field galaxy
redshift survey (2dFGRS). The 2dFGRS is over an order of magnitude larger than
previous redshift surveys used to address this issue. We measure the projected
two-point correlation function of galaxies in a series of volume-limited
samples. The projected correlation function is free from any distortion of the
clustering pattern induced by peculiar motions and is well described by a
power-law in pair separation over the range 0.1 < r /h Mpc < 10. The clustering
of L* galaxies in real space is well fit by a correlation length r0 = 4.9 +/-
0.3 /h Mpc and power-law slope gamma = 1.71 +/- 0.06. The clustering amplitude
increases slowly with absolute magnitude for galaxies fainter than M*, but
rises more strongly at higher luminosities. At low luminosities, our results
agree with measurements from the SSRS2 by Benoist et al. However, we find a
weaker dependence of clustering strength on luminosity at the highest
luminosities. The correlation function amplitude increases by a factor of 4.0
between $M_{b_{J}} -5\log_{10}h = -18$ and -22.5, and the most luminous
galaxies are 3.0 times more strongly clustered than L* galaxies. The power-law
slope of the correlation function shows remarkably little variation for samples
spanning a factor of 20 in luminosity. Our measurements are in very good
agreement with the predictions of the hierarchical galaxy formation models of
Benson et al.


title: Lumpy Structures in Self-Gravitating Disks

abstract:
  Following Toomre & Kalnajs (1991), local models of slightly dissipative
self-gravitating disks show how inhomogeneous structures can be maintained over
several galaxy rotations. Their basic physical ingredients are self-gravity,
dissipation and differential rotation. In order to explore the structures
resulting from these processes on the kpc scale, local simulation of
self-gravitating disks are performed in this paper in 2D as well as in 3D. The
third dimension becomes a priori important as soon as matter clumping causes a
tight coupling of the 3D equations of motion. The physically simple and general
framework of the model permits to make conclusions beyond the here considered
scales. A time dependent affine coordinate system is used, allowing to
calculate the gravitational forces via a particle-mesh FFT-method, increasing
the performance with respect to previous direct force calculations. Persistent
patterns, formed by transient structures, whose intensity and morphological
characteristic depend on the dissipation rate are obtained and described. Some
of our simulations reveal first signs of mass-size and velocity dispersion-size
power-law relations, but a clear scale invariant behavior will require more
powerful computer techniques.


title:
Classification of Power Density Spectrum Features and Estimation of the
  Delta-Invariant Value for the Z Source GX 340+0

abstract:
  We present a theoretical analysis of Rossi X-ray Timing Explorer data of Z
source GX 340+0 obtained by Jonker et al. In the frameworks of the recently
formulated the transition layer model the delta-angle is an angle between the
neutron star (NS) magnetospheric axis and the disk (presumably NS rotational)
axis.We determine the angle delta=6^o.3+/- 0.^o 3 which is a combination of the
simultaneously observed kHz QPO and HBO frequencies. While these three
frequencies change by a factor of three or more their delta-combination stays
almost constant. GX 340+0 is the fourth source (in addition to 4U 0614+09, Sco
X-1 and 4U 1702-42) for which delta-angle has been determined. With at most one
(constrained) parameter we make a complete classification of six observed power
spectral features, including the two kHz frequencies, the first and second
harmonics of the HBO frequency, low-frequency noise component and break
frequencies. We demonstrate that a new component discovered by Jonker et al. in
the GX 340+0 power spectrum is related to the viscous frequency branch which
has been, in fact reported earlier in 4U 1728-34 by Ford and van der Klis
(1998). Finally, we re-classify several previously misidentified features in
the power spectrum.


title: Lyman alpha emitters with red colors at z~=2.4

abstract:
  We have carried out a search for Lyman alpha emission from galaxies at z~=2.4
over a field of 1200 sq. arcmin using the CFH12K camera at the CFHT and a
custom medium band filter. The search has uncovered 58 candidates,
corresponding to a completeness-corrected source density of ~0.07 sq.
arcmin^-2. Our sources have red colors (B-I~=1.8) which imply either that a
large fraction of the light is highly reddened and we are detecting Lyman alpha
through special lines of sight, or that these objects contain an underlying
older stellar population. While for each individual object we cannot
discriminate between these alternatives, we conclude that most of the objects
actually contain an older component because the star formation rates inferred
from the picture based on reddening, applied to all candidates, would imply an
exceedingly high star formation rate, i.e. more than two orders of magnitude
above the peak cosmic star formation rate (e.g. Lilly et al. 1996).


title:
Microwave Background Signatures of a Primordial Stochastic Magnetic
  Field

abstract:
  A stochastic magnetic field in the early Universe will produce anisotropies
in the temperature and polarization of the cosmic microwave background. We
derive analytic expressions for the microwave background temperature and
polarization power spectra induced by vector and tensor perturbations from a
power-law magnetic field. For a scale-invariant stochastic magnetic field
smoothed over a comoving scale of $1 {\rm Mpc}$, the MAP satellite has the
potential to constrain the comoving mean-field amplitude to be no greater than
approximately $2\times10^{-9}$ G. Limits improve as the power-law slope
increases: for causally-generated power-law magnetic fields, the comoving
mean-field amplitude has an upper bound of approximately $4\times10^{-13} {\rm
G}$. Such constraints will surpass all current limits on galactic-scale
primordial stochastic magnetic fields at decoupling.


title: Accretion Flows in X--Ray Binaries and Active Galactic Nuclei

abstract:
  Hard X-ray emission is ubiquitous in accreting black holes, both in Galactic
binary systems and in Active Galactic Nuclei. I review the different spectra
which can be seen from these systems, and possible ways of producing this
emission. X-ray reflection should give an observational test of these
scenarios, but we need better models before this can give an unambiguous
diagnostic of the accretion geometry.


title: Identification and Analysis of Young Star Cluster Candidates in M31

abstract:
  We present a method for finding clusters of young stars in M31 using
broadband WFPC2 data from the HST data archive. Applying our identification
method to 13 WFPC2 fields, covering an area of ~60 arcmin^2, has revealed 79
new candidate young star clusters in these portions of the M31 disk. Most of
these clusters are small (~<5 pc) young (~10-200 Myr) star groups located
within large OB associations. We have estimated the reddening values and the
ages of each candidate individually by fitting isochrones to the stellar
photometry. We provide a catalog of the candidates including rough
approximations of their reddenings and ages. We also look for patterns of
cluster formation with galactocentric distance, but our rough estimates are not
precise enough to reveal any clear patterns.


title:
Non-LTE Models and Theoretical Spectra of Accretion Disks in Active
  Galactic Nuclei. IV. Effects of Compton Scattering and Metal Opacities

abstract:
  We extend our models of the vertical structure and emergent radiation field
of accretion disks around supermassive black holes described in previous papers
of this series. Our models now include both a self-consistent treatment of
Compton scattering and the effects of continuum opacities of the most important
metal species (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni). With these new effects
incorporated, we compute the predicted spectrum from black holes accreting at
nearly the Eddington luminosity (L/L_Edd = 0.3) and central masses of 10^6,
10^7, and 10^8 M_sun. We also consider two values of the Shakura-Sunyaev alpha
parameter, 0.1 and 0.01. Although it has little effect when M > 10^8 M_sun,
Comptonization grows in importance as the central mass decreases and the
central temperature rises. It generally produces an increase in temperature
with height in the uppermost layers of hot atmospheres. Compared to models with
coherent electron scattering, Comptonized models have enhanced EUV/soft X-ray
emission, but they also have a more sharply declining spectrum at very high
frequencies. Comptonization also smears the hydrogen and the He II Lyman edges.
The effects of metals on the overall spectral energy distribution are smaller
than the effects of Comptonization for these parameters. Compared to pure
hydrogen-helium models, models with metal continuum opacities have reduced flux
in the high frequency tail, except at the highest frequencies, where the flux
is very low. Metal photoionization edges are not present in the overall
disk-integrated model spectra. In addition to our new grid of models, we also
present a simple analytic prescription for the vertical temperature structure
of the disk in the presence of Comptonization, and show under what conditions a
hot outer layer (a corona) is formed.


title: Characterizing the QPO Behavior of the X-ray Nova XTE J1550--564

abstract:
  For all 209 RXTE observations of the X-ray nova XTE J1550-564 during its
major outburst of 1998-1999, we have analyzed the X-ray power spectra, phase
lags, and coherence functions. The phase lags and coherence measures are used
to distinguish three types of low-frequency QPOs (one more than those reported
by Wijnands, Homan, & van der Klis 1999). For the most common type, the phase
lag is correlated with both the QPO frequency and the amplitude. The physical
significance of the QPO types is evident in the relationships between QPO
properties and the apparent temperature and flux from the accretion disk. There
is also a clear pattern in how the QPO types relate to the presence of
high-frequency QPOs. In general, both the amplitude and the Q value
(freq./FWHM) of low-frequency QPOs decrease as the high-frequency oscillations
increase in frequency (100 to 284 Hz) and in Q value. We speculate that the
antagonism between low-frequency and high-frequency QPOs arises from competing
structures in a perturbed accretion disk. However, we find that the frequencies
of slow (< 20 Hz) and fast (> 100 Hz) QPOs are not correlated. In addition, we
encounter systematic problems in attempting to reliably compare the QPO
frequencies with broad features in the power continuum, since there are a
variable number of such features in the power spectra. These results cast some
doubt on the reported global relationship between QPOs from neutrons stars and
those from black hole systems.


title: On the MBM12 Young Association

abstract:
  I present a comprehensive study of the MBM12 young association (MBM12A). By
combining infrared (IR) photometry from the Two-Micron All-Sky Survey (2MASS)
survey with new optical imaging and spectroscopy, I have performed a census of
the MBM12A membership that is complete to 0.03 Msun (H~15) for a 1.75deg X
1.4deg field encompassing the MBM12 cloud. I find five new members with masses
of 0.1-0.4 Msun and a few additional candidates that have not been observed
spectroscopically. From an analysis of optical and IR photometry for stars in
the direction of MBM12, I identify M dwarfs in the foreground and background of
the cloud. By comparing the magnitudes of these stars to those of local field
dwarfs, I arrive at a distance modulus 7.2+/-0.5 (275 pc) to the MBM12 cloud;
it is not the nearest molecular cloud and is not inside the local bubble of hot
ionized gas as had been implied by previous distance estimates of 50-100 pc. I
have also used Li strengths and H-R diagrams to constrain the absolute and
relative ages of MBM12A and other young populations; these data indicate ages
of 2 +3/-1 Myr for MBM12A and 10 Myr for the TW Hya and Eta Cha associations.
MBM12A may be a slightly evolved version of the aggregates of young stars
within the Taurus dark clouds (~1 Myr) near the age of the IC 348 cluster (~2
Myr).


title:
Excess Hard X-ray Emission from the Obscured Low Luminosity AGN In the
  Nearby Galaxy M 51 (NGC 5194)

abstract:
  We observed the nearby galaxy M~51 (NGC 5194) with BeppoSAX. The X-ray
properties of the nucleus below 10 keV are almost the same as the ASCA results
regarding the hard component and the neutral Fe K$\alpha$ line, but the
intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS
data, we detected a bright hard X-ray emission component which dominates above
10 keV. The 10 -- 100 keV flux and luminosity of this component are
respectively $2\times10^{-11}$ erg s$^{-1}$ cm$^{-2}$ and $2\times10^{41}$ erg
s$^{-1}$. These are about 10 times higher than the extrapolation from the soft
X-ray band, and similar to the flux observed with Ginga, which found a bright
power law component in 2 -- 20 keV band. Considering other wavelength
properties and the X-ray luminosity, together with strong neutral Fe K line,
the hard X-ray emission most likely arises from a low luminosity active
nucleus, which is obscured with a column density of $\sim10^{24}$cm$^{-2}$.
This suggests that hidden low luminosity AGNs may well be present in other
nearby galaxies. We interpret the discrepancy between Ginga and other X-ray
satellites to be due to a large variability of absorption column density toward
the line of sight over several years, suggesting that the Compton thick
absorption material may be present on a spatial scale of a parsec. Apart from
the nucleus, several ultra-luminous off-nuclear X-ray sources detected in M~51
exhibit long-term time variability, suggesting the state transition similar to
that observed in Galactic black hole candidates.


title: Solar System Objects Observed in the SDSS Commissioning Data

abstract:
  We discuss measurements of the properties of about 10,000 asteroids detected
in 500 deg2 of sky in the Sloan Digital Sky Survey (SDSS) commissioning data.
The moving objects are detected in the magnitude range 14 < r < 21.5, with a
baseline of 5 minutes. Extensive tests show that the sample is at least 98%
complete, with the contamination rate of less than 3%.
  We find that the size distribution of asteroids resembles a broken power-law,
independent of the heliocentric distance: D^{-2.3} for 0.4 km < D < 5 km, and
D^{-4} for 5 km < D < 40 km. As a consequence of this break, the number of
asteroids with r < 21.5 is ten times smaller than predicted by extrapolating
the power-law relation observed for brighter asteroids (r < 18). The observed
counts imply that there are about 530,000 objects with D>1 km in the asteroid
belt, or about four times less than previous estimates.
  The distribution of main belt asteroids in the 4-dimensional SDSS color space
is bimodal, and the two groups can be associated with S (rocky) and C
(carbonaceous) asteroids. A strong bimodality is also seen in the heliocentric
distribution of asteroids and suggests the existence of two distinct belts: the
inner rocky belt, about 1 AU wide (FWHM) and centered at R~2.8 AU, and the
outer carbonaceous belt, about 0.5 AU wide and centered at R~3.2 AU. The colors
of Hungarias, Mars crossers, and near-Earth objects are more similar to the
C-type than to S-type asteroids, suggesting that they originate in the outer
belt. (abridged).


title: The Anomalous Infrared Emission of Abell 58

abstract:
  We present a new model to explain the excess in mid and near infrared
emission of the central, hydrogen poor dust knot in the planetary nebula (PN)
Abell 58. Current models disagree with ISO measurement because they apply an
average grain size and equilibrium conditions only. We investigate grain size
distributions and temperature fluctuations affecting infrared emission using a
new radiative transfer code and discuss in detail the conditions requiring an
extension of the classical description. The peculiar infrared emission of V605
Aql, the central dust knot in Abell 58, has been modeled with our code. V605
Aql is of special interest as it is one of only three stars ever observed to
move from the evolutionary track of a central PN star back to the post-AGB
state.


title:
A Possible Solution to the Cosmological Constant Problem By Discrete
  Space-time Hypothesis

abstract:
  The cosmological constant problem is explained by a theory based on the
discrete space-time hypothesis. The calculated cosmological constant value is
of the order of 10^-52[m]^-2 or equivalent to about 0.7 of the critical mass
density. It is in excellent agreement with Type Ia SN observational data and
recent results of BOOMERANG and MAXIMA. Our theory also implies that the
quantization of the space-time metric is not necessary since it is not a
fundamental field. The divergence problem of quantum gravity is then of no
interest. Cosmic inflation is given out as a consequence of the theory and the
universe is found to be alternatively dominated by the cosmological constant
and the mass density at different cosmic time period. Our calculation also
shows that the mass energy density is of similar order of magnitude as the
vacuum energy density in the present universe but it is just a coincidence.
This result supports the anthropic principle.


title: Using Correlation Integrals to Characterize 3D Stellar Orbits

abstract:
  In an effort to more fully understand the variety of stellar distribution
functions that can be used to construct models of realistic galaxies, the
correlation integral method for orbit characterization that previously has been
introduced by Grassberger & Procaccia (1983) and Carnevali & Santangelo (1984)
is examined in considerable detail. The broad utility of the method is
validated and demonstrated by applying it to orbits in a number of different,
previously studied test cases (1D, 2D, and 3D; nonrotating and rotating). At
the same time, the correlation integral method is compared and contrasted with
other more traditional characterization tools such as Lyapunov exponents and
surfaces of section. The method is then extended to orbits in a previously
unexamined rotating, 3D bar potential. The correlation integral method is found
to be a simple and reliable way to quantitatively categorize orbits in
virtually any potential. It is recommended that it be broadly adopted as a tool
for characterizing the properties of orbits and, by extension, stellar
distribution functions, in all Hamiltonian dynamical systems.


title:
A Survey for Infall Motions toward Starless Cores. II. $CS (2-1)$ and
  $N_2H^+ (1-0)$ Mapping Observations

abstract:
  We present the results of an extensive mapping survey of 53 `starless' cores
in the optically thick line of CS 2-1 and the optically thin lines of N2H+ 1-0
and C18O 1-0. The purpose of this survey was to search for signatures of
extended inward motions.
  This study finds 10 `strong' and 9 `probable' infall candidates, based on
$\delta V_{CS}$ analysis and on the spectral shapes of CS lines.
  From our analysis of the blue-skewed CS spectra and the $\delta V_{CS}$
parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple
two layer radiative transfer model to fit the profiles, we derive
one-dimensional infall speeds, half of whose values lie in the range of
0.05-0.09 km s$^{-1}$. These values are similar to those found in L1544 by
Tafalla et al., and this result confirms that infall speeds in starless cores
are generally faster than expected from ambipolar diffusion in a strongly
sub-critical core. In addition, the observed infall regions are too extended to
be consistent with the `inside-out' collapse model applied to a very low-mass
star. In the largest cores, the spatial extent of the CS spectra with infall
asymmetry is larger than the extent of the $\rm N_2H^+$ core by a factor of
2-3. All these results suggest that extended inward motions are a common
feature in starless cores, and that they could represent a necessary stage in
the condensation of a star-forming dense core.


title:
Metallicity distribution of halo stars and minor merger processes of the
  Galaxy

abstract:
  A possible relation between the high dispersion in metallicity of metal-poor
halo stars and the minor merger processes in the history of the Galaxy is
presented. The foreign populations of stars in the satellites through minor
merger processes, make considerable contributions to the Galactic halo,
therefore alter the intrinsic distribution of metallicities of the halo stars.
Our model reproduces the observed trends in the metallicity distribution of the
extremely metal-poor halo field stars in the Galaxy. Taken all the parameters
for our model, a density of $1.9\times 10^{-3}kpc^{-3}$ of extremely metal-poor
halo stars in $-5<[Fe/H]<-4$ is predicted, which explains why no such stars
have been observed so far.


title: The Long-Period Orbit of the Dwarf Nova V630 Cassiopaeia

abstract:
  We present extensive spectroscopy and photometry of the dwarf nova V630
Cassiopeiae. A late-type (K4-5) absorption spectrum is easily detectable, from
which we derive the orbital parameters. We find a spectroscopic period of
P=2.56387 +/- (4 times 10^{-5}) days and a semiamplitude of K_2=132.9 +/- 4.0
km/s. The resulting mass function, which is a firm lower limit on the mass of
the white dwarf, is then f(M)=0.624 +/- 0.056 solar masses. The secondary star
is a ``stripped giant'', and using relations between the core mass and the
luminosity and the core mass and the radius we derive a lower limit of M_2 >
0.165 solar masses for the secondary star. The rotational velocity of the
secondary star is not resolved in our spectra and we place a limit of
V_rot*sin(i) < 40 km/s. The long-term light curve shows variations of up to 0.4
mag on short (1-5 days) time scales, and variations of 0.2-0.4 mag on longer
(3-9 months) time scales. In spite of these variations, the ellipsoidal light
curve of the secondary star is easily seen when the data are folded on the
spectroscopic ephemeris. Ellipsoidal models fit to the mean light curve give an
inclination in the range 66.96 < i < 78.08 degrees (90 per cent confidence).
This inclination range, and the requirement that M_2 > 0.165 solar masses and
V_rot*sin(i) < 40 km/s yields a white dwarf mass of M_1=0.977^{+0.168}_{-0.098}
solar masses and a secondary star mass of M_2=0.172^{+0.029}_{-0.012} solar
masses (90 per cent confidence limits). Our findings confirm the suggestion of
Warner (1994), namely that V630 Cas is rare example of a dwarf nova with a long
orbital period.


title:
From brane assisted inflation to quintessence through a single scalar
  field

abstract:
  We explore within the context of brane cosmology whether it is possible to
obtain both early inflation and accelerated expansion during the present epoch
through the dynamics of the same scalar field in an exponential potential.
Considerations from successful inflation and viable radiation and matter
dominated eras impose constraints on the parameters of the potential. We find
that the additional requirement of late time quintessence behaviour in
conformity with present observations necessitates the inclusion of two
exponential terms in the potential.


title:
Relativistic corrections to the multiple scattering effect on the
  Sunyaev-Zel'dovich effect in the isotropic approximation

abstract:
  We extend the formalism for the calculation of the relativistic corrections
to the Sunyaev-Zel'dovich effect for clusters of galaxies and include the
multiple scattering effects in the isotropic approximation. We present the
results of the calculations by the Fokker-Planck expansion method as well as by
the direct numerical integration of the collision term of the Boltzmann
equation. The multiple scattering contribution is found to be very small
compared with the single scattering contribution. For high-temperature galaxy
clusters of kT=15keV, the ratio of the both contributions is -0.2% in the Wien
region. In the Rayleigh--Jeans region the ratio is -0.03%. Therefore the
multiple scattering contribution is safely neglected for the observed galaxy
clusters.


title: The baryon fraction distribution in X-rays galaxy clusters

abstract:
  The baryon fraction in galaxy clusters is one of the most direct way to
constrain $\omo$. The baryonic fraction is estimated to be in the range fb =
(0.15 - 0 .20) h^{-3/2} which is several times higher than expected from the
observed light element abundances in $\omo=1$ universe, leading to the
conclusion that we live in low density universe. In this work I will first
present the various steps which lead to these results and then foccus on the
distribution of the baryon fraction inside clusters. I will show that while the
baryon fraction in clusters follows a scaling law, its theoretical expected
shape does not agree with what is inferred from observations.
  I will show that when various systematics entering into the determination of
clusters gas fraction are taken into account the observed baryon fraction
profile matches better the predictions, provided that the average baryon
fraction at the virial radius is lower than previously found fb~10%.


title: Two-sided radio emission in ON231 (W Comae)

abstract:
  Recent radio images of the BL Lac object ON231 (W Com, 1219+285) show
remarkable new features in the source structure compared to those previously
published. The images were obtained from observations made with the European
VLBI Network plus MERLIN at 1.6 GHz and 5 GHz after the exceptional optical
outburst occurred in Spring 1998. The up-to-date B band historic light curve of
ON231 is also presented together with the R band luminosity evolution in the
period 1994--1999. We identify the source core in the radio images with the
brightest component having the flattest spectrum. A consequence of this
assumption is the existence of a two--sided emission in ON231 not detected in
previous VLBI images. A further new feature is a large bend in the jet at about
10 mas from the core. The emission extends for about 20 mas after the bend,
which might be due to strong interaction with the environment surrounding the
nucleus. We suggest some possible interpretations to relate the changes in the
source structure with the optical and radio flux density variation in the frame
of the unification model.


title: The Cluster Substructure - Alignment Connection

abstract:
  Using the APM cluster data we investigate whether the dynamical status of
clusters is related to the large-scale structure of the Universe. We find that
cluster substructure is strongly correlated with the tendency of clusters to be
aligned with their nearest neighbour and in general with the nearby clusters
that belong to the same supercluster. Furthermore, dynamically young clusters
are more clustered than the overall cluster population. These are strong
indications that cluster develop in a hierarchical fashion by anisotropy
merging along the large-scale filamentary superclusters within which they are
embedded.


title: Origin of light nuclei in near earth orbit

abstract:
  The possible sources of light nuclei populations observed recently below the
geomagnetic cutoff by the AMS experiment are discussed in terms of nuclear
processes: fragmentation of the incoming flux of cosmic helium on atmospheric
nuclei, and nuclear coalescence from proton and helium induced reactions.
Results of simulations for deuterium, tritium, helium 3 and 4, are presented.


title:
The BeppoSAX High Energy Large Area Survey HELLAS, II: Number counts and
  X-ray spectral properties

abstract:
  The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed about 85
deg^2 of sky in the 5-10 keV band down to a flux of 4-5 10^-14 erg cm-2 s-1.
The source surface density of 16.9+-6.4 deg$^{-2}$ at the survey limit
corresponds to a resolved fraction of the 5-10 keV X-ray background (XRB) of
the order of 20-30 %. Hardness ratios analysis indicates that the spectra of a
substantial fraction of the HELLAS sources (at least one third) are harder than
a alpha_E = 0.6 power law. This hardness may be due to large absorbing columns.
The hardness ratio analysis also indicates that many HELLAS sources may have a
spectrum more complex than a single absorbed power law. A soft component,
superimposed to a strongly cut-off power law, is likely to be present in
several sources.


title:
The BeppoSAX High Energy Large Area Survey HELLAS, III: testing
  synthesis models for the X-ray background

abstract:
  The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens
of square degrees of the sky in the 5--10 keV band down to a flux of about 5
10^-14 erg cm-2 s-1. The extrapolation of the HELLAS logN--logS towards fainter
fluxes with an euclidean slope is consistent with the first XMM measurements,
in the same energy band, which are a factor 20 more sensitive. The source
counts in the hardest band so far surveyed by X-ray satellites are used to
constrain XRB models. It is shown that in order to reproduce the 5--10 keV
counts over the range of fluxes covered by BeppoSAX and XMM a large fraction of
highly absorbed (logN_H = 23--24 cm-2), luminous (L_X > 10^44 erg s-1) AGN is
needed. A sizeable number of more heavily obscured, Compton thick, objects
cannot be ruled out but it is not required by the present data. The model
predicts an absorption distribution consistent with that found from the
hardness ratios analysis of the so far identified HELLAS sources. Interestingly
enough, there is evidence of a decoupling between X-ray absorption and optical
reddening indicators especially at high redshifts/luminosities where several
broad line quasars show hardness ratios typical of absorbed power law models
with logN_H=22--24 cm-2.


title: Atomic Carbon and CO Isotope Emission in the Vicinity of DR15

abstract:
  We present observations of the 3P1-3P0 fine structure transition of atomic
carbon [CI], the J=3-2 transition of CO, as well as of the J=1-0 transitions of
13CO and C18O toward DR15, an HII region associated with two mid-infrared dark
clouds (IRDCs). The 13CO and C18O J=1-0 emissions closely follow the dark
patches seen in optical wavelength, showing two self-gravitating molecular
cores with masses of 2000 Msun and 900 Msun, respectively, at the positions of
the catalogued IRDCs.
  Our data show a rough spatial correlation between [CI] and 13CO J=1-0. Bright
[CI] emission occurs in relatively cold gas behind the molecular cores, neither
in highly excited gas traced by CO J=3-2 emission nor in HII region/molecular
cloud interface. These results are inconsistent with those predicted by
standard photodissociation region (PDR) models, suggesting an origin for
interstellar atomic carbon unrelated to photodissociation processes.