title: Large-Scale Regular Morphological Patterns in the Radio Jet of NGC 6251

abstract:
  We report on large-scale, regular morphological patterns found in the radio
jet of the nearby radio galaxy NGC 6251. Investigating morphological properties
of this radio jet from the nucleus to a radial distance of $\sim$ 300 arcsec
($\approx$ 140 kpc) mapped at 1662 MHz and 4885 MHz by Perley, Bridle, &
Willis, we find three chains, each of which consists of five radio knots. We
also find that eight radio knots in the first two chains consist of three small
sub-knots (the triple-knotty substructures). We discuss the observational
properties of these regular morphological patterns.


title: Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows

abstract:
  We investigate the effect of a hybrid electron population, consisting of both
thermal and non-thermal particles, on the synchrotron spectrum, image size, and
image shape of a hot accretion flow onto a supermassive black hole. We find two
universal features in the emitted synchrotron spectrum: (i) a prominent
shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape
of the electron energy distribution, and (ii) an extended tail of emission at
high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the
power-law energy distribution of the electrons. In the low-frequency shoulder,
the luminosity can be up to two orders of magnitude greater than with a purely
thermal plasma even if only a small fraction (< 1%) of the steady-state
electron energy is in the non-thermal electrons. We apply the hybrid model to
the Galactic center source, Sgr A*. The observed radio and IR spectra imply
that at most 1% of the steady-state electron energy is present in a power-law
tail in this source. This corresponds to no more than 10% of the electron
energy injected into the non-thermal electrons and hence 90% into the thermal
electrons. We show that such a hybrid distribution can be sustained in the flow
because thermalization via Coulomb collisions and synchrotron self-absorption
are both inefficient. The presence of non-thermal electrons enlarges the size
of the radio image at low frequencies and alters the frequency dependence of
the brightness temperature. A purely thermal electron distributions produces a
sharp-edged image while a hybrid distribution causes strong limb brightening.
These effects can be seen up to frequencies ~10^11 Hz and are accessible to
radio interferometers.


title:
Observational constraints upon quintessence models arising from moduli
  fields

abstract:
  We study observational constraints on cosmological models with a quintessence
arising from moduli fields. The scalar field potential is given by a double
exponential potential V=V_0 exp(-A e^{sqrt{2} kappa phi}). After reviewing the
properties of the solutions, from a dynamical systems phase space analysis, we
consider the constraints on parameter values imposed by luminosity distances
from the 60 Type IA supernovae published by Perlmutter et al., and also from
gravitational lensing statistics of distant quasars. We also update the
constraints on models with a single exponential potential V=V_0 e^{-lambda
kappa phi}.


title:
Possible evidence for the disk origin for the powering of jets in Sgr A*
  and nearby elliptical galaxies

abstract:
  The predicted radio flux for nearby ellipticals by the canonical
ADAF(advection-dominated accretion flow) model is well above the observations.
If the mass accretion rate favored by the numerical simulation were adopted,
the ADAF model would also significantly overpredict the radio flux of Sgr A*.
On the observational side, strong evidence for the existence of jets in Sgr
A$^*$ and these ellipticals has been obtained. Based on these facts, we propose
a truncated ADAF model for these sources. We assume that the accretion disk is
truncated at a certain radius within which a jet forms by extracting the energy
of the disk therefore the radio flux is greatly suppressed due to the radiative
truncation of the disk and the fits to the observational data are excellent.
For example, for Sgr A*, the model fits the spectrum very well from radio
including the ``excess'' below the break frequency to hard X-ray under a high
accretion rate near the simulation value; for M87, the predicted upper limit of
the jet location is 24 $R_g$, in excellent agreement with the observational
result of < 30 $R_g$. The success of the model supplies a possible piece of
evidence for the disk rather than the hole origin for the powering of jets.


title: Limits on the optical magnitude of PSR 1821-24 in M28

abstract:
  The detection of a pulsed X-ray counterpart (RX J1824.2-2R52P) of the 3.05 ms
pulsar PSR 1821-24, suggests the possibility of a part of the rotational energy
loss of this high spindown rate pulsar being in the optical band. Archival HST
data for M28 is used here to set upper limits on the optical V-band magnitude
of PSR 1821-24. The optical limit extends the multiwavelength observations for
this source and provides a constraint for theoretical models of pulsar
emission.


title: A TiO study of the dwarf nova IP Pegasi

abstract:
  We present red spectra in the region 7000-8300A of the eclipsing dwarf nova
IP Peg, with simultaneous narrow-band photometry centered at 7322A. We show
that by placing a second star on the slit we can correct for the telluric
absorption bands which have hitherto made the TiO features from the secondary
star unusable. We use these TiO features to carry out a radial velocity study
of the secondary star, and find this gives an improvement in signal-to-noise of
a factor two compared with using the Na{I} doublet. In contrast with previous
results, we find no apparent ellipticity in the radial velocity curve. As a
result we revise the semi-amplitude to 331.3+/-5.8km/s, and thus the primary
and secondary star masses to 1.05Mo and 0.33Mo respectively. Although this is
the lowest mass yet derived for the secondary star, it is still over-massive
for its observed spectral type. However, the revised mass and radius bring IP
Peg into line with other CVs in the mass-radius-period relationships.
  By fitting the phase resolved spectra around the TiO bands to a mean
spectrum, we attempt to isolate the lightcurve of the secondary star. The
resulting light curve has marked deviations from the expected ellipsoidal
shape. The largest difference is at phase 0.5, and can be explained as an
eclipse of the secondary star by the disc, indicating that the disc is
optically thick when viewed at high inclination angles.


title: Source Reconstruction as an Inverse Problem

abstract:
  Inverse Problem techniques offer powerful tools which deal naturally with
marginal data and asymmetric or strongly smoothing kernels, in cases where
parameter-fitting methods may be used only with some caution. Although they are
typically subject to some bias, they can invert data without requiring one to
assume a particular model for the source. The Backus-Gilbert method in
particular concentrates on the tradeoff between resolution and stability, and
allows one to select an optimal compromise between them. We use these tools to
analyse the problem of reconstructing features of the source star in a
microlensing event, show that it should be possible to obtain useful
information about the star with reasonably obtainable data, and note that the
quality of the reconstruction is more sensitive to the number of data points
than to the quality of individual ones.


title: The Parameter Space Occupied by Galaxies: The Optical Window

abstract:
  We discuss the optical parameter space (luminosity and surface brightness)
over which galaxies are known to exist and report on the results from three
recent datasets: the Two-degree Field Galaxy Redshift Survey, the Hubble Deep
Field north, and the Local Group. These data are combined to provide a unique
and comprehensive insight into the luminosity distribution of galaxies over the
range -24 < M(B) < -8 mags, and the surface brightness distribution over the
range 18 < SB(Effective) < 28 mags/sq arcsec. The main conclusions are: (1)
luminous low surface brightness galaxies are rare; (2) there exists a universal
luminosity-surface brightness relation; (3) the most numerous galaxies are low
luminosity low surface brightness dwarfs.


title: Testing the Gaussianity of the COBE-DMR data with spherical wavelets

abstract:
  We investigate the Gaussianity of the 4-year COBE-DMR data (in HEALPix
pixelisation) using an analysis based on spherical Haar wavelets. We use all
the pixels lying outside the Galactic cut and compute the skewness, kurtosis
and scale-scale correlation spectra for the wavelet coefficients at each scale.
We also take into account the sensitivity of the method to the orientation of
the input signal. We find a detection of non-Gaussianity at $> 99$ per cent
level in just one of our statistics. Taking into account the total number of
statistics computed, we estimate that the probability of obtaining such a
detection by chance for an underlying Gaussian field is 0.69. Therefore, we
conclude that the spherical wavelet technique shows no strong evidence of
non-Gaussianity in the COBE-DMR data.


title:
Large scale star formation in galaxies. I. The spirals NGC 7217, NGC
  1058 and UGC 12732

abstract:
  We present multiband photometric observations of three spiral galaxies
selected in a sample suited for the study of young stellar groupings and their
relationship with the parent galaxy and the galactic environment. Star forming
regions have been identified using an objective technique based on a
multivariate statistical analysis. Maps of young star groupings are given for
each galaxy. The luminosity functions of the young star group populations show
a remarkable similarity with a slope in the range -1.52 to -1.33 range. The
size distributions peak around the classical 100pc value of the Local Group
associations for two out of the three galaxies. NGC 1058 shows smaller
associations (peak at about 50pc). The total number of young groups per unit B
absolute luminosity of the galaxy is significantly greater in UGC 12732. The
activity of star formation is in all three galaxies clearly stronger in the
central regions.


title: The evolution of rotating stars

abstract:
  First, we review the main physical effects to be considered in the building
of evolutionary models of rotating stars on the Upper Main-Sequence (MS). The
internal rotation law evolves as a result of contraction and expansion,
meridional circulation, diffusion processes and mass loss. In turn,
differential rotation and mixing exert a feedback on circulation and diffusion,
so that a consistent treatment is necessary.
  We review recent results on the evolution of internal rotation and the
surface rotational velocities for stars on the Upper MS, for red giants,
supergiants and W-R stars. A fast rotation is enhancing the mass loss by
stellar winds and reciprocally high mass loss is removing a lot of angular
momentum. The problem of the ``break-up'' or $\Omega$-limit is critically
examined in connection with the origin of Be and LBV stars. The effects of
rotation on the tracks in the HR diagram, the lifetimes, the isochrones, the
blue to red supergiant ratios, the formation of W-R stars, the chemical
abundances in massive stars as well as in red giants and AGB stars, are
reviewed in relation to recent observations for stars in the Galaxy and
Magellanic Clouds. The effects of rotation on the final stages and on the
chemical yields are examined, as well as the constraints placed by the periods
of pulsars. On the whole, this review points out that stellar evolution is not
only a function of mass M and metallicity Z, but of angular velocity $\Omega$
as well.


title: The Arecibo Dual-Beam Survey: Arecibo and VLA Observations

abstract:
  The Arecibo Dual-Beam Survey is a "blind" 21 cm search for galaxies covering
\~430 deg^2 of sky. We present the data from the detection survey as well as
from the follow-up observations to confirm detections and improve positions and
flux measurements. We find 265 galaxies, many of which are extremely low
surface brightness. Some of these previously uncataloged galaxies lie within
the zone of avoidance where they are obscured by the gas and dust in our
Galaxy. 81 of these sources are not previously cataloged optically and there
are 11 galaxies that have no associated optical counterpart or are only
tentatively associated with faint wisps of nebulosity on the Digitized Sky
Survey images. We discuss the properties of the survey and in particular we
make direct determinations of the completeness and reliability of the sample.
The behavior of the completeness and its dependencies is essential for
determining the HI mass function. We leave the discussion of the mass function
for a later paper, but do note that we find many low surface brightness
galaxies and 7 sources with M_HI < 10^8 Msolar.


title: The Redshift Distribution of FIRST Radio Sources at 1 mJy

abstract:
  We present spectra for a sample of radio sources from the FIRST survey, and
use them to define the form of the redshift distribution of radio sources at
mJy levels.We targeted 365 sources and obtained 46 redshifts (13 per cent of
the sample). We find that our sample is complete in redshift measurement to R
$\sim 18.6$, corresponding to $z\sim 0.2$. Early-type galaxies represent the
largest subset (45 per cent) of the sample and have redshifts $0.15\la z \la
0.5$ ; late-type galaxies make up 15 per cent of the sample and have redshifts
$0.05\la z \la 0.2$; starbursting galaxies are a small fraction ($\sim 6$ per
cent), and are very nearby ($z\la 0.05$). Some 9 per cent of the population
have Seyfert1/quasar-type spectra, all at $z\ga 0.8$, and there are 4 per cent
are Seyfert2 type galaxies at intermediate redshifts ($z\sim 0.2$). Using our
measurements and data from the Phoenix survey, we obtain an estimate for $N(z)$
at $S_{1.4 \rm {GHz}}\ge 1$ mJy and compare this with model predictions. At
variance with previous conclusions, we find that the population of starbursting
objects makes up $\la 5 $ per cent of the radio population at S $\sim 1$ mJy.


title: A robust method for fitting peculiar velocity field models

abstract:
  We present a new method for fitting peculiar velocity models to complete flux
limited magnitude-redshifts catalogues, using the luminosity function of the
sources as a distance indicator.The method is characterised by its robustness.
In particular, no assumptions are made concerning the spatial distribution of
sources and their luminosity function. Moreover, selection effects in redshift
are allowed. Furthermore the inclusion of additional observables correlated
with the absolute magnitude -- such as for example rotation velocity
information as described by the Tully-Fisher relation -- is straightforward.
  As an illustration of the method, the predicted IRAS peculiar velocity model
characterised by the density parameter beta is tested on two samples. The
application of our method to the Tully-Fisher MarkIII MAT sample leads to a
value of beta=0.6 \pm 0.125, fully consistent with the results obtained
previously by the VELMOD and ITF methods on similar datasets. Unlike these
methods however, we make a very conservative use of the Tully-Fisher
information. Specifically, we require to assume neither the linearity of the
Tully-Fisher relation nor a gaussian distribution of its residuals. Moreover,
the robustness of the method implies that no Malmquist corrections are
required.
  A second application is carried out, using the fluxes of the IRAS 1.2 Jy
sample as the distance indicator. In this case the effective depth of the
volume in which the velocity model is compared to the data is almost twice the
effective depth of the MarkIII MAT sample. The results suggest that the
predicted IRAS velocity model, while successful in reproducing locally the
cosmic flow, fails to describe the kinematics on larger scales.


title: Studies of binary star cluster candidates in the bar of the LMC. II

abstract:
  Binary clusters account for more than 10 % of the cluster population in the
Magellanic Clouds. Statistically fewer than 50 % of the found pairs are
expected to be chance superpositions. We estimated the cluster encounter rate
and suggest that tidal capture is an unlikely formation scenario for the
formation of binary clusters. Thus, most true binary clusters can be expected
to have formed together. Here we present a study of three binary cluster
candidates which are located in the bar of the LMC. NGC 1971 & NGC 1972 are
situated in the association LH 59 in the eastern part of the bar. A third star
cluster, NGC 1969, is close enough to this pair that all three objects may
constitute a triple system. We present the first age determination that is
based on CMDs for these star clusters. Our findings suggest that all three
clusters are young (40-70 Myr) and may have been formed in the same GMC. It
cannot clearly decided whether the clusters are physically interacting or not.
NGC 1894 & SL 341 are located at the south-western rim of the LMC bar. This
pair is studied in detail for the first time: The isopleths of both clusters
reveal an elliptical shape. Whether this might be interpreted as a sign of
interaction or is a peculiarity which is shared with a large amount of LMC star
clusters which show higher ellipticities than their counterparts in the Milky
Way remains unclear. From our age determination we find that both clusters are
coeval with an age of 55+/-5 Myr. This makes a formation from the same GMC a
likely scenario. SL 385 & SL 387 are a close pair in the western part of the
LMC bar. We derived ages of 170+/-30 Myr for SL 385 and >=250 Myr for SL 387.
The large age difference makes it unlikely that these two clusters formed in
the same GMC.


title: Large-scale CMB anisotropies: wrinkles in the Galaxy rather than in time

abstract:
  Withdrawm.
  Public data of COBE-DMR are not perfectly subtracted from dipole. Therefore,
the Galactic latitude dependence is affected by the remnants of the dipole and
the results previously announced in this paper about this are not valid.
  Anyway, the suspect of a high Galactic contamination announced in another
paper (Lopez-Corredoira 1999; A&A 346, 369) is still valid and to be
considered.
  Thanks are given to Ted Bunn for helpful discussions which made me realize of
the problem with the dipole.

categories: astro-ph gr-qc physics.data-an physics.space-ph


title:
The saturation of disk heating in the solar neighborhood and evidence
  for a merger 9 Gyrs ago

abstract:
  We re-examine the age-velocity dispersion relation in the solar neighborhood
using improved stellar age estimates that are based on Hipparcos parallaxes and
recent stellar evolution calculations. The resulting relation shows that the
Milky Way stellar disk was relatively quiescent, suffering little heating or
dispersion increase between 3 and 9 Gyrs. However, at an age of 9 Gyrs there is
an abrupt increase in the stellar velocity dispersions. To explain the abrupt
increase (by almost a factor of 2) in dispersions we propose that the Milky Way
suffered a minor merger 9 Gyrs ago that created the thick disk. The quiescent
phase is consistent with inefficient heating caused by scattering from tightly
wound transient spiral structure. It is possible that many galaxy disks have
radial velocity dispersions at the point where heating from spiral structure
becomes inefficient. Surface brightness profiles and dispersion measurements
can be used to predict spiral arm winding numbers at this saturation point. A
comparison between the observed and predicted winding numbers could test this
hypothesis and also search for evidence of merger induced disk heating in
nearby galaxies.


title:
The Compton Microscope: Using the Energy Dependence of QPO Amplitudes to
  Probe Their Origin in Accretion Disks

abstract:
  We report the development of a new tool to determine the origin of
quasi-periodic oscillations (QPOs) in accretion disk systems. The technique
uses the source energy spectrum and the energy dependence of the QPO fractional
amplitude to restrict the location of the emission region of the modulated
photons, which are assumed to originate in the inner accretion disk. Both Monte
Carlo and semi-analytical methods are presented. We assume the accretion disk
is enshrouded by a slab atmosphere of hot electrons in which unsaturated
Compton scattering produces the high-energy spectrum. Properties of the
atmosphere, in particular the electron temperature, are assumed functions of
radius from the central compact object. We show that our model reproduces the
observed energy dependence of the fractional amplitude of the 67 Hz QPO in GRS
1915+105 if the QPO is assumed to originate within five gravitational radii
from the innermost stable circular orbit.


title: GRBs from unstable Poynting dominated outflows

abstract:
  Poynting flux driven outflows from magnetized rotators are a plausible
explanation for gamma-ray burst engines. We suggest a new possibility for how
such outflows might transfer energy into radiating particles. We argue that the
Poynting flux drives non-linearly unstable large amplitude electromagnetic
waves (LAEMW) which ``break'' at radii $r_t \sim 10^{14} $ cm where the MHD
approximation becomes inapplicable. In the ``foaming'' (relativisticly
reconnecting) regions formed during the wave breaks the random electric fields
stochastically accelerate particles to ultrarelativistic energies which then
radiate in turbulent electromagnetic fields. The typical energy of the emitted
photons is a fraction of the fundamental Compton energy $ \epsilon \sim f \hbar
c/r_e $ with $f \sim 10^{-3}$ plus additional boosting due to the bulk motion
of the medium. The emission properties are similar to synchrotron radiation,
with a typical cooling time $\sim 10^{-4}$ sec. During the wave break, the
plasma is also bulk accelerated in the outward radial direction and at larger
radii can produce afterglows due to the interactions with external medium. The
near equipartition fields required by afterglow models maybe due to magnetic
field regeneration in the outflowing plasma (similarly to the field generation
by LAEMW of laser-plasma interactions) and mixing with the upstream plasma.


title:
The First VLT FORS1 spectra of Lyman-break candidates in the HDF-S and
  AXAF Deep Field

abstract:
  We report on low-resolution multi-object spectroscopy of 30 faint targets (R
\~ 24-25) in the HDF-S and AXAF deep field obtained with the VLT Focal
Reducer/low dispersion Spectrograph (FORS1). Eight high-redshift galaxies with
2.75< z < 4 have been identified. The spectroscopic redshifts are in good
agreement with the photometric ones with a dispersion $\sigma_z = 0.07$ at z<2
and $\sigma_z = 0.16$ at z>2. The inferred star formation rates of the
individual objects are moderate, ranging from a few to a few tens solar masses
per year. Five out of the eight high-z objects do not show prominent emission
lines. One object has a spectrum typical of an AGN. In the AXAF field two
relatively close pairs of galaxies have been identified, with separations of
8.7 and 3.1 proper Mpc and mean redshifts of 3.11 and 3.93, respectively.


title:
Central cusp due to a super-massive black hole in axisymmetric models of
  elliptical galaxies

abstract:
  We use numerical simulations to investigate the cusp at the centre of
elliptical galaxies, due to the slow growth of a super-massive black hole. We
study this problem for axisymmetric models of galaxies, with or without
rotation. The numerical simulations are based on the `Perturbation Particles'
method, and use GRAPEs to compute the force due to the cusp. We study how the
density cusp is affected by the initial flattening of the model, as well as the
role played by initial rotation. The logarithmic slope of the density cusp is
found to be very much insensitive to flattening; as a consequence, we deduce
that tangential velocity anisotropy -which supports the flattening- is also of
little influence on the final cusp. We investigate via two different kinds of
rotating models the efficiency with which a rotation velocity component builds
within the cusp. A cusp in rotation develops only for models where a net
rotation component is initially present at high energy levels. The eventual
observation of a central rotational velocity peak in E galaxies has therefore
some implications for the galaxy dynamical history.


title:
Characterizing the Low-Frequency QPO behavior of the X-ray Nova XTE
  J1550-564

abstract:
  For all 209 RXTE observations of the 7-Crab X-ray nova XTE J1550-564, we
analyzed the X-ray power spectra, phase lags, and coherence functions. These
observations cover the entire 250 day outburst cycle of the source and are the
most complete and arguably the richest data set for any black hole X-ray nova.
We find that there are three fundamental types of QPO behavior--one more than
reported by Wijnands, Homan, & van der Klis (1999, ApJ, 526, 33). The new type
occurred during the first half of the outburst. These three types of QPO
behavior can be grouped according to the relative contributions of the disk and
power-law components to the total flux.


title:
Local Luminosity Function at 15$\mu m$ and Galaxy Evolution Seen by
  ISOCAM 15$\mu m$ Surveys

abstract:
  A local luminosity function at 15$\mu m$ is derived using the bivariate
(15$\mu m$ vs. 60$\mu m$ luminosity) method, based on the newly published
ISOCAM LW3-band (15$\mu m$) survey of the very deep IRAS 60$\mu m$ sample in
the north ecliptic pole region (NEPR). New IRAS 60$\mu m$ fluxes are obtained
using the SCANPI/SUPERSCANPI software at the new ISOCAM positions of the
sources in the sample. It is found to be in excellent agreement with the 15$\mu
m$ local luminosity function published by Xu et al (1998), which is derived
from the {\it predicted} 15$\mu m$ luminosities of a sample of IRAS 25$\mu m$
selected galaxies. Model predictions of number counts and redshift
distributions based on the local luminosity function and assumptions of its
evolution with the redshift are calculated and compared with the data of ISOCAM
15$\mu m$ surveys. Strong luminosity evolution on the order of $L\propto
(1+z)^{4.5}$ is suggested in these comparisons, while pure density evolution
can be ruled out with high confidence. The sharp peak at about 0.4mJy in the
Euclidean normalized differential counts at 15$\mu m$ can be explained by the
effects of MIR broadband emission features, eliminating the need for any
hypothesis for a 'new population'. It is found that the contribution from the
population represented by ISOCAM 15$\mu m$ sources can account for the entire
IR/submm background, leaving little room for any missing 'new population' which
can be significant energy sources of the IR/submm sky.


title: Grad-Shafranov equation with anisotropic pressure

abstract:
  The most general form of the nonrelativistic Grad-Shafranov equation
describing anisotropic pressure effects is formulated within the double
adiabatic approximation. It gives a possibility to analyze quantitatively how
the anisotropic pressure affects the 2D structure of the ideal
magnetohydrodynamical flows.


title: Space-Based Infrared Near-Earth Asteroid Survey Simulation

abstract:
  We demonstrate the efficiency and effectiveness of using a satellite-based
sensor with visual and infrared focal plane arrays to search for NEAs with
orbits largely interior to the Earth's orbit. Such a space-based system could
detect approximately 97% of the Atens and 64% of the, as yet hypothetical,
objects with orbits entirely Interior to Earth's Orbit with diameters greater
than 1 km in a five-year mission and obtain orbits, albedos and diameters for
all of them. These are the results of a feasibility study; optimizing the
concept presented would result in greater levels of completion. The cost of
such a space-based system is estimated to be within a factor of two of the cost
of a ground-based system capable of about 21st magnitude, which would provide
only orbits and absolute magnitudes and require decades to reach these
completeness levels. In addition to obtaining albedos and diameters for the
asteroids discovered in the space-based survey, a space-based system would
obtain the same information on virtually all known NEOs of interest. A combined
space-based and ground-based survey would be highly synergistic in that each
can concentrate on what it does best and each complements the strengths of the
other. An integrated ground-based and space-based system could accomplish the
Spaceguard goal in less time than the ground-based system alone. In addition,
the result would be a catalog containing well-determined orbits, diameters, and
albedos for the majority of NEAs with diameters greater than 500 m.


title: Structure and Star Formation in NGC 925

abstract:
  We present the results from an optical study of the stellar & star formation
properties of NGC 925 using the WIYN 3.5m telescope. Images in B,V,R, & H-alpha
reveal a galaxy that is fraught with asymmetries. From isophote fits we
discover that the bar center is not coincident with the center of the outer
isophotes nor with the dynamical center (from Pisano et al. 1998). Cuts across
the spiral arms reveal that the northern arms are distinctly different from the
southern arm. The southern arm not only appears more coherent, but the peaks in
stellar and H-alpha emission are found to be coincident with those of the HI
distribution, while no such consistency is present in the northern disk. We
also examine the gas surface density criterion for massive star formation in
NGC 925, and find that its behavior is more consistent with that for irregular
galaxies, than with late-type spirals. In particular, star formation persists
beyond the radius at which the gas surface density falls below the predicted
critical value for star formation for late-type spirals. Such properties are
characteristic of Magellanic spirals, but are present at a less dramatic level
in NGC 925, a late-type spiral.


title:
J-Net Galactic Plane Survey of VLBI Radio Sources for VLBI Exploration
  of Radio Astrometry (VERA)

abstract:
  In order to search for new VLBI sources in the Galactic plane that can be
used as phase reference sources in differential VLBI, we have conducted 22 GHz
observations of radio sources in the Galactic plane using the Japanese VLBI
Network (J-Net). We have observed 267 VLBI source candidates selected from
existing radio surveys and have detected 93 sources at the signal-to-noise
ratio larger than 5. While 42 of the 93 detected sources had already been
detected with VLBI at relatively lower frequency (typically 2 to 8 GHz), the
remaining 51 are found to be new VLBI sources detected for the first time.
These VLBI sources are located within $|b|\le 5^\circ$, and have a large number
of Galactic maser sources around them. Thus, they are potential candidates for
phase reference sources for VLBI Exploration of Radio Astrometry (VERA), which
is the first VLBI array dedicated to the phase referencing VLBI astrometry
aiming at measuring the parallax and proper motion of maser sources in the
whole Galaxy.


title:
Color Gradients in Early-Type Galaxies in Clusters at the Redshift from
  0.37 to 0.56

abstract:
  Color gradients in elliptical galaxies in distant clusters ($z=0.37-0.56$)
are examined by using the archival deep imaging data of Wide Field Planetary
Camera 2 (WFPC2) on-board the Hubble Space Telescope (HST). Obtained color
gradients are compared with the two model gradients to examine the origin of
the color gradients. In one model, a color gradient is assumed to be caused by
a metallicity gradient of stellar populations, while in the other one, it is
caused by an age gradient. Both of these model color gradients reproduce the
average color gradient seen in nearby ellipticals, but predict significantly
different gradients at a redshift larger than $\sim$0.3. Comparison between the
observed gradients and the model gradients reveals that the metallicity
gradient is much more favorable as the primary origin of color gradients in
elliptical galaxies in clusters. The same conclusion has been obtained for
field ellipticals by using those at the redshift from 0.1 to 1.0 in the Hubble
Deep Field-North by Tamura et al. (2000). Thus, it is also suggested that the
primary origin of the color gradients in elliptical galaxies does not depend on
galaxy environment.


title:
Extrapolation of the solar magnetic field within the potential-field
  approximation from full-disk magnetograms

abstract:
  This paper is concerned with the Laplace boundary-value problem with the
directional derivative, corresponding to the specific nature of measurements of
the longitudinal component of the photospheric magnetic field. Boundary
conditions are specified by a distribution on the sphere of projection of the
magnetic field vector unto a given direction. It is shown that the solution of
this problem exists in the form of a spherical harmonic expansion, and
uniqueness of this solution is proved. A conceptual sketch of numerical
determination of the harmonic series coefficients is given. The field of
application of the method is analyzed having regard to the peculiarities of
actual data. Finally, we present differences in results derived from
extrapolating the magnetic field from a synoptic map and a full-disk
magnetogram.


title: GRB000301C with peculiar afterglow emission

abstract:
  The CCD magnitudes in Johnson V and Cousins R and I photometric passbands are
determined for GRB 000301C afterglow starting ~ 1.5 day after the gamma-ray
burst. In fact we provide the earliest optical observations for this burst.
Light curves of the afterglow emissions in U, B, V, R, I, J and K' passbands
are obtained by combining the present measurements with the published data.
Flux decay shows a very uncommon variation relative to other well observed
GRBs. Overall, there is a steepening of the optical and near-infrared flux
decay caused by a geometric and sideways expanding jet. This is superimposed by
a short term variability especially during early time (Delta t < 8 days). The
cause of variability is not well understood, though it has occurred
simultaneously with similar amplitude in all the filters. We derive the early
and late time flux decay constants using jet model. The late time flux decay is
the steepest amongst the GRB OTs observed so far with alpha ~ 3. Steepening in
the flux decay seems to have started simultaneously around Delta t ~ 7.6 day in
all passbands. The value of spectral index in the optical-near IR region is ~
-1.0. Redshift determination with z=2.0335 indicates cosmological origin of the
GRB having a luminosity distance of 16.6 Gpc. Thus it becomes the second
farthest amongst the GRBs with known distances. An indirect estimate of the
fluence > 20 keV indicates, if isotropic,> =10^53 ergs of release of energy.
The enormous amount of released energy will be reduced, if the radiation is
beamed which is the case for this event. Using a jet break time of 7.6 days, we
infer a jet opening angle of ~ 0.15 radian. This means the energy released is
reduced by a factor of ~ 90 relative to the isotropic value.


title: Wide-field J- and K-band galaxy counts in the ELAIS fields

abstract:
  New near-infrared galaxy counts in the J and K bands are presented over a
total area of 0.70 and 0.97 degrees^2, respectively. The limiting magnitudes of
the deepest regions are 19.5 in J and 18.0 in K. At J>16 and K>15 our J and
K-band counts number counts agree well with existing surveys provided all data
are corrected to a common magnitude scale. There are real differences from
field to field, and the ELAIS N1 and N2 fields show an overdensity of J<16,
K<15 galaxies. The slopes of log N(m) / dm are 0.40 to 0.45 at 15<K<18 and
16<J<19.5. Our counts favor galaxy models with a high normalization of the
local luminosity function and without strong evolution.


title: The Quest for Quantum Gravity: Testing Times for Theories?

abstract:
  I discuss some theoretical ideas concerning the representation of quantum
gravity as a Lorentz-symmetry-violating `medium' with non-trivial optical
properties, which include a refractive index in `vacuo' and stochastic effects
associated with a spread in the arrival times of photons, growing linearly with
the photon energy. Some of these properties may be experimentally detectable in
future satellite facilities (e.g. GLAST or AMS), using as probes light from
distant astrophysical sources such as gamma ray bursters. I also argue that
such linear violations of Lorentz symmetry may not always be constrained by
ultra-high-energy cosmic-ray data, as seems to be the case with a specific
(stringy) model of space-time foam.


title: Determining the reality of X-ray filaments

abstract:
  A number of authors have reported filaments connecting bright structures in
high-resolution X-ray images, and in some cases these have been taken as
evidence for a physical connection between the structures, which might be
thought to provide support for a model with non-cosmological redshifts. In this
paper I point out two problems which are inherent in the interpretation of
smoothed photon-limited data of this kind, and develop some simple techniques
for the assessment of the reality of X-ray filaments, which can be applied to
either simply smoothed or adaptively smoothed data. To illustrate the
usefulness of these techniques, I apply them to archival ROSAT observations of
galaxies and quasars previously analysed by others. I show that several
reported filamentary structures connecting X-ray sources are not in fact
significantly detected.


title: Can our Universe be inhomogeneous on large sub-horizon scales ?

abstract:
  We show that our Universe may be inhomogeneous on large sub-horizon scales
without us being able to realise it. We assume that a network of domain walls
permeates the universe dividing it in domains with slightly different vacuum
energy densities. We require that the energy scale of the phase transition
which produced the domain walls is sufficiently low so that the walls have a
negligible effect on structure formation. Nevertheless, the different vacuum
densities of different domains will lead to different values of the
cosmological parameters $\Omega_\Lambda^0$, $\Omega_m^0$ and $h$, in each patch
thus affecting the growth of cosmological perturbations at recent times. Hence,
if our local patch of the universe (with uniform vacuum density) is big enough
-- which is likely to happen given that we should have on average about one
domain per horizon volume -- we might not notice these large-scale
inhomogeneities. This happens because in order to see a patch with a different
vacuum density one may have to look back at a time when the universe was still
very homogeneous.


title: EROs in the EIS Fields. I: The AXAF (Chandra) Deep Field

abstract:
  The publicly available EIS-DEEP optical-NIR data for the AXAF (Chandra) Deep
Field have been used to construct samples of Extremely Red Objects (EROs) using
various single-band and multi-band color criteria. In this work we define as
EROs objects with colors consistent with passively evolving elliptical galaxies
at z $\geq$ 1. The EROs surface densities we derive are intermediate between
previous published values, emphasizing again the need for larger survey areas
to constrain the effects of possible large-scale structure. Although various
single-color selected samples can be derived, the EROs sample selected using
R-Ks > 5, I-Ks > 4, J-Ks > 1.8 jointly is likely to contain the highest
fraction of passively evolving luminous field elliptical galaxies at z >= 1, or
conversely, the lowest fraction of lower redshift interlopers. The surface
density of this multi-band selected EROs sample is consistent with the
conclusion that little or no field elliptical volume density evolution has
occurred in the redshift range 0 > z > 1.5. However, extensive spectroscopic
followup is necessary to confirm this conclusion.


title:
The Relation Between Activity and Environment in Compact Groups of
  Galaxies

abstract:
  We present the results of the classification of spectral activity types for
193 galaxies from a new sample of 49 compact groups of galaxies in the southern
hemisphere (SCGs). This sample was selected in automated fashion from a
digitized galaxy catalogue, covering an area of ~5200 sq deg, around the South
Galactic Pole. It is complete up to m ~14.5 in b_j for the brightest galaxy of
the group.
  The spectral analysis of the SCG galaxies confirms the results previously
obtained for a smaller sample of Hickson's compact groups (HCG). We confirm the
luminosity-activity and morphology-activity relations, as well as the
predominance of AGNs (41% of SCGs galaxies). We verified also that the number
of early-type non-emission-line galaxies increases with the number of members
in the group.
  The SCGs contain more star-forming galaxies (SFGs) and less non-emission-line
galaxies than HCGs, which suggests that they probe a wider range of physical
properties. The SFGs are composed in majority of HII Nucleus Galaxies, which
have less intense star formation than starburst galaxies. The star formation
activity in SCGs is, consequently, remarkably low. The SFGs show also evidence
for nuclear activity. If these results are further confirmed, 70% of the
galaxies in SCGs would then have an active nucleus, making these systems
remarkably rich in AGNs. Curiously, however, this characteristic of CGs
generally excludes Seyfert 1 galaxies.(Abridged)


title:
Exploration of the BaSeL stellar library for 9 F-type stars COROT
  potential targets

abstract:
  The Basel Stellar Library (BaSeL models) is constituted of the merging of
various synthetic stellar spectra libraries, with the purpose of giving the
most comprehensive coverage of stellar parameters. It has been corrected for
systematic deviations detected in respect to UBVRIJHKLM photometry at solar
metallicity, and can then be considered as the state-of-the-art knowledge of
the broad band content of stellar spectra. In this paper, we consider a sample
of 9 F-type stars with detailed spectroscopic analysis to investigate the Basel
Stellar Library in two photometric systems simultaneously, Johnson (B-V, U-B)
and Stromgren (b-y, m_1, and c_1). The sample corresponds to potential targets
of the central seismology programme of the COROT space experiment, which have
been recently observed at OHP. The atmospheric parameters T_eff, [Fe/H], and
log g obtained from the BaSeL models are compared with spectroscopic
determinations as well as with results of other photometric calibrations. For a
careful interpretation of the BaSeL solutions, we computed confidence regions
around the best $\chi$^2-estimates and projected them on T_eff-[Fe/H],
T_eff-log g, and log g-[Fe/H] diagrams. (Abridged)


title:
Deep HST+STIS Color-Magnitude Diagrams of the Dwarf Irregular Galaxy
  WLM: Detection of the Horizontal Branch

abstract:
  We have obtained V and I-band photometry for 1886 stars down to I=27 and V=28
in the field of the dwarf irregular galaxy WLM, using deep STIS CL and LP-band
images taken in the parallel mode with the Hubble Space Telescope. The
photometry is used to study the horizontal branch identified in WLM. The
horizontal branch, extending blueward from the red giant clump, is an
unambiguous signature of an old population. We demonstrate that it is possible
to reach the horizontal branch of an old population at a distance of ~1 Mpc
using STIS, with relatively short exposure times. From the VI color-magnitude
diagram we obtain an accurate distance modulus (m-M)_0 = 24.95 \pm 0.13 for WLM
by using the V-magnitude of the horizontal branch, and by adopting E(V-I)=0.03.
The implications are: 1) WLM formed stars at high redshift, and 2) the old
population of WLM can be representative of a protogalactic fragment, related to
those accreted to form the Milky Way halo.


title:
Detailed Analysis of the Pulsations During and After Bursts from the
  Bursting Pulsar (GRO J1744-28)

abstract:
  The hard X-ray bursts observed during both major outbursts of the Bursting
Pulsar (GRO J1744-28) show pulsations near the neutron star spin frequency with
an enhanced amplitude relative to that of the persistent emission. Consistent
with previous work, we find that the pulsations within bursts lag behind their
expected arrival times based upon the persistent pulsar ephemeris. For an
ensemble of 1293 bursts recorded with the Burst and Transient Source
Experiment, the average burst pulse time delay is 61.0 +/- 0.8 ms in the 25 -
50 keV energy range and 72 +/- 5 ms in the 50 - 100 keV band. The residual time
delay from 10 to 240 s following the start of the burst is 18.1 +/- 0.7 ms (25
- 50 keV). A significant correlation of the average burst time delay with burst
peak flux is found. Our results are consistent with the model of the pulse time
lags presented by Miller (1996).


title:
Electron, Muon, and Hadron Lateral Distributions Measured in Air-Showers
  by the KASCADE Experiment

abstract:
  Measurements of electron, muon, and hadron lateral distributions of extensive
air showers as recorded by the KASCADE experiment are presented. The data cover
the energy range from about 5x10^14 eV up to almost 10^17 eV and extend from
the inner core region to distances of 200 m. The electron and muon
distributions are corrected for mutual contaminations by taking into account
the detector properties in the experiment. All distributions are well described
by NKG-functions. The scale radii describing the electron and hadron data best
are approx. 30 m and 10 m, respectively. We discuss the correlation between
scale radii and `age' parameter as well as their dependence on shower size,
zenith angle, and particle energy threshold.


title: Black Hole Emergence in Supernovae

abstract:
  If a black hole formed in a core-collapse supernova is accreting material
from the base of the envelope, the accretion luminosity could be observable in
the supernova light curve. Here we continue the study of matter fall back onto
a black hole in the wake of a supernova and examine realistic supernovae models
which allow for an early emergence of the accretion luminosity. Such cases may
provide a direct observational identification of the black hole formed in the
aftermath of the explosion. Our approach combines analytic estimates and fully
relativistic, radiation-hydrodynamic numerical computations. We employ a
numerical hydrodynamical scaling technique to accommodate the diverse range of
dynamical time scales in a single simulation. We find that while in typical
Type II supernovae heating by radioactive decays dominates the late-time light
curve, low-energy explosions of more massive stars should provide an important
exception where the accretion luminosity will emerge while it is still
relatively large. Our main focus is on the only current candidate for such an
observation, the very unusual SN1997D. Due to the low energy of the explosion
and the very small ($2\times10^{-3} M_\sun$) inferred mass of Co56 in the
ejected envelope, we find that accretion should become the dominant source of
its luminosity during the year 2000. The total luminosity at emergence is
expected to lie in the range $0.5-3\times10^{36} $ ergs/s, potentially
detectable with HST. We also discuss the more favorable case of explosions
which eject negligible amounts of radioactive isotopes and find that the black
hole is likely to emerge a few tens of days after the explosion, with a
luminosity of $\sim 10^{37} $\ergss.


title: A TiO study of the black-hole binary GRO J0422+32 in a very low state

abstract:
  We present 53 simultaneous photometric (I band) and spectroscopic (6900-9500
Angstroms) observations of J0422+32, taken during December 1997. From these we
determine that J0422+32 was in its lowest state yet observed, at
I=20.44+/-0.08. Using relative spectrophotometry, we show that it is possible
to correct very accurately for telluric absorption. Following this, we use the
TiO bands at 7055 Angstroms and 7589 Angstroms for a radial velocity study and
thereby obtain a semi-amplitude of 378+/-16kms-1, which yields
f(M)=1.191+/-0.021M_solar and q=9.0+2.2-2.7, consistent with previous
observations. We further demonstrate that this little explored method is very
powerful for such systems. We also determine a new orbital ephemeris of
HJD=2450274.4156+/-0.0009 + 0.2121600+/-0.0000002E.
  We see some evidence for an ellipsoidal modulation, from which we determine
the orbital inclination of J0422+32 to be less than 45 degrees. We therefore
calculate a minimum mass for the primary of 2.22M_solar, consistent with a
black hole, but not necessarily the super-massive one proposed by Beekman et al
(1997). We obtain an M4-5 spectral type for the secondary star and determine
that the secondary contributes 38+/-2% of the flux that we observe from
J0422+32 over the range 6950-8400 Angstroms. From this we calculate the
distance to the system to be 1.39+/-0.15kpc.


title: Discovery of High-Frequency QPOs in Black Hole Candidate XTE J1859+226

abstract:
  We report the discovery of quasi-periodic oscillations (QPOs) at roughly 187
Hz and 150 Hz in the X-ray intensity of X-ray nova XTE J1859+226. The source
was observed during a recent outburst with RXTE. Besides these high-frequency
QPOs, we have also detected QPOs (and sometimes their harmonics) at 6--7 Hz,
and significant broad-band variability at low frequencies. These properties, as
well as the observed hard X-ray spectrum, make XTE J1859+226 a black hole
candidate (BHC).
  The detection of QPOs at two distinct frequencies $\gtrsim 100$ Hz in two
observations separated by about 4 hours provide additional insights into the
high-frequency QPO phenomenon in BHCs. The importance lies in the proposed
interpretations which invariably involve the effects of strong gravity near a
black hole. We compare our results to those of other BHCs, and discuss the
impact of the observational data on the models in a global context.


title: Microlensing and the Physics of Stellar Atmospheres

abstract:
  The simple physics of microlensing provides a well-understood tool with which
to probe the atmospheres of distant stars in the Galaxy and Local Group with
high magnification and resolution. Recent results in measuring stellar surface
structure through broad band photometry and spectroscopy of high amplification
microlensing events are reviewed, with emphasis on the dramatic expectations
for future contributions of microlensing to the field of stellar atmospheres.


title: Perfect fluid cosmologies with varying light speed

abstract:
  We have found exact constant solutions for the cosmological density parameter
using a generalization of general relativity that incorporates a cosmic
time-variation of the velocity of light in vacuum and the Newtonian gravitation
constant. We have determined the conditions when these solutions are attractors
for an expanding universe and solved the problems of the Standard Big Bang
model for perfect fluids.


title: Discovery of a Bright Field Methane (T-type) Brown Dwarf by 2MASS

abstract:
  We report the discovery of a bright (J = 13.83$\pm$0.03) methane brown dwarf,
or T dwarf, by the Two Micron All Sky Survey. This object, 2MASSI
J0559191-140448, is the first brown dwarf identified by the newly commissioned
CorMASS instrument mounted on the Palomar 60-inch Telescope. Near-infrared
spectra from 0.9 - 2.35 $\micron$ show characteristic CH$_4$ bands at 1.1, 1.3,
1.6, and 2.2 $\micron$, which are significantly shallower than those seen in
other T dwarfs discovered to date. Coupled with the detection of an FeH band at
0.9896 $\micron$ and two sets of K I doublets at J-band, we propose that 2MASS
J0559-14 is a warm T dwarf, close to the transition between L and T spectral
classes. The brightness of this object makes it a good candidate for detailed
investigation over a broad wavelength regime and at higher resolution.


title: Tidal spin-up of stars in dense stellar cusps around massive black holes

abstract:
  We show that main-sequence stars in dense stellar cusps around massive black
holes are likely to rotate at a significant fraction of the centrifugal breakup
velocity due to spin-up by hyperbolic tidal encounters. We use realistic
stellar structure models to calculate analytically the tidal spin-up in soft
encounters, and extrapolate these results to close and penetrating collisions
using smoothed particle hydrodynamics simulations. We find that the spin-up
falls off only slowly with distance from the black hole because the increased
tidal coupling in slower collisions at larger distances compensates for the
decrease in the stellar density. We apply our results to the stars near the
massive black hole in the Galactic Center. Over their lifetime, ~1 Msol main
sequence stars in the inner 0.3 pc of the Galactic Center are spun-up on
average to ~10%--30% of the centrifugal breakup limit. Such rotation is ~20--60
times higher than is usual for such stars and may affect their subsequent
evolution and their observed properties.


title: Constraining Galaxy Mass Profiles with Strong Gravitational Lensing

abstract:
  Measured time delays between the images of a gravitationally lensed source
can lead to a determination of the Hubble constant ($H_o$), but only if the
lensing mass distribution is well understood. The inability to sufficiently
constrain galaxy mass models results in large uncertainties on the derived
$H_o$, and severely hampers the cosmological application of this otherwise
elegant method. At the same time, lensing must compete with new techniques that
have the potential to measure the Hubble constant to within a few percent by
the middle of the decade. In this letter we re-evaluate the role of strong
gravitational lensing in the age of precision cosmology, and present a series
of Monte Carlo simulations that demonstrate the effect of the galaxy mass
distribution on Hubble constant determination. Though most gravitational lens
systems are unlikely to contribute significantly to precision studies of the
cosmological distance scale, the strong dependence of predicted time delays on
the galaxy mass profile suggests that the most useful and interesting results
may be obtained by running the traditional lensing problem in reverse --
namely, combining measured time delays with a well-constrained value of $H_o$
to study galaxy mass distributions.


title:
The X-ray Size-Temperature Relation for Intermediate Redshift Galaxy
  Clusters

abstract:
  We present the first measurements of the X-ray size-temperature (ST) relation
in intermediate redshift (z~0.30) galaxy clusters. We interpret the local ST
relation (z~0.06) in terms of underlying scaling relations in the cluster dark
matter properties, and then we use standard models for the redshift evolution
of those dark matter properties to show that the ST relation does not evolve
with redshift. We then use ROSAT HRI observations of 11 clusters to examine the
intermediate redshift ST relation; for currently favored cosmological
parameters, the intermediate redshift ST relation is consistent with that of
local clusters. Finally, we use the ST relation and our evolution model to
measure angular diameter distances; with these 11 distances we evaluate
constraints on Omega_M and Omega_L which are consistent with those derived from
studies of Type Ia supernovae. The data rule out a model with Omega_M=1 and
Omega_L=0 with 2.5 sigma confidence. When limited to models where
Omega_M+Omega_L=1, these data are inconsistent with Omega_M=1 with 3 sigma
confidence.


title:
PLANET observations of microlensing event OGLE-1999-BUL-23: limb
  darkening measurement of the source star

abstract:
  We present PLANET observations of OGLE-1999-BUL-23, a binary-lens
microlensing event towards the Galactic bulge. PLANET observations in the I and
V bands cover the event from just before the first caustic crossing until the
end of the event. In particular, a densely-sampled second caustic crossing
enables us to derive the linear limb-darkening coefficients of the source star;
c_V = 0.786^{+0.080}_{-0.078} and c_I = 0.632^{+0.047}_{-0.037}. Combined
analysis of the light curve and the color-magnitude diagram suggests that the
source star is a G/K subgiant in the Galactic bulge (T_eff = 4800 K). The
resulting linear limb-darkening coefficient of the source is consistent with
theoretical predictions, although it is likely that non-linearity of the
stellar surface brightness profile complicates the interpretation, especially
for the I band. The global light curve fit to the data indicates that the event
is due to a binary lens of a mass ratio q = 0.39 and a projected separation d =
2.42. The lens/source relative proper motion is (22.8 +_ 1.5) km/s/kpc, typical
of bulge/bulge or bulge/disk events.


title: Galaxy Cluster Baryon Fractions, Cluster Surveys and Cosmology

abstract:
  The properties of nearby galaxy clusters limit the range of cosmological
parameters consistent with our universe. We describe the limits which arise
from studies of the intracluster medium (ICM) mass fraction fICM and
consideration of the possible sources of systematic error:
Omega_M<0.44h_{50}^{-1/2} at 95% confidence. We emphasize that independent of
Type Ia supernovae (SNe Ia) observations, this cluster study, taken together
with published cosmic microwave background (CMB) anisotropy studies, indicates
a non-zero quintessence or dark energy component Omega_Q>0.
  We then discuss future galaxy cluster surveys which will probe the abundance
of galaxy clusters to intermediate and high redshift. We investigate the
sensitivity of these surveys to the cosmological density parameter Omega_M and
the equation of state parameter w of any quintessence component. In particular,
we show that cluster survey constraints from a proposed large solid angle X-ray
survey are comparable in precision and complementary in nature to constraints
expected from future CMB anisotropy and SNe Ia studies.


title:
Discovery and Photometric Observation of the Optical Counterpart in a
  Possible Galactic Halo X-ray Transient, XTE J1118+480

abstract:
  We discovered the optical counterpart about 13 mag of a soft X-ray transient,
XTE J1118+480 on 2000 March 30. We perform astrometry and provide the accurate
position as R.A. = 11h18m10s.85, Decl. = +48o02'12".9. The outbursting object
is identified with a 18.8 mag star in USNO catalog. Our pre-discovery data
shows another outburst during 2000 January, again coinciding with an outburst
detected in X-rays. Through the CCD time-series photometry, we found the
presence of a periodic variation with the amplitude of 0.055 mag and the period
of 0.17078(0.00004)d which we consider as promising candidate of orbital
period. Because of the high galactic latitude and faint quiescence magnitude of
18.8, XTE J1118+480 is the possible first firmly identified black hole
candidate (BHC) X-ray transient in the galactic halo.


title: Excitation of Oscillations in the Magnetic Network on the Sun

abstract:
  We examine the excitation of oscillations in the magnetic network of the Sun
through the footpoint motion of photospheric magnetic flux tubes located in
intergranular lanes. The motion is derived from a time series of
high-resolution G band and continuum filtergrams using an object-tracking
technique. We model the response of the flux tube to the footpoint motion in
terms of the Klein-Gordon equation, which is solved analytically as an initial
value problem for transverse (kink) waves. We compute the wave energy flux in
upward propagating transverse waves. In general we find that the injection of
energy into the chromosphere occurs in short-duration pulses, which would lead
to a time variability in chromospheric emission that is incompatible with
observations. Therefore, we consider the effects of turbulent convective flows
on flux tubes in intergranular lanes. The turbulent flows are simulated by
adding high-frequency motions (periods 5-50 s) with an amplitude of 1 km
s^{-1}. The latter are simulated by adding random velocity fluctuations to the
observationally determined velocities. In this case we find that the energy
flux is much less intermittent and can in principle carry adequate energy for
chromospheric heating.


title:
The H Beta Index as an Age Indicator of Old Stellar Systems: The Effects
  of Horizontal-Branch Stars

abstract:
  The strength of the H$\beta$ index has been computed for the integrated
spectra of model globular clusters from the evolutionary population synthesis.
These models take into account, for the first time, the detailed systematic
variation of horizontal-branch (HB) morphology with age and metallicity. Our
models show that the H$\beta$ index is significantly affected by the presence
of blue HB stars. Because of the contribution from blue HB stars, the H$\beta$
does not monotonically decrease as metallicity increases at a given age.
Instead, it reaches a maximum strength when the distribution of HB stars is
centered around 9500 K, the temperature where the H$\beta$ index becomes
strongest. Our models indicate that the strength of the H$\beta$ index
increases as much as 0.75 {\AA} due to the presence of blue HB stars.
  The comparison of the recent Keck observations of the globular cluster system
in the Milky Way Galaxy with those in giant elliptical galaxies, NGC 1399 and
M87, shows a systematic shift in the H$\beta$ against metallicity plane. Our
models suggest that this systematic difference is understood if the globular
cluster systems in giant elliptical galaxies are several billion years older,
in the mean, than the Galactic counterpart. Further observations of globular
cluster systems in the external galaxies from the large ground-based telescopes
and space UV facilities will enable to clarify whether this difference is
indeed due to the age difference or other explanations are also possible.


title: IC3328: a "dwarf elliptical galaxy" with spiral structure

abstract:
  We present the 2-D photometric decomposition of the Virgo galaxy IC3328. The
analysis of the global light distribution of this morphologically classified
nucleated dwarf elliptical galaxy (dE1,N) reveals a tightly wound, bi-symmetric
spiral structure with a diameter of 4.5 kpc, precisely centered on the nucleus
of the dwarf. The amplitude of the spiral is only three percent of the dwarf's
surface brightness making it the faintest and smallest spiral ever found in a
galaxy. In terms of pitch angle and arm winding the spiral is similar to the
intermediate-type galaxy M51, but it lacks the dust and prominent HII regions
which signal the presence of gas. The visual evidence of a spiral pattern in an
early-type dwarf galaxy reopens the question on whether these dwarfs are
genuine rotationally supported or anisotropic stellar systems. In the case of
IC3328, we argue for a nearly face-on disk (dS0) galaxy with an estimated
maximum rotation velocity of v_c,max = 55kms-1. The faintness of the spiral and
the small motions within it, suggests that we could be seeing swing-amplified
noise. The other possibility is a tidal origin, caused by the near passage of a
small companion.


title:
A modified parallel tree code for N-body simulation of the Large Scale
  Structure of the Universe

abstract:
  N-body codes to perform simulations of the origin and evolution of the Large
Scale Structure of the Universe have improved significantly over the past
decade both in terms of the resolution achieved and of reduction of the CPU
time. However, state-of-the-art N-body codes hardly allow one to deal with
particle numbers larger than a few 10^7, even on the largest parallel systems.
In order to allow simulations with larger resolution, we have first
re-considered the grouping strategy as described in Barnes (1990) (hereafter
B90) and applied it with some modifications to our WDSH-PT (Work and Data
SHaring - Parallel Tree) code. In the first part of this paper we will give a
short description of the code adopting the Barnes and Hut algorithm
\cite{barh86} (hereafter BH), and in particular of the memory and work
distribution strategy applied to describe the {\it data distribution} on a
CC-NUMA machine like the CRAY-T3E system. In the second part of the paper we
describe the modification to the Barnes grouping strategy we have devised to
improve the performance of the WDSH-PT code. We will use the property that
nearby particles have similar interaction list. This idea has been checked in
B90, where an interaction list is builded which applies everywhere within a
cell C_{group} containing a little number of particles N_{crit}. B90 reuses
this interaction list for each particle $ p \in C_{group}$ in the cell in turn.
We will assume each particle p to have the same interaction list.
  Thus it has been possible to reduce the CPU time increasing the performances.
This leads us to run simulations with a large number of particles (N ~
10^7/10^9) in non-prohibitive times.


title: Microlensing extended stellar sources

abstract:
  We have developed a code to compute multi-colour microlensing lightcurves for
extended sources, including the effects of limb darkening and photospheric star
spots as a function of spot temperature, position, size and lens trajectory.
Our model also includes the effect of structure within the spot and rotation of
the stellar source.
  Our results indicate that star spots generally give a clear signature only
for transit events. Moreover, this signature is strongly suppressed by limb
darkening for spots close to the limb -- although such spots can be detected by
favourable lens trajectories.


title:
The lithium isotope ratio in the metal-poor halo star G271-162 from
  VLT/UVES observations

abstract:
  A high resolution (R = 110.000), very high S/N (>600) spectrum of the
metal-poor turnoff star G271-162 has been obtained in connection with the
commissioning of UVES at VLT/Kueyen. Using both 1D hydrostatic and 3D
hydrodynamical model atmospheres, the lithium isotope ratio has been estimated
from the LiI 670.8 nm line by means of spectral synthesis. The necessary
stellar line broadening (1D: macroturbulence + rotation, 3D: rotation) has been
determined from unblended KI, CaI and FeI lines. The 3D line profiles agree
very well with the observed profiles, including the characteristic line
asymmetries. Both the 1D and 3D analyses reveal a possible detection of 6Li in
G271-162, 6Li/7Li = 0.02 +-0.01 (one sigma). It is discussed if the smaller
amount of 6Li in G271-162 than in the similar halo star HD84937 could be due to
differences in stellar mass and/or metallicity or whether it may reflect an
intrinsic scatter of the Li isotope ratio in the ISM at a given metallicity.


title: A Systematic Search for Short-term Variability of EGRET Sources

abstract:
  The 3rd EGRET Catalog of High-energy Gamma-ray Sources contains 170
unidentified sources, and there is great interest in the nature of these
sources. One means of determining source class is the study of flux variability
on time scales of days; pulsars are believed to be stable on these time scales
while blazars are known to be highly variable. In addition, previous work has
demonstrated that 3EG J0241-6103 and 3EG J1837-0606 are candidates for a new
gamma-ray source class. These sources near the Galactic plane display transient
behavior but cannot be associated with any known blazars. Although many
instances of flaring AGN have been reported, the EGRET database has not been
systematically searched for occurrences of short-timescale ($\sim1$ day)
variability. These considerations have led us to conduct a systematic search
for short-term variability in EGRET data, covering all viewing periods through
proposal cycle 4. Six 3EG catalog sources are reported here to display
variability on short time scales; four of them are unidentified. In addition,
three non-catalog variable sources are discussed.


title: The SOFIA Telescope

abstract:
  The SOFIA telescope as the heart of the observatory is a major technological
challenge. I present an overview on the astro-nomical and scientific
requirements for such a big airborne observatory and demonstrate the impact of
these requirements on the layout of SOFIA, in particular on the telescope
design as it is now. Selected components of the telescope will be de-scribed in
their context and functionality. The current status of the telescope is
presented.


title: Pixel lensing towards M31 in principle and in practice

abstract:
  The Andromeda galaxy (M31) provides a new line of sight for Galactic MACHO
studies and also a signature, near-far asymmetry, which may establish the
existence of MACHOs in the M31 halo. We outline the principles behind the
so-called ``pixel-lensing'' experiments monitoring unresolved stars in M31. We
present detailed simulations of the POINT-AGAPE survey now underway, which is
using the INT wide-field camera to map the microlensing distribution over a
substantial fraction of the M31 disc. We address the extent to which
pixel-lensing observables, which differ from those of classical microlensing,
can be used to determine the contribution and mass of M31 MACHOs. We also
present some preliminary light-curves obtained from the first season
(1999/2000) of INT data.


title:
Galactic Halo substructure in the Sloan Digital Sky Survey: the ancient
  tidal stream from the Sagittarius dwarf galaxy

abstract:
  Two studies have recently reported the discovery of pronounced Halo
substructure in the Sloan Digital Sky Survey (SDSS) commissioning data. Here we
show that this Halo substructure is almost in its entirety due to the expected
tidal stream torn off the Sagittarius dwarf galaxy during the course of its
many close encounters with the Milky Way. This interpretation makes strong
predictions on the kinematics and distances of these stream stars. Comparison
of the structure in old horizontal branch stars, detected by the SDSS team,
with the carbon star structure discovered in our own survey, indicates that
this halo stream is of comparable age to the Milky Way. It would appear that
the Milky Way and the Sagittarius dwarf galaxy have been a strongly interacting
system for most of their existence. Once complete, the SDSS will provide a
unique dataset with which to constrain the dynamical evolution of the
Sagittarius dwarf galaxy, it will also strongly constrain the mass distribution
of the outer Milky Way.


title: III Zw 2, the first superluminal jet in a Seyfert galaxy

abstract:
  So far all relativistically boosted jets with superluminal motion have only
been detected in typical radio galaxies with early type host galaxies. We have
now discovered superluminal motion in the Seyfert I galaxy III Zw 2, classified
as a spiral. Superluminal motion was first inferred from the spectral evolution
of the source and then confirmed by VLBI observations. The lower limit for the
apparent expansion speed is 1.25 +/- 0.09 c. The fact that the spectral and
spatial evolution are closely linked demonstrates that we are dealing with real
physical expansion. Prior to this rapid expansion we have seen a period of
virtually no expansion with an expansion speed less than 0.04 c. Since III Zw 2
is also part of a sample of so called radio-intermediate quasars (RIQ), it
confirms earlier predictions of superluminal motion for this source, based on
the argument that RIQs could be relativistically boosted jets in radio-weak
quasars and Seyfert galaxies.


title:
On the pair-electromagnetic pulse from an electromagnetic Black Hole
  surrounded by a Baryonic Remnant

abstract:
  The interaction of an expanding Pair-Electromagnetic pulse (PEM pulse) with a
shell of baryonic matter surrounding a Black Hole with electromagnetic
structure (EMBH) is analyzed for selected values of the baryonic mass at
selected distances well outside the dyadosphere of an EMBH. The dyadosphere,
the region in which a super critical field exists for the creation of
electron-positron pairs, is here considered in the special case of a
Reissner-Nordstrom geometry. The interaction of the PEM pulse with the baryonic
matter is described using a simplified model of a slab of constant thickness in
the laboratory frame (constant-thickness approximation) as well as performing
the integration of the general relativistic hydrodynamical equations. The
validation of the constant-thickness approximation, already presented in a
previous paper Ruffini, et al.(1999) for a PEM pulse in vacuum, is here
generalized to the presence of baryonic matter. It is found that for a baryonic
shell of mass-energy less than 1% of the total energy of the dyadosphere, the
constant-thickness approximation is in excellent agreement with full general
relativistic computations. The approximation breaks down for larger values of
the baryonic shell mass, however such cases are of less interest for observed
Gamma Ray Bursts (GRBs). On the basis of numerical computations of the slab
model for PEM pulses, we describe (i) the properties of relativistic evolution
of a PEM pulse colliding with a baryonic shell; (ii) the details of the
expected emission energy and observed temperature of the associated GRBs for a
given value of the EMBH mass; 10^3 solar masses, and for baryonic mass-energies
in the range 10^{-8} to 10^{-2} the total energy of the dyadosphere.


title: The Orbit of X Per and Its Neutron Star Companion

abstract:
  We have observed the Be/X-ray pulsar binary system X Per/4U 0352+30 on 61
occasions spanning an interval of 600 days with the PCA instrument on RXTE.
Pulse timing analyses of the 837-s pulsations yield strong evidence for the
presence of orbital Doppler delays. We confirm the Doppler delays by using
measurements made with the RXTE All-Sky Monitor. We obtain an orbital period of
250 days, a projected semimajor axis of the neutron star of 454 lt-s, a mass
function of 1.61 solar masses, and a modest eccentricity of 0.11. We discuss
the formation of the system in the context of the standard evolutionary
scenario for Be/X-ray binaries with consideration of the possibility that the
birth of the neutron star was accompanied by a kick of the type often inferred
from the velocity distribution of isolated radio pulsars. The orbital
eccentricity just after the supernova explosion was almost certainly virtually
the same as at present, because the Be star is much smaller than the orbital
separation. We find that the system most likely formed from a pair of massive
progenitor stars, and probably involved a quasi-stable and nearly conservative
transfer of mass from the primary to the secondary. We find that the He star
remnant of the primary most likely had a mass less than 6 solar masses after
mass transfer. Finally, we speculate that there may be a substantial population
of neutron stars formed with little or no kick.


title: Identification and astrometry of variables in M3

abstract:
  We present identification and astrometry of all previously known or suspected
variables along with the discovery of six new variables in the globular cluster
M3. The number of the catalogized variables increased to 274 by including all
the confirmed, previously known variables and the new discoveries. The precise
and homogeneous astrometry, as well as the clarification of misapprehensions in
the preceding identifications are done by using overlapping fields from a
wide-field Schmidt-camera, a 1m RCC telescope, and HST archive observations
from the center of the cluster. The astrometric positions can serve as a direct
input to any photometry which needs the accurate centers of the variables.


title: X-ray Sources in the Hubble Deep Field Detected by Chandra

abstract:
  We present first results from an X-ray study of the Hubble Deep Field North
(HDF-N) and its environs obtained using 166 ks of data collected by the
Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray
Observatory. This is the deepest X-ray observation ever reported, and in the
HDF-N itself we detect six X-ray sources down to a 0.5--8 keV flux limit of
4E-16 erg cm^-2 s^-1. Comparing these sources with objects seen in
multiwavelength HDF-N studies shows positional coincidences with the extremely
red object NICMOS J123651.74 +621221.4, an active galactic nucleus (AGN), three
elliptical galaxies, and one nearby spiral galaxy. The X-ray emission from the
ellipticals is consistent with that expected from a hot interstellar medium,
and the spiral galaxy emission may arise from a `super-Eddington' X-ray binary
or ultraluminous supernova remnant. Four of the X-ray sources have been
detected at radio wavelengths. We also place X-ray upper limits on AGN
candidates found in the HDF-N, and we present the tightest constraints yet on
X-ray emission from the SCUBA submillimeter source population. None of the 10
high-significance submillimeter sources reported in the HDF-N and its vicinity
is detected with Chandra ACIS. These sources appear to be dominated by star
formation or have AGN with Compton-thick tori and little circumnuclear X-ray
scattering.


title:
Astrometry of the Omega Centauri Hubble Space Telescope Calibration
  Field

abstract:
  Astrometry, on the International Celestial Reference Frame (epoch J2000.0),
is presented for the Walker (1994, PASP, 106, 828) stars in the Omega Centauri
(= NGC 5139 = C1323-1472) Hubble Space Telescope Wide Field/Planetary Camera
(WF/PC) calibration field of Harris et al. (1993, AJ, 105, 1196). Harris et al.
stars were first identified on a WFPC2 observation of the omega Cen HST
calibration field. Relative astrometry of the Walker stars in this field was
then obtained using Walker's CCD positions and astrometry derived using the
STSDAS METRIC task on the positions of the Harris et al. stars on the WFPC2
observation. Finally, the relative astrometry, which was based on the HST Guide
Star Catalog, is placed on the International Celestial Reference Frame with
astrometry from the USNO-A2.0 catalog. An ASCII text version of the astrometric
data of the Walker stars in the omega Cen HST calibration field is available
electronically in the online version of the article.


title:
On the Role of Minor Galaxy Mergers in the Formation of Active Galactic
  Nuclei

abstract:
  The large scale (~ 100 kpc) environments of Seyfert galaxies are not
significantly different from those of non-Seyfert galaxies. In the context of
the interaction model of the formation of active galactic nuclei (AGN), it has
been proposed that AGN form via "minor mergers" of large disk galaxies with
smaller companions. We test this hypothesis by comparing the nuclear spectra of
105 bright nearby galaxies with measurements of their R or r band morphological
asymmetries at three successive radii. We find no significant differences in
these asymmetries between the 13 Seyfert galaxies in the sample and galaxies
having other nuclear spectral types (absorption, H II-region like, LINER), nor
is there strong qualitative evidence that such mergers have occured among any
of the Seyferts or LINERs. Thus either any minor mergers began > 1 Gyr ago and
are essentially complete, or they did not occur at all, and AGN form
independently of any type of interaction. Support for the latter interpretation
is provided by the growing evidence that supermassive black holes exist in the
cores of most elliptical and early-type spiral galaxies, which in turn suggests
that nuclear activity represents a normal phase in the evolution of the bulges
of massive galaxies. Galaxy mergers may increase the luminosity of Seyfert
nuclei to the level of QSOs, which could explain why the latter objects appear
to be found in rich environments and in interacting systems.


title: A Merger Scenario for the Dynamics of Abell 665

abstract:
  We present new redshift measurements for 55 galaxies in the vicinity of the
rich galaxy cluster Abell 665. When combined with results from the literature,
we have good velocity measurements for a sample of 77 confirmed cluster members
from which we derive the cluster's redshift z=0.1829 +/- 0.0005 and
line-of-sight velocity dispersion of 1390 +/- 120 km/s. Our analysis of the
kinematical and spatial data for the subset of galaxies located within the
central 750 kpc reveals only subtle evidence for substructure and
non-Gaussianity in the velocity distribution. We find that the brightest
cluster member is not moving significantly relative to the other galaxies near
the center of the cluster. On the other hand, our deep ROSAT high resolution
image of A665 shows strong evidence for isophotal twisting and centroid
variation, thereby confirming previous suggestions of significant substructure
in the hot X-ray--emitting intracluster gas. In light of this evident
substructure, we have compared the optical velocity data with N-body
simulations of head-on cluster mergers. We find that a merger of two similar
mass subclusters (mass ratios of 1:1 or 1:2) seen close to the time of
core-crossing produces velocity distributions that are consistent with that
observed.


title:
Unavoidable Selection Effects in the Analysis of Faint Galaxies in the
  Hubble Deep Field: Probing the Cosmology and Merger History of Galaxies

abstract:
  (Abridged) We present a detailed analysis of the number count and photometric
redshift distribution of faint galaxies in the Hubble Deep Field (HDF), paying
a special attention to the selection effects including the cosmological dimming
of surface brightness of galaxies. We find a considerably different result from
previous studies ignoring the selection effects, and these effects should
therefore be taken into account in the analysis. We find that the model of pure
luminosity evolution (PLE) of galaxies in the Einstein-de Sitter (EdS) universe
predicts much smaller counts than those observed at faint magnitude limits by a
factor of more than 10, so that a very strong number evolution of galaxies with
\eta > 3-4 must be invoked to reproduce the I_{814} counts, when parametrized
as \phi^* \propto (1+z)^\eta. However we show that such a strong number
evolution under realistic merging processes of galaxies can not explain the
steep slope of the B_{450} and V_{606} counts, and it is seriously inconsistent
with their photometric redshift distribution. We find that these difficulties
still persist in an open universe with \Omega_0 > 0.2, but are resolved only
when we invoke a $\Lambda$-dominated flat universe, after examining various
systematic uncertainties in modeling the formation and evolution of galaxies.
The present analysis revitalizes the practice of using faint number counts as
an important cosmological test, giving one of the arguments against the EdS
universe and suggests acceleration of the cosmic expansion by vacuum energy
density. While a modest number evolution of galaxies with \eta ~ 1 is still
necessary even in a Lambda-dominated universe, a stronger number evolution with
\eta > 1 is rejected from the HDF data, giving a strong constraint on the
merger history of galaxies.


title: The distance to the nearest star-forming clouds: MBM12 and MBM20

abstract:
  We present high-resolution spectra (R ~ 49,000) of stars that have parallax
measurements from the Hipparcos satellite and are projected along the line of
sight to the two nearest known star forming clouds to the Sun: MBM12 and MBM20.
The spectra were obtained with the FOCES Echelle Spectrograph at the 2.2 meter
telescope in Calar Alto, Spain and the wavelength range was chosen to include
the interstellar NaI D lines at 5889.950 Angstroms and 5895.924 Angstroms.
Since the stars are at a range of distances, we use their spectra along with
their parallaxes from Hipparcos to determine the distance to the molecular gas.
The stars in front of the cloud do not show interstellar NaI D absorption
features while the stars behind the cloud do show interstellar absorption
features. We find that both clouds are somewhat more distant than previously
estimated. The revised distance to MBM12 is 58+-5 pc < d < 90+-12 pc and the
distance to MBM20 is 112+-15 pc < d < 161+-21 pc.


title:
General Relativistic Constraints on Emission Models of Anomalous X-ray
  Pulsars

abstract:
  Most models of anomalous X-ray pulsars (AXPs) account for the observed X-ray
spectra and pulsations by means of radiation processes that occur on the
surfaces of neutron stars. For any such model, general relativistic deflection
of light severely suppresses the amplitude of the observed pulsations. We
calculate the expected pulsation amplitudes of AXPs according to various models
and compare the results with observations. We show that the high (<= 70%) pulse
amplitudes observed in some AXPs can be accounted for only if the surface
emission is localized (spot radius <40 degrees) and strongly beamed
(cos^n[theta'] with n>2, where theta' is the angle to the normal). These
constraints are incompatible with those cooling and magnetar models in which
the observed X-rays originate as thermal emission from the neutron-star
surface. Accretion models, on the other hand, are compatible with observations
for a wide range of parameters. Finally, definitive conclusions cannot be
reached on magnetospheric models, since their localization and beaming
properties are not well understood.


title: X-ray Images of Hot Accretion Flows

abstract:
  We consider the X-ray emission due to bremsstrahlung processes from hot, low
radiative-efficiency accretion flows around supermassive and galactic black
holes. We calculate surface brightness profiles and Michelson visibility
functions for a range of density profiles, rho ~ r^(-3/2+p), with 0 < p < 1, to
allow for the presence of outflows. We find that although the 1 keV emitting
region in these flows can always extend up to 10^6 Schwarzschild radii (R_S),
their surface brightness profiles and visibility functions are strongly
affected by the specific density profile. The advection-dominated solutions
with no outflows (p=0) lead to centrally peaked profiles with characteristic
sizes of only a few tens of R_S. Solutions with strong outflows (p~1) lead to
flat intensity profiles with significantly larger characteristic sizes of up to
10^6 R_S. This implies that low luminosity galactic nuclei, such as M87, may
appear as extended X-ray sources when observed with current X-ray imaging
instruments. We show that X-ray brightness profiles and their associated
visibility functions may be powerful probes for determining the relevant mode
of accretion and, in turn, the properties of hot accretion flows. We discuss
the implications of our results for observations with the Chandra X-ray
Observatory and the planned X-ray interferometer MAXIM.


title:
Near/Mid Infrared-Wavelength Study of the Interacting Galaxy Pair UGC
  12914/5 -- "Taffy"

abstract:
  We report on an infrared 1 to 17 micron study of the nearby (cz = 4600 km/s)
interacting spiral galaxy system, UGC 12914/12915, using the ground-based
Palomar 200" telescope and PFIRCAM near-infrared detector and space-based
mid-infrared imaging and spectral observations using ISOCAM and Phot-S on the
Infrared Space Observatory (ISO). Strong line emission from aromatic band
features at 6.2, 7.7 and 11.3 microns are seen in both the mid-IR imaging and
Phot-S spectro-photometry centered on the nuclei. In the connecting bridge, the
mid-infrared imaging reveals dust grains intermixed with the atomic hydrogen
gas. The heating mechanism for the hot dust is likely to be UV photons
diffusing out from the galactic disks and the H II complex located along the
extreme northeastern portion of the bridge: The dust emission, or mid-infrared
intensity per atomic hydrogen column density ratio, is consistent with heating
from the local (bridge) interstellar radiation field.


title: Microlensing Constraints on the Frequency of Jupiter Mass Planets

abstract:
  Microlensing is the only technique likely, within the next 5 years, to
constrain the frequency of Jupiter-analogs. The PLANET collaboration has
monitored nearly 100 microlensing events of which more than 20 have sensitivity
to the perturbations that would be caused by a Jovian-mass companion to the
primary lens. No clear signatures of such planets have been detected. These
null results indicate that Jupiter mass planets with separations of 1.5-3 AU
occur in less than 1/3 of systems. A similar limit applies to planets of 3
Jupiter masses between 1-4 AU.


title: Hidden Galaxies in the Fornax Cluster

abstract:
  We are using the Multibeam 21cm receiver on the Parkes Telescope combined
with the optical Two degree Field spectrograph (2dF) of the Anglo-Australian
Telescope to obtain the first complete spectroscopic sample of the Fornax
cluster. In the optical the survey is unique in that all objects (both
``stars'' and ``galaxies'') within our magnitude limits (Bj=16.5 to 19.7) are
measured, producing the most complete survey of cluster members irrespective of
surface brightness. We have detected two new classes of high surface brightness
dwarf galaxy in the cluster. With 2dF we have discovered a population of very
low luminosity (Mb approx -12) objects which are unresolved from the ground and
may be the stripped nuclei of dwarf galaxies; they are unlike any known
galaxies. In a survey of the brighter (Bj=16.5 to 18) galaxies with the
FLAIR-II spectrograph we have found a number of new high surface brightness
dwarf galaxies and show that the fraction of star-forming dwarf galaxies in the
cluster is about 30 per cent, about twice that implied by earlier morphological
classifications. Our radio observations have greatly improved upon the
sensitivity of the standard Multibeam survey by using a new ``basket weave''
scanning pattern. Our initial analysis shows that we are detecting new cluster
members with HI masses of order 10-to-the-8 Msun and HI mass-to-light ratios of
1-2 Msun/Lsun.


title: Hot Horizontal Branch Stars in the Galactic Bulge. I

abstract:
  We present the first results of a survey of blue horizontal branch (BHB)
stars in the Galactic bulge. 164 candidates with 15 < V < 17.5 in a field
7.5deg from the Galactic Center were observed in the blue at 2.4A FWHM
resolution with the AAT 2dF spectrograph. Radial velocities were measured for
all stars. For stars with strong Balmer lines, their profiles were matched to
theoretical spectrum calculations to determine stellar temperature Teff and
gravity log g; matches to metal lines yielded abundances. CTIO UBV photometry
then gave the reddening and distance to each hot star. Reddening was found to
be highly variable, with E(B-V) from 0.0 to 0.55 around a mean of 0.28.
Forty-seven BHB candidates were identified with Teff >= 7250K, of which seven
have the gravities of young stars, three are ambiguous, and 37 are HB stars.
They span a wide metallicity range, from solar to 1/300 solar. The warmer BHB's
are more metal-poor and loosely concentrated towards the Galactic center, while
the cooler ones are of somewhat higher metallicity and closer to the center.
Their red B-V colors overlap main-sequence stars, but the U-B vs. B-V diagram
separates them until E(B-V) > 0.5. We detect two cool solar-metallicity HB
stars in the bulge of our own Galaxy, the first such stars known. Still elusive
are their hot counterparts, the metal-rich sdB/O stars causing excess UV light
in metal-rich galaxies; they have V ~ 20.5 in the Bulge.


title: An Extended View of the Fornax Cluster

abstract:
  The Multibeam survey at the Parkes Radio Telescope has provided a wealth of
21 cm HI data. We have mosaicked ten standard HIPASS cubes to produce a
datacube approximately 25 deg x 25 deg in size, centred on NGC1399, the optical
centre of the Fornax Cluster. Some properties of the initial approx. 80
galaxies identified in HI are described.
  In the optical, Ferguson's Fornax Cluster Catalogue (FCC) identified 340
likely member galaxies in the central 40 sq.deg. In the radio we have detected
a more uniform sheet of about 80 galaxies at the cluster velocity extending up
to 15 deg from the cluster centre. At the cluster distance of about 15 Mpc,
this corresponds to an elongated large-scale structure (LSS) more than 7 Mpc in
extent.
  Galaxies were detected to a lower mass limit of 1x10^8M_sun and fewer than 25
of these were within the central 40 sq.deg, suggesting considerable HI
depletion of galaxies in the centre of the cluster. Further, these results
strongly indicate that HI surveys do not readily identify galaxy clusters.


title: Limits on radio emission from pulsar wind nebulae

abstract:
  We report on a sensitive survey for radio pulsar wind nebulae (PWN) towards
27 energetic and/or high velocity pulsars. Observations were carried out at 1.4
GHz using the Very Large Array and the Australia Telescope Compact Array, and
utilised pulsar-gating to search for off-pulse emission. These observing
parameters resulted in a considerably more sensitive search than previous
surveys, and could detect PWN over a much wider range of spatial scales (and
hence ambient densities and pulsar velocities). However, no emission clearly
corresponding to a PWN was discovered. Based on these non-detections we argue
that the young and energetic pulsars in our sample have winds typical of young
pulsars, but produce unobservable PWN because they reside in low density (n
approx 0.003 cm^-3) regions of the ISM. However, non-detections of PWN around
older and less energetic pulsars can only be explained if the radio luminosity
of their winds is less than 1e-5 of their spin-down luminosity, implying an
efficiency at least an order of magnitude smaller than that seen for young
pulsars.


title: Large scale magnetic fields and their dissipation in GRB fireballs

abstract:
  We consider possible geometries of magnetic fields in GRB outflows, and their
evolution with distance from the source. For magnetically driven outflows, with
an assumed ratio of magnetic to kinetic energy density of order unity, the
field strengths are sufficient for efficient production of gamma-rays by
synchrotron emission in the standard internal shock scenario, without the need
for local generation of small scale fields. In these conditions, the MHD
approximation is valid to large distances (>10^19cm). In outflows driven by
nonaxisymmetric magnetic fields, changes of direction of the field cause
dissipation of magnetic energy by reconnection. This dissipation takes place
outside the photosphere of the outflow, and can convert a significant fraction
of the magnetic energy flux into radiation.


title: Turmoil on the accretion disk of GRO J1655-40

abstract:
  During the 1996/1997 outburst of the X-ray transient GRO J1655-40 the Wide
Field Cameras onboard BeppoSAX observed the system with nearly continuous
coverage during some stages of this outburst. On two occasions we find clear
evidence for frequent dipping behaviour, characterized by several absorption
dips occurring within one binary orbit. However, during preceding and following
binary orbits there is no sign of such dips at the same range of orbital
phases. We also collected light curves of absorption dips as seen with other
X-ray instruments. Although the 65 dips that were collected occur preferably
between orbital phases 0.68 and 0.92, there is evidence that the morphology of
the material causing the dips is changing over just one binary orbit. The short
duration (~minutes) and phasing of the dips are discussed in the framework of
models where the stream from the companion star encounters a hot outer rim on
the accretion disk.


title:
Stringent constraints on single-star evolution theory from masses and
  radii of well-detached binaries in star clusters

abstract:
  Which sample of objects can give strong constraints on single-star evolution
theory ? Whilst star cluster members share the same age and the same
metallicity, many questions (e.g. field stars contamination, stellar rotation,
presence of unresolved binaries) are difficult to clarify properly when using
their colour-magnitude diagram to compare with theoretical isochrones.
Alternatively, binary stars can be used to put constraints on theoretical
predictions. However, while the stellar mass is accurately known for some
sample of well-detached binary stars, their metallicities are often poorly
known. Then it appears that a better test could be obtained by combining both
advantages (well-detached binaries members of a star cluster). This idea is
applied in this work to binaries in the Hyades and one binary in the Cepheus
OB3 association to test the validity of independent sets of theoretical tracks.
A detailed comparison of theoretical vs. observational masses (and radii when
possible) is presented.


title: The Population of Faint Transients in the Galactic Centre

abstract:
  BeppoSAX has detected a population of faint transient X-ray sources in the
Galactic Centre. I show that a simple irradiated-disc picture gives a
consistent fit to the properties of this population, and that it probably
consists of low-mass X-ray binaries which have evolved beyond their minimum
periods ~80 min. Since all post-minimum systems are transient, and neutron-star
LMXBs are more common than black-hole LMXBs in the Galaxy, the majority of
these systems should contain neutron stars, as observed. This picture predicts
that the Galactic Centre transients should have orbital periods in the range
\~80-120 minutes, and that most of them should repeat in the next few years. In
this case, the total number of post-minimum transients in the Galaxy would be
considerably smaller than the usual estimates of its total LMXB population. I
discuss possible reasons for this.


title:
On the mass of moderately rotating strange stars in the MIT bag model
  and LMXBs

abstract:
  We compute the maximum mass of moderately rotating strange stars as a
function of the strange quark mass, of the QCD coupling constant, $\alpha_c$,
and of the bag constant (vacuum energy density), $B$, in the MIT bag model of
quark matter with lowest order quark-gluon interactions. For a fixed value of
$B$, the maximum stellar mass depends only weakly on $\alpha_c$, and is
independent of this coupling in the limit of massless quarks. However, if it is
the value of the chemical potential of quark matter at zero pressure which is
held constant, for example at the value corresponding to the stability limit of
nucleons against conversion to quark matter, the maximum mass of the strange
star is higher by up to 25% for $\alpha_c=0.6$, than for non-interacting
quarks, and this may be relevant in the discussion of kHz QPO sources. The
maximum mass of a non-rotating strange star could be sufficiently high to allow
an orbital frequency as low as 1.0 kHz in the marginally stable orbit. However,
for all $\alpha_c<0.6$, the stellar mass cannot exceed $2.6M_\odot$ at any
rotational period $\ge1.6 $ms.