title: WFPC2 Stellar Photometry with HSTphot

abstract:
  HSTphot, a photometry package designed to handle the undersampled PSFs found
in WFPC2 images, is introduced and described, as well as some of the
considerations that have to be made in order to obtain accurate PSF-fitting
stellar photometry with WFPC2 data. Tests of HSTphot's internal reliability are
made using multiple observations of the same field, and tests of external
reliability are made by comparing with DoPHOT reductions of the same data.


title:
Halo Properties in Cosmological Simulations of Self-Interacting Cold
  Dark Matter

abstract:
  We present a comparison of halo properties in cosmological simulations of
collisionless cold dark matter (CDM) and self-interacting dark matter (SIDM)
for a range of dark matter cross sections. We find, in agreement with various
authors, that CDM yields cuspy halos that are too centrally concentrated as
compared to observations. Conversely, SIDM simulations using a Monte Carlo
N-body technique produce halos with significantly reduced central densities and
flatter cores with increasing cross section. We introduce a concentration
parameter based on enclosed mass that we expect will be straightforward to
determine observationally, unlike that of Navarro, Frenk & White, and provide
predictions for SIDM and CDM. SIDM also produces more spherical halos than CDM,
providing possibly the strongest observational test of SIDM. We discuss our
findings in relation to various relevant observations as well as SIDM
simulations of other groups. Taking proper account of simulation limitations,
we find that a dark matter cross section per unit mass of sigma_DM ~=
10^{-23}-10^{-24} cm^2/GeV is consistent with all current observational
constraints.


title: A Near-Infrared L Band Survey of the Young Embedded Cluster NGC 2024

abstract:
  We present the results of the first sensitive L band (3.4 micron) imaging
study of the nearby young embedded cluster NGC 2024. Two separate surveys of
the cluster were acquired in order to obtain a census of the circumstellar disk
fraction in the cluster. From an analysis of the JHKL colors of all sources in
our largest area, we find an infrared excess fraction of > 86%. The JHKL colors
suggest that the infrared excesses arise in circumstellar disks, indicating
that the majority of the sources which formed in the NGC 2024 cluster are
currently surrounded by, and likely formed with circumstellar disks. The excess
fractions remain very high, within the errors, even at the faintest L
magnitudes from our deeper surveys suggesting that disks form around the
majority of the stars in very young clusters such as NGC 2024 independent of
mass. From comparison with published JHKL observations of Taurus, we find the K
- L excess fraction in NGC 2024 to be consistent with a high initial incidence
of circumstellar disks in both NGC 2024 and Taurus. Because NGC 2024 represents
a region of much higher stellar density than Taurus, this suggests that disks
may form around most of the YSOs in star forming regions independent of
environment. We find a relatively constant JHKL excess fraction with increasing
cluster radius, indicating that the disk fraction is independent of location in
the cluster. In contrast, the JHK excess fraction increases rapidly toward the
central region of the cluster, and is most likely due to contamination of the K
band measurements by bright nebulosity in the central regions of the cluster.
We identify 45 candidate protostellar sources in the central regions of the NGC
2024 cluster, and find a lower limit on the protostellar phase of early stellar
evolution of 0.4 - 1.4 X 10^5 yr, similar to that in Taurus.


title:
A Plasma Prism Model for an Anomalous Dispersion Event in the Crab
  Pulsar

abstract:
  In 1997 October daily monitoring observations of the Crab pulsar at 327 MHz
and 610 MHz with an 85ft telescope in Green Bank, WV showed a jump in the
dispersion measure by 0.1 cm$^{-3}$ pc. Pulses were seen simultaneously at both
old and new dispersions over the course of several days. During the dispersion
jump the pulsed flux diminished by an order of magnitude. In the months before
this event the average pulse profiles contained faint ``ghost'' pulse
components offset in phase from the regular main pulse and interpulse
components by a nearly frequency independent time delay that quadratically
diminished to zero over a month. After the dispersion event there was an order
of magnitude increase in the level of scattering, as measured by pulse
broadening at 327 MHz. There was also a curious shift in the rotational phase,
a slowing down, at both frequencies at the time of the dispersion jump which we
associate with intrinsic timing noise. All of the observed phenomena except
this slowing down can be explained by the variable perturbing optics of a
triangular plasma prism that is located in the filamentary interface between
the synchrotron nebula and the supernova ejecta and which crosses the line of
sight over a period of months. The required density, scale length and velocity
are reasonable given previous observations and analysis of these filaments. Our
study thus provides a probe of the plasma column on scales of 30 microarcsecond
to 3 milliarcsecond ($10^{12-14}$ cm) which complements scales accessible to
optical emission line studies with HST resolution ($10^{16-18}$ cm). In
combination both observations provide a detailed look at a sample of the
interface region that can be matched statistically to the results of numerical
simulations.


title: Cosmic Ray Generation by Quasar Remnants: Constraints and Implications

abstract:
  The quasar remnant cores of nearby giant elliptical galaxies NGC 4486 (M87),
NGC 1399, NGC 4649 and NGC 4472 are the sites of supermassive (greater than one
billion solar masses) black holes. These objects are investigated as to the
viability of the conjecture that they could harbor compact dynamos capable of
generating the highest energy cosmic rays. For an accretion process involving
an equipartition magnetic field near the event horizons of the underlying
putative spun-up black holes, the energy achievable in accelerating protons
could well be greater or equal than 100 EeV for all these when only considering
the drag induced by curvature radiation. Estimates of the SED (spectral energy
distribution) of ambient core photons lead to the conclusion that the energy
losses arising from photo-pion production in proton collisions with these
target photons are relatively small for all but M87. For M87, the ambient
photon field is likely to be a limiting factor. Accretion rates on the order of
one solar mass per year, comparable to the Bondi rates and to the stellar mass
loss rates, are associated with (greater than 100 EeV) cosmic ray generation in
the other (electromagnetically dark) galactic core sites. If these sites are
found to be sources of such cosmic rays, it would suggest the presence of a
global inflow of interstellar gas all the way into the center of the host
galaxy.


title:
Removing radio interference from contaminated astronomical spectra using
  an independent reference signal and closure relations

abstract:
  The growing level of radio frequency interference (RFI) is a recognized
problem for research in radio astronomy. This paper describes an intuitive but
powerful RFI cancellation technique that is suitable for radio spectroscopy
where time-averages are recorded. An RFI ``reference signal,'' is constructed
from the cross power spectrum of the signals from the two polarizations of a
reference horn pointed at the source of the RFI signal. The RFI signal paths
obey simple phase and amplitude closure relations, which allows computation of
the RFI contamination in the astronomical data and the corrections to be
applied to the astronomical spectra.
  Since the method is immune to the effects of multipath scattering in both the
astronomy and reference signal channels, ``clean copies'' of the RFI signal are
not required.
  The method could be generalized (1) to interferometer arrays, (2) to correct
for scattered solar radiation that causes spectral ``standing waves'' in
single-dish spectroscopy, and (3) to pulsar survey and timing applications
where a digital correlator plays an important role in broadband pulse
dedispersion.
  Future large radio telescopes, such as the proposed LOFAR and SKA arrays,
will require a high degree of RFI suppression and could implement the technique
proposed here with the benefit of faster electronics, greater digital precision
and higher data rates.


title:
Nonlinear Evolution of Very Small Scale Cosmological Baryon
  Perturbations at Recombination

abstract:
  The evolution of baryon density perturbations on very small scales is
investigated. In particular, the nonlinear growth induced by the radiation drag
force from the shear velocity field on larger scales during the recombination
epoch, which is originally proposed by Shaviv in 1998, is studied in detail. It
is found that inclusion of the diffusion term which Shaviv neglected in his
analysis results in rather mild growth whose growth rate is $\ll 100$ instead
of enormous amplification $\sim 10^4$ of Shaviv's original claim since the
diffusion suppresses the growth. The growth factor strongly depends on the
amplitude of the large scale velocity field. The nonlinear growth mechanism is
applied to density perturbations of general adiabatic cold dark matter (CDM)
models. In these models, it has been found in the previous works that the
baryon density perturbations are not completely erased by diffusion damping if
there exists gravitational potential of CDM. With employing the perturbed rate
equation which is derived in this paper, the nonlinear evolution of baryon
density perturbations is investigated. It is found that: (1) The nonlinear
growth is larger for smaller scales. This mechanism only affects the
perturbations whose scales are smaller than $\sim 10^2M_\odot$, which are
coincident with the stellar scales. (2) The maximum growth factors of baryon
density fluctuations for various COBE normalized CDM models are typically less
than factor 10 for $3-\sigma$ large scale velocity peaks. (3) The growth factor
depends on $\Omega_{\rm b}$.


title: ASCA Observation of the Lyman-limit Quasar PKS2145+067

abstract:
  X-ray observation of a famous Lyman-limit quasar PKS2145+067at z_em= 0.990
was carried with ASCA. The source showed a 2-10 keV flux of 1.3 E-11 erg cm^-2
sec^-1 (L_X = 2.5 E46 erg sec^-1 for H_0=50 km s^-1 Mpc^-1) described by a
power-law spectrum with a photon index Gamma=1.63 +/- 0.04 . In the ASCA energy
band, no excess absorption was detected implying the absorption column density
at z_ ab=0.791 was less than 1.6 E21 cm^-2 if absorbing medium had a metal
abundance of 0.5 solar. Comparison with previous Einstein and ROSAT
observations shows that PKS2145+067 has increased its luminosity by a factor of
2--3 between 1991 and 1998.


title: 150 keV Emission from PKS2149-306 with BeppoSAX

abstract:
  A BeppoSAX observation of the z=2.34 quasar PKS2149-306 produced a strong
signal in the high energy PDS instrument up to a maximum observed energy of
nearly 50 keV, 150 keV in the quasar frame. The Beppo-SAX spectrum spans almost
3 decades (0.3-150 keV, quasar frame) and shows an extremely hard
(alpha=0.4+/-0.05) X-ray spectrum above 3 keV (comparable to the X-ray
background slope), and either a softer (alpha=1.0(+0.6, -0.3)) low energy
component, or an ionized absorber at zero redshift. No evidence is seen of an
Fe-K emission line (EW<167 eV at 6.5 keV quasar frame) or a Compton hump
(R<0.3). A bremsstrahlung fit gives kT(rest)=46(+32, -16) keV, similar to the
X-ray background value, and a high energy cut-off power law requires E(cut)>120
keV (quasar frame).
  The SED of PKS 2149-306 shows two peaks at ~ 10(12+/-0.5}Hz and
\~10(21+/-1.0)Hz (~ 0.3 mm and ~ 4 MeV), strongly resembling a low energy
cutoff BL~Lac object (LBL). The ratio of the two peaks shows an extreme Compton
dominance (C_D=1.4+/- 0.4), as in flat spectrum radio quasars (FSRQs). The
presence of an additional `optical/UV big bump' component may provide photons
that cool the jet, suppressing the radio emission.


title:
Can Rotational Properties of General Relativistic Compact Objects Be
  Predicted from Static Ones?

abstract:
  We propose an approximate description of basic parameters (radius, mass and
oblateness) of general relativistic compact rotating objects in terms of the
parameters of the static configuration and of the angular velocity only. The
representation in terms of static properties is derived using the condition of
stationary equilibrium together with some phenomenological assumptions. The
predicted radius and mass of rotating neutron star (described by some realistic
equations of state) and strange star (described by the bag model equation of
state) are compared with data obtained by numerical integration of
gravitational field equation. The obtained formulae also allow a simple
derivation of the "empirical" equation relating the maximum rotation frequency
$\Omega_{max}$ of uniformly rotating star models to the mass and radius of the
maximum allowable mass configuration of the non-rotating model.


title: Soft state of Cygnus X-1: stable disk and unstable corona

abstract:
  Two component X-ray spectra (soft multicolor black body plus harder power
law) are frequently observed from accreting black holes. These components are
presumably associated with the different parts of the accretion flow (optically
thick and optically thin respectively) in the vicinity of the compact source.
Most of the aperiodic variability of the X-ray flux on the short time scales is
associated with the harder component. We suggest that drastically different
amplitudes of variability of these two components are simply related to the
very different viscous time scales in the geometrically thin and geometrically
thick parts of the accretion flow. In the geometrically thin disks variations
of viscosity or mass accretion rate occurring at large radius from the black
hole on the local dynamical or thermal time scales do not cause any significant
variations of the mass accretion rate at smaller radii due to a very long
diffusion time. Any variations on the time scales shorter than the diffusion
time scale are effectively dampened. On the contrary such variations can easily
survive in the geometrically thick flows and as a result the mass accretion
rate in the innermost region of the flow will reflect modulations of the mass
accretion rate added to the flow at any distance from the black hole. Therefore
if primary instabilities operate on the short time scales then the stability of
the soft component (originating from the geometrically thin and optically thick
flow) and variability of the hard component (coming from the geometrically
thick and optically thin flow) are naturally explained.


title:
Theoretical Models for Classical Cepheids: VII. Metallicity effects on
  the Cepheid distance scale

abstract:
  We use theoretical Period-Luminosity and Period-Luminosity-Color relations in
the VI passbands, as based on nonlinear, nonlocal and time-dependent convective
pulsating models, to predict the reddening and true distance modulus of distant
Cepheids observed with the Hubble Space Telescope. By relying on the pulsating
models with metal content Z=0.008, we find that the theoretical predictions
agree to the values obtained by the Extragalactic Distance Scale Key Project on
the basis of empirical Period-Luminosity relations referenced to LMC variables.
In the meantime, from the theoretical relations with Z=0.004 and 0.02 we find
that the predicted E(B-V) and mu_0 decrease as the adopted metal content
increases. This suggests a metallicity correction to LMC-based distances as
given by about -0.27 mag/dex. Such a theoretical correction appears supported
by an existing, although weak, correlation between the Cepheid distance and the
[O/H] metallicity of galaxies within a given group or cluster, as well as by a
similar correlation between the H_0 estimate and the [O/H] metallicity of the
galaxies which calibrate the SNIa luminosity. On the contrary, the metallicity
correction earlier suggested on empirical grounds seem to be excluded.
Eventually we suggest that the average value <H_0> ~ 67 km/s/Mpc provided by
the Key Project team should increase at least up to ~ 69 km/s/Mpc.


title: Classical Cepheid pulsation models. VI. The Hertzsprung progression

abstract:
  We present the results of an extensive theoretical investigation on the
pulsation behavior of Bump Cepheids. We constructed several sequences of full
amplitude, nonlinear, convective models by adopting a chemical composition
typical of Large Magellanic Cloud (LMC) Cepheids (Y=0.25, Z=0.008) and stellar
masses ranging from 6.55 to 7.45 solar masses. We find that theoretical light
and velocity curves reproduce the HP, and indeed close to the blue edge the
bump is located along the descending branch, toward longer periods it crosses
at first the luminosity/velocity maximum and then it appears along the rising
branch. In particular, we find that the predicted period at the HP center is
11.24 +- 0.46 d and that such a value is in very good agreement with the
empirical value estimated by adopting the Fourier parameters of LMC Cepheid
light curves. We also find that the models at the HP center are located within
the resonance region but not on the 2:1 resonance line. Interestingly enough,
the predicted Bump Cepheid masses, based on a Mass-Luminosity (ML) relation
which neglects the convective core overshooting, are in good agreement with the
empirical masses of Galactic Cepheids estimated by adopting the Baade-Wesselink
method. Even by accounting for the metallicity difference between Galactic and
LMC Cepheids, this result seems to settle down the long-standing problem of the
Bump mass discrepancy. Finally, the dynamical behavior of a cool Bump Cepheid
model provides a plain explanation of an ill-understood empirical evidence.


title: The Very Low Mass Component of the Gliese 105 System

abstract:
  Multiple-epoch, multicolor images of the astrometric binary Gliese 105A and
its very low mass companion Gliese 105C have been obtained using the Hubble
Space Telescope's Wide Field Planetary Camera 2 (WFPC2) and Near-Infrared
Camera and Multi-Object Spectrometer (NICMOS). The optical and near-infrared
colors of Gl 105C strongly suggest a spectral type of M7V for that star.
Relative astrometric measurements spanning 3 yr reveal the first evidence of Gl
105C's orbital motion. Previous long-term astrometric studies at Sproul and
McCormick Observatories have shown that the period of Gl 105A's perturbation is
approximately 60 yr. To satisfy both the observed orbital motion and Gl 105A's
astrometric period, Gl 105C's orbit must have an eccentricity of approximately
0.75 and a semimajor axis of approximately 15 AU. Measurements of Gl 105A's
radial velocity over 12 yr show a linear trend with a slope of 11.3 m/s/yr,
which is consistent with these orbital constraints and a nearly face-on orbit.
As no other faint companions to Gl 105A have been detected, we conclude that Gl
105C is probably the source of the 60-yr astrometric perturbation.


title:
Theoretical Models of Young Open Star Clusters: Effects of a Gaseous
  Component and Gas Removal

abstract:
  We construct a family of semi-analytic models for young open clusters,
including a gaseous component and varying assumptions about the distribution
function for the stellar component. The parameters of these models are informed
by observed open clusters and general theoretical considerations regarding
cluster formation. We use this framework to estimate the fraction of the
stellar component that remains gravitationally bound after the gaseous
component disperses. The remaining stellar fraction is a smooth function of the
star formation efficiency, and depends on the distribution function of the
stars. We calculate the fraction of stars remaining for this class of open
cluster models and provide fitting formulae for representative cases.


title:
New identification of the near infrared Source in the "born-again"
  planetary nebula A58 (=V605 Aql)

abstract:
  The central dust knot V605 Aql (= Nova Aql 1919) in the planetary nebula
  A58 is the result of a late-helium flash. Previous near infrared (NIR)
measurements yield very bright fluxes. This exceeds model predictions, based on
other wavelengths and estimates of the visual extinction within the object, by
a factor of 100. Using NIR imaging by the DENIS instrument, we found that the
source was misidentified in previous (single channel photometer) observations.
We present the NIR flux of A58 and identify an unrelated stellar field source,
which in fact represents the NIR source attributed to A58, according to the
literature. The new identification is consistent with model predictions for the
source. An accurate astrometry for the core of A58 is also provided.


title: Sub-mm and X-ray background: two unrelated phenomena?

abstract:
  Obscured AGNs are thought to contribute a large fraction of the hard X-ray
background (2-10 keV), and have also been proposed as the powerhouse of a
fraction of the SCUBA sources which make most of the background at 850um, thus
providing a link between the two spectral windows. We have tackled this issue
by comparing data at 2-10 keV and at 850um for a sample of 34 sources at fluxes
(or limiting fluxes) which resolve most of the background in the two bands. We
present here new SCUBA observations, and new correlations between separate data
sets retrieved from the literature. Similar correlations presented by others
are added for completeness. None of the 11 hard X-ray (2-10 keV) sources has a
counterpart at 850um, with the exception of a Chandra source in the SSA13
field, which is a candidate type 2, heavily absorbed QSO at high redshift. The
ratios F(850um)/F(5keV) (mostly upper limits) of the X-ray sources are
significantly lower than the value observed for the cosmic background. In
particular, we obtain that 2-10 keV sources brighter than 10^-15 erg s^-1
cm^-2, which make at least 75% of the background in this band, contribute for
less than 7% to the submillimeter background. Out of the 24 SCUBA sources, 23
are undetected by Chandra. The ratios F(850um)/F(5keV) (mostly lower limits) of
these SCUBA sources indicate that most of them must be powered either by
starburst activity, or by an AGN which is obscured by a column Nh > 10^25
cm^-2, with a reflection efficiency in the hard X rays significantly lower than
1% in most cases. However, AGNs of this type could not contribute significantly
to the 2-10 keV background.


title:
USA and RXTE Observations of a Variable Low-Frequency QPO in XTE
  J1118+480

abstract:
  The USA experiment on ARGOS and RXTE have exensively observed the X-ray
transient XTE J1118+480 during its recent outburst in 2000 April--June. We
present detailed monitoring of the evolution of a low frequency QPO which
drifts from 0.07 Hz to 0.15 Hz during the outburst. We examine possible
correlations of the QPO frequency with the flux and spectral characteristics of
the source, and compare this QPO to low frequency QPOs observed in other black
hole candidates.


title:
Photospheric Metals in the FUSE Spectrum of the Subdwarf B Star
  PG0749+658

abstract:
  We present an abundance analysis of the far-ultraviolet (905--1187 A)
spectrum of the subdwarf B star PG0749+658, obtained by the Far Ultraviolet
Spectroscopic Explorer (FUSE). The data have a resolution of about R = lambda /
Delta lambda = 12000--15000 (20--25 km/s). We determine C, N, Si, P, S, Cr, Mn,
Fe, Co, and Ni abundances, and upper limits on the abundance of Cl and V, using
a grid of synthetic spectra based on a LTE stellar atmosphere model. He, C, N,
Si, and Cl are depleted by a factor of >~10 with respect to solar, while P, S,
and Fe are diminished by less than a factor of 10. We measure a solar abundance
of Cr, Mn, and Co and a Ni enhancement of ~0.6 dex. We compare these values to
predictions based on radiative levitation theory. The radial velocity of the
foreground interstellar material has one dominant component, and coincides with
that of the photospheric lines.


title:
Eclipsing Binaries in the OGLE Variable Star Catalogs.V. Long-Period
  Beta Lyrae-type Systems in the Small Magellanic Cloud and the PLC-beta
  Relation

abstract:
  Thirty eight long-period (P>10 days), apparently contact binary stars
discovered by the OGLE-II project in the SMC appear to be Beta Lyrae-type
systems with ellipsoidal variations of the cool components dominating over
eclipse effects in the systemic light variations and in the total luminosity. A
new period-luminosity- color (PLC) relation has been established for these
systems; we call it the PLC-beta relation, to distinguish it from the Cepheid
relation. Two versions of the PLC-beta relation - based on the (B-V)0 or (V-I)0
color indices - have been calibrated for 33 systems with (V-I)0>0.25 spanning
the orbital period range of 11 to 181 days. The relations can provide
maximum-light, absolute-magnitude estimates accurate to epsilon-M_V~0.35 mag.
within the approximate range -3<M_V<+1. In terms of their number in the SMC,
the long-period Beta Lyrae-type binaries are about 50 times less common than
the Cepheids. Nevertheless, their large luminosities coupled with continuous
light variations make these binaries very easy to spot in nearby galaxies, so
that the PLC-beta relation can offer an auxiliary and entirely independent
method of distance determination to nearby stellar systems rich in massive
stars. The sample of the long-period Beta Lyrae systems in the SMC analyzed in
this paper is currently the best defined and uniform known sequence of such
binaries.


title: The Charge Transfer Efficiency and Calibration of WFPC2

abstract:
  A new determination of WFPC2 photometric corrections is presented, using
HSTphot reduction of the WFPC2 Omega Centauri and NGC 2419 observations from
January 1994 through March 2000 and a comparison with ground-based photometry.
No evidence is seen for any position-independent photometric offsets (the
"long-short anomaly"); all systematic errors appear to be corrected with the
CTE and zero point solution. The CTE loss time dependence is determined to be
very significant in the Y direction, causing time-independent CTE solutions
(Stetson 1998; Saha, Lambert, & Prosser 2000) to be valid only for a small
range of times. On average, the present solution produces corrections similar
to Whitmore, Heyer, & Casertano (1999), although with an improved functional
form that produces less scatter in the residuals and determined with roughly a
year of additional data. In addition to the CTE loss characterization, zero
point corrections are also determined as functions of chip, gain, filter, and
temperature. Of interest, there are chip-to-chip differences of order 0.01-0.02
magnitudes relative to the Holtzman et al. (1995) calibrations, and the present
study provides empirical zero point determinations for the non-standard filters
such as the frequently-used F450W, F606W, and F702W.


title: The Highest Redshift Radio Galaxies

abstract:
  At low redshifts powerful radio sources are uniquely associated with massive
galaxies, and are thought to be powered by supermassive black holes. Modern 8m
-- 10m telescopes may be used used to find their likely progenitors at very
high redshifts to study their formation and evolution.


title: Physical parameters and emission mechanism in Gamma-Ray Bursts

abstract:
  Detailed information on the physical parameters in the sources of
cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions
consistent with observations. Model-independent requirements posed by these
assumptions on the emission mechanism in GRBs are formulated. It is found that
the observed radiation in sub-MeV energy range is generated by the synchrotron
emission mechanism, though about ten per cent of the total GRB energy should be
converted via the inverse Compton process into ultra-hard spectral domain
(above 100 GeV). We estimate the magnetic field strength in the emitting
region, the Lorentz factor of accelerated electrons, and the typical energy of
IC photons.
  We show that there is a "line-of-death" relation for GRBs and derive from
this relation the lower limits on both GRB duration and GRB variability
timescale. The upper limit on the Lorentz factor of GRB fireballs is also
found. We demonstrate that steady-state electron distribution consistent with
the Compton losses may produce different spectral indices, e.g., 3/4 as opposed
to the figure 1/2 widely discussed in the literature. It is suggested that the
changes in the decline rate observed in the lightcurves of several GRB
afterglows may be due to the time evolution of spectral break, which appears in
the synchrotron emission generated by steady-state self-consistent electron
distribution.


title: 3-D MHD Numerical Simulations of Cloud-Wind Interactions

abstract:
  We present results from three-dimensional (3-D) numerical simulations
investigating the magnetohydrodynamics of cloud-wind interactions. The initial
cloud is spherical while the magnetic field is uniform and transverse to the
cloud motion. A simplified analytical model that describes the magnetic energy
evolution in front of the cloud is developed and compared with simulation
results. In addition, it is found the interaction of the cloud with a
magnetized interstellar medium (ISM) results in the formation of a highly
structured magnetotail. The magnetic flux in the wake of the cloud organizes
into flux ropes and a reconnection, current sheet is developed, as field lines
of opposite polarity are brought close together near the symmetry axis. At the
same time, magnetic pressure is strongly enhanced at the leading edge of the
cloud from the stretching of the field lines that occurs there. This has an
important dynamical effect on the subsequent evolution of the cloud, since some
unstable modes tend to be strongly enhanced.


title:
Implications of mean field accretion disc theory for vorticity and
  magnetic field growth

abstract:
  In addition to the scalar Shakura-Sunyaev $\alpha_{ss}$ turbulent viscosity
transport term used in simple analytic accretion disc modeling, a pseudoscalar
transport term also arises. The essence of this term can be captured even in
simple models for which vertical averaging is interpreted as integration over a
half-thickness and one separately studies each hemisphere. The additional term
highlights a complementarity between mean field magnetic dynamo theory and
accretion disc theory treated as a mean field theory. Such pseudoscalar terms
have been studied, and can lead to large scale magnetic field and vorticity
growth. Here it is shown that vorticity can grow even in the simplest azimuthal
and half-height integrated disc model, for which mean quantities depend only on
radius. The simplest vorticity growth solutions seem to have scales and vortex
survival times consistent those required for facilitating planet formation.
Also it is shown that when the magnetic back-reaction is included to lowest
order, the pseudoscalar driving the magnetic field growth and that driving the
vorticity growth will behave differently with respect to shearing and
non-shearing flows: the former can reverse sign in the two cases, while the
latter will have the same sign.


title: Effect of a neutron-star crust on the r-mode instability

abstract:
  The presence of a viscous boundary layer under the solid crust of a neutron
star dramatically increases the viscous damping rate of the fluid r-modes. We
improve previous estimates of this damping rate by including the effect of the
Coriolis force on the boundary-layer eigenfunction and by using more realistic
neutron-star models. If the crust is assumed to be perfectly rigid, the
gravitational radiation driven instability in the r-modes is completely
suppressed in neutron stars colder than about 1.5 x 10^8 K. Energy generation
in the boundary layer will heat the star, and will even melt the crust if the
amplitude of the r-mode is large enough. We solve the heat equation explicitly
(including the effects of thermal conduction and neutrino emission) and find
that the r-mode amplitude needed to melt the crust is approximately a_c = 5 x
10^{-3} for maximally rotating neutron stars. If the r-mode saturates at an
amplitude larger than a_c, the heat generated is sufficient to maintain the
outer layers of the star in a mixed fluid-solid state analogous to the pack ice
on the fringes of the Arctic Ocean. We argue that in young, rapidly rotating
neutron stars this effect considerably delays the formation of the crust. By
considering the dissipation in the ice flow, we show that the final spin
frequency of stars with r-mode amplitude of order unity is close to the value
estimated for fluid stars without a crust.


title:
The Harmonic and Sideband Structure of the Kilohertz Quasi-Periodic
  Oscillations in Sco X-1

abstract:
  We use data from the Rossi X-ray Timing Explorer to search for harmonics and
sidebands of the two simultaneous kilohertz quasi-periodic oscillations (kHz
QPOs) in Sco X-1. We do not detect any of these harmonics or sidebands, with
95% confidence upper limits to their power between ~ 1% and ~ 10% of the power
of the upper kHz QPO. The oscillations produced at these frequencies may be
attenuated in a scattering corona around the neutron star. We find that upper
limits to the unattenuated power of some of the strongest theoretically
predicted harmonics and sidebands are as low as ~ 2% of the unattenuated power
of the high-frequency QPO in Sco X-1.


title:
Timing the Kilohertz Quasi-Periodic Oscillations in Low-mass X-ray
  Binaries

abstract:
  I describe a new technique that we have been using in the past few years to
get precise measurements of the frequency separation of the kHz QPOs in some
Low-mass X-ray binaries. I show how this technique (that we call
"shift-and-add") works, and I present some of the results we obtained using it.


title:
The Amplitude of the Kilohertz Quasi-periodic Oscillations in 4U
  1728-34, 4U 1608-52, and Aql X-1, as a Function of X-ray Intensity

abstract:
  We study the kilohertz quasi-periodic oscillations (kHz QPOs) in the low-mass
X-ray binaries 4U 1728-34, 4U 1608-52, and Aql X-1. Each source traces out a
set of nearly parallel lines in a frequency vs. X-ray count rate diagram. We
find that between two of these tracks, for similar QPO frequency, the source
count rate can differ by up to a factor of ~ 4, whereas at the same time the
rms amplitude of the kHz QPOs is only a factor of ~ 1.1 different. We also find
that, for 4U 1608-52 and Aql X-1, the rms spectrum of the kHz QPOs does not
depend upon which track the source occupies in the frequency vs. X-ray count
rate diagram. Our results for 4U 1728-34, 4U 1608-52, and Aql X-1 are
inconsistent with scenarios in which the properties of the kHz QPOs are only
determined by the mass accretion rate through the disk, whereas X-ray count
rate also depends upon other sources of energy that do not affect the QPOs.


title: Limits on the cosmic-ray ionization rate toward massive young stars

abstract:
  Recent models of the envelopes of seven massive protostars are used to
analyze observations of H3+ infrared absorption and H13CO+ submillimeter
emission lines toward these stars, and to constrain the cosmic-ray ionization
rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 s^-1,
in good agreement with diffuse cloud models and with recent Voyager/Pioneer
data but factors of up to 7 lower than found from the H3+ data. No relation of
zeta with luminosity or total column density is found, so that local (X-ray)
ionization and shielding against cosmic rays appear unimportant for these
sources. The difference between the H3+ and H13CO+ results and the correlation
of N(H3+) with heliocentric distance suggest that intervening clouds contribute
significantly to the H3+ absorptions in the more distant regions. The most
likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in
neutral form or in CO.


title:
Near-infrared photometry of isolated spirals with and without an AGN.
  II: Photometric properties of the host galaxies

abstract:
  We present the analysis of morphological and photometric properties of a
sample of isolated spirals with (18) and without (11) an active nucleus, based
on near-infrared imaging in the J and K' bands. The aim of this analysis is to
find the differential properties that could be directly connected with the
phenomenon of nuclear activity. We stress the importance of using isolated
objects for that purpose. Our study shows that both sets of galaxies are
similar in their global properties: they define the same Kormendy relation,
their disk components share the same properties, the bulge and disk scale
lengths are correlated in a similar way, bar strengths and lengths are similar
for primary bars. Hosts of isolated Seyfert galaxies have bulge and disk
properties comparable to those of isolated non active spirals. Central colors
(the innermost 200 pc) of active galaxies are redder than the centers of non
active spirals, most probably due to AGN light being re-emitted by the hot dust
and/or due to circumnuclear star formation, through the contribution of
giants/supergiants. Only one of the Seyfert galaxies in our sample does not
present a primary bar,but both primary and secondary bars are equally present
in active and control objects. Secondary central elongations (associated with
secondary bars, lenses, rings or disks) may be somewhat different. Numerical
models indicate that such secondary bars are not strictly necessary to feed the
central engine when a primary bar is present. Our results show that down to
scales of 100-300 pc, there are no obvious differences between active and non
active spiral galaxies.


title: Time-scales of Radio Emission in PSR J0437-4715 at 327 MHz

abstract:
  Time-scales of radio emission are studied in PSR J0437-4715 at 327 MHz using
almost half a million periods of high quality data from Ooty Radio Telescope.
The radio emission in this milli second pulsar occurs on a short (s) time-scale
of approximately 0.026 +- 0.001 periods, and on a (l) time-scale that is much
longer than the widths of the components of the integrated profile
(approximately 0.05 periods). The width of the s emission increases with its
increasing relative contribution to the total radio emission. This may provide
constraints for the details of discharge of vacuum gaps above pulsar polar
caps. The s emission occasionally takes place in the form of intense spikes,
which are confined to the main component of the integrated profile for 90 per
cent of the time. The positions of spikes within a component of the integrated
profile have no simple relation to the shape of that component. This may have
impact on the interpretation of the integrated profile components in terms of
independent regions of emission on the polar cap.


title: BeppoSAX Observations of the Soft Gamma-Ray Repeater SGR 1806-20

abstract:
  We have observed with the BeppoSAX satellite the quiescent counterpart of the
Soft Gamma-ray Repeater SGR 1806-20. Observations performed in October 1998 and
in March 1999 showed that this pulsar continued its long term spin-down trend
at an average rate of ~8 10e-11 s/s while its flux and spectrum remained
remarkably constant between the two observations, despite the soft gamma-ray
bursting activity that occurred in this period. We also reanalyzed archival
ASCA data, that when compared with the new BeppoSAX observations, show evidence
for a long term variation in luminosity.


title:
Testing Dispersion Relations of Quantum $\kappa$-Poincar\'e Algebra on
  Cosmological Ground

abstract:
  Following the procedure proposed recently by Martin and Brandenberger we
investigate the spectrum of the cosmological perturbations in the case when the
``trans-Plackian'' dispersion relations are motivated by the quantum
$\kappa$-Poincar\'e algebra. We find that depending on the choice of initial
conditions of the perturbations, the spectrum either differs from the flat one
for for instantaneous Minkowski vacuum or, in the case of initial conditions
minimizing energy density, leads to the observed scale-invariant
Harrison-Zel'dovich spectrum in the Friedmann epoch.


title: Intermediate-mass star models with different helium and metal contents

abstract:
  We present a comprehensive theoretical investigation of the evolutionary
properties of intermediate-mass stars. The evolutionary sequences were computed
from the Zero Age Main Sequence up to the central He exhaustion and often up to
the phases which precede the carbon ignition or to the reignition of the
H-shell which marks the beginning of the thermal pulse phase. The evolutionary
tracks were constructed by adopting a wide range of stellar masses
($3\leq$\msun$\leq15$) and chemical compositions. In order to account for
current uncertainties on the He to heavy elements enrichment ratio, the stellar
models were computed by adopting at Z=0.02 two different He contents (Y=0.27,
0.289) and at Z=0.04 three different He contents (Y=0.29, 0.34, and 0.37). To
supply a homogeneous evolutionary scenario which accounts for young Magellanic
stellar systems the calculations were also extended toward lower metallicities
(Z=0.004, Z=0.01), by adopting different initial He abundances. We evaluated
for both solar (Z=0.02) and super-metal-rich (SMR, Z=0.04) models the
transition mass $M^{up}$ between the stellar structures igniting carbon and
those which develop a full electron degeneracy inside the CO core. This
evolutionary scenario allows us to investigate in detail the properties of
classical Cepheids. In particular, we find that the range of stellar masses
which perform the blue loop during the central He-burning phase narrows when
moving toward metal-rich and SMR structures.


title: Origin of classical structure from inflation

abstract:
  According to the inflationary scenario, all structure in the Universe can be
traced back to quantum fluctuations of the metric and scalar field(s) during
inflation. The seeds of this structure can be observed as classical
anisotropies in the cosmic microwave background. I briefly review how the
transition from the inherent quantum nature of these fluctuations to classical
behaviour comes about. Two features play a crucial role: Firstly, the quantum
state of the fluctuations becomes highly squeezed for wavelengths that exceed
the Hubble radius. Secondly, decoherence due to other fields distinguishes the
field-amplitude basis as the classical pointer basis. I also discuss the
entropy of the fluctuations and make a brief comparison with chaotic systems.


title: Gas, Stars and Baryons in Low Surface Brightness Galaxies

abstract:
  Recent surveys have discovered hundreds of low surface brightness galaxies in
the local (z < 0.1) Universe. Plots of the surface brightness distribution (the
space density of galaxies plotted against central surface brightness) show a
flat distribution from the bright-end cutoff of 21.65 through the current
observational limit of 25.0 B mag arcsec^{-2}. As no trend is seen to indicate
the size or mass of galaxies decreases with decreasing central surface
brightness, it is likely that a significant percentage of the baryon content in
the universe is contained in these diffuse systems. In this paper I briefly
review the known properties of low surface brightness galaxies, and describe
some current theories on the baryonic mass fraction of low surface brightness
systems and their consequences.


title: Preliminary results and perspectives in the Archeops experiment

abstract:
  Observations of the Cosmic Microwave Background (CMB) temperature
fluctuations are a powerful tool for testing theories of the early Universe and
for measuring cosmological parameters. We present basics of CMB physics, review
some of the most recent results and discuss their implications for cosmology.
The Archeops balloon-borne experiment is designed to map the CMB with an
angular resolution of 10 arcminutes and a precision of 30 $\mu\mathrm{K}$ per
pixel. This will allow the measurement of the CMB fluctuation power spectrum
from large to small angular scales. We describe preliminary results from the
test flight which took place in July 1999 and present perspectives for upcoming
scientific flights (January 2001).


title:
Four Years of Extreme Ultraviolet Observations of Markarian 421: II.
  Temporal Analysis

abstract:
  The Extreme Ultraviolet Explorer (EUVE) satellite accumulated ~one million
seconds of public data between 1994 and 1997 for the BL Lacertae object
Markarian 421. This is the second of two papers in which we present the results
of spectral and temporal analysis of this EUVE data set. We analyze in the
present paper the imaging data by means of power spectrum and structure
function techniques, while the spectral analysis is presented in a companion
paper. We find for MRK 421 a power spectrum with slope -2.14 +- 0.28 with a
break at ~3 days. This is the first time that a break in the power spectrum of
a BL Lacertae object has been found. We also find evidence of non-stationarity
for MRK 421 EUV emission


title: Approximate expressions for polar gap electric field of pulsars

abstract:
  We derive easy-to-handle approximations for the polar gap electric field due
to inertial frame dragging as derived by Harding & Muslimov 1998. A simple
formula valid for polar gap height comparable to the polar cap radius is
presented.


title: Blank Field X-Ray Sources

abstract:
  The X-ray sky is not as well known as is sometimes thought. We report on our
search of minority populations (Kim & Elvis 1999). One of the most intriguing
is that of `blank field sources', i.e. bright ROSAT sources (F(X)>1e-13
erg/cm2/s) with no optical counterpart on the Palomar Sky Survey (to O=21.5)
within their 39"(99%) radius error circle. The nature of Blank Field sources is
unknown: no known extragalactic population has such extreme X-ray to optical
ratios (f(X)/f(V) >60). Moreover blank field source tend to have much flatter
PSPC spectra compared to radio-quiet AGN. Both properties suggest obscuration.
`Blank field sources' could be: Quasar-2s, low-mass AGNs, isolated neutron
stars, high redshift clusters of galaxies, failed clusters, AGNs with no big
blue bump. Identification with any of these populations would be an interesting
discovery.


title: SISSA Searching for blazars: the BLEIS project

abstract:
  We present the aim and the status of the BLEIS project currently under
development at SISSA. This project consists in selecting a complete Blazar
sample from deep optical data, down to B=24.6, V=24.4 and I=23.7 (80%
completness). The optical images are taken from the ESO Imaging Survey (EIS -
Wide), a public survey covering ~16 deg2 of the Southern Emisphere. This new
Blazar sample, thanks to the different energy band of the selection and to the
faintness of EIS images, will be useful not only in understanding the physics
of such objects, but also in evolutive and cosmological studies. It will also
be a new test for the unified models of AGNs and will be helpful in making
predictions for future deep surveys.


title: The location of the dense and ionized gas in the NGC 2023 PDR

abstract:
  The VLA and the BIMA array were used to obtain high resolution (10-20")
observations of C+, traced by the C91a recombination line at 8.6 GHz, and the
dense molecular gas, traced by HCN and HCO+(1-0), of the photon dominated
region (PDR) associated with the reflection nebula NGC 2023. Using the VLA,
continuum emission is detected at 8.6 GHz from a faint HII region associated
with HD 37903. The C91a emission originates from a 0.4 pc long filament,
extending from the east to the south of the exciting star HD 37903. Within the
filament three C91a clumps can be distinguished, each associated with
filamentary vibrationally excited H2 emission in the direction toward HD 37903.
The HCO+ emission has a clumpy appearance superimposed on a more extended
component. C91a is, in general, closer to the exciting star than HCO+ emission
as expected from PDR models. The morphologies of HCO+ and HCN are quite
similar. Based on the C91a linewidth towards one of the clumps a limit of 170 K
on the kinetic temperature in the ionized carbon layer can be derived. This
value is consistent with PDR models with H2 densities of about 10^5/cm^3.
However, this result suggests surprisingly low limits on the turbulence in the
PDR. We detected a compact 3 mm continuum source in the PDR, which appears to
be a cold "core" of density 10^7/cm^3, 0.03 parsec diameter, and 6 solar
masses. We conclude that it may have formed within the PDR. In an appendix,
observations of the C91a recombination line toward five additional PDRs using
the Effelsberg 100m telescope are described.


title:
Abundances and Kinematics of Field Halo and Disk Stars I: Observational
  Data and Abundance Analysis

abstract:
  We describe observations and abundance analysis of a high-resolution,
high-S/N survey of 168 stars, most of which are metal-poor dwarfs. We follow a
self-consistent LTE analysis technique to determine the stellar parameters and
abundances, and estimate the effects of random and systematic uncertainties on
the resulting abundances. Element-to-iron ratios are derived for key alpha,
odd, Fe-peak, r- and s-process elements. Effects of Non-LTE on the analysis of
Fe I lines are shown to be very small on the average. Spectroscopically
determined surface gravities are derived that are generally close to those
obtained from Hipparcos parallaxes.


title:
Probing the Atmospheres of Planets Orbiting Microlensed Stars via
  Polarization Variability

abstract:
  We present a new method to identify and probe planetary companions of stars
in the Galactic Bulge and Magellanic Clouds using gravitational microlensing.
While spectroscopic studies of these planets is well beyond current
observational techniques, monitoring polarization fluctuations during high
magnification events induced by binary microlensing events will probe the
composition of the planetary atmospheres, an observation which otherwise is
currently unattainable even for nearby planetary systems.


title:
Serendipitous 2MASS Discoveries Near the Galactic Plane: A Spiral Galaxy
  and Two Globular Clusters

abstract:
  We present the basic properties of three objects near the Galactic Plane - a
large galaxy and two candidate globular clusters - discovered in the Two Micron
All Sky Survey (2MASS) dataset. All were noted during spot-checks of the data
during 2MASS quality assurance reviews. The galaxy is a late-type spiral galaxy
(Sc-Sd), ~11 Mpc distant. We estimate an extinction-corrected optical diameter
of ~9.5', making it larger than most Messier galaxies. The candidate globular
clusters are ~2-3' in extent and are hidden optically behind foreground
extinctions of Av ~18-21 mag at l ~ 10 deg, b ~ 0 deg. These chance discoveries
were not the result of any kind of systematic search but they do hint at the
wealth of obscured sources of all kinds, many previously unknown, that are in
the 2MASS dataset.


title: Optical Spectroscopy of Supernova 1993J During Its First 2500 Days

abstract:
  We present 42 low-resolution spectra of Supernova (SN) 1993J, our complete
collection from the Lick and Keck Observatories, from day 3 after explosion to
day 2454, as well as one Keck high-dispersion spectrum from day 383. SN 1993J
began as an apparent SN II, albeit an unusual one. After a few weeks, a
dramatic transition took place, as prominent helium lines emerged in the
spectrum. SN 1993J had metamorphosed from a SN II to a SN IIb. Nebular spectra
of SN 1993J closely resemble those of SNe Ib and Ic, but with a persistent
H_alpha line. At very late times, the H_alpha emission line dominated the
spectrum, but with an unusual, box-like profile. This is interpreted as an
indication of circumstellar interaction.


title: Detailed Analysis of Early to Late-Time Spectra of Supernova 1993J

abstract:
  We present a detailed study of line structure in early to late-time spectra
of Supernova (SN) 1993J. Spectra during the nebular phase, but within the first
two years after explosion, exhibit small-scale structure in the emission lines
of some species, notably oxygen and magnesium, showing that the ejecta of SN
1993J are clumpy. On the other hand, a lack of structure in emission lines of
calcium implies that the source of calcium emission is uniformly distributed
throughout the ejecta. These results are interpreted as evidence that oxygen
emission originates in clumpy, newly synthesized material, while calcium
emission arises from material pre-existing in the atmosphere of the progenitor.
Spectra spanning the range 433-2454 days after the explosion show box-like
profiles for the emission lines, clearly indicating circumstellar interaction
in a roughly spherical shell. This is interpreted within the Chevalier &
Fransson (1994) model for SNe interacting with mass lost during prior stellar
winds. At very late times, the emission lines have a two-horned profile,
implying the formation of a somewhat flattened or disk-like structure that is a
significant source of emission. The very high signal-to-noise ratio spectra are
used to demonstrate the potential significance of misinterpretation of telluric
absorption lines in the spectra of bright SNe.


title:
Consequences of Interstellar Absorption for Models of Anomalous X-ray
  Pulsars

abstract:
  We examine properties of thermal radiation emitted by strongly magnetized
neutron stars (NSs). In particular, we show that the pulsation amplitudes of
the energy-integrated flux are an increasing function of the absorption column
density to the source. This is especially important for the interpretation of
the Anomalous $X$-ray Pulsars (AXPs) as cooling neutron stars with high
magnetic fields. We show that the high-pulsation amplitudes observed in these
objects are consistent with cooling models, if the large amount of absorption
to these sources is taken into account. We also show that cooling models imply
inferred radii of the emitting regions on the order of $\sim 5-6$ times smaller
than the actual NS radii, again in agreement with observations.


title: Three-dimensional structure of an alpha accretion disk

abstract:
  An analytic solution is presented to the three-dimensional problem of steady
axisymmetric fluid flow through an accretion disk. The solution has been
obtained through a systematic expansion in the small parameter epsilon =H/R
(the ratio of disk thickness to its radial dimension) of the equations of
viscous hydrodynamics. The equation of state was assumed to be polytropic. For
all values alpha< 0.685 of the viscosity parameter, we find significant
backflow in the midplane of the disk occuring at all radii larger than a
certain value; however, in the inner regions of the disk the fluid always flows
toward the accreting object. The region of backflow is separated from the
region of inflow by a surface flaring outwards from a circular locus of
stagnation points situated in the midplane of the disk.


title:
On Batchelor passive advection by a finite-time correlated random
  velocity field

abstract:
  The Batchelor passive advection is an advection by a smooth velocity field.
If the velocity field is a delta-correlated in time random Gaussian process,
then the problem is reduced to quantum mechanics of fluctuating velocity
gradient u'(t). For the finite-time correlated velocity field, such a reduction
does not exist. To illustrate this point, the second moment of a passively
advected magnetic field is considered, and the stochastic calculus is used to
find finite-time corrections to its growth rate. The growth rate depends on
large scale properties of the velocity field. Moreover, the problem is not
universal with respect to the short-time regularization: different
regularizations give different answers for the growth rate.


title:
The profile of an emission line from relativistic outflows around a
  black hole

abstract:
  Recent observations show strong evidence for the presence of Doppler-shifted
emission lines in the spectrum of both black hole candidates and active
galactic nuclei. These lines are likely to originate from relativistic outflows
(or jets) in the vicinity of the central black hole. Consequently, the profile
of such a line should be distorted by strong gravitational effects near the
black hole, as well as special relativistic effects. In this paper, we present
results from a detailed study on how each process affects the observed line
profile. We found that the profile is sensitive to the intrinsic properties of
the jets (Lorentz factor, velocity profile, and emissivity law), as well as to
the spin of the black hole and the viewing angle (with respect to the axis of
the jets). More specifically, in the case of approaching jets, an intrisically
narrow line (blue-shifted) is seen as simply broadened at small viewing angles,
but it shows a doubly peaked profile at large viewing angles for extreme Kerr
black holes (due to the combination of gravitational focusing and Doppler
effects); the profile is always singly peaked for Schwarzschild black holes.
For receding jets, however, the line profile becomes quite complicated owing to
complicated photon trajectories. To facilitate comparison with observations, we
searched a large parameter space to derive representative line profiles. We
show the results and discuss how to use emission lines as a potential tool for
probing the inner region of a black hole jet system.


title:
Statistics of smoothed cosmic fields in perturbation theory I:
  Formulation and useful formulas in second-order perturbation theory

abstract:
  We formulate a general method for perturbative evaluations of statistics of
smoothed cosmic fields, and provide useful formulas in application of the
perturbation theory to various statistics. This formalism is an extensive
generalization of the method used by Matsubara (1994) who derived a weakly
nonlinear formula of the genus statistic in a 3D density field. After
describing the general method, we apply the formalism to a series of
statistics, including genus statistics, level-crossing statistics, Minkowski
functionals, and a density extrema statistic, regardless of the dimensions in
which each statistic is defined. The relation between the Minkowski functionals
and other geometrical statistics is clarified. These statistics can be applied
to several cosmic fields, including 3D density field, 3D velocity field, 2D
projected density field, and so forth. The results are detailed for second
order theory of the formalism. The effect of the bias is discussed. The
statistics of smoothed cosmic fields as functions of rescaled threshold by
volume-fraction are discussed in the framework of second-order perturbation
theory. In CDM-like models, their functional deviations from linear predictions
plotted against the rescaled threshold are generally much smaller than that
plotted against the direct threshold. There is still slight meat-ball shift
against rescaled threshold, which is characterized by asymmetry in depths of
troughs in the genus curve. A theory-motivated asymmetry factor in genus curve
is proposed.


title:
Construction of the one-point PDF of the local aperture mass in weak
  lensing maps

abstract:
  We present a general method for the reconstruction of the one-point
Probability Distribution Function of the local aperture mass in weak lensing
maps. Exact results, that neglect the lens-lens coupling and departure form the
Born approximation, are derived for both the quasilinear regime at leading
order and the strongly nonlinear regime assuming the tree hierarchical model is
valid. We describe in details the projection effects on the properties of the
PDF and the associated generating functions. In particular, we show how the
generic features which are common to both the quasilinear and nonlinear regimes
lead to two exponential tails for $P(\Map)$. We briefly investigate the
dependence of the PDF with cosmology and with the shape of the angular filter.
Our predictions are seen to agree reasonably well with the results of numerical
simulations and should be able to serve as foundations for alternative methods
to measure the cosmological parameters that take advantage of the full shape of
the PDF.


title:
The Earliest Luminous Sources and the Damping Wing of the Gunn-Peterson
  Trough

abstract:
  Recent observations of high-redshift galaxies and quasars indicate that the
hydrogen component of the intergalactic medium (IGM) must have been reionized
at some redshift z>6. Prior to complete reionization, sources of ultraviolet
radiation will be seen behind intervening gas that is still neutral, and their
spectra should show the red damping wing of the Gunn-Peterson trough. While
this characteristic feature may, in principle, totally suppress the
Lyman-alphaa emission line in the spectra of the first generation of objects in
the universe, we show here that the IGM in the vicinity of luminous quasars
will be highly photoionized on several Mpc scales due to the source emission of
Lyman-continuum photons. If the quasar lifetime is shorter than the expansion
and gas recombination timescales, the volume ionized will be proportional to
the total number of photons produced above 13.6 eV: the effect of this local
photoionization is to greatly reduce the scattering opacity between the
redshift of the quasar and the boundary of its HII region. We find that the
transmission on the red side of the Lyman-alpha resonance is always greater
than 50% for sources radiating a total of 1e69.5 ionizing photons into the IGM.
The detection of a strong Lyman-alpha emission line in the spectra of bright
QSOs shining for >1e7 yr cannot then be used, by itself, as a constraint on the
reionization epoch. The first signs of an object radiating prior to the
transition from a neutral to an ionized universe may be best searched for in
the spectra of luminous sources with a small escape fraction of Lyman-continuum
photons into the IGM, or sources with a short duty cycle.


title:
The Effect of Diffusion on Pulsations of Stars on the Upper Main
  Sequence. delta Scuti and Metallic A Stars

abstract:
  Recent dramatic improvements in the modeling of abundance evolution due to
diffusion in A stars have been achieved with the help of monochromatic opacity
tables from the OPAL group. An important result in the context of stellar
pulsations is the substantial helium abundance shown to be left over in the
driving region of delta Scuti-type pulsations in chemically peculiar Am stars.
An accurate opacity profile in the entire stellar envelope including the full
effect of heavy elements is also now available for the first time.
  Pulsations are shown to be excluded for young Am stars but occur naturally
when these stars evolve off the ZAMS. The predicted variable metallic A stars
all lie towards the red edge of the instability strip, in qualitative agreement
with the observed variable delta Delphini and mild Am stars.
  Results show little direct excitation from iron-peak elements in A-type
stars. The main abundance effect is due to the settling of helium, along with a
marginal effect due to the enhancement of hydrogen.


title: SNIa data to probe the Cosmological Principle

abstract:
  Recently collected SNIa data have been used to address the problem of
measuring the cosmological parameters of the universe. Analysed in the
framework of homogeneous models, they have yielded, as a primary result, a
strictly positive cosmological constant. However, a straight reading of the
published measurements, conducted with no a priori idea of which model would
best describe our universe, at least up to redshifts z=1, does not exclude the
possibility of ruling out the Cosmological Principle - and cosmological
constant - hypotheses. It is here shown how the large scale (in)homogeneity of
this part of the universe can be tested on our past light cone, using the
magnitude-redshift relation, provided sufficiently accurate data from sources
at redshifts approaching z=1 would be available.


title: On the Seyfert 2 warm and cool infrared dichotomy

abstract:
  An optical spectropolarimetric study has shown that the detectability of
polarised broad H-alpha in Seyfert 2 galaxies is correlated to the IRAS f60/f25
flux ratio where only those Seyfert 2s with "warm" IRAS colours show polarised
broad line emission. It was suggested that those Seyfert 2s with "cool" IRAS
colours have highly inclined tori which obscure the broad line scattering
screen.
  I present here hard X-ray observations inconsistent with this picture showing
that the derived column densities of warm and cool Seyfert 2 galaxies are
statistically the same. I suggest that the IRAS f60/f25 flux ratio is more
consistent with implying the relative strength of galactic to Seyfert emission
and provide supporting evidence for this view.


title:
Constraints from Dynamical Friction on the Dark Matter Content of Barred
  Galaxies

abstract:
  We show that bars in galaxy models having halos of moderate density and a
variety of velocity distributions all experience a strong drag from dynamical
friction unless the halo has large angular momentum in the same sense as the
disk. The frictional drag decreases the bar pattern speed, driving the
corotation point out to distances well in excess of those estimated in barred
galaxies. The halo angular momentum required to avoid strong braking is
unrealistically large, even when rotation is confined to the inner halo only.
We conclude, therefore, that bars are able to maintain their observed high
pattern speeds only if the halo has a central density low enough for the disk
to provide most of the central attraction in the inner galaxy. We present
evidence that this conclusion holds for all bright galaxies.


title:
Curvature Dependence of Peaks in the Cosmic Microwave Background
  Distribution

abstract:
  The widely cited formula $\ell_1\simeq 200 \Omega_0^{-1/2}$ for the multipole
number of the first Doppler peak is not even a crude approximation in the case
of greatest current interest, in which the cosmic mass density is less than the
vacuum energy density. For instance, with $\Omega_M$ fixed at 0.3, the position
of any Doppler peak varies as $\Omega_0^{-1.58}$ near $\Omega_0=1$.


title: Non-equilibrium Ionization and FIP Effect Diagnostics

abstract:
  We examine the accuracy of a common FIP effect diagnostic, the ratio of Ne VI
to Mg VI lines in the solar transition region. Since the two ions have quite
similar contribution functions near their maxima in equilibrium, the ratio of
these two ions is often taken to be the abundance ratio of Ne and Mg. First we
compute non-equilibrium ionization effects on the ratio f(Ne+5)/f(Mg+5) of ion
fractions for a variety of simple flows through the transition region.
Calculating the spectral line ratios for a few examples, we then show that
non-equilibrium effects as well as temperature and density dependence must be
evaluated for each line ratio used in the diagnostics.


title: Giant repeated ejections from GRS 1915+105

abstract:
  We report simultaneous millimetre and infrared observations of a sequence of
very large amplitude quasi-periodic oscillations from the black hole X-ray
binary GRS 1915+105. These oscillations are near the end of a sequence of over
700 repeated events as observed at 15 GHz, and are simultaneous at the mm and
infrared wavelengths to within our time resolution (<4 min), consistent with
the respective emitting regions being physically close near the base of the
outflow. One infrared event appears to have no mm counterpart, perhaps due to
highly variable absorption. The overall radio-mm-infrared spectrum around the
time of the observations does suggest some absorption at lower frequencies. We
calculate the energy and mass-flow into the outflow for a number of different
assumptions, and find that the time-averaged power required to produce the
observed synchrotron emission cannot be much less than 3e38 erg/s, and is
likely to be much larger. This minimum power requirement is found regardless of
whether the observed emission arises in discrete ejections or in an internal
shock in a quasi-continuous flow. Depending on the similarity of the physical
conditions in the two types of ejection, GRS 1915+105 may be supplying more
power (and mass, if both have the same baryonic component) to the jet during
periods of repeated oscillations than during the more obvious larger events.


title: Sinks of Light Elements in Stars - Part II

abstract:
  The fragile light elements lithium, beryllium, and boron are easily destroyed
in stellar interiors, and are thus superb probes of physical processes occuring
in the outer stellar layers. The light elements are also excellent tracers of
the chemical evolution of the Galaxy, and can test big bang nucleosynthesis
(BBN). These inter-related topics are reviewed with an emphasis on stellar
physics.
  In Part I (presented by CPD), an overview is given of the physical processes
which can modify the surface abundances of the light elements, with emphasis on
Population I dwarfs - convection; gravitational settling, thermal diffusion,
and radiative levitation; slow mixing induced by gravity waves or rotation. We
will discuss the increasingly large body of data which begin to enable us to
discern the relative importance of these mechanisms in Population I main
sequence stars.
  In Part II (presented by MHP), discussion is extended to the issue of whether
or not the halo Li plateau is depleted, and includes the following topics: Li
dispersion in field and globular cluster stars, Li production vs. destruction
in Li-rich halo stars, and constraints from 6Li. Also discussed are trends with
metal abundance and Teff and implications for chemical evolution and BBN.
  In Part III (presented by CC), evidence is reviewed that suggests that in
situ mixing occurs in evolved low mass Population I and Population II stars.
Theoretical mechanisms that can create such mixing are discussed, as well as
their implications in stellar yields.


title: Sinks of Light Elements in Stars - Part III


title: Anisotropic Galaxy-Galaxy Lensing

abstract:
  We investigate the weak lensing shear due to dark matter galaxy halos whose
mass distributions, as projected on the sky, are nearly elliptical. The shear
pattern due to these halos is anisotropic about the lens centers and we
quantify the level of anisotropy by comparing the mean shear experienced by
sources located closest to the major axes of the lenses $<\gamma>_{major}$, to
that experienced by sources located closest to the minor axes,
$<\gamma>_{minor}$. We also compute the degree to which the anisotropy in the
lensing signal is degraded due to a noisy determination of the position angles
of the lens galaxies and find that provided the typical error on the
orientation of the lenses is less than $15^\circ$, more than 90% of the true
lensing signal is recovered in the mean. We discuss our our results in the
context of detecting anisotropic galaxy-galaxy lensing in large, ground-based
data sets and conclude that a modest net flattening of dark matter halos should
be detectable at a statistically significant level. The forthcoming Sloan
Digital Sky Survey (SDSS) data will necessarily provide a very useful data set
for this analysis, but a detection of anisotropic galaxy-galaxy lensing is not
dependent upon the very large sky coverage of the SDSS. Rather, we argue that a
significant detection of this effect can also be obtained from an imaging
survey that is of order two magnitudes fainter than SDSS and which covers only
a relatively small area of sky, of order one square degree. [Abridged]


title: Microlensing in M31 - The MEGA Survey's Prospects and Initial Results

abstract:
  January 2000 completes the first season of intensive, wide-field observations
of microlensing and stellar variability in M31 by MEGA (Microlensing
Exploration of the Galaxy and Andromeda) at the Isaac Newton 2.5m Telescope,
the KPNO 4m, and the 1.3m and 2.5m telescopes of MDM Observatory. In
preliminary analysis, we detect ~50000 variable objects, including some
consistent with microlensing events. We present the level of sensitivity to be
reached in our planned three-year program to test for the presence of a
significant halo microlensing population in M31, as well as its spatial
distribution and mass-function. We also discuss our application of image
subtraction to these wide fields and HST WFPC2 Snapshot followup observations
to confirm candidates identified from previous years' surveys.
  We present intermediate results from our smaller-field survey, on the MDM
1.3m and Vatican Advanced Technology 1.8m Telescope, from 1994-1998, wherein we
have discovered 8 additional probable microlensing events, over about one-half
the time base of the project, in addition to confirming three of our original 6
microlensing candidates from 1995.


title: The central part of the young open cluster NGC 6383

abstract:
  The spectral and extinction properties of 14 pre-main sequence candidates in
the central part of the very young open cluster NGC 6383 were investigated.
None of these stars shows evidence for anomalous circumstellar extinction.
However, six out of 14 programme stars do show an infrared excess, indicative
of the presence of circumstellar dust, heated up by the central star. One of
these stars (number 4), also shows Halpha in emission and shows some
indications for the presence of circumstellar gas in its spectrum, and might
therefore be a newly identified Herbig Ae star.


title: The Caltech Faint Galaxy Redshift Survey XII: Clustering of Galaxies

abstract:
  A clustering analysis is performed on two samples of $\sim 600$ faint
galaxies each, in two widely separated regions of the sky, including the Hubble
Deep Field. One of the survey regions is configured so that some galaxy pairs
span angular separations of up to 1 deg. The median redshift is $z_{med}\approx
0.55$. Strong clustering is obvious, with every pencil-beam field containing a
handful of narrow redshift-space features, corresponding to galaxy structures
with sizes of 5 to 20 Mpc. The structures are not obviously organized on
planes, though one prominent, colinear triplet of structures is observed,
spanning $\sim 20$ Mpc. This may be evidence of a filament. A galaxy--galaxy
correlation function calculation is performed. No significant evolution of
clustering (relative to stable clustering) is found in the redshift range
0.3<z<1.0. This is not surprising, since uncertainties in the correlation
amplitude estimated from surveys like these are large; field-to-field
variations and covariances between data points are both shown to be
significant. Consistent with other studies in this redshift range, the
galaxy--galaxy correlation length is found to be somewhat smaller than that
predicted from local measurements and an assumption of no evolution. Galaxies
with absorption-line-dominated spectra show much stronger clustering at
distances of <2 Mpc than typical field galaxies. There is some evidence for
weaker clustering at intermediate redshift than at low redshift, when the
results presented here are compared with surveys of the local Universe. In
subsets of the data, the measured pairwise velocity dispersion of galaxies
ranges from 200 to $600 km s^{-1}$, depending on the properties of the dominant
redshift structures in each subset.


title: Pulsar acceleration by asymmetric emission of sterile neutrinos

abstract:
  A convincing explanation for the observed pulsar large peculiar velocities is
still missing. We argue that any viable particle physics solution would most
likely involve the resonant production of a non-interacting neutrino $\nu_s$ of
mass $m_{\nu_s}\sim 20$--50 keV. We propose a model where anisotropic magnetic
field configurations strongly bias the resonant spin flavour precession of tau
antineutrinos into $\nu_s$. For internal magnetic fields $B_{int} \gsim
10^{15}$ G a $\bar\nu_\tau$-$\nu_s$ transition magnetic moment of the order of
$10^{-12}$ Bohr magnetons is required. The asymmetric emission of $\nu_s$ from
the core can produce sizeable natal kicks and account for recoil velocities of
several hundred kilometers per second.


title: Theoretical Calculations of Atomic Data for Spectroscopy

abstract:
  Several different approximations and techniques have been developed for the
calculation of atomic structure, ionization, and excitation of atoms and ions.
These techniques have been used to compute large amounts of spectroscopic data
of various levels of accuracy. This paper presents a review of these
theoretical methods to help non-experts in atomic physics to better understand
the qualities and limitations of various data sources and assess how reliable
are spectral models based on those data.


title: Rotation Curve Measurement using Cross-Correlation

abstract:
  Longslit spectroscopy is entering an era of increased spatial and spectral
resolution and increased sample size. Improved instruments reveal complex
velocity structure that cannot be described with a one-dimensional rotation
curve, yet samples are too numerous to examine each galaxy in detail.
Therefore, one goal of rotation curve measurement techniques is to flag cases
in which the kinematic structure of the galaxy is more complex than a
single-valued curve.
  We examine cross-correlation as a technique that is easily automated and
works for low signal-to-noise spectra. We show that the technique yields
well-defined errors which increase when the simple spectral model (template) is
a poor match to the data, flagging those cases for later inspection.
  We compare the technique to the more traditional, parametric technique of
simultaneous emission line fitting. When the line profile at a single slit
position is non-Gaussian, the techniques disagree. For our model spectra with
two well-separated velocity components, assigned velocities from the two
techniques differ by up to ~52% of the velocity separation of the model
components. However, careful use of the error statistics for either technique
allows one to flag these non-Gaussian spectra.


title: Implications of Binary Properties for Theories of Star Formation

abstract:
  The overall frequency and other statistical properties of binary systems
suggest that star formation is intrinsically a complex and chaotic process, and
that most binaries and single stars actually originate from the decay of
multiple systems. Interactions between stars forming in close proximity to each
other may play an important role in the star formation process itself, for
example via tidally induced accretion from disks. Some of the energetic
activity of newly formed stars could be due to bursts of rapid accretion
triggered by interactions with close companions.


title:
A Relationship Between Nuclear Black Hole Mass and Galaxy Velocity
  Dispersion

abstract:
  We describe a correlation between the mass M_BH of a galaxy's central black
hole and the luminosity-weighted line-of-sight velocity dispersion sigma_e
within the half-light radius. The result is based on a sample of 26 galaxies,
including 13 galaxies with new determinations of black hole masses from Hubble
Space Telescope measurements of stellar kinematics. The best-fit correlation is
M_BH = 1.2 (+-0.2) x 10^8 M_sun (sigma_e/200 km/s)^(3.75 (+-0.3))over almost
three orders of magnitude in M_BH; the scatter in M_BH at fixed sigma_e is only
0.30 dex and most of this is due to observational errors. The M_BH-sigma_e
relation is of interest not only for its strong predictive power but also
because it implies that central black hole mass is constrained by and closely
related to properties of the host galaxy's bulge.


title: Gas Rich Dwarfs from the PSS-II III. HI Profiles and Dynamical Masses

abstract:
  We present Arecibo neutral hydrogen data on a sample of optically selected
dwarf galaxies. The sample ranges in HI mass from 10^6 M_sun to 5x10^9 M_sun,
with a mean of 7.9x10^8 M_sun. Using estimated HI radii, the HI surface
densities range from 0.6 to 20 M_sun pc^-2, all well below the critical
threshold for star formation (Kennicutt 1998). M_HI/L values of the LSB dwarfs
range from 0.3 to 12 with a mean value of 2.0. Dynamical masses, calculated
from the HI profile widths, range from 10^8 M_sun to 10^11 M_sun. There is a
strong correlation between optical luminosity and dynamical mass for LSB dwarfs
implying that the dark matter (whether baryonic or non-baryonic) follows the
detectable baryonic matter.


title:
Monte Carlo Simulations of Type Ia Supernova Observations in Supernova
  Surveys

abstract:
  We have performed Monte Carlo simulations of type Ia supernova (SN Ia)
surveys to quantify their efficiency in discovering peculiar overluminous and
underluminous SNe Ia. We determined how the type of survey (magnitude-limited,
distance-limited, or a hybrid) and its characteristics (observations frequency
and detection limit) affect the discovery of peculiar SNe Ia. We find that
there are strong biases against the discovery of peculiar SNe Ia introduced by
at least four observational effects: the Malmquist bias, the age of the SN Ia
at the time of its discovery, the shape of its light curve, and its degree of
extinction. Surveys with low observation frequency (less than once per 10 days)
tend to discover SNe Ia which are too old for observers to easily recognize
their peculi arity. Subluminous SNe Ia are underrepresented in
magnitude-limited surveys because they can only be found within a small volume
and they remain above the detection limit for less time. Conversely,
overluminous SNe Ia are more easily found in magnitude-limited surveys although
their likely association with dusty regions reduces the volume in which they
can be discovered. The unbiased rate of peculiar SNe Ia can be recovered only
in distance-limited surveys with high observation frequencies and with
detection limits which are fainter than the peak magnitude of a subluminous SN
Ia in the farthest potential host.


title: A High Intrinsic Peculiarity Rate Among Type Ia Supernovae

abstract:
  We have compiled a sample of 45 Type Ia supernovae (SNe Ia) discovered by the
Lick Observatory Supernova Search (LOSS) and the Beijing Astronomical
Observatory Supernova Survey (BAOSS), and determined the rate of
spectroscopically peculiar SNe Ia (i.e., SN 1986G-like, SN 1991bg-like, and SN
1991T-like objects) and the luminosity function of SNe Ia. Because of the
nature of the two surveys (distance-limited with small baselines and deep
limiting magnitudes), nearly all SNe Ia have been discovered in the sample
galaxies of LOSS and BAOSS; thus, the observed peculiarity rate and luminosity
function of SNe Ia are intrinsic. We find that 36$\pm$9% of nearby SNe Ia are
peculiar; specifically, the luminosity function of SNe Ia consists of 20% SN
1991T-like, 64% normal, and 16% SN 1991bg-like objects. We have compared our
results to those found by earlier studies, and to those found at high redshift.
The apparent dearth of SN 1991T-like objects at high redshift may be due to
extinction, and especially to the difficulty of recognizing them from spectra
obtained past maximum brightness or from spectra with low signal-to-noise
ratios. Implications of the high peculiarity rate for the progenitor systems of
SNe Ia are also briefly discussed.


title:
Substructures revealed by the Sunyaev-Zel'dovich effect at 150GHz in the
  high resolution map of RXJ1347-1145

abstract:
  We report on mapping observations toward the region of the most luminous
X-ray cluster RXJ1347-1145 (z=0.45) through the Sunyaev-Zel'dovich effect at
21GHz and 150GHz with the Nobeyama 45-m telescope. While a low angular
resolution image at 21GHz (beam-size of 76'') shows a consistent feature with
the ROSAT/HRI X-ray image, a higher angular resolution image (13'') at 150GHz
reveals complex morphological structures of the cluster region, which cannot be
simply described by the spherical isothermal beta-model. If such inhomogeneous
morphological features prove to be generic for high redshift clusters, distance
measurements to the clusters based on their Sunyaev-Zel'dovich data with low
angular resolution imaging should be interpreted with caution.


title:
The ORFEUS II Echelle Spectrum of HD 93521: A reference for interstellar
  molecular hydrogen

abstract:
  During the second flight of the ORFEUS-SPAS mission in November/December
1996, the Echelle spectrometer was used extensively by the Principal and Guest
Investigator teams as one of the two focal plane instruments of the ORFEUS
telescope. The spectrum of HD 93521 was obtained during this mission with a
total integration time of 1740 s. This spectrum shows numerous sharp
interstellar absorption lines. We identified 198 lines of molecular hydrogen
including at least 7 lines with a high velocity component. Also most of the 67
identified interstellar metal lines are visible with a high velocity component.
  We present plots of the complete ORFEUS Echelle spectrum together with tables
of all identified interstellar absorption lines including all 14 detectable HI
lines. In addition several identified stellar lines, partially with narrow
absorption components, and stellar wind lines are given in a separate table.


title:
The ORFEUS II Echelle Spectrometer: Instrument description, performance
  and data reduction

abstract:
  During the second flight of the ORFEUS-SPAS mission in November/December
1996, the Echelle spectrometer was used extensively by the Principal and Guest
Investigator teams as one of the two focal plane instruments of the ORFEUS
telescope. We present the in-flight performance and the principles of the data
reduction for this instrument. The wavelength range is 90 nm to 140 nm, the
spectral resolution is significantly better than lambda/(Delta lambda) = 10000,
where Delta lambda is measured as FWHM of the instrumental profile. The
effective area peaks at 1.3 cm^2 near 110 nm. The background is dominated by
straylight from the Echelle grating and is about 15% in an extracted spectrum
for spectra with a rather flat continuum. The internal accuracy of the
wavelength calibration is better than +/- 0.005 nm.


title:
NGC 4672: a new case of an early-type disk galaxy with an orthogonally
  decoupled core

abstract:
  We report the case of the early-type disk galaxy NGC 4672 as a new example of
a galaxy characterized by the orthogonal geometrical decoupling between bulge
and disk. The morphological features of this galaxy are discussed as well as
the velocity curves and velocity dispersion profiles of stars and ionized gas
along both its major and minor axis. We conclude that NGC 4672 has structural
(i.e., a bulge elongated perpendicularly to the disk) and kinematical (i.e., a
stellar core rotating perpendicularly to the disk) properties similar to those
of the Sa NGC 4698. The presence of the isolated core suggests that the disk
component is the end result of the acquisition of external material in polar
orbits around a pre-existing oblate spheroid as in the case of the ring
component of AM 2020-504, the prototype of polar ring ellipticals.


title:
Line Formation in the Atmosphere of Brown Dwarf Gliese 229B: $CH_4$ at
  2.3 micron

abstract:
  We investigate the formation of methane line at 2.3 ${\rm \mu m}$ in Brown
Dwarf Gliese 229B. Two sets of model parameters with (a) $T_{\rm eff}=940 $K
and $\log (g) =5.0$, (b) $T_{\rm eff}=1030 $K and $ \log(g)=5.5$ are adopted
both of which provide excellent fit for the synthetic continuum spectra with
the observed flux at a wide range of wavelengths. In the absence of
observational data for individual molecular lines, we set the additional
parameters that are needed in order to model the individual lines by fitting
the calculated flux with the observed flux at the continuum. A significant
difference in the amount of flux at the core of the line is found with the two
different models although the flux at the ontinuum remains the same. Hence, we
show that if spectroscopic observation at $2.3{\rm \mu m}$ with a resolution as
high as $R \simeq 200,000$ is possible then a much better constraint on the
surface gravity and on the metallicity of the object could be obtained by
fitting the theoretical model of individual molecular line with the observed
data.


title:
Extragalactic 2-10 keV source counts from a fluctuation analysis of deep
  BeppoSAX MECS images

abstract:
  We present an analysis of the spatial fluctuations of the 2-10 keV Cosmic
X-ray Background (CXB) as measured from 22 high galactic latitude (|b| > 25
deg) fields observed with the MECS instrument on-board BeppoSAX. This technique
allowed us to probe extragalactic source counts a factor 3-4 fainter than is
possible with direct measurements of pointlike sources in MECS deep fields. The
slope of the 2-10 keV logN-logS relationship is found to be still close to the
``Euclidean'' one (gamma = 1.5) down to our flux limit of 1.5 10^-14 erg cm^-2
s^-1, where the contribution of discrete sources to the 2-10 keV CXB amounts to
40-50%. Source counts derived from the analysis presented in this letter are in
very good agreement both with those directly measured with ASCA and BeppoSAX
deep surveys at bright fluxes and with a first estimation of the 2-10 keV
Chandra logN-logS at fainter fluxes.


title: Photometric catalog of nearby globular clusters (II)

abstract:
  In this paper we present the second and final part of a large and
photometrically homogeneous CCD color-magnitude diagram (CMD) data base,
comprising 52 nearby Galactic globular clusters (GGC) imaged in the V and I
bands.
  The catalog has been collected using only two telescopes (one for each
hemisphere). The observed clusters represent 75% of the known Galactic
globulars with (m-M)_v <= 16.15 mag, cover most of the globular cluster
metallicity range (-2.2 <= [Fe/H] <= -0.4), and span Galactocentric distances
from ~1.2 to ~18.5 kpc.
  In particular, here we present the CMDs for the 13 GGCs observed in the
Northern hemisphere. The remaining 39 Southern hemisphere clusters of the
catalog have been presented in a companion paper (Rosenberg et al., 2000, A&AS
144, 5). We present the first CCD color magnitude diagram for NGC6779 (M56).
  All the CMDs extend from at least ~2 magnitudes below the turn-off (i.e.
V_lim ~22) to the tip of the red giant branch. The calibration has been done
using a large number of standard stars, and the absolute calibration is
reliable to a ~0.02 mag level in both filters.
  This catalog, because of its homogeneity, is expected to represent a useful
data base for the measurement of the main absolute and relative parameters
characterizing the CMD of GGCs.


title: Dark energy and the Boomerang data

abstract:
  The recent high-quality Boomerang data allow to test many competing
cosmological models. Here I present a seven-parameter likelihood analysis of
dark energy models with exponential potential and explicit coupling to dark
matter. Such a model is conformally equivalent to a scalar field with
non-minimal coupling to gravity. So far, the constraints on a dark energy -
dark matter coupling were extremely weak. The Boomerang data constrain the
dimensionless coupling $\beta $ to be smaller than 0.1, an order of magnitude
better than previous limits. In terms of the constant $\xi $ of non-minimally
coupled theories, this amounts to $\xi <0.01.$
  On the other hand, Boomerang has not enough sensitivity to put constraints on
the potential slope.


title:
Extraplanar diffuse ionized gas in a small sample of nearby edge-on
  galaxies

abstract:
  We present narrowband H-alpha imaging data of a small survey of nearby
edge-on spiral galaxies, aiming at the detection of `extraplanar' diffuse
ionized gas (DIG). A few of our studied edge-on spirals show signs of disk-halo
interaction (DHI), where extended line emission far above the galactic plane of
these galaxies is detected. In some cases an extraplanar diffuse ionized gas
(eDIG) layer is discovered, e.g., NGC4634, NGC 3044, while other galaxies show
only filamentary features reaching into the halo (e.g., IC 2531) and some
galaxies show no sign of eDIG at all. The extraplanar distances of the DIG
layer in our narrowband H-alpha images reach values of z<= 2 kpc above the
galactic plane. The derived star formation rates (SFRs) from the H-alpha flux
of the studied galaxies range from 0.05-0.7 M_{sun}/yr, neglecting a correction
for internal absorption. The variation of the SFR values among our sample
galaxies reflects the diversity of star formation within this sample. A
diagnostic diagram is introduced, which allows to predict the existence of gas
halos in `quiescent' galaxies based on the ratio S_{60}/S_{100} versus L_{FIR}
/ D^2_{25} in this diagram. We compare the positions of the non--starburst
galaxies with starburst galaxies, since these galaxies populate distinct
positions in these diagrams.


title: Is there ERE in diffuse galactic light at high latitude?

abstract:
  In 1998 Gordon, Witt and Friedmann wrote of detection of ERE in diffuse
galactic light at high latitudes. The comparaison of diffuse galactic light and
starlight over a few regions of the sky cannot, according to the authors, be
fully explained by extinction of starlight. 50% of the red interstellar light
needs to come from an interstellar emitting process.
  Different aspects of Gordon et al.'s data reduction work, and of the model
which supports their assertion, will be considered here. Analysis of the 100
micron emission of the regions Gordon et al. have studied will prove that the
color of the diffuse galactic light is explained by scattering of the
background starlight by dust embedded in nearby cirrus.
  ERE may not be present in diffuse galactic light, and, more generally, in
interstellar space.


title: Primordial Gaussian Fluctuations from Cosmic Defects

abstract:
  We extend our recent work on ``two-metric'' theories of gravity by showing
how in such models cosmic defects can produce a spectrum of primordial Gaussian
density perturbations. This will happen when the speed characterising the decay
products of the defect network is much larger than the speed characterising
gravity and all standard model particles. This model will exactly mimic all the
standard predictions of inflationary models, and the only way of distinguishing
the two will be via the detection of the decay products of the network.


title:
High Energy Gamma-Ray Observations of the Crab Nebula and Pulsar with
  the Solar Tower Atmospheric Cherenkov Effect Experiment

abstract:
  The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new
ground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEE
uses the large mirror area of a solar heliostat facility to achieve a low
energy threshold. A prototype experiment which uses 32 heliostat mirrors with a
total mirror area of ~ 1200\unit{m^2} has been constructed. This prototype,
called STACEE-32, was used to search for high energy gamma-ray emission from
the Crab Nebula and Pulsar. Observations taken between November 1998 and
February 1999 yield a strong statistical excess of gamma-like events from the
Crab, with a significance of $+6.75\sigma$ in 43 hours of on-source observing
time. No evidence for pulsed emission from the Crab Pulsar was found, and the
upper limit on the pulsed fraction of the observed excess was < 5.5% at the 90%
confidence level. A subset of the data was used to determine the integral flux
of gamma rays from the Crab. We report an energy threshold of E_{th} = 190 \pm
60\unit{GeV}, and a measured integral flux of I (E > E_{th}) = (2.2 \pm 0.6 \pm
0.2) \times 10^{-10}\unit{photons cm^{-2} s^{-1}}. The observed flux is in
agreement with a continuation to lower energies of the power law spectrum seen
at TeV energies.


title: Type Ia Supernova Explosion Models

abstract:
  Because calibrated light curves of Type Ia supernovae have become a major
tool to determine the local expansion rate of the Universe and also its
geometrical structure, considerable attention has been given to models of these
events over the past couple of years. There are good reasons to believe that
perhaps most Type Ia supernovae are the explosions of white dwarfs that have
approached the Chandrasekhar mass, M_ch ~ 1.39 M_sun, and are disrupted by
thermonuclear fusion of carbon and oxygen. However, the mechanism whereby such
accreting carbon-oxygen white dwarfs explode continues to be uncertain. Recent
progress in modeling Type Ia supernovae as well as several of the still open
questions are addressed in this review. Although the main emphasis will be on
studies of the explosion mechanism itself and on the related physical
processes, including the physics of turbulent nuclear combustion in degenerate
stars, we also discuss observational constraints.


title:
The HST Survey of BL~Lacertae Objects. IV. Infrared Imaging of Host
  Galaxies

abstract:
  The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects
taken from the larger sample observed with the WFPC2 in the R band (Urry et al.
2000; Scarpa et al. 2000). Ten of the 12 BL Lacs are clearly resolved, and the
detected host galaxies are large, bright ellipticals with average H-band
absolute magnitude M=-26.2+-0.45 mag and effective radius 10+-5 kpc. The
rest-frame integrated color of the host galaxies is on average R-H=2.3+-0.3,
consistent with the value for both radio galaxies and normal, non-active
elliptical galaxies, and indicating the dominant stellar population is old. The
host galaxies tend to be bluer in their outer regions than in their cores, with
average color gradient Delta(R-H)/Delta(log r)=-0.2 mag, again consistent with
results for normal non-active elliptical galaxies. The infrared Kormendy
relation, derived for the first time for BL Lac host galaxies, is m(e) =
3.8*log(R)+14.8 (where m(e) is the surface brightness at the effective radius
R), fully in agreement with the relation for normal ellipticals. The close
similarity between BL Lac host galaxies and normal ellipticals suggests the
active nucleus has surprisingly little effect on the host galaxy. This supports
a picture in which all elliptical galaxies harbor black holes which can be
actively accreting for some fraction of their lifetime.