title: The radio luminosity of persistent X-ray binaries

abstract:
  We summarise all the reported detections of, and upper limits to, the radio
emission from persistent (i.e. non-transient) X-ray binaries. A striking result
is a common mean observed radio luminosity from the black hole candidates
(BHCs) in the Low/Hard X-ray state and the neutron-star Z sources on the
horizontal X-ray branch. This implies a common mean intrinsic radio luminosity
to within a factor of twenty five (or less, if there is significant Doppler
boosting of the radio emission). Unless coincidental, these results imply a
physical mechanism for jet formation which requires neither a black hole event
horizon or a neutron star surface. As a whole the populations of Atoll and
X-ray pulsar systems are less luminous by factors of >5 and >10 at radio
wavelengths than the BHCs and Z sources (while some Atoll sources have been
detected, no high-field X-ray pulsar has ever been reliably detected as a radio
source). We suggest that all of the persistent BHCs and the Z sources generate,
at least sporadically, an outflow with physical dimensions > 1e12cm, i.e.
significantly larger than the binary separations of most of the systems. We
compare the physical conditions of accretion in each of the types of persistent
X-ray binary and conclude that a relatively low (<1e10 G) magnetic field
associated with the accreting object, and a high (>0.1 Eddington) accretion
rate and/or dramatic physical change in the accretion flow, are required for
formation of a radio-emitting outflow or jet.


title: A GPS-based method to model the plasma effects in VLBI observations

abstract:
  Global Positioning System (GPS) satellites broadcast at frequencies of
1,575.42 MHz (L1) and of 1,227.60 MHz (L2). The dispersive property of the
ionosphere can be used to combine independent measurements at the two
frequencies to estimate the total electron content (TEC) between a GPS receiver
site and a broadcasting satellite. Such measurements, made at sites near to
Very Long Baseline Interferometry (VLBI) sites, can be used to estimate the
ionospheric contribution to VLBI observables. For our 1991.9 astrometric VLBI
experiment in which we obtained group-delay observations in the 8.4 and 2.3 GHz
bands simultaneously, we found that the GPS and VLBI determinations of the
ionosphere delays agreed with root-mean-square differences below 0.15 ns for
intercontinental baselines and 0.10 ns for continental ones. We also
successfully applied the GPS-based procedure to reduce the ionospheric effect
in phase delays used for high precision differenced astrometry at 8.4 GHz for
this same experiment.


title: Detection of the first X-ray selected large AGN group

abstract:
  We have examined the spatial distribution of 856 AGN detected by the ROSAT
All-Sky Survey (RASS) using a direct search for structures with the minimal
spanning tree. The AGNs were compiled from an area of 7000 deg^2, in which
optical identifications of RASS sources were made with the help of the
digitized objective prism plates of the Hamburg Quasar Survey (HQS). Redshifts
were taken from the literature or from own follow-up observations. The sample
probes the spatial distribution at low redshifts, since the redshift
distribution peaks at z=0.1. The application of the minimal spanning tree led
to a 1.8 sigma discovery of an AGN group with 7 members in a volume V=140*75*75
h^-3Mpc^3 in the Pisces constellation. With a mean redshift z=0.27 this group
is only the third discovered group at redshifts z<0.5. The RASS offers
excellent possibilities to study large scale structure with AGNs at low
redshifts, once these redshifts are determined.


title: ROSAC: Studying the clustering properties of X-ray selected AGNs

abstract:
  We present the first ROSAC results of an AGN clustering analysis. This study
comprises a sample of 200 AGNs, 75% of which being at low redshifts z<0.5, in
the Ursa Major constellation. The spatial 2-point-correlation function (SCF) as
well as the minimal spanning tree (MST) technique were applied. Some evidence
for clustering is found in the SCF, although with low significance. Using the
MST technique, we could find two AGN groups. This result is preliminary and the
exact significance will be tested with careful simulations.


title:
The Distribution of Stellar Orbits in the Giant Elliptical Galaxy NGC
  2320

abstract:
  We present direct observational constraints on the orbital distribution of
the stars in the giant elliptical NGC 2320. Long-slit spectra along multiple
position angles are used to derive the stellar line-of-sight velocity
distribution within one effective radius. In addition, the rotation curve and
dispersion profile of an ionized gas disk are measured from the [OIII] emission
lines. After correcting for the asymmetric drift, we derive the circular
velocity of the gas, which provides an independent constraint on the
gravitational potential.
  To interpret the stellar motions, we build axisymmetric three-integral
dynamical models based on an extension of the Schwarzschild orbit-
superposition technique. We consider two families of gravitational potential,
one in which the mass follows the light (i.e. no dark matter) and one with a
logarithmic gravitational potential. Using chi^2-statistics, we compare our
models to both the stellar and gas data to constrain the value of the V-band
mass-to-light ratio Upsilon-V. We find Upsilon-V = 15.0 \pm 0.6 h75 for the
mass-follows-light models and Upsilon-V = 17.0 \pm 0.7 h75 for the logarithmic
models. For the latter, Upsilon-V is defined within a sphere of 15'' radius.
  Models with radially constant Upsilon-V and logarithmic models with dark
matter provide comparably good fits to the data and possess similar dynamical
structure. Across the full range of Upsilon-V permitted by the observational
constraints, the models are radially anisotropic in the equatorial plane over
the radial range of our kinematical data (1'' < r < 40''). Along the true minor
axis, they are more nearly isotropic. (abridged)


title: ISO observational campaign of the BL Lac object PKS 2155-304

abstract:
  The BL Lacertae object PKS 2155-304 was observed by the Infrared Space
Observatory in May and June 1996, during a multiwavelength campaign. These are
the first observations in the mid- and far-infrared bands since IRAS. In the
observing period, the source showed no detectable time variability at 4.0,
14.3, 60 and 90 micron. The spectrum from 2.8 to 100 micron is well fitted by a
single power law with energy spectral index alpha = 0.4, intermediate between
the flatter radio spectrum and the steeper simultaneous optical spectrum. The
overall infrared to X-ray spectral energy distribution can be well explained by
optically thin synchrotron emission, with negligible contributions from thermal
sources. We also show that the host galaxy flux is negligible in this spectral
range.


title: The Near-Ultraviolet Continuum of Late-Type Stars

abstract:
  Analyses of the near-ultraviolet continuum of late-type stars have led to
controversial results regarding the performance of state-of-the-art model
atmospheres. The release of the homogeneous IUE final archive and the
availability of the high-accuracy Hipparcos parallaxes provide an opportunity
to revisit this issue, as accurate stellar distances make it possible to
compare observed absolute fluxes with the predictions of model atmospheres.
  The near-UV continuum is highly sensitive to Teff and [Fe/H], and once the
gravity is constrained from the parallax, these parameters may be derived from
the analysis of low-dispersion "long-wavelength" (2000-3000 A) IUE spectra for
stars previously studied by Alonso et al. (1996; A&AS 117, 227) using the
Infrared Flux Method (IRFM). A second comparison is carried out against the
stars spectroscopically investigated by Gratton et al. (1996; A&A 314, 191). It
is shown that there is a good agreement between Teffs obtained from the IRFM
and from the near-UV continuum, and a remarkable correspondence between
observed and synthetic fluxes for stars with 4000 <= Teff <= 6000 K of any
metallicity and gravity. These facts suggest that model atmospheres provide an
adequate description of the near-UV continuum forming region and that the
opacities involved are essentially understood. For cooler stars, the results of
the IRFM are no longer reliable, as shown by Alonso et al., but the discrepancy
noticed for stars hotter than 6000 K may reflect problems in the model
atmospheres and/or the opacities at these higher temperatures.


title:
Particle Acceleration at Ultra-Relativistic Shocks and the Spectra of
  Relativistic Fireballs

abstract:
  We examine Fermi-type acceleration at relativistic shocks, and distinguish
between the initial boost of the first shock crossing cycle, where the energy
gain per particle can be very large, and the Fermi process proper with repeated
shock crossings, in which the typical energy gain is of order unity. We
calculate by means of numerical simulations the spectrum and angular
distribution of particles accelerated by this Fermi process, in particular in
the case where particle dynamics can be approximated as small-angle scattering.
We show that synchrotron emission from electrons or positrons accelerated by
this process can account remarkably well for the observed power-law spectra of
GRB afterglows and Crab-like supernova remnants. In the context of a
decelerating relativistic fireball, we calculate the maximum particle energy
attainable by acceleration at the external blast wave, and discuss the minimum
energy for this acceleration process and its consequences for the observed
spectrum.


title: Influence of the Tachocline on Solar Evolution

abstract:
  Recently helioseismic observations have revealed the presence of a shear
layer at the base of the convective zone related to the transition from
differential rotation in the convection zone to almost uniform rotation in the
radiative interior, the tachocline. At present, this layer extends only over a
few percent of the solar radius and no definitive explanations have been given
for this thiness. Following Spiegel and Zahn (1992, Astron. Astrophys.), who
invoke anisotropic turbulence to stop the spread of the tachocline deeper in
the radiative zone as the Sun evolves, we give some justifications for their
hypothesis by taking into account recent results on rotating shear instability
(Richard and Zahn 1999, Astron. Astrophys.). We study the impact of the
macroscopic motions present in this layer on the Sun's structure and evolution
by introducing a macroscopic diffusivity $D_T$ in updated solar models. We find
that a time dependent treatment of the tachocline significantly improves the
agreement between computed and observed surface chemical species, such as the
$^7$Li and modify the internal structure of the Sun (Brun, Turck-Chi\`eze and
Zahn, 1999, in Astrophys. J.).


title:
First Dark Matter Limits from a Large-Mass, Low-Background Superheated
  Droplet Detector

abstract:
  We report on the fabrication aspects and calibration of the first large
active mass ($\sim15$ g) modules of SIMPLE, a search for particle dark matter
using Superheated Droplet Detectors (SDDs). While still limited by the
statistical uncertainty of the small data sample on hand, the first weeks of
operation in the new underground laboratory of Rustrel-Pays d'Apt already
provide a sensitivity to axially-coupled Weakly Interacting Massive Particles
(WIMPs) competitive with leading experiments, confirming SDDs as a convenient,
low-cost alternative for WIMP detection.

categories: astro-ph cond-mat hep-ex hep-ph nucl-ex physics.ins-det


title: The Local Interstellar Medium in Puppis-Vela

abstract:
  The first study of the local interstellar medium (LISM) toward Puppis-Vela (l
= 245\degr to 275\degr, b = -15\degr to +5\degr, d < 200 pc) is presented in
this paper. A study of the locations, sizes, and physical characteristics of
local interstellar gas, i.e. ``astronephography,'' is included, and relies upon
the improved distance measurements provided by Hipparcos parallax measurements.
All spectra of more distant sight lines contain absorption features due to
intervening local gas, and more distant structures can only be studied
accurately if components due to the LISM have been isolated. Towards this end,
high resolution (R ~ 95,000), high signal-to-noise (S/N ~ 110 to 250) Na I
spectra of 11 nearby stars in the direction of Puppis-Vela have been obtained.
Toward Puppis-Vela, absorption due to the LIC was not observed, but components
at three distinct velocities were found, and the extent of the local gas
producing the features was estimated. The conclusions regarding the UV spectrum
of gamma^2 Vel presented by Fitzpatrick & Spitzer (1994) were re-examined in
light of this new LISM data, and the ambiguity in their conclusions about
several absorption components is resolved. The stars in Puppis-Vela flank the
region of the apparent extension of the Local Bubble (or Cavity) known as the
beta CMa tunnel,and measurements of the Na I column density towards the sample
stars have been used to modify existing estimates of the extent of the tunnel.
A compilation of all existing Na I observations of < 200 pc sight lines around
the tunnel reveal that low column densities have been exclusively detected
within l ~ 210\degr to 250\degr,and b ~ -21\degr to -9\degr. Near the Galactic
plane,at latitudes -10\degr < b < 0\degr and d ~< 150 pc, the tunnel is
confined to l < 270\degr, a lower longitude than was previously reported.


title:
Strong and weak lensing analysis of cluster Abell 2219 based on optical
  and near infrared data

abstract:
  We present a gravitational lensing study of the massive galaxy cluster A2219
(redshift 0.22). This investigation is based on multicolour images from U
through H, which allows photometric redshifts to be estimated for the
background sources. The redshifts provide useful extra information for the
lensing models: we show how they can be used to identify a new multiple-image
system (and rule out an old one), how this information can be used to anchor
the mass model for the cluster, and how the redshifts can be used to construct
optimal samples of background galaxies for a weak lensing analysis. Combining
all results, we obtain the mass distribution in this cluster from the inner,
strong lensing region, out to a radius of 1.5 Mpc. The mass profile is
consistent with a singular isothermal model over this radius range. Parametric
and non-parametric reconstructions of the mass distribution in the cluster are
compared. The main features (elongation, sub-clumps, radial mass profile) are
in good agreement.


title: Dynamics of Warm-Absorbing Gas in Seyfert Galaxies: NGC 5548

abstract:
  A hydromagnetic (MHD) wind from a clumpy molecular accretion disk is invoked
to explain observations of warm absorbing (WA) gas in UVX from Sy galaxies.
This paper focuses on two issues: (1) compatibility of kinematics and dynamics
of MHD wind with the observed properties of WAs; and (2) relationship between
the UVX absorptions. We provide an in-depth comparison between the MHD model
and the Sy 1 galaxy NGC 5548, which at high spectral resolution exhibits a
number of discrete UV absorption components. We find that: (1) the total column
densities of Ovii, Oviii and H, are reproduced by constraining the UV ion
column densities of Civ and Nv in each component to lie within a factor of 2 of
their observed values and optimizing over the possible sets of component
ionization states and Civ column densities; (2) the WA exists in the outer part
of the wind and is not a continuation of the flow in the BLR; and (3) the WA
extends in radial and polar directions and is ionization-stratified. X-ray
absorption is found to be heavily biased towards smaller r, and UV absorption
originates at larger distances from the central continuum source. We show that
the discrete absorption components along the line-of-sight are intrinsically
clumpy. Density differences between kinematic components result in a range of
ionization and recombination timescales. We further test the applicability of
the MHD wind to WAs in general, by constructing a quasi-continuous flow model,
and extending it to arbitrary aspect angles. We estimate the fraction of Sy 1s
having detectable WAs with larger Ovii column density than Oviii, and the range
of total H column densities. We also find that the ratio of Ovii to Oviii
optical depths can serve as a new diagnostic of AGN aspect angle.


title: Toy model of obscurational variability in active galactic nuclei

abstract:
  We propose an explanation of the variability of AGN based on a cloud model of
accretion onto a black hole. These clouds, present at a distance of 10-100
Schwarzschild radii, possibly come from the disrupted innermost disk and they
can partially obscure the central X-ray source due to their extreme Compton
optical depth. We consider the implications of this scenario using a toy model
of AGN spectra and support the results with full radiative transfer
computations from the codes TITAN and NOAR. We show that small random
rearrangements of the cloud distribution can happen on timescales of the order
of 10^2 - 10^6 s and may lead to relatively high variability amplitude in the
X-ray band if the mean covering factor is large. The normalized variability
amplitude in the UV band is either the same as in the X-ray band, if the
contribution from the dark sides of the clouds to the UV band is negligible, or
it is smaller. The X-ray spectrum should basically preserve its spectral shape
even during the high amplitude variability, within the frame of our model.


title: Physics of Extremely High Energy Cosmic Rays

abstract:
  Over the last third of the century, a few tens of events, detected by
ground-based cosmic ray detectors, have opened a new window in the field of
high-energy astrophysics. These events have macroscopic energies, unobserved
sources, an unknown chemical composition and a production and transport
mechanism yet to be explained. With a flux as low as one particle per century
per square kilometer, only dedicated detectors with huge apertures can bring in
the high-quality and statistically significant data needed to answer those
questions. In this article, we review the present status of the field both from
an experimental and theoretical point of view. Special attention is given to
the next generation of detectors devoted to the thorough exploration of the
highest energy ranges


title: A Young Globular Cluster in the Galaxy NGC 6946

abstract:
  A globular cluster ~15 My old that contains 5x10^5 Msun of stars inside an 11
pc radius has been found in the nearby spiral galaxy NGC 6946, surrounded by
clouds of dust and smaller young clusters inside a giant circular bubble 300 pc
in radius. At the edge of the bubble is an arc of regularly-spaced clusters
that could have been triggered during the bubble's formation. The region is at
the end of a spiral arm, suggesting an origin by the asymmetric collapse of
spiral arm gas. The globular is one of the nearest examples of a cluster that
is similar to the massive old globulars in the Milky Way. We consider the
energetics of the bubble and possible formation mechanisms for the globular
cluster, including the coalescence of smaller clusters.


title: Molecules as Tracers of PN Structure

abstract:
  Molecular gas plays an important role in the structure of planetary nebulae:
it is a major component of the equatorial tori of bipolar nebulae, it forms the
cores of globules and related mircostructures, and is the likely origin of
multiple arcs. It is also a key component during the early stages of formation
where interactions with outflows or jets provide an important shaping
mechanism.


title: Early spectroscopic observations of Nova (V1494) Aquilae 1999 No.2

abstract:
  Low- and medium resolution spectra of the fast nova, Nova (V1494) Aql 1999
No.2 obtained approximately 6, 7, 19 and 28 days after the maximum brightness
are presented and discussed. The spectrum covering the whole optical range at
day 6 shows the principal plus diffuse-enhanced spectrum. The presence of
strong Fe II multiplets with P-Cyg profiles suggest that V1494 Aql belongs to
the ``Fe II'' class defined by Williams (1992). The medium-resolution profiles
(lambda/Delta lambda \approx 7000) of the Hgamma and Hdelta lines show
well-defined sharp absorption features with the same radial velocities, while
the Halpha split into two distinct emission peaks in the last two spectra
(Delta t=19 and 28 days). The observed behaviour suggests an expanding
equatorial ring with possible small-scale clumpiness in the nova shell.
  The visual lightcurve is used to deduce M_V by the maximum magnitude versus
rate of decline relationship. The resulting parameters are: t_2=6.6+/-0.5 days,
t_3=16+/-0.5 days, M_V=-8.8+/-0.2 mag. Adopting this value, a distance
d=3.6+/-0.3 kpc is determined.


title:
The Fornax Spectroscopic Survey I. Survey Strategy and Preliminary
  Results on the Redshift Distribution of a Complete Sample of Stars and
  Galaxies

abstract:
  The Fornax Spectroscopic Survey will use the Two degree Field spectrograph
(2dF) of the Anglo-Australian Telescope to obtain spectra for a complete sample
of all 14000 objects with 16.5<=Bj<=19.7 in a 12 square degree area centred on
the Fornax Cluster. By selecting all objects---both stars and
galaxies---independent of morphology, we cover a much larger range of surface
brightness and scale size than previous surveys. In this paper we present
results from the first 2dF field. Redshift distributions and velocity
structures are shown for all observed objects in the direction of Fornax,
including Galactic stars, galaxies in and around the Fornax Cluster, and for
the background galaxy population. The velocity data for the stars show the
contributions from the different Galactic components, plus a small tail to high
velocities. We find no galaxies in the foreground to the cluster in our 2dF
field. The Fornax Cluster is clearly defined kinematically. The mean velocity
from the 26 cluster members having reliable redshifts is 1560+/-80 km/s. They
show a velocity dispersion of 380+/-50 km/s. Large-scale structure can be
traced behind the cluster to a redshift beyond z=0.3. Background compact
galaxies and low surface brightness galaxies are found to follow the general
galaxy distribution.


title:
Evolution of the Blue Luminosity-to-Baryon Mass Ratio of Clusters of
  Galaxies

abstract:
  We derive the ratio of total blue luminosity to total baryon mass, LB/Mb, for
massive (Mgas at the Abell radius is \ge 1 \times 10^{13} h^{-2.5} \Msolar)
clusters of galaxies up to z \simeq 1 from the literature. Twenty-two clusters
in our sample are at z > 0.1. Assuming that the relative mix of hot gas and
galaxies in clusters does not change during cluster evolution, we use LB/Mb to
probe the star formation history of the galaxy population as a whole in
clusters. We find that LB/Mb of clusters increases with redshift from
LB/Mb=0.024 (solar units) at z = 0 to \simeq 0.06 at z=1, indicating a factor
of 2-3 brightening (we assume H0=70 km/s/Mpc). This amount of brightening is
almost identical to the brightening of the M/LB ratio of early-type galaxies in
clusters at 0.02 \le z \le 0.83 reported by van Dokkum et al. (1998). We
compare the observed brightening of LB/Mb with luminosity evolution models for
the galaxy population as a whole, changing the e-folding time of star formation
\tau by 0.1 \le \tau \le 5 Gyr and the formation redshift \zF by 2 \le \zF <
\infty. We find that \tau=0.1 Gyr 'single burst' models with \zF \ge 3 and
\tau=5 Gyr 'disk' models with arbitrary \zF are consistent with the observed
brightening, while models with \tau=1-2 Gyr tend to predict too steep
brightening. We also derive the ratio of blue luminosity density to baryon
density for field galaxies, adopting \Omega_b h^2 = 0.02, and find that blue
luminosity per unit baryon is similar in clusters and in fields up to z \simeq
1 within the observational uncertainties.


title: Superwind model of extended Lyman alpha emitters at high redshift

abstract:
  We propose a new model for the extended Ly alpha blobs found recently at high
redshift z ~ 3. The observational properties of these blobs are; 1) the
observed Ly alpha luminosities are ~ 10^{43} h^{-2} ergs s^{-1}, 2) they appear
elongated morphologically, 3) their sizes amount to ~ 100 kpc, 4) the observed
line widths amount to ~ 1000 km s^{-1}, and 5) they are not associated with
strong radio-continuum sources. All these observational properties seem to be
explained in terms of galactic winds driven by successive supernova explosions
shortly after the initial burst of massive star formation in the galactic
centers. The observed number density of Ly alpha blobs (~ 3.4 x 10^{-5} h^3
Mpc^{-3}) may be explained if their present-day counterparts are elliptical
galaxies with a luminosity above ~ 1 L^*.


title: WSRT 1.4 GHz Observations of the Hubble Deep Field

abstract:
  We present WSRT 1.38 GHz observations of the Hubble Deep Field (and flanking
fields). 72 hours of data were combined to produce the WSRT's deepest image
yet, achieving an r.m.s. noise level of 8 microJy per beam. We detect radio
emission from galaxies both in the HDF and HFF which have not been previously
detected by recent MERLIN or VLA studies of the field.


title: Steady dynamos in finite domains: an integral equation approach

abstract:
  The paper deals with the integral equation approach to steady kinematic
dynamo models in finite domains based on Biot-Savart's law. The role of the
electric potential at the boundary is worked out explicitly. As an example, a
modified version of the simple spherical $\alpha$-effect dynamo model proposed
by Krause and Steenbeck is considered in which the $\alpha$-coefficient is no
longer constant but may vary with the radial coordinate. In particular, the
results for the original model are re-derived. Possible applications of this
integral equation approach for numerical simulations of dynamos in arbitrary
geometry and for an ''inverse dynamo theory'' are sketched.


title:
Infrared spectroscopy of cataclysmic variables: III. Dwarf novae below
  the period gap and novalike variables

abstract:
  We present K-band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK
Lyn, T Leo, SW UMa and WZ Sge, the novalike variables DW UMa, V1315 Aql, RW
Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral
types ranging from K2-M6.
  The spectra of the dwarf novae are dominated by emission lines of HI and HeI.
The large velocity and equivalent widths of these lines, in conjunction with
the fact that the lines are double-peaked in the highest inclination systems,
indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which
we obtained time-resolved data covering a complete orbital cycle, the emission
lines show modulations in their equivalent widths which are most probably
associated with the bright spot (the region where the gas stream collides with
the accretion disc). There are no clear detections of the secondary star in any
of the dwarf novae below the period gap, yielding upper limits of 10-30% for
the contribution of the secondary star to the observed K-band flux. In
conjunction with the K-band magnitudes of the dwarf novae, we use the derived
secondary star contributions to calculate lower limits to the distances to
these systems.
  The spectra of the novalike variables are dominated by broad, single-peaked
emission lines of HI and HeI - even the eclipsing systems we observed do not
show the double-peaked profiles predicted by standard accretion disc theory.
With the exception of RW Tri, which exhibits NaI, CaI and 12CO absorption
features consistent with a M0V secondary contributing 65% of the observed
K-band flux, we find no evidence for the secondary star in any of the novalike
variables. The implications of this result are discussed.


title:
On the Existence of Differences in Luminosity between Horizontal Branch
  Stars in Globular Clusters and in the Field

abstract:
  The discrepancy between a LONG distance scale derived from Hipparcos based
distances to globular clusters via main sequence fitting to local subdwarfs,
and a SHORT distance scale derived from the absolute magnitude of field RR
Lyraes via statistical parallaxes and the Baade-Wesselink method could be
accounted for whether an intrinsic difference of about 0.1-0.2 mag was found to
exist between horizontal branch (HB) stars populating the "sparse" general
field and the "dense" globular clusters. In this paper we discuss the possible
existence of such a systematic difference comparing the "period-shifts"
observed for field and cluster RR Lyraes. Various approaches based on different
parameters and data-sets for both cluster and field variables were used in
order to establish the size of such a hypothetical difference, if any. We find
that on the whole very small not significant differences exist between the
period-metallicity distributions of field and cluster RR Lyraes, thus
confirming with a more quantitative approach, the qualitative conclusions by
Catelan (1998). This observational evidence translates into a very small
difference between the horizontal branch luminosity of field and cluster stars,
unless RR Lyraes in Globular Clusters are about 0.06 Mo more massive than field
RR Lyrae at same metallicity, which is to be proven.


title:
Structure and Evolution of the Envelopes of Deeply Embedded Massive
  Young Stars

abstract:
  The physical structure of the envelopes around a sample of fourteen massive
(1000-100,000 solar L) young stars is investigated on 100- 100,000 AU scales
using maps and spectra in submillimeter continuum and lines of C17O, CS and
H2CO. The total column densities and the temperature profiles are obtained by
fitting self-consistent dust models to submillimeter photometry. Both the
molecular line and dust emission data indicate density gradients ~r^{-alpha},
with alpha=1.0-1.5, significantly flatter than the alpha=2.0 generally found
for low-mass objects. This flattening may indicate that in massive young
stellar objects, nonthermal pressure is more important for the support against
gravitational collapse, while thermal pressure dominates for low-mass sources.
We find alpha=2 for two hot core-type sources, but regard this as an upper
limit since in these objects, the CS abundance may be enhanced in the warm gas
close to the star.


title: A broadband X-ray view of NGC4945

abstract:
  We present the results of a BeppoSAX observation of the nearby spiral galaxy
NGC4945 in the 0.1-200 keV energy band. The nuclear X-ray emission emerges
above ~7 keV, through an absorber with column density N_H ~ a few 10^24/cm/cm.
Its remarkable variability (doubling/halving timescale ~10^4 s) is not
associated with any appreciable spectral changes, ruling out variations of the
interposed absorber or changes in the primary continuum shape. The intense iron
K-ALPHA fluorescent emission line is likely to be produced by the same nuclear
absorbing matter. Our estimate of the intrinsic nuclear power suggests that the
active nucleus dominates the bolometric energy output of NGC4945, if its
nucleus has a typical quasar L_X/L(bol) ratio. This supports the idea that
X-rays are the best energy band to search for absorbed AGN. The 0.1-5 keV
emission is extended along the plane of the galaxy, and most likely due to a
population of unresolved binaries, as believed to happen in several early-type
galaxies. Alternatively, hot gas associated with a starburst-driven superwind
outflow could substantially contribute to the extended emission above 1 keV.


title: The Properties of Young Clusters in M82

abstract:
  We present a detailed study of two luminous super star clusters in the
starburst galaxy M82. Spectra, covering 3250-8790 A at a resolution of 1.6 A,
were obtained at the 4.2 m William Herschel Telescope (WHT) for cluster F and
the highly reddened cluster L. We compare the strengths of the observed Balmer
absorption lines and the Balmer jump in the blue spectrum of F with theoretical
model cluster spectra using the PEGASE spectral synthesis code to derive an age
of 60+/-20 Myr. For cluster L, we find that the similarities in the strength of
the Ca II triplet and overall spectral appearance with cluster F suggest a
similar age. The brightness and compactness of cluster F make it an ideal
candidate for determining its dynamical mass by measuring the velocity
dispersion. We present the results of such an investigation based on echelle
spectra at a resolution of 8 km/s obtained at the WHT from 5760-9140 A. By
cross-correlating various wavelength regions in the spectrum of cluster F with
cool giant and supergiant template stars, we derive a velocity dispersion and,
by application of the virial theorem, determine a dynamical mass of 2 million
solar masses. We compare our derived mass with those determined for other young
super star clusters and discuss whether our derived parameters are consistent
with cluster F being able to survive to become a globular cluster.


title: X-ray/optical observations of XTE J0421+560/CI Cam in quiescence

abstract:
  We report on a BeppoSAX observation of the transient X-ray source XTE
J0421+560 during quiescence performed ~150 days after the 1998 April outburst.
The source had an unabsorbed 0.5-10 keV flux of 6.7x10^{-12} erg/cm2/s and was
still remarkably soft with most of the emission below 2 keV. The X-ray spectrum
can be fit with the same two-temperature model as the outburst observations.
There is evidence for the presence of an Iron emission feature at ~7 keV. We
report also on a series of optical observations performed using the 72 cm
Teramo-Normale Telescope (TNT) of the Teramo Observatory, and the 1.5 m Loiano
Telescope of the Bologna Observatory. The optical spectrum includes very strong
Balmer emission lines, He I, [Fe II], and [O I] features. From the observed
L_x/Lopt~10^{-3} the quiescent optical emission cannot be due to re-processing
of the X-rays, but has to be generated in the optical companion or the
circumstellar material. Moreover, the quiescent X-ray luminosity cannot be due
to the optical star if it is of spectral type OB. Although the nature of the
compact object present in the XTE J0421+560/CI Cam system cannot be firmly
established, we speculate that it is most probably a white dwarf.


title:
Super star clusters as probes of massive star evolution and the IMF at
  extreme metallicities

abstract:
  Young super star clusters and young compact massive star forming regions can
provide useful information on their burst properties (age, burst duration,
SFR), the upper end of the IMF and yield new constraints on the evolution of
massive stars. Through the study of their stellar populations we can in
particular extend our knowledge on massive stars to extreme metallicities
unavailable for such stars in the Local Group.
  Here we summarise the main results from recent studies on two metallicity
extremes: Wolf-Rayet and O star populations in very metal-poor BCD, and
metal-rich compact nuclear SF regions.


title: Magnetic Confinement, MHD Waves, and Smooth Line Profiles in AGN

abstract:
  In this paper, we show that if the broad line region clouds are in
approximate energy equipartition between the magnetic field and gravity, as
hypothesized by Rees, there will be a significant effect on the shape and
smoothness of broad emission line profiles in active galactic nuclei. Line
widths of contributing clouds or flow elements are much wider than their
thermal widths, due to the presence of non-dissipative MHD waves, and their
collective contribution produce emission line profiles broader and smoother
than would be expected if a magnetic field were not present. As an
illustration, a simple model of isotropically emitting clouds, normally
distributed in velocity, is used to show that smoothness can be achieved for
less than 80,000 clouds and may even be as low as a few hundred. We conclude
that magnetic confinement has far reaching consequences for observing and
modeling active galactic nuclei.


title:
A time-delay determination from VLA light curves of the CLASS
  gravitational lens B1600+434

abstract:
  We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images
of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find
a nearly linear decrease of 18-19% in the flux densities of both lens images
over a period of eight months (February-October) in 1998. Additionally, the
brightest image A shows modulations up to 11% peak-to-peak on scales of days to
weeks over a large part of the observing period. Image B varies significantly
less on this time scale. We conclude that most of the short-term variability in
image A is not intrinsic source variability, but is most likely caused by
microlensing in the lens galaxy. The alternative, scintillation by the ionized
Galactic ISM, is shown to be implausible based on its strong opposite frequency
dependent behavior compared with results from multi-frequency WSRT monitoring
observations (Koopmans & de Bruyn 1999). From these VLA light curves we
determine a median time delay between the lens images of 47^{+5}_{-6} d (68%)
or 47^{+12}_{-9} d (95%). We use two different methods to derive the time
delay; both give the same result within the errors. We estimate an additional
systematic error between -8 and +7 d. If the mass distribution of lens galaxy
can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998),
this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11}
(95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and
Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give
H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For
Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)


title: The rise and fall of V4334 Sgr (Sakurai's Object)

abstract:
  CCD UBVRi photometry of the final helium flash object V4334 Sgr (Sakurai's
Object), carried out during 1997 - 1999, is presented, and the light curve from
its pre-discovery rise to the dust obscuration phase is constructed. The
optical light curve can be divided into four sections, the rise to maximum, the
maximum, the dust onset, and the massive dust shell phase. The color indices
show a general increase with time, first because of the photospheric expansion
and cooling, and later because of the dust forming events. The energy
distributions for the years 1996 - 1999 show that an increasing part of the
energy is radiated at infrared wavelengths. In 1996, the infrared excess is
likely caused by free-free radiation in the stellar wind. Starting from 1997 or
1998 at the latest, carbon dust grains are responsible for the more and more
dramatic decrease of optical radiation and the growing infrared excess. Its
photometric behavior in 1998 - 1999 mimics the ``red declines'' of R CrB
variables, the amplitude, however, is more extreme than any fading ever
observed in an R CrB star. Evidence is given that a complete dust shell has
formed around V4334 Sgr. It therefore shows similarities with dust-forming
classical novae, although evolving ~20 times more slowly. Its luminosity
increased by a factor 4 between 1996 and 1998. A comparison of time scales of
the final helium flash objects FG Sge, V605 Aql and V4334 Sgr shows that the
observed photometric and spectroscopic features are similar, while V4334 Sgr is
the most rapidly evolving object to date.


title: The Evolution of He4 and LiBeB

abstract:
  Our understanding of the evolution of He4 and Li7 depends critically on the
available data for these two elements at low metallicity. In particular, the
degree to which there is a slope in an abundance vs metallicity regression can
help determine the evolution of He, C, N, and O in dwarf galaxies in the case
of He, and cosmic-ray induced nucleosynthesis of LiBeB in our own galaxy in the
case of Li. Recent data and their implications will be discussed.


title: Chandra Observations of the Crab-like Supernova Remnant G21.5-0.9

abstract:
  Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a
compact central core and spectral variations indicative of synchrotron burn-off
of higher energy electrons in the inner nebula. The central core is slightly
extended, perhaps indicating the presence of an inner wind-shock nebula
surrounding the pulsar. No pulsations are observed from the central region,
yielding an upper limit of ~40% for the pulsed fraction. A faint outer shell
may be the first evidence of the expanding ejecta and blast wave formed in the
initial explosion, indicating a composite nature for G21.5-0.9.


title: The Equation of State of Neutron-Star Matter in Strong Magnetic Fields

abstract:
  We study the effects of very strong magnetic fields on the equation of state
(EOS) in multicomponent, interacting matter by developing a covariant
description for the inclusion of the anomalous magnetic moments of nucleons.
For the description of neutron star matter, we employ a field-theoretical
approach which permits the study of several models which differ in their
behavior at high density. Effects of Landau quantization in ultra-strong
magnetic fields ($B>10^{14}$ Gauss) lead to a reduction in the electron
chemical potential and a substantial increase in the proton fraction. We find
the generic result for $B>10^{18}$ Gauss that the softening of the EOS caused
by Landau quantization is overwhelmed by stiffening due to the incorporation of
the anomalous magnetic moments of the nucleons. In addition, the neutrons
become completely spin polarized. The inclusion of ultra-strong magnetic fields
leads to a dramatic increase in the proton fraction, with consequences for the
direct Urca process and neutron star cooling. The magnetization of the matter
never appears to become very large, as the value of $|H/B|$ never deviates from
unity by more than a few percent. Our findings have implications for the
structure of neutron stars in the presence of large frozen-in magnetic fields.


title: Quasar Candidate Multicolor Selection Technique: a different approach

abstract:
  We present a quasar candidate identification technique based on multicolor
photometry. The traditional multi-dimensional method (2 $\times$ N dimensions,
where N is the number of the color-color diagrams) is reduced to a
one-dimensional technique, which consists in a standard fitting procedure,
where the observed spectral energy distributions are compared to quasar
simulated spectra and stellar templates. This new multicolor approach is
firstly applied to simulated catalogues and its {\it efficiency} is examined in
various redshift ranges, as a function of the filter combination and the
available observing time for spectroscopy. We conclude that this method is
better suited than the usual multicolor selection techniques to quasar
identification, especially for high-redshift quasars. The application of the
method to real quasar samples found in the literature results in an {\it
efficiency} comparable to the one obtained from the use of color-color
diagrams. The major advantage of the new method is the estimation of the {\it
photometric} redshift of quasar candidates, enabling, in almost all cases,
spectroscopy to be targeted to best suited wavelength ranges.


title:
Linear and non-linear theory of a parametric instability of hydrodynamic
  warps in Keplerian discs

abstract:
  We consider the stability of warping modes in Keplerian discs. We find them
to be parametrically unstable using two lines of attack, one based on
three-mode couplings and the other on Floquet theory. We confirm the existence
of the instability, and investigate its nonlinear development in three
dimensions, via numerical experiment. The most rapidly growing non-axisymmetric
disturbances are the most nearly axisymmetric (low m) ones. Finally, we offer a
simple, somewhat speculative model for the interaction of the parametric
instability with the warp. We apply this model to the masing disc in NGC 4258
and show that, provided the warp is not forced too strongly, parametric
instability can fix the amplitude of the warp.


title:
The Number of Publications Used As A Metric of the NOAO WIYN Queue
  Experiment

abstract:
  We use the number of papers published in 1998 and 1999 to test the hypothesis
that the queue observing mode at WIYN leads to a significantly higher
scientific throughput than classical mode observing. We use the papers
published from the 4-m, and papers published from the non-queue WIYN time as
controls, requiring only that the data be obtained after 1996 August 1, at
which time the WIYN queue was in its third full semester of operation, and the
WIYN instruments functional and stable. The number of papers published from the
queue data is actually 1.5 times smaller (on a per night basis) than from the
4-m, and roughly comparable to (but lower than) the number published from
non-queue WIYN time. Thus neither comparison offers any support for the
hypothesis that queue leads to a higher scientific throughput. The number of
papers is relatively small, but the statistics are sufficiently robust to
reject the possibility that queue observing at WIYN leads to a factor of 1.5
enhancement in publication rate with a 99.3% confidence in comparison to the
4-m, and with an 89.9% confidence in comparison with non-queue WIYN time. We
consider several explanations, and urge other observatories planning to employ
the queue mode include some controls to provide an objective evaluation of its
success.


title:
ALTAI: Computational code for the simulations of TeV air showers as
  observed with the ground-based imaging atmospheric Cherenkov telescopes

abstract:
  Ground-based atmospheric Cherenkov telescopes are proven to be effective
instruments for observations of very high energy (VHE) gamma-radiation from
celestial objects. For effective use of such technique one needs detailed Monte
Carlo simulations of gamma-ray- and proton/nuclei-induced air showers in Earth
atmosphere. Here we discuss in detail the algorithms used in the numerical code
ALTAI developed particularly for the simulations of Cherenkov light emission
from air showers of energy below 50 TeV. The specific scheme of sampling the
charged particle transport in the atmosphere allows the performance of very
fast and accurate simulations used for interpretation of the VHE gamma-ray
observations.


title:
Structure and Evolution of Circumstellar Disks Around Young Stars: New
  Views from ISO

abstract:
  A question central to understanding the origin of our solar system is: how do
planets form in circumstellar disks around young stars? Because of the complex
nature of the physical processes involved, multi-wavelength observations of
large samples will be required in order to obtain a complete answer to this
question. Surveys undertaken with ISO have helped to solve pieces of this
puzzle in addition to uncovering new mysteries. We review a variety of studies
aimed at understanding; i) the physical structure and composition of
circumstellar disks commonly found surrounding young stellar objects; and ii)
the evolution of circumstellar disks from the active accretion phase to
post-planet building debris disks.


title: Accelerations of Water Masers in NGC4258

abstract:
  The water masers in NGC4258 delineate the structure and dynamics of a
sub-parsec-diameter accretion disk around a supermassive black hole. We report
acceleration measurements for 12 systemic-velocity spectral features and 19
high-velocity spectral features, using a total of 25 epochs of observations
from Effelsberg (5 epochs), the VLA (15 epochs), and the VLBA (5 epochs)
spanning the years 1994 to 1997. The measured accelerations of the
systemic-velocity features are between 7.5 and 10.4 km/s/yr, and there is no
evidence for a dip in the spectrum at the systemic velocity. Such a dip has
been attributed in the past to an absorbing layer of non-inverted H2O (Watson &
Wallin 1994; Maoz & McKee 1998). The accelerations of the high-velocity
features, measured here for the first time, range from -0.77 to 0.38 km/s/yr.
  From the line-of-sight accelerations and velocities, we infer the positions
of these high-velocity masers with a simple edge-on disk model. The resulting
positions fall between -13.6 and 9.3 degrees in azimuth (measured from the
midline, the diameter perpendicular to the line of sight). A model that
suggests a spiral shock origin of the masers (Maoz & McKee 1998), in which
changes in maser velocity are due to the outward motion of the shock wave,
predicts apparent accelerations of -0.05 times theta_p/2.5 degrees km/s/yr,
where theta_p is the pitch angle of the spiral arms. Our data are not
consistent with these predictions. We also discuss the physical properties of
the high-velocity masers. Most notably, the strongest high-velocity masers lie
near the midline, where the velocity gradient is smallest, thereby providing
the longest amplification path lengths. (Abridged)


title: Oscillation Waveforms and Amplitudes from Hot Spots on Neutron Stars

abstract:
  The discovery of high-amplitude brightness oscillations during type I X-ray
bursts from six low-mass X-ray binaries has provided a powerful new tool to
study the properties of matter at supranuclear densities, the effects of strong
gravity, and the propagation of thermonuclear burning. There is substantial
evidence that these brightness oscillations are produced by spin modulation of
one or two localized hot spots confined to the stellar surface. It is therefore
important to calculate the expected light curves produced by such hot spots
under various physical assumptions, so that comparison with the observed light
curves may most sensitively yield information about the underlying physical
quantities. In this paper we make general relativistic calculations of the
light curves and oscillation amplitudes produced by a rotating neutron star
with one or two hot spots as a function of spot size, stellar compactness,
rotational velocity at the stellar surface, spot location, orientation of the
line of sight of the observer, and the angular dependence of the surface
specific intensity. We find that stellar rotation and beaming of the emission
tend to increase the observed oscillation amplitudes whereas greater
compactness and larger spot size tend to decrease them. By applying these
results to 4U 1636--536, we show that this source must have two emitting spots
and place strong constraints on the neutron star's magnetic field geometry. We
also show that the data on the phase lags between photons of different energies
in the persistent pulsations in SAX J1808--58 can be fit well with a model in
which the observed hard leads are due to Doppler beaming.


title: The Distance to the Large Magellanic Cloud

abstract:
  I demonstrate that the two unexpected results in the local Universe:
anomalous intrinsic (V-I)_0 colors of RR Lyrae stars and clump giants in the
Galactic center, and very short distances to Magellanic Clouds inferred from
clump giants, can be at least partially resolved with a modified coefficient of
selective extinction A_V/E(V-I). With this modification, I find a new
clump-giant distance modulus to the Large Magellanic Cloud, mu_{LMC} = 18.27
+/- 0.07, which is 0.09 larger than the Udalski (1998b) result. When distance
estimates from the red clump, RR Lyrae stars and the eclipsing binary HV2274
are combined, one obtains mu_{LMC} = 18.31 +/- 0.04 (internal).


title:
Near-Infrared Classification Spectroscopy: J-Band Spectra of Fundamental
  MK Standards

abstract:
  We present a catalog of J-band spectra for 88 fundamental MK standard stars
observed at a resolving power of R ~ 3000. This contribution serves as a
companion atlas to the K-band spectra published by Wallace and Hinkle (1997)
and the H-band atlas of Meyer et al. (1998). We report data from 7400-9550 1/cm
(1.05-1.34 microns) for stars of spectral types O7-M6 and luminosity classes
I-V as defined in the MK system. In reducing these data, special care has been
taken to remove time-variable telluric features of water vapor. We identify
atomic and molecular indices which are both temperature and luminosity
sensitive that aid in the classification of stellar spectra in the J-band. In
addition to being useful in the classification of late-type stars, the J-band
contains several features of interest in the study of early-type stellar
photospheres. These data are available electronically for anonymous FTP in
addition to being served on the World Wide Web.


title: QSO Absorption Line Constraints on Intragroup High-Velocity Clouds

abstract:
  We show that the number statistics of moderate redshift MgII and Lyman limit
absorbers may rule out the hypothesis that high velocity clouds are infalling
intragroup material.


title:
Point source models for the gravitational lens B1608+656: Indeterminacy
  in the prediction of the Hubble constant

abstract:
  We apply elliptical isothermal mass models to reproduce the point source
properties, i.e. image positions, flux density ratios and time delay ratios, of
the quadruple lens B1608+656. A wide set of suitable solutions is found,
showing that models that only fit the properties of point sources are
under-constrained and can lead to a large uncertainty in the prediction of Ho.
We present two examples of models predicting Ho=100 km/(s Mpc) (chi^2=4) and
Ho=69 km/(s Mpc) (chi^2=24).


title:
Compact Stellar Systems in the Fornax Cluster: Super-massive Star
  Clusters or Extremely Compact Dwarf Galaxies?

abstract:
  We describe a population of compact objects in the centre of the Fornax
Cluster which were discovered as part of our 2dF Fornax Spectroscopic Survey.
These objects have spectra typical of old stellar systems, but are unresolved
on photographic sky survey plates. They have absolute magnitudes -13<Mb<-11, so
they are 10 times more luminous than any Galactic globular clusters, but
fainter than any known compact dwarf galaxies. These objects are all within 30
arcminutes of the central galaxy of the cluster, NGC 1399, but are distributed
over larger radii than the globular cluster system of that galaxy. We suggest
that these objects are either super-massive star clusters (intra-cluster
globular clusters or tidally stripped nuclei of dwarf galaxies) or a new type
of low-luminosity compact elliptical dwarf (M32-type) galaxy. The best way to
test these hypotheses will be to obtain high resolution imaging and
high-dispersion spectroscopy to determine their structures and mass-to-light
ratios. This will allow us to compare them to known compact objects and
establish if they represent a new class of hitherto unknown stellar system.


title: Extracting Energy from a Black Hole through Its Disk

abstract:
  When some magnetic field lines connect a Kerr black hole with a disk rotating
around it, energy and angular momentum are transferred between them. If the
black hole rotates faster than the disk, $ca/GM_H>0.36$ for a thin Keplerian
disk, then energy and angular momentum are extracted from the black hole and
transferred to the disk ($M_H$ is the mass and $a M_H$ is the angular momentum
of the black hole). This way the energy originating in the black hole may be
radiated away by the disk.
  The total amount of energy that can be extracted from the black hole spun
down from $ca/GM_H = 0.998$ to $ca/GM_H = 0.36$ by a thin Keplerian disk is
$\approx 0.15 M_Hc^2$. This is larger than $\approx 0.09 M_Hc^2$ which can be
extracted by the Blandford-Znajek mechanism.


title:
A Comparison of Ultraviolet, Optical, and X-Ray Imagery of Selected
  Fields in the Cygnus Loop

abstract:
  During the Astro-1 and Astro-2 Space Shuttle missions in 1990 and 1995, far
ultraviolet (FUV) images of five 40' diameter fields around the rim of the
Cygnus Loop supernova remnant were observed with the Ultraviolet Imaging
Telescope (UIT). These fields sampled a broad range of conditions including
both radiative and nonradiative shocks in various geometries and physical
scales. In these shocks, the UIT B5 band samples predominantly CIV 1550 and the
hydrogen two-photon recombination continuum. Smaller contri- butions are made
by emission lines of HeII 1640 and OIII] 1665. We present these new FUV images
and compare them with optical Halpha and [OIII], and ROSAT HRI X-ray images.
Comparing the UIT images with those from the other bands provides new insights
into the spatial variations and locations of these different types of emission.
By comparing against shock model calculations and published FUV spectroscopy at
select locations, we surmise that resonance scattering in the strong FUV
permitted lines is widespread in the Cygnus Loop, especially in the bright
optical filaments typically selected for observation in most previous studies.


title: Ultra High Energy Cosmic Rays: The theoretical challenge

abstract:
  The origin of the highest-energy cosmic rays remains a mystery. The lack of a
high energy cutoff in the cosmic ray spectrum together with an apparently
isotropic distribution of arrival directions have strongly constrained most
models proposed for the generation of these particles. An overview of the
present state of theoretical proposals is presented. Astrophysical accelerators
as well as top-down scenarios are reviewed along with their most general
signatures. The origin and nature of these ultra-high energy particles will be
tested by future observations and may indicate as well as constrain physics
beyond the standard model of particle physics.


title:
The Cosmological Quark-Hadron Transition and Massive Compact Halo
  Objects

abstract:
  One of the abiding mysteries in the so-called standard cosmological model is
the nature of the dark matter. It is universally accepted that there is an
abundance of matter in the universe which is non-luminous, due to their very
weak interaction, if at all, with the other forms of matter, excepting of
course the gravitational attraction. Speculations as to the nature of dark
matter are numerous, often bordering on exotics, and searches for such exotic
matter is a very active field of astroparticle physics at the dawn of the new
century. Nevertheless, in recent years, there has been experimental evidence
for at least one form of dark matter - the massive compact halo objects
detected through gravitational microlensing effects proposed by Paczynski some
years ago. To date, no clear consensus as to what these objects, referred to in
the literature as well as in the following by the acronym MACHO, are made of;
for a brief discussion of some of the suggestions, see below. In this work, we
show that they find a natural explanation as leftover relics from the putative
first order cosmic quark - hadron phase transition that is predicted by the
standard model of particle interactions to have occurred during the microsecond
epoch of the early universe.


title: ISO-SWS spectroscopy of NGC 1068

abstract:
  We present ISO-SWS spectroscopy of NGC 1068 for the wavelength range 2.4 to
45um, detecting a total of 36 emission lines. Most of the observed transitions
are fine structure and recombination lines originating in the narrow line
region. We compare the line profiles of optical lines and reddening-insensitive
infrared lines to constrain the dynamical structure and extinction properties
of the NLR. The considerable differences found are most likely explained by two
effects. (1) The spatial structure of the NLR is a combination of a highly
ionized outflow cone and lower excitation extended emission. (2) Parts of the
NLR, mainly in the receding part at velocities above systemic, are subject to
extinction that is significantly suppressing optical emission. Line asymmetries
and net blueshifts remain, however, even for infrared fine structure lines
suffering very little obscuration. This may be either due to an intrinsic
asymmetry of the NLR, or due to a very high column density obscuring component
which is hiding part of the NLR even from infrared view. Mid-infrared emission
of molecular hydrogen in NGC 1068 arises in a dense molecular medium at
temperatures of a few hundred Kelvin that is most likely closely related to the
warm and dense components seen in the near-infrared H2 transitions, and in
millimeter wave tracers of molecular gas. Any emission of the putative pc-scale
molecular torus is likely overwhelmed by this larger scale emission.


title: The giant radio galaxy 8C0821+695 and its environment

abstract:
  We present new VLA and Effelsberg observations of the radio galaxy
8C0821+695. We have obtained detailed images in total intensity and
polarization of this 2 Mpc sized giant. The magnetic field has a configuration
predominantly parallel to the source main axis. We observe Faraday rotation at
low frequencies, most probably produced by an ionized medium external to the
radio source. The spectral index distribution is that typical of FR II radio
galaxies, with spectral indices gradually steepening from the source extremes
towards the core. Modeling the spectrum in the lobes using standard synchrotron
loss models yields the spectral age of the source and the mean velocity of the
jet-head with respect to the lobe material. The existence of a possible
backflow in the lobe is considered to relate spectral with dynamical
determinations of the age and the velocity with respect to the external medium.
Through a very simple model, we obtain a physical characterization of the jets
and the external medium in which the radio galaxy expands. The results in
8C0821+695 are consistent with a relativistic jet nourishing the lobes which
expand in a hot, low density halo. We infer a deceleration of the source
expansion velocity which we explain through a progressive increase in the
hot-spot size.


title:
Radiation Spectra from Advection-Dominated Accretion Flows in a Global
  Magnetic Field

abstract:
  We calculate the radiation spectra from advection-dominated accretion flows
(ADAFs), taking into account the effects of a global magnetic field.
Calculation is based on the analytic model for magnetized ADAFs proposed by
Kaburaki, where a large-scale magnetic field controls the accretion process.
Adjusting a few parameters, we find that our model can well reproduce the
observed spectrum of Sagittarius A$^{*}$. The result is discussed in comparison
with those of well-known ADAF models, where the turbulent viscosity controls
the accretion process.


title:
Gravitational lens magnification by Abell 1689: Distortion of the
  background galaxy luminosity function

abstract:
  Gravitational lensing magnifies the luminosity of galaxies behind the lens.
We use this effect to constrain the total mass in the cluster Abell 1689 by
comparing the lensed luminosities of background galaxies with the luminosity
function of an undistorted field. Since galaxies are assumed to be a random
sampling of luminosity space, this method is not limited by clustering noise.
We use photometric redshift information to estimate galaxy distance and
intrinsic luminosity. Knowing the redshift distribution of the background
population allows us to lift the mass/background degeneracy common to lensing
analysis. In this paper we use 9 filters observed over 12 hours with the Calar
Alto 3.5m telescope to determine the redshifts of 1000 galaxies in the field of
Abell 1689. Using a complete sample of 151 background galaxies we measure the
cluster mass profile. We find that the total projected mass interior to
0.25h^(-1)Mpc is (0.48 +/- 0.16) * 10^(15)h^(-1) solar masses, where our error
budget includes uncertainties from the photometric redshift determination, the
uncertainty in the off-set calibration and finite sampling. This result is in
good agreement with that found by number count and shear-based methods and
provides a new and independent method to determine cluster masses.


title: Integrable models of galactic discs with double nuclei

abstract:
  We introduce a new class of 2-D mass models, whose potentials are of
St\"ackel form in elliptic coordinates. Our model galaxies have two separate
strong cusps that form double nuclei. The potential and surface density
distributions are locally axisymmetric near the nuclei and become {\it highly}
non-axisymmetric outside the nucleus. The surface density diverges toward the
cuspy nuclei with the law $\Sigma \propto r^{-2}$. Our model is sustained by
four general types of regular orbits: {\it butterfly}, {\it nucleuphilic
banana}, {\it horseshoe} and {\it aligned loop} orbits. Horseshoes and
nucleuphilic bananas support the existence of cuspy regions. Butterflies and
aligned loops control the non-axisymmetric shape of outer regions. Without any
need for central black holes, our distributed mass models resemble the nuclei
of M31 and NGC4486B. It is also shown that the self-gravity of the stellar disc
can prevent the double nucleus to collapse.


title: Clustering of mass and galaxies

abstract:
  These lectures cover various aspects of the statistical description of
cosmological density fields. Observationally, this consists of the point
process defined by galaxies, and the challenge is to relate this to the
continuous density field generated by gravitational instability in dark matter.
The main topics discussed are (1) nonlinear structure in CDM models; (2)
statistical measures of clustering; (3) redshift-space distortions; (4)
small-scale clustering and bias. The overall message is optimistic, in that
simple assumptions for where galaxies should form in the mass density field
allow one to understand the systematic differences between galaxy data and the
predictions of CDM models.


title: SGRs and AXPs - Magnetars or Young Quark Stars ?

abstract:
  Recent measurements of the spin-down rates of soft gamma ray repeaters (SGRs)
and anomalous X-ray pulsars (AXPs) have been interpreted as evidence that these
objects are ``magnetars'': neutron stars spinning down by magnetic dipole
radiation, but with a magnetic field two orders of magnitude larger than that
of ordinary neutron stars. We discuss the evidence disfavouring this
interpretation. We argue that, instead, the observations support the hypothesis
that SGRs and AXPs are neutron stars that have suffered a transition into a
denser form of nuclear matter to become, presumably, strange stars or quark
stars. Their gravitational energy release while they are cooling and
contracting can power both their quiescent X-ray emission and their star quakes
which produce `soft' gamma ray bursts.


title: An extended multi-zone model for the MCG-6-30-15 warm absorber

abstract:
  The variable warm absorber seen with {\em ASCA} in the X-ray spectrum of
MCG$-$6-30-15 shows complex time behaviour in which the optical depth of OVIII
anticorrelates with the flux whereas that of OVII is unchanging. The
explanation in terms of a two zone absorber has since been challenged by {\em
BeppoSAX} observations. These present a more complicated behaviour for the
OVIII edge. We demonstrate here that the presence of a third, intermediate,
zone can explain all the observations. In practice, warm absorbers are likely
to be extended, multi-zone regions of which only part causes directly
observable absorption edges at any given time.


title:
Understanding NLR in Seyfert Galaxies: numerical simulation of jet-cloud
  interaction

abstract:
  Recent HST observations suggest that the NLR in Seyfert Galaxies can be the
result of interaction between jet and external inhomogeneous medium; following
this suggestion we perform numerical simulations considering the impact of a
radiative jet on a dense cloud. We approach the problem adopting a
hydrodynamical code, that consents us to study in detail the jet hydrodynamics,
while we choose a more simplified treatment of radiative processes, in order to
give a qualitatively good interpretation of the emission processes. Our three
main purposes are: i) to reproduce in our simulations the physical conditions
observed in the NLR of Seyfert Galaxies, ii) to obtain physical constraints of
the jet parameters and iii) to study the jet capability to photoionize the
surrounding medium.


title: X-Ray Wakes in Abell 160

abstract:
  `Wakes' of X-ray emission have now been detected trailing behind a few (at
least seven) elliptical galaxies in clusters. To quantify how widespread this
phenomenon is, and what its nature might be, we have obtained a deep (70 ksec)
X-ray image of the poor cluster Abell 160 using the ROSAT HRI. Combining the
X-ray data with optical positions of confirmed cluster members, and applying a
statistic designed to search for wake-like excesses, we confirm that this
phenomenon is observed in galaxies in this cluster. The probability that the
detections arise from chance is less than 0.0038. Further, the wakes are not
randomly distributed in direction, but are preferentially oriented pointing
away from the cluster centre. This arrangement can be explained by a simple
model in which wakes arise from the stripping of their host galaxies'
interstellar media due to ram pressure against the intracluster medium through
which they travel.


title: The HST view of the FR I / FR II dichotomy

abstract:
  In order to explore how the FR I / FR II dichotomy is related to the nuclear
properties of radio galaxies, we studied a complete sample of 26 nearby FR II
radio galaxies using Hubble Space Telescope (HST) images and compared them with
a sample of FR I previously analyzed. FR I nuclei lie in the radio-optical
luminosity plane along a tight linear correlation, which argues for a common
synchrotron origin. FR II show a more complex behavior, which is however
clearly related to their optical spectral classification.
  Broad line FR II radio galaxies (BLRG) are located overall well above the FR
I correlation, suggesting that a contribution from thermal (disc) emission is
present. Three narrow line (NLRG) and one weak line radio galaxy (WLRG), in
which no nuclear source is seen, can be interpreted as the obscured
counter-parts of BLRG, in agreement with the current unification schemes.
  Conversely, in 5 sources of the sample, all of them NLRG or WLRG, optical
cores are located on the same correlation defined by FR I and with similar
radio and optical luminosities. This suggests that, in analogy to FR I, the
emission is dominated by synchrotron radiation and represents the optical
counter-part of the non-thermal radio cores. Interestingly, all these galaxies
are located in clusters, an environment typical of FR I.
  These results imply that, at least at low redshifts, the FR II population is
not homogeneous. Furthermore, the traditional dichotomy between edge darkened
and brightened radio morphology is not univocally connected with the innermost
nuclear structure, as we find FR~II with FR I-like nuclei and this has
interesting bearings from the point of view of the AGN unified models.


title: Magnetized Atmospheres around Accreting Neutron Stars

abstract:
  We present a detailed investigation of atmospheres around accreting neutron
stars with high magnetic field ($B\gtrsim 10^{12}$ G) and low luminosity
($L\lesssim 10^{33}$ erg/s). We compute the atmospheric structure, intensity
and emergent spectrum for a plane-parallel, pure hydrogen medium by solving the
transfer equations for the normal modes coupled to the hydrostatic and energy
balance equations. The hard tail found in previous investigations for
accreting, non-magnetic neutron stars with comparable luminosity is suppressed
and the X-ray spectrum, although still harder than a blackbody at the star
effective temperature, is nearly planckian in shape. Spectra from accreting
atmospheres, both with high and low fields, are found to exhibit a significant
excess at optical wavelengths above the Rayleigh-Jeans tail of the X-ray
continuum.


title: The ages of quasar host galaxies

abstract:
  We present the results of fitting deep off-nuclear optical spectroscopy of
radio-quiet quasars, radio-loud quasars and radio galaxies at z ~ 0.2 with
evolutionary synthesis models of galaxy evolution. Our aim was to determine the
age of the dynamically dominant stellar populations in the hos t galaxies of
these three classes of powerful AGN. Some of our spectra display residual
nuclear contamination at the shortest wavelengths, but the detailed quality of
the fits longward of the 4000A break provide unequivocal proof, if further
proof were needed, that quasars lie in massive galaxies with (at least at z ~
0.2) evolved stellar populations. By fitting a two-component model we have
separated the very blue (starburst and/or AGN contamination) from the redder
underlying spectral energy distribution, and find that the hosts of all three
classes of AGN are dominated by old stars of age 8 - 14 Gyr. If the blue
component is attributed to young stars, we find that, at most, 1% of the
baryonic mass of these galaxies is involved in star-formation activity at the
epoch of observation. These results strongly support the conclusion reached by
McLure et al. (1999) that the host galaxies of luminous quasars are massive
ellipticals which formed prior to the peak epoch of quasar activity at z ~ 2.5.


title: Optical off-nuclear spectra of quasar hosts and radio galaxies

abstract:
  We present optical (~3200A to ~9000A) off-nuclear spectra of 26 powerful
active galaxies in the redshift range 0.1 < z < 0.3, obtained with the Mayall
and William Herschel 4-meter class telescopes. The sample consists of
radio-quiet quasars, radio-loud quasars (all with -23 > M_V > -26) and radio
galaxies of Fanaroff & Riley Type II (with extended radio luminosities and
spectral indices comparable to those of the radio-loud quasars). The spectra
were all taken approximately 5 arcseconds off-nucleus, with offsets carefully
selected so as to maximise the amount of galaxy light falling into the slit,
whilst simultaneously minimising the amount of scattered nuclear light. The
majority of the resulting spectra appear to be dominated by the integrated
stellar continuum of the underlying galaxies rather than by light from the
non-stellar processes occurring in the active nuclei, and in many cases a 4000A
break feature can be identified. The individual spectra are described in
detail, and the importance of the various spectral components is discussed.
Stellar population synthesis modelling of the spectra will follow in a
subsequent paper (Nolan et al. 2000).


title: The power spectral properties of the Z-source GX 340+0

abstract:
  We present an analysis of ~390 ksec of data of the Z source GX 340+0 taken
during 24 observations with the Rossi X-ray Timing Explorer satellite. We
report the discovery of a new broad component in the power spectra. The
frequency of this component varied between 9 and 14 Hz, and remained close to
half that of the horizontal branch quasi-periodic oscillations (HBO). Its rms
amplitude was consistent with being constant around ~5%, while its FWHM
increased with frequency from 7 to 18 Hz. If this sub-HBO component is the
fundamental frequency, then the HBO and its second harmonic are the second and
fourth harmonic component, while the third harmonic was not detected. This is
similar to what was recently found for the black hole candidate XTE J1550-564.
The profiles of both the horizontal- and the normal branch quasi-periodic
oscillation peaks were asymmetric when they were strongest. We describe this in
terms of a shoulder component at the high frequency side of the quasi-periodic
oscillation peak, whose rms amplitudes were approximately constant at ~4% and
\~3%, respectively. The peak separation between the twin kHz quasi-periodic
oscillations was consistent with being constant at 339+-8 Hz but a trend
similar to that seen in, e.g. Sco X-1 could not be excluded. We discuss our
results within the framework of the various models which have been proposed for
the kHz QPOs and low frequency peaks.


title: The relativistic iron line profile in the Seyfert 1 Galaxy IC4329a

abstract:
  We present simultaneous ASCA and RXTE data on the bright Seyfert 1 galaxy
IC4329a. The iron line is significantly broadened, but not to the extent
expected from an accretion disk which extends down to the last stable orbit
around a black hole. We marginally detect a narrow line component, presumably
from the molecular torus, but even including this gives a line profile from the
accretion disk which is significantly narrower that that seen in MCG-6-30-15,
and is much more like that seen from the low/hard state galactic black hole
candidates. This is consistent with the inner disk being truncated before the
last stable orbit, forming a hot flow at small radii as in the ADAF models.
However, we cannot rule out the presence of an inner disk which does not
contribute to the reflected spectrum, either because of extreme ionisation
suppressing the characteristic atomic features of the reflected spectrum or
because the X-ray source is intrinsically anisotropic, so it does not
illuminate the inner disk.
  The source was monitored by RXTE every 2 days for 2 months, and these
snapshot spectra show that there is intrinsic spectral variability. The data
are good enough to disentangle the power law from the reflected continuum and
we see that the power law softens as the source brightens. The lack of a
corresponding increase in the observed reflected spectrum implies that either
the changes in disk inner radial extent/ionisation structure are small, or that
the variability is actually driven by changes in the seed photons which are
decoupled from the hard X-ray mechanism.


title: ISO-LWS spectroscopy of Centaurus A: extended star formation

abstract:
  We present the first full FIR spectrum of Centaurus A (NGC 5128) from 43 -
196.7 um. The data was obtained with the ISO Long Wavelength Spectrometer
(LWS). We conclude that the FIR emission in a 70 arcsec beam centred on the
nucleus is dominated by star formation rather than AGN activity. The flux in
the far-infrared lines is ~ 1 % of the total FIR: the [CII] line flux is ~ 0.4
% FIR and the [OI] line is ~ 0.2 %, with the remainder arising from [OIII],
[NII] and [NIII] lines. These are typical values for starburst galaxies.
  The ratio of the [NIII]/[NII] line intensities from the HII regions in the
dust lane can be modelled as a ~ 6 million year old starburst. This suggests
that the galaxy underwent either a recent merger or a merger which triggered a
series of bursts.
  We estimate that < 5 % of the observed [CII] arises in the cold neutral
medium (CNM) and that ~ 10 % arises in the warm ionized medium (WIM). The main
contributors to the [CII] emission are the PDRs, which are located throughout
the dust lane and in regions beyond where the bulk of the molecular material
lies. On scales of ~ 1 kpc the average physical properties of the PDRs are
modelled with a gas density, n ~ 1000 cm^-3, an incident far-UV field, G ~ 100
times the local Galactic field, and a gas temperature of ~ 250 K.


title: First results of the air shower experiment KASCADE

abstract:
  The main goals of the KASCADE (KArlsruhe Shower Core and Array DEtector)
experiment are the determination of the energy spectrum and elemental
composition of the charged cosmic rays in the energy range around the knee at
ca. 5 PeV. Due to the large number of measured observables per single shower a
variety of different approaches are applied to the data, preferably on an
event-by-event basis. First results are presented and the influence of the
high-energy interaction models underlying the analyses is discussed.


title: Mass-loss induced instabilities in fast rotating stars

abstract:
  To explain the origin of Herbig Ae/Be stars activity, it has been recently
proposed that strong mass-losses trigger rotational instabilities in the
envelope of fast rotating stars. The kinetic energy transferred to turbulent
motions would then be the energy source of the active phenomena observed in the
outer atmosphere of Herbig Ae/Be stars (Vigneron et al. 1990; Lignieres et al.
1996).
  In this paper, we present a one-dimensional model of angular momentum
transport which allows to estimate the degree of differential rotation induced
by mass-loss. Gradients of angular velocity are very close to - 2 \Omega / R
(\Omega being the surface rotation rate and R the stellar radius). For strong
mass-loss, this process occurs in a short time scale as compared to other
processes of angular momentum transport. Application of existing stability
criteria indicates that rotational instabilities should develop for fast
rotating star. Thus, in fast rotating stars with strong winds, shear
instabilities are expected to develop and to generate subphotospheric turbulent
motions. Albeit very simple, this model gives strong support to the assumption
made by Vigneron et al. 1990 and Lignieres et al. 1996.


title: Overmerging in N-body simulations

abstract:
  The aim of this paper is to clarify the notion and cause of overmerging in
N-body simulations, and to present analytical estimates for its timescale.
Overmerging is the disruption of subhaloes within embedding haloes due to {\it
numerical} problems connected with the discreteness of N-body dynamics. It is
shown that the process responsible for overmerging is particle-subhalo two-body
heating. Various solutions to the overmerging problem are discussed


title: ChaMP and the High Redshift Quasars in X-rays

abstract:
  Chandra X-ray Observatory, (formerly known as AXAF), will observe down to the
flux limit of 2\times 10^{-16} erg~s^{-1} cm^{-2}. In its first year of
operation Chandra's CCD detectors will observe over 1500 quasars
serendipitously in the soft (0.5--3.5 keV) band. Over 200 quasars will be
detected in X-rays in the redshift range 3<z<4 and over 400 quasars in 2<z<3.
This will enable us to determine the high redshift X-ray luminosity function.
This is the contribution by unabsorbed sources only. The total numbers would be
larger by about 60%


title: High-resolution simulations and visualization of protoplanetary disks

abstract:
  A problem of mass flow in the immediate vicinity of a planet embedded in a
protoplanetary disk is studied numerically in two dimensions. Large differences
in temporal and spatial scales involved suggest that a specialized
discretization method for solution of hydrodynamical equations may offer great
savings in computational resources, and can make extensive parameter studies
feasible. Preliminary results obtained with help of Adaptive Mesh Refinement
technique and high-order explicit Eulerian solver are presented. This
combination of numerical techniques appears to be an excellent tool which
allows for direct simulations of mass flow in vicinity of the accretor at
moderate computational cost. In particular, it is possible to resolve the
surface of the planet and to model the process of planet growth with minimal
set of assumptions. Some issues related to visualization of the results and
future prospects are discussed briefly.


title: Cosmic defects and cosmology

abstract:
  We provide a pedagogical overview of defect models of structure formation. We
first introduce the concept of topological defect, and describe how to classify
them. We then show how defects might be produced in phase transitions in the
Early Universe and approach non-pathological scaling solutions. A very
heuristic account of structure formation with defects is then provided,
following which we introduce the tool box required for high precision
calculations of CMB and LSS power spectra in these theories. The decomposition
into scalar vector and tensor modes is reviewed, and then we introduce the
concept of unequal-time correlator. We use isotropy and causality to constrain
the form of these correlators. We finally show how these correlators may be
decomposed into eigenmodes, thereby reducing a defect problem to a series of
``inflation'' problems. We conclude with a short description of results in
these theories and how they fare against observations. We finally describe yet
another application of topological defects in cosmology: baryogenesis.


title: The Role of H2 Molecules in Cosmological Structure Formation

abstract:
  We review the relevance of H2 molecules for structure formation in cosmology.
Molecules are important at high redshifts, when the first collapsed structures
appear with typical temperatures of a few hundred Kelvin. In these chemically
pristine clouds, radiative cooling is dominated H2 molecules. As a result, H2
``astro-chemistry'' is likely to determine the epoch when the first
astrophysical objects appear. We summarize results of recent three-dimensional
simulations. A discussion of the effects of feedback, and implications for the
reionization of the universe is also given.


title:
The Fading of Supernova Remnant Cassiopeia A from 38 MHz to 16.5 GHz
  from 1949 to 1999 with New Observations at 1405 MHz

abstract:
  We report 1405 MHz measurements of the flux density of the approximately 320
year old supernova remnant Cassiopeia A, relative to the flux density of Cygnus
A, made between 1995 and 1999. When compared to measurements made between 1957
and 1976, we find that the rate at which Cassiopeia A has been fading at this
and nearby frequencies has changed from approximately 0.9 % yr^-1 in the 1960s
to approximately 0.6 - 0.7 % yr^-1 now. Furthermore, we have collected from the
literature measurements of this fading rate at lower (38 - 300 MHz) and higher
(7.8 - 16.5 GHz) frequencies. We show that the fading rate has dropped by a
factor of approximately 3 over the past 50 years at the lower frequencies,
while remaining relatively constant at the higher frequencies, which is in
agreement with the findings of others. Our findings at 1405 MHz, in conjunction
with a measurement of the fading rate at the nearby frequency of 927 MHz by
Vinyajkin (1997), show an intermediate behavior at intermediate frequencies. We
also find that Cassiopeia A, as of approximately 1990, was fading at about the
same rate, approximately 0.6 - 0.7 % yr^-1, at all of these frequencies. Future
measurements are required to determine whether the fading rate will continue to
decrease at the lower frequencies, or whether Cassiopeia A will now fade at a
relatively constant rate at all of these frequencies.


title:
Multiwavelength Observations of the Second Largest Known FR II Radio
  Galaxy, NVSS 2146+82

abstract:
  We present multi-frequency VLA, multicolor CCD imaging, optical spectroscopy,
and ROSAT HRI observations of the giant FR II radio galaxy NVSS 2146+82. This
galaxy, which was discovered by the NRAO VLA Sky Survey (NVSS), has an angular
extent of nearly 20' from lobe to lobe. The radio structure is normal for an FR
II source except for its large size and regions in the lobes with unusually
flat radio spectra. Our spectroscopy indicates that the optical counterpart of
the radio core is at a redshift of z=0.145, so the linear size of the radio
structure is ~4 h_50^-1 Mpc. This object is therefore the second largest FR II
known (3C 236 is ~6 h_50^-1 Mpc). Optical imaging of the field surrounding the
host galaxy reveals an excess number of candidate galaxy cluster members above
the number typically found in the field surrounding a giant radio galaxy. WIYN
HYDRA spectra of a sample of the candidate cluster members reveal that six
share the same redshift as NVSS 2146+82, indicating the presence of at least a
``rich group'' containing the FR II host galaxy. ROSAT HRI observations of NVSS
2146+82 place upper limits on the X-ray flux of 1.33 x 10^-13 ergs cm^-2 s^-1
for any hot IGM and 3.52 x 10^-14 ergs cm^-2 s^-1 for an X-ray AGN, thereby
limiting any X-ray emission at the distance of the radio galaxy to that typical
of a poor group or weak AGN. Several other giant radio galaxies have been found
in regions with overdensities of nearby galaxies, and a separate study has
shown that groups containing FR IIs are underluminous in X-rays compared to
groups without radio sources. We speculate that the presence of the host galaxy
in an optically rich group of galaxies that is underluminous in X-rays may be
related to the giant radio galaxy phenomenon.


title:
The Detectability of Gamma-Ray Bursts and Their Afterglows at Very High
  Redshifts

abstract:
  There is increasingly strong evidence that gamma-ray bursts (GRBs) are
associated with star-forming galaxies, and occur near or in the star-forming
regions of these galaxies. These associations provide indirect evidence that at
least the long GRBs detected by BeppoSAX are a result of the collapse of
massive stars. The recent evidence that the light curves and the spectra of the
afterglows of GRB 970228 and GRB 980326 appear to contain a supernova
component, in addition to a relativistic shock wave component, provide more
direct clues that this is the case. Here we establish that GRBs and their
afterglows are both detectable out to very high redshifts (z > 5).


title: Gamma-Ray Bursts as a Probe of the Very High Redshift Universe

abstract:
  We show that, if many GRBs are indeed produced by the collapse of massive
stars, GRBs and their afterglows provide a powerful probe of the very high
redshift (z > 5) universe.


title:
The Merger Rate to Redshift One from Kinematic Pairs: Caltech Faint
  Galaxy Redshift Survey XI

abstract:
  The rate of mass accumulation due to galaxy merging depends on the mass,
density, and velocity distribution of galaxies in the near neighborhood of a
host galaxy. The fractional luminosity in kinematic pairs combines all of these
effects in a single estimator which is relatively insensitive to population
evolution. Here we use a k-corrected and evolution compensated volume-limited
sample drawing about 300 redshifts from CFGRS and 3000 from CNOC2 to measure
the rate and redshift evolution of merging. We identify kinematic pairs with
projected separations less than either 50 or 100 \hkpc and rest-frame velocity
differences of less than 1000\kms. The fractional luminosity in pairs is
modeled as f_L(Delta v,r_p,M_r^{ke})(1+z)^{m_L} where [f_L,m_L] are
[0.14+/-0.07,0+/-1.4] and [0.37+/-0.7,0.1+/-0.5] for r_p<= 50 and 100\hkpc,
respectively (Omega_M=0.2, Omega_Lambda=0.8). The value of m_L is about 0.6
larger if Lambda=0. To convert these redshift space statistics to a merger rate
we use the data to derive a conversion factor to physical space pair density, a
merger probability and a mean in-spiral time. The resulting mass accretion rate
per galaxy (M_1,M_2>= 0.2 M*) is 0.02+/-0.01(1+z)^{0.1+/-0.5} M*~Gyr^{-1}.
Present day high-luminosity galaxies therefore have accreted approximately
0.15M* of their mass over the approximately 7 Gyr to redshift one. (abridged)


title: Gravitational Clustering from Chi^2 Initial Conditions

abstract:
  We consider gravitational clustering from primoridal non-Gaussian
fluctuations provided by a $\chi^2$ model, as motivated by some models of
inflation. The emphasis is in signatures that can be used to constrain this
type of models from large-scale structure galaxy surveys. Non-Gaussian initial
conditions provide additional non-linear couplings otherwise forbidden by
symmetry that cause non-linear gravitational corrections to become important at
larger scales than in the Gaussian case. In fact, the lack of hierarchical
scaling in the initial conditions is partially restored by gravitational
evolution at scales $k> 0.1$ h/Mpc. However, the bispectrum shows much larger
amplitude and residual scale dependence not present in evolution from Gaussian
initial conditions that can be used to test this model against observations. We
include the effects of biasing and redshift distortions essential to compare
this model with galaxy redshift surveys. We also discuss the effects of
primordial non-Gaussianity on the redshift-space power spectrum and show that
it changes the shape of the quadrupole to monopole ratio through non-linear
corrections to infall velocities.


title:
A Search for Resonant Structures in the Zodiacal Cloud with COBE DIRBE:
  The Mars Wake and Jupiter's Trojan Clouds

abstract:
  We searched the COBE DIRBE Sky and Zodi Atlas for a wake of dust trailing
Mars and for Trojan dust near Jupiter's L5 Lagrange point. We compare the DIRBE
images to a model Mars wake based on the empirical model of the Earth's wake as
seen by the DIRBE and place a 3-sigma upper limit on the fractional overdensity
of particles in the Mars wake of 18% of the fractional overdensity trailing the
Earth. We place a 3-sigma upper limit on the effective emitting area of large
(10-100 micron diameter) particles trapped at Jupiter's L5 Lagrange point of 6
x 10^17 cm^2, assuming that these large dust grains are distributed in space
like the Trojan asteroids. We would have detected the Mars wake if the surface
area of dust in the wake scaled simply as the mass of the planet times the
Poynting-Robertson time scale.


title:
Keck Speckle Imaging of the White Dwarf G29-38: No Brown Dwarf Companion
  Detected

abstract:
  The white dwarf Giclas 29-38 has attracted much attention due to its large
infrared excess and the suggestion that excess might be due to a companion
brown dwarf. We observed this object using speckle interferometry at the Keck
telescope, obtaining diffraction-limited resolution (55 milliarcseconds) at K
band, and found it unresolved. Assuming the entire K band excess is due to a
single point-like companion, we place an upper limit on the binary separation
of 30 milliarcseconds, or 0.42 AU at the star's distance of 14.1 pc. This
result, combined with astroseismological data and other images of G29-38,
supports the hypothesis that the source of the near-infrared excess is not a
cool companion but a dust cloud.


title: An 11.6 Micron Keck Search For Exozodiacal Dust

abstract:
  We have begun an observational program to search nearby stars for dust disks
that are analogous to the disk of zodiacal dust that fills the interior of our
solar system. We imaged six nearby main-sequence stars with the Keck telescope
at 11.6 microns, correcting for atmosphere-induced wavefront aberrations and
deconvolving the point spread function via classical speckle analysis. We
compare our data to a simple model of the zodiacal dust in our own system based
on COBE/DIRBE observations and place upper limits on the density of exozodiacal
dust in these systems.


title: Cosmic Ray Rejection by Linear Filtering of Single Images

abstract:
  We present a convolution-based algorithm for finding cosmic rays in single
well-sampled astronomical images. The spatial filter used is the point spread
function (approximated by a Gaussian) minus a scaled delta function, and cosmic
rays are identified by thresholding the filtered image. This filter searches
for features with significant power at spatial frequencies too high for
legitimate objects. Noise properties of the filtered image are readily
calculated, which allows us to compute the probability of rejecting a pixel not
contaminated by a cosmic ray (the false alarm probability). We demonstrate that
the false alarm probability for a pixel containing object flux will never
exceed the corresponding probability for a blank sky pixel, provided we choose
the convolution kernel appropriately. This allows confident rejection of cosmic
rays superposed on real objects. Identification of multiple-pixel cosmic ray
hits can be enhanced by running the algorithm iteratively, replacing flagged
pixels with the background level at each iteration.


title:
The Variation of Gas Mass Distribution in Galaxy Clusters: Effects of
  Preheating and Shocks

abstract:
  We investigate the origin of the variation of the gas mass fraction in the
core of galaxy clusters, which was indicated by our work on the X-ray
fundamental plane. The adopted model supposes that the gas distribution
characterized by the slope parameter is related to the preheated temperature.
Comparison with observations of relatively hot (~> 3 keV) and low redshift
clusters suggests that the preheated temperature is about 0.5-2 keV, which is
higher than expected from the conventional galactic wind model and possibly
suggests the need for additional heating such as quasars or gravitational
heating on the largest scales at high redshift. The dispersion of the preheated
temperature may be attributed to the gravitational heating in subclusters. We
calculate the central gas fraction of a cluster from the gas distribution,
assuming that the global gas mass fraction is constant within a virial radius
at the time of the cluster collapse. We find that the central gas density thus
calculated is in good agreement with the observed one, which suggests that the
variation of gas mass fraction in cluster cores appears to be explained by
breaking the self-similarity in clusters due to preheated gas. We also find
that this model does not change major conclusions on the fundamental plane and
its cosmological implications obtained in previous papers, which strongly
suggests that not only for the dark halo but also for the intracluster gas the
core structure preserves information about the cluster formation.


title:
A Search for Exozodiacal Dust and Faint Companions Near Sirius, Procyon,
  and Altair with the NICMOS Coronagraph

abstract:
  We observed Sirius, Altair, and Procyon with the NICMOS Coronagraph on the
Hubble Space Telescope to look for scattered light from exozodiacal dust and
faint companions within 10 AU from these stars. We did not achieve enough
dynamic range to surpass the upper limits set by IRAS on the amount of
exo-zodiacal dust in these systems, but we did set strong upper limits on the
presence of nearby late-type and sub-stellar companions.


title: The Cosmic Microwave Background Radiation

abstract:
  We summarize the theoretical and observational status of the study of the
Cosmic Microwave Background radiation. Its thermodynamic spectrum is a robust
prediction of the Hot Big Bang cosmology and has been confirmed
observationally. There are now 76 observations of Cosmic Microwave Background
anisotropy, which we present in a table with references. We discuss the
theoretical origins of these anisotropies and explain the standard jargon
associated with their observation.


title: Conservation of Circulation in Magnetohydrodynamics

abstract:
  We demonstrate, both at the Newtonian and (general) relativistic levels, the
existence of a generalization of Kelvin's circulation theorem (for pure fluids)
which is applicable to perfect magnetohydrodynamics. The argument is based on
the least action principle for magnetohydrodynamic flow. Examples of the new
conservation law are furnished. The new theorem should be helpful in
identifying new kinds of vortex phenomena distinct from magnetic ropes or fluid
vortices.

categories: astro-ph gr-qc physics.flu-dyn physics.plasm-ph