title: Four Extreme Relic Radio Sources in Clusters of Galaxies

abstract:
  (Abridged) We describe the results of the highest-resolution radio
observations yet made of four relic radio sources in the Abell clusters A13,
A85, A133 and A4038. Our VLA images at 1.4 GHz with 4" resolution show a
remarkable variety of fine structure in the form of spectacular arcs, wisps,
plumes and loops. Their integrated radio flux densities fall very rapidly with
frequency, with power-law slopes between 2.1 and 4.4 near 1.4 GHz The relics
possess linear polarization levels ranging between 2.3 % (A133) and 35 % (A85);
the higher polarization fractions imply a highly ordered magnetic field in the
fine structure and low differential Faraday rotation in the intervening cluster
gas. The optical identification of host galaxies remains problematic. In A85,
A133 and A4038 the travel times for the brightest cluster galaxies are
significantly longer than the modeled ages of the relics and nearby bright
ellipticals provide a better match. Excess X-ray emission in the 0.5 keV-to-2
keV band was found near the relics in A85 and A133. The surface brightness was
too high to be attributed to the inverse-Compton mechanism alone. We found
excellent fits to the broad-band radio spectra using the anisotropic (KGKP)
model of spectral ageing, and we have extended the model to include diffusion
of particles between regions of different field strength (the Murgia-JP, or
MJP, model). The steep radio spectra imply ages for the relics of ~ 10^8 yr, at
the start of which period their radio luminosities would have been ~ 10^25 W/Hz
at 1.4 GHz.


title: Trajectories of the images in binary microlensing

abstract:
  We study in detail the trajectories followed by single images during binary
microlensing events. Starting from perturbative resolutions of the lens
equation, we explore the full parameter space by continuity arguments. We see
that the images created during the caustic crossing can recombine with the
others in different ways. This leads to a new classification of the
microlensing events according to the behaviour of the images. We show that the
images involved in these combinations depend on the folds that are crossed at
the entry and at the exit from the caustic. Non-trivial trajectories can be
classified into four main types. Some consequences for the motion of the center
of light of the source in astrometric measurements are also examined.


title:
A search for very high energy neutrinos with the Baikal Neutrino
  Telescope

abstract:
  We present the results of a search for high energy neutrinos with the Baikal
underwater Cherenkov detector {\it NT-200.} An upper limit on the
($\nu_e+\tilde{\nu_e}$) diffuse flux of $E^2 \Phi_{\nu}(E)<(1.3 \div 1.9)\cdot
10^{-6} {cm}^{-2} {s}^{-1} {sr}^{-1} {GeV}$ within a neutrino energy range
$10^4 \div 10^7 {GeV}$ is obtained, assuming an $E^{-2}$ behaviour of the
neutrino spectrum and flavor ratio
$(\nu_e+\tilde{\nu_e}):(\nu_{\mu}+\tilde{\nu_{\mu}})$=1:2.


title: Window Function for Non-Circular Beam CMB Anisotropy Experiment

abstract:
  We develop computationally rapid methods to compute the window function for a
cosmic microwave background anisotropy experiment with a non-circular beam
which scans over large angles on the sky. To concretely illustrate these
methods we compute the window function for the Python V experiment which scans
over large angles on the sky with an elliptical Gaussian beam.


title: HST Imaging in the Chandra Deep Field South: I. Multiple AGN Populations

abstract:
  We present preliminary results from imaging three HST/WFPC2 fields in V and I
within the Chandra Deep Field South (CDFS). HST's sensitivity and resolution
are sufficient to reveal optical counterparts for 24 of the 26 CDFS X-ray
sources detected in the 300 ksec X-ray catalog and to determine the
morphologies of most of these. We find that the X-ray sources comprise two
apparently distinct populations of optical candidates: one optically faint (I >
24) with V - I colors consistent with the I > 24 field population; the other
significantly brighter (I < 22) with colors redder than the I < 22 field
population. More than 2/3 of the X-ray source counterparts are resolved
galaxies. The brighter sources are mostly AGN, based on their high X-ray
luminosity. The optically resolved sources in the brighter population have a
very narrow range of V - I color and appear to be a mix of both late and early
type morphologies at low to moderate redshift. We show that the second
population, with fainter optical counterparts, can be explained as higher
redshift Type 2 AGN.


title: Spectrophotometric Evolutionary Models for Tidal Dwarfs

abstract:
  We describe our procedure to model Tidal Dwarf Galaxies in the context of our
evolutionary synthesis code. Our analysis shows strong contributions of gaseous
emission lines to optical colors and of continuum emission to NIR colors during
the burst phase. This underlines the importance of including both types of
emission when modeling any type of star-bursting galaxies.


title: Chandra Observations of Diffuse X-Rays from the Sagittarius B2 Cloud

abstract:
  We present the first Chandra results of the giant molecular cloud Sagittarius
B2 (Sgr B2), located about 100 pc away from the Galactic center. Diffuse X-rays
are clearly separated from one-and-a-half dozen resolved point sources. The
X-ray spectrum exhibits pronounced iron K-shell transition lines at 6.40 keV (K
alpha) and 7.06 keV (K beta), deep iron K-edge at 7.11 keV and large
photo-electric absorption at low energy. The absorption-corrected X-ray
luminosity is \sim 1 times 10^{35} erg s^{-1}, two orders of magnitude larger
than the integrated luminosity of all the resolved point sources. The diffuse
X-rays come mainly from the south-west half of the cloud with a concave-shape
pointing to the Galactic center direction. These results strongly support the
ASCA model that Sgr B2 is irradiated by an X-ray source at the Galactic center
side.


title: A study of the H2O maser emission from R Cas

abstract:
  We describe the monitoring program we have been carrying out during the past
10 years on a sample of late-type stars, with the Medicina 32-m and Effelsberg
100-m antennas. We discuss the observations on the Mira-variable R Cas in some
detail.


title: Spectrophotometric Investigation of a Sample of Tidal Dwarf Galaxies

abstract:
  We define a Tidal Dwarf Galaxy (TDG) as a self-gravitating entity of
dwarf-galaxy mass built from tidal material expelled during interactions. We
then summarize our findings on broad-band imaging and spectroscopy of a sample
of TDG candidates in a sequence of interacting systems. Evidence for decoupled
kinematics in the ionized gas have been found in several objects. This could
indicate that they are bound galaxies and therefore genuine TDGs. As a detailed
example we analyze the system AM 1159-530, where surprisingly high velocity
gradients have been measured.


title:
NH3 in the Central 10 pc of the Galaxy I: General Morphology and
  Kinematic Connections Between the CND and GMCs

abstract:
  New VLA images of NH3 (1,1), (2,2), and (3,3) emission in the central 10
parsecs of the Galaxy trace filamentary streams of gas, several of which appear
to feed the circumnuclear disk (CND). The NH3 images have a spatial resolution
of 16.5''x14.5'' and have better spatial sampling than previous NH3
observations. The images show the ``southern streamer,'' ``50 km/s cloud,'' and
new features including a ``western streamer'', 6 parsecs in length, and a
``northern ridge'' which connects to the CND. NH3(3,3) emission is very similar
to 1.2 mm dust emission indicating that NH3 traces column density well. Ratios
of the NH3(2,2) to (1,1) line intensities give an estimate of the temperature
of the gas and indicate high temperatures close to the nucleus and CND. The new
data cover a velocity range of 270 km/s, including all velocities observed in
the CND, with a resolution of 9.8 km/s. Previous NH3 observations with higher
resolution did not cover the entire range of velocities seen in the CND. The
large-scale kinematics of the CND do not resemble a coherent ring or disk. We
see evidence for a high velocity cloud within a projected distance of 50'' (2
pc) which is only seen in NH3(3,3) and is likely to be hot. Comparison to 6 cm
continuum emission reveals that much of the NH3 emission traces the outer edges
of Sgr A East and was probably pushed outward by this expanding shell. The
connection between the northern ridge (which appears to be swept up by Sgr A
East) and the CND indicates that Sgr A East and the CND are in close proximity
to each other. Kinematic evidence for these connections is presented in this
paper, while the full kinematic analysis of the central 10 pc will be presented
in Paper II.


title:
Shape of the 4.438 MeV gamma-ray line of ^12C from proton and
  alpha-particle induced reactions on ^12C and ^16O

abstract:
  We calculated in detail the angular distribution of gamma-rays and the
resulting shape of the gamma-ray line produced by the nuclear deexcitation of
the 4.439 MeV state of ^12C following proton and alpha-particle interactions
with ^12C and ^16O in the energy range from threshold to 100 MeV per nucleon,
making use of available experimental data. In the proton energy range from 8.6
to 20 MeV, the extensive data set of a recent accelerator experiment on
gamma-ray line shapes and angular distributions was used to deduce
parameterizations for the gamma-ray emission of the 2^+, 4.439 MeV state of
^12C following inelastic proton scattering off ^12C and proton induced
spallation of ^16O. At higher proton energies and for alpha-particle induced
reactions, optical model calculations were the main source to obtain the needed
reaction parameters for the calculation of gamma-ray line shapes and angular
distributions. Line shapes are predicted for various interaction scenarios of
accelerated protons and alpha-particles in solar flares.


title: Observations of O VI Emission from the Diffuse Interstellar Medium

abstract:
  We report the first Far Ultraviolet Spectroscopic Explorer (FUSE)
measurements of diffuse O VI (lambda,lambda 1032,1038) emission from the
general diffuse interstellar medium outside of supernova remnants or
superbubbles. We observed a 30arcsec x 30arcsec region of the sky centered at l
= 315 and b = -41. From the observed intensities
(2930+/-290(random)+/-410(systematic) and 1790+/-260(random)+/-250(systematic)
photons/cm/cm/s/sr in the 1032 and 1038 Angstrom emission lines, respectively),
derived equations, and assumptions about the source location, we calculate the
intrinsic intensity, electron density, thermal pressure, and emitting depth.
The intensities are too large for the emission to originate solely in the Local
Bubble. Thus, we conclude that the Galactic thick disk and lower halo also
contribute. High velocity clouds are ruled out because there are none near the
pointing direction. The calculated emitting depth is small, indicating that the
O VI-bearing gas fills a small volume. The observations can also be used to
estimate the cooling rate of the hot interstellar medium and constrain models.
The data also yield the first intensity measurement of the C II 3s2 S1/2 to 2p2
P3/2 emission line at 1037 Angstroms and place upper limits on the intensities
of ultraviolet line emission from C I, C III, Si II, S III, S IV, S VI, and Fe
III.


title: X-Ray Absorption Associated with High-Velocity UV Absorbers

abstract:
  We present Chandra observations of two radio-quiet QSOs, PG 2302+029 and PG
1254+047. PG 2302+029 has an ultra high-velocity UV absorption system -56,000
km/s, while PG 1254+047 is a Broad Absorption Line (BAL) QSO with detached
troughs. Both objects are X-ray weak, consistent with the known correlation
between alpha_ox and the strength of the UV absorption lines. The data suggest
that there is evidence that both objects are intrinsically weak X-ray sources,
in addition to being heavily X-ray absorbed. The X-ray absorption column
densities are N_H > 10^22 cm^-2 for neutral gas and the intrinsic emission
spectra have alpha_ox > 2. The data are fit best by including ionized (rather
than neutral) absorbers, with column densities N_H(PG2302) > 2.98 x 10^22 cm^-2
and N_H(PG1254) > 17.3 x 10^22 cm^-2. The degrees of ionization are consistent
with the UV lines, as are the total column densities if the strongest lines are
saturated.


title: A meta-analysis of cosmic star-formation history

abstract:
  A meta-analysis is performed of the literature on evolution in cosmic
star-formation rate density from redshift unity to the present day. The
measurements are extremely diverse, including radio, infrared, and ultraviolet
broad-band photometric indicators, and visible and near-ultraviolet
line-emission indicators. Although there is large scatter among indicators at
any given redshift, virtually all studies find a significant decrease from
redshift unity to the present day. This is the most heterogeneously confirmed
result in the study of galaxy evolution. When comoving star-formation rate
density is treated as being proportional to $(1+z)^{\beta}$, the meta-analysis
gives a best-fit exponent and conservative confidence interval of $\beta=
2.7\pm 0.7$ in a world model with $(\Omega_M,\Omega_{\Lambda})=(0.3,0.7)$ and
$\beta= 3.3\pm 0.8$ in $(\Omega_M,\Omega_{\Lambda})=(1.0,0.0)$. In either case
these evolutionary trends are strong enough that the bulk of the stellar mass
at the present day ought to be in old ($>6 \mathrm{Gyr}$) populations.


title: X Ray Precursors in SGRs: Precessing Gamma Jet Tails

abstract:
  Weak isolated X-ray precursor events before the main Gamma Ray Burst, GRB,
and also rare Soft Gamma Repeaters, SGR, events are in complete disagreement
with any Fireball, or Magnetar, one-shoot explosive scenarios. Fireball model
in last two years has been deeply modified into a fountain beamed Jet exploding
and interacting on external shells to explain GRB fine time structure. On the
contrary earlier we proposed a unified scenario for both GRBs-SGRs where a
precessing Gamma Jet (of different intensity) and its geometrical beaming is
the source of both GRB and SGRs wide morphology. GRBs are peaked SNs Jet
spinning and precessing observed along the thin Jet axis. Their mysterious weak
X precursors bursts, corresponding to non-negligible energy powers, up to
million Supernova ones for GRB, are gamma Jet tails beamed off-axis, observed
at X-Ray tails. They are rare, about (3-6)% of all GRBs, but not unique at all.
Comparable brief X-ray precursor flashes occurred in rarest and most detailed
SGRs events as the 27 and the 29 August 1998 event from SGR 1900+14. The same
source has been in very power-full activity on recent 18 April 2001 once again
preceded by X-Ray precursors. These events are inconsistent with any Fireball
or Magnetar-Mini-Fireball models. We interpret them naturally as earlier
marginal blazing of outlying X conical precessing Jet, an off-axis tails
surrounding a narrower gamma precessing Jet. Only when the light-house Jet is
in on-axis blazing mode toward the Earth we observe the harder power-full SGR
event. We predict such a rich X-Ray precursor signals (more numerous then gamma
ones) during Soft Gamma Repeater peak activities; they should be abundant and
within detection threshold by a permanent monitoring SGRs by Beppo-Sax WFC or
Chandra X ray satellites while at peak activity.


title: On Star Formation Rates in Dwarf Galaxies

abstract:
  We present evolutionary synthesis models of starbursts on top of old stellar
populations to investigate in detailed time evolution the relation between Ha
luminosity and star formation rate (SFR). The models show that several effects
have an impact on the ratio between L(Ha) and SFR. Metallicity different from
solar abundance, a time delay between star formation and maximum Ha-luminosity,
and a varying stellar initial mass function give rise to strong variations in
the ratio of Ha luminosity to SFR and can cause large errors in the
determination of the SFR when employing well-known calibrations. When studying
star-bursting dwarf galaxies, and sub-galactic fragments at high redshift,
which show SFR fluctuating on short timescales, these effects can add up to
errors of two orders of magnitude compared with the calibrations. To accurately
determine the true current SFR additional data in combination with models for
the spectral energy distribution are needed.


title:
Multiwavelength Monitoring of the Narrow-Line Seyfert 1 Galaxy Akn 564.
  I. ASCA Observations and the Variability of the X-ray Spectral Components

abstract:
  We present a 35 day ASCA observation of the NLS1 Akn 564, which was part of a
multiwavelength AGN Watch monitoring campaign. Akn 564 shows a photon index
varying across the range 2.45--2.72. The presence of the soft hump component
below 1 keV, previously detected in ASCA data, is confirmed. Time-resolved
spectroscopy with ~daily sampling reveals a distinction in the variability of
the soft hump and power-law components over a timescale of weeks, with the hump
varying by a factor of 6 across the 35-day observation compared to a factor 4
in the power-law. Flux variations in the power-law component are measured down
to a timescale of ~1000s and accompanying spectral variability suggests the
soft hump is not well-correlated with the power-law on such short timescales.
We detect Fe Ka and a blend of Fe Kb plus Ni Ka, indicating an origin in highly
ionized gas. Variability measurements constrain the bulk of the Fe Ka to
originate within a light week of the nucleus. The large EW of the emission
lines may be due to high metallicity in NLS1s, supporting some evolutionary
models for AGN.


title: Search for Nemesis Encounters with Vega, epsilon Eridani, and Fomalhaut

abstract:
  We calculate the space motions of 21,497 stars to search for close stellar
encounters with Vega, epsilon Eridani and Fomalhaut during the past 1 Myr. We
discover that epsilon Eridani experienced three <2 pc encounters over the past
100,000 yr. Within the uncertainties, epsilon Eridani is having a close
encounter with Kapteyn's star near the present epoch, with a 42.2 percent
probability that the closest approach distance is <1 pc. Vega and Fomalhaut
experienced four and six <2 pc encounters, respectively, over the past 1 Myr.
Each had one encounter with a roughly 2 percent probability that the closest
approach distance is less than 0.5 pc. These encounters will not directly
influence the debris disks observed around Vega, epsilon Eridani and Fomalhaut,
but they may pass through hypothetical Oort clouds surrounding these stars. We
find that two other Vega-like stars, HD 17848 and HD 20010, experienced rare,
<0.1 pc stellar encounters that are more likely to directly perturb their
circumstellar disks.


title: VLBA Polarimetry of Three Powerful Radio Galaxy Cores

abstract:
  We present sensitive, high dynamic range, Very Long Baseline Array (VLBA)
polarimetric observations of the cores of three powerful radio galaxies: 3C166,
3C236 and 3C390.3. Significant polarization is detected in one source (3C166)
allowing us to map out the Faraday Rotation Measure (RM) distribution and
projected magnetic field direction. The inner jet of 3C166 is found to have a
rest frame RM of -2300 rad/m**2, similar to those found in quasars cores. No
polarized flux was detected from the other two sources, but in both counterjets
are seen. The counterjet in 3C236 was previously known, but the detection in
3C390.3 is a new discovery. We suggest that the low fractional polarization in
radio galaxy cores is the result of Faraday depolarization by ionized gas
associated with the accretion disk. The lower polarization of radio galaxy
cores compared to quasars is then naturally explained by unified models as a
result of the viewing angle.


title: Long-lived lop-sided modes of annular disks orbiting a central mass

abstract:
  We demonstrate the existence of long-lived, large-amplitude eccentric modes
in massive, annular particle disks in orbit about a central mass. The lopsided
modes we have found precess slowly in the prograde direction at a rate which
increases with disk mass and decreases with the amplitude of the distortion.
The lopsidedness generally survives for as long as we run the calculations and
may last indefinitely; we found no significant decay in over 700 disk-particle
orbits in one case. These strongly non-linear modes are readily created using a
number of different perturbing rules, but we find no evidence for linear
instabilities in unperturbed disks. Our results suggest that Tremaine's
eccentric disk model for the nucleus of M31 may be viable, but none of our
models matches all aspects of the observational data.


title:
UV (IUE) spectra of the central stars of high latitude planetary nebulae
  Hb7 and Sp3

abstract:
  We present an analysis of the UV (IUE) spectra of the central stars of Hb7
and Sp3. Comparison with the IUE spectrum of the standard star HD 93205 leads
to a spectral classification of O3V for these stars, with an effective
temperature of 50,000 K. From the P-Cygni profiles of CIV (1550 A), we derive
stellar wind velocities and mass loss rates of -1317 km/s +/- 300 km/s and
2.9X10^{-8} solar mass yr^{-1} and -1603 km/s +/- 400 km/s and 7X10^{-9} solar
mass yr^{-1} for Hb7 and Sp3 respectively. From all the available data, we
reconstruct the spectral energy distribution of Hb7 and Sp3.


title: New Southern Groups

abstract:
  Since the eighties we have begun, in Brazil, a search for Post-T Tauri stars.
Here we describe the main steps of this research that resulted in the discovery
of the nearby TW Hya and Horologium associations. A very recent survey resulted
in the detection of three different kinematical groups: 1) the Great Austral
Young Association, which is a very extended complex region involving the
Horologium and Tucana associations, 2) a CrA extend association and 3) a new
group in Pisces.


title: The Eta Chamaeleontis Cluster: Recent Results

abstract:
  The eta Chamaeleontis star cluster is one of the most important nearby
groupings of pre-main sequence (PMS) stars. With accurately known distance,
compact structure and highly-coeval intermediate-aged population, the eta Cha
cluster is an important laboratory for enhancing our understanding of all PMS
star evolutionary issues. In this paper, recent results concerning the stellar
population are discussed including the discovery of a rare example of a ~10
Myr-old Classical T Tauri (CTT) star. This and other objects point to an
X-ray-faint late-type population residing amongst the ROSAT-detected
membership.


title: The Dispersal of Young Stars and the Greater Sco-Cen Association

abstract:
  We review topics related to the dispersal of young stars from their
birth-sites, and focus in particular on the entourage of young stars related to
the ongoing star-formation event in the Sco-Cen OB association. We conduct a
follow-up kinematic study to that presented in Mamajek, Lawson, & Feigelson
(2000; ApJ 544, 356) amongst nearby, isolated, young stars. In addition to the
eta Cha and TW Hya groups, we find several more intriguing Sco-Cen outlier
candidates: most notably beta Pic, PZ Tel, HD 199143, and HD 100546. We discuss
the connection between Sco-Cen and the southern ``150 pc Conspiracy'' molecular
clouds, and in particular, Corona Australis. The kinematic evidence suggests
that many of the nearby, isolated ~10 Myr-old stars were born near Sco-Cen
during the UCL and LCC starbursts 10-15 Myr ago. We hypothesize that these
stars inherited 5-10 km/s velocities moving away from Sco-Cen, either through
molecular cloud turbulence, or through formation in molecular clouds associated
with the expanding Sco-Cen superbubbles (e.g. Loop I).


title: The Great Austral Nearby Young Association

abstract:
  Observing ROSAT sources in an area covering $\sim$30% of the Southern
Hemisphere, we found evidences for a great nearby association (GAYA),
comprising the proposed associations of Horologium (HorA) and the eastern part
of Tucana (TucA), formed by at least 44 Post-T Tauri stars. The stars of the
GAYA have similar space velocity components relative to the Sun (U, V, W) =
($-9.8 \pm 1.2, -21.7 \pm 1.1, -2.0 \pm 2.2$) km/s and their Li line
intensities are between those of the classical T Tauri stars and the ones of
the Local Association stars. The distances of the members of the GAYA cover an
interval of $\sim$70 pc, compatible with the angular size of $\sim60\deg$ and
in agreement with an initial velocity dispersion of $\sim$1.5 km/s and its
evolutive age. We found many other young stars, not members of the GAYA. We
also observed a control region near the equator, covering 700 square degrees,
where we found only four young stars. The overabundance of young stars near the
South Pole (by a factor of five) seems to show that there may be other young
associations not yet characterized.


title: The Extended R CrA Young Association

abstract:
  Observing ROSAT sources in an area covering ~30% of the Southern Hemisphere,
including the R CrA Association, we found evidences that this nearby
association may be much larger than previously thought. Although in the survey
we found many young stars near the R CrA, there are 20 young stars with
properties that characterize them as possibly belonging to the R CrA
association. From the nine known members with measured proper motions and
radial velocities, we obtain the mean space velocity components for the
Association relative to the Sun: (U, V, W) = (-3.8 +/- 1.2, -14.3 +/- 1.7, -8.3
+/- 2.0$)km/s. The new young stars with similar space velocities are in a
projected diameter of ~35deg. At a distance of ~100pc, this represents a size
of ~60pc, similar to the spread in the kinematical distances obtained assuming
the above space velocity components. If the original velocity dispersion during
star formation was equal to the dispersion of the velocity vector moduli
(~3km/s), then the age of the association should be ~10Myr to reach this size.
In this way, the classical T Tauri spectroscopic binary V4046 Sgr could be a
member of the association.


title:
Criteria for the Formation of Population III Objects in the Ultraviolet
  Background Radiation

abstract:
  We explore possibilities of collapse and star formation in Population III
objects exposed to the external ultraviolet background (UVB) radiation.
Assuming spherical symmetry, we solve self-consistently radiative transfer of
photons, non-equilibrium H2 chemistry, and gas hydrodynamics. Although the UVB
does suppress the formation of low mass objects, the negative feedback turns
out to be weaker than previously suggested. In particular, the cut-off scale of
collapse drops significantly below the virial temperature 10^4 K at weak UV
intensities, due to both self-shielding of the gas and H2 cooling. Clouds above
this cut-off tend to contract highly dynamically, further promoting
self-shielding and H2 formation. For plausible radiation intensities and
spectra, the collapsing gas can cool efficiently to temperatures well below
10^4 K before rotationally supported and the final H2 fraction reaches 10^{-3}.
Our results imply that star formation can take place in low mass objects
collapsing in the UVB. The threshold baryon mass for star formation is \sim
10^9 solar mass for clouds collapsing at redshifts z \simlt 3, but drops
significantly at higher redshifts. In a conventional cold dark matter universe,
the latter coincides roughly with that of the 1 \sigma density fluctuations.
Objects near and above this threshold can thus constitute `building blocks' of
luminous structures, and we discuss their links to dwarf spheroidal/elliptical
galaxies and faint blue objects. These results suggest that the UVB can play a
key role in regulating the star formation history of the Universe.


title:
Star formation history in the solar neighborhood: the link between stars
  and cosmology

abstract:
  Using a cosmological galactic evolutionary approach to model the Milky Way,
we calculate the star formation history (SFH) of the solar neighborhood. The
good agreement we obtain with the observational inferences suggests that our
physical model describes accurately the long term/large spatial trends of the
local and global Milky Way SFH. In this model, star formation is triggered by
disk gravitational instabilities and self-regulated by an energy balance in the
ISM. The drivers of the SFH are the cosmological gas infall rate and the gas
surface density determined by the primordial spin parameter. A LambdaCDM
cosmology was used throughout.


title: Spectropolarimetry of the Type II Supernovae 1997ds, 1998A, and 1999gi

abstract:
  We present single-epoch spectropolarimetry of the Type II supernovae (SNe II)
1997ds, 1998A, and 1999gi. SN 1997ds and SN 1998A were both observed during the
early photospheric phase, while spectropolarimetry of SN 1999gi was obtained
near the start of the transition to the nebular phase. Uncorrected for
interstellar polarization (ISP), SN 1997ds is characterized by p_V = 0.85 +/-
0.02%, SN 1998A has p_V = 0.24 +/- 0.05%, and SN 1999gi is polarized at p_V =
5.72 +/- 0.01%. The low continuum polarization inferred for SN 1997ds and SN
1998A and the amplitude of polarization modulations across strong line features
are consistent with those measured at similar epochs for SN 1987A and the Type
II-plateau SN 1999em, and supports the growing consensus that core-collapse
events with hydrogen envelopes substantially intact at the time of explosion
are not significantly aspherical during the early photospheric phase.
  The spectral shape of the high continuum polarization of SN 1999gi closely
resembles a ``Serkowski'' ISP curve, and is inconsistent with the
wavelength-independent nature of electron scattering expected for an aspherical
SN atmosphere. Since Galactic reddening is minimal along this line-of-sight,
the majority of the observed polarization in SN 1999gi is believed to be due to
ISP of the host galaxy, although significant (up to p = 2%) intrinsic
polarization cannot be ruled out. The potential power of SN spectropolarimetry
to study the properties of interstellar dust in external galaxies is described
and applied to the SN 1999gi data, where it is shown that if the polarization
is indeed predominantly interstellar in origin, then R_V = 3.0 +/- 0.2 for the
dust along this line-of-sight in NGC 3184.


title:
Multiple Peaks in the Angular Power Spectrum of the Cosmic Microwave
  Background: Significance and Consequences for Cosmology

abstract:
  Three peaks and two dips have been detected in the power spectrum of the
cosmic microwave background from the BOOMERANG experiment, at $\ell \sim 210,
540, 840$ and $\ell \sim 420, 750$, respectively. Using model-independent
analyses, we find that all five features are statistically significant and we
measure their location and amplitude. These are consistent with the adiabatic
inflationary model. We also calculate the mean and variance of the peak and dip
locations and amplitudes in a large 7-dimensional parameter space of such
models, which gives good agreement with the model-independent estimates, and
forecast where the next few peaks and dips should be found if the basic
paradigm is correct. We test the robustness of our results by comparing
Bayesian marginalization techniques on this space with likelihood maximization
techniques applied to a second 7-dimensional cosmological parameter space,
using an independent computational pipeline, and find excellent agreement:
$\Omega_{\rm tot} = 1.02^{+0.06}_{-0.05}$ {\it vs.} $1.04 \pm 0.05$, $\Omega_b
h^2 = 0.022^{+0.004}_{-0.003}$ {\it vs.} $0.019^{+0.005}_{-0.004}$, and $n_s =
0.96^{+0.10}_{-0.09}$ {\it vs.} $0.90 \pm 0.08$. The deviation in primordial
spectral index $n_s$ is a consequence of the strong correlation with the
optical depth.


title:
Effects of SNe II and SNe Ia Feedback on the Chemo-Dynamical Evolution
  of Elliptical Galaxies

abstract:
  We numerically investigate the dynamical and chemical processes of the
formation of elliptical galaxies in a cold dark matter (CDM) universe, in order
to understand the origin of the mass-dependence of the photometric properties
of elliptical galaxies. Our three-dimensional TREE N-body/SPH numerical
simulations of elliptical galaxy formation take into account both Type II (SNe
II) and Type Ia (SNe Ia) supernovae (SNe) and follow the time evolution of the
abundances of several chemical elements (C, O, Ne, Mg, Si, and Fe). Moreover we
compare different strengths of SNe feedback.In combination with stellar
population synthesis, we derive the photometric properties of simulation
end-products, including the magnitude, color, half-light radius, and abundance
ratios, and compare them with the observed scaling relations directly and
quantitatively. We find that the extremely strong influence of SNe is required
to reproduce the observed color-magnitude relation (CMR), where we assume each
SN yields energy of 4x10^51 ergs and that 90% of this energy is ejected as
kinetic feedback. The feedback affects the evolution of lower mass systems more
strongly and induces the galactic wind by which a larger fraction of gas is
blown out in a lower mass system. Finally higher mass systems become more metal
rich and have redder colors than lower mass systems. We emphasize based on our
simulation results that the galactic wind is triggered mainly by SNe Ia rather
than SNe II. In addition we examined the Kormendy relation, which prescribes
the size of elliptical galaxies, and the [Mg/Fe]--magnitude relation, which
provides a strong constraint on the star formation history.


title: Star Formation in Turbulent Molecular Clouds

abstract:
  Recent progress in the understanding of star formation is summarized. A
consistent picture is emerging where molecular clouds form with turbulent
velocity fields and clumpy substructure, imprinted already during their
formation. The clouds are initially supported by supersonic turbulence which
dissipates however within massive clumps on short timescales, of order their
local dynamical timescales. As a result, the clumps collapse and fragment into
stellar clusters. Subsequent energetic feedback processes by the newly formed
stars will change the thermal and kinematical state of the surrounding
molecular gas, triggering or suppressing star formation and leading eventually
to the disruption of the molecular clouds. More work is required to understand
these processes in greater details.


title: The Formation of the Milky Way in the Cosmological Context

abstract:
  The formation of the Milky Way is discussed within the context of the cold
dark matter scenario. Several problems arise which can be solved if the Galaxy
experienced an early phase of gas heating and decoupling from the dark matter
substructure. This model combines the Eggen, Lynden-Bell and Sandage picture of
a monolithic protogalactic collapses with the Searle and Zinn picture of an
early merging phase of substructures into one consistent scenario of Galactic
formation.


title: On the gas temperature in circumstellar disks around A stars

abstract:
  In circumstellar disks or shells it is often assumed that gas and dust
temperatures are equal where the latter is determined by radiative equilibrium.
This paper deals with the question whether this assumption is applicable for
tenous circumstellar disks around young A stars. In this paper the thin
hydrostatic equilibrium models described by Kamp & Bertoldi (2000) are combined
with a detailed heating/cooling balance for the gas. The most important heating
and cooling processes are heating through infrared pumping, heating due to the
drift velocity of dust grains, and fine structure and molecular line cooling.
Throughout the whole disk gas and dust are not efficiently coupled by
collisions and hence their temperatures are quite different. Most of the gas in
the disk models considered here stays well below 300 K. In the temperature
range below 300 K the gas chemistry is not much affected by T_gas and therefore
the simplifying approximation T_gas = T_dust can be used for calculating the
chemical structure of the disk. Nevertheless the gas temperature is important
for the quantitative interpretation of observations, like fine structure and
molecular lines.


title: The distribution of exoplanet masses

abstract:
  The present study derives the distribution of secondary masses M2 for the 67
exoplanets and very low-mass brown dwarf companions of solar-type stars, known
as of April 4, 2001. This distribution is related to the distribution of M2 sin
i through an integral equation of Abel's type. Although a formal solution
exists for this equation, it is known to be ill-behaved, and thus very
sensitive to the statistical noise present in the input M2 sin i distribution.
To overcome that difficulty, we present two robust, independent approaches: (i)
the formal solution of the integral equation is numerically computed after
performing an optimal smoothing of the input distribution, (ii) the
Lucy-Richardson algorithm is used to invert the integral equation. Both
approaches give consistent results. The resulting statistical distribution of
exoplanet true masses reveals that there is no reason to ascribe the transition
between giant planets and brown dwarfs to the threshold mass for deuterium
ignition (about 13 MJ). The M2 distribution shows instead that all the objects
have M2 < 10 MJ, except the heavier candidates which cluster around 15 MJ.


title:
MASTER of the CMB Anisotropy Power Spectrum: A Fast Method for
  Statistical Analysis of Large and Complex CMB Data Sets

abstract:
  We describe a fast and accurate method for estimation of the cosmic microwave
background (CMB) anisotropy angular power spectrum --- Monte Carlo Apodised
Spherical Transform EstimatoR. Originally devised for use in the interpretation
of the Boomerang experimental data, MASTER is both a computationally efficient
method suitable for use with the currently available CMB data sets (already
large in size, despite covering small fractions of the sky, and affected by
inhomogeneous and correlated noise), and a very promising application for the
analysis of very large future CMB satellite mission products.


title: An analysis of the observed radio emission from planetary nebulae

abstract:
  We have analysed the radio fluxes for 264 planetary nebulae for which
reliable measurements of fluxes at 1.4 and 5 GHz, and of nebular diameters are
available. For many of the investigated nebulae the optical thickness is
important, especially at 1.4 GHz. Simple models like the one specified only by
a single optical thickness or spherical, constant density shells do not account
satisfactorily for the observations. Also an r^{-2} density distribution is
ruled out. A reasonable representation of the observations can be obtained by a
two-component model having regions of two different values of optical
thickness. We show that the nebular diameters smaller than 10 arcsec are
uncertain, particularly if they come from photographic plates or gaussian
fitting to the radio profile. While determining the interstellar extinction
from an optical to radio flux ratio caution should be paid to optical thickness
effects in the radio. We have developed a method for estimating the value of
self absorption. At 1.4 GHz self absorption of the flux is usually important
and can exceed a factor of 10. At 5 GHz self absorption is negligible for most
of the objects although in some cases it can reach a factor of 2. The Galactic
bulge planetary nebulae when used to calibrate the Shklovsky method give the
mean nebular mass of 0.14 solar mass. The statistical uncertainty of the
Shklovsky distances is smaller than factor 1.5.


title: The X-ray spectrum of LSI+61o303

abstract:
  It had been proposed earlier that the hard X-ray and gamma-ray radiation of
the Be/X-ray system LSI+61o303 could be due to inverse Compton scattering of
optical photons from the Be star by the same electron population which also
produces the radio emission. Recently, Apparao (2001) has calculated this
inverse Compton emission in more detail, and predicted that the X-ray spectrum
should show a break at around 20 keV. We investigated archival RXTE data, but
do not find such a break in the 2--25 keV range. The implications of this
finding are shortly discussed.


title: Quasars, Starbursts, and the Cosmic Energy Budget

abstract:
  Observations in the far-infrared are the best way to make an unbiased survey
of active galaxies in the loosest sense - but separating the true quasars from
the starburst may be difficult. There is in fact much evidence to suggest that
starburst and AGN activity are intimately connected, and FIRST will help us to
explore this link. Since the last major FIRST conference in Grenoble, this
issue has become even more central to modern astrophysics, with the realisation
of the dominance of the FIR background, the discovery of high redshift SCUBA
sources, and the emergence of the black hole mass deficit problem. What
dominates the cosmic energy budget - nuclear fusion or accretion ?


title: The Influence of Quintessence on the Separation of CMB Peaks

abstract:
  A hypothetical dark energy component may have an equation of state that is
different from a cosmological constant and possibly even changing in time. The
spacing of the cosmic microwave background peaks is sensitive to the ratio of
the horizon sizes today and at last scattering and therefore to the details of
the cosmological evolution. Together with independent measurements of today's
cosmological parameters, this restricts quintessence models in the epoch before
and during decoupling. For fixed \Omega^b_0, \Omega^{cdm}_0 and h, we give an
analytic estimate of the spacing that depends on \Omega^{\phi}_{ls},
\Omega^{\phi}_0 and an effective equation of state of the dark energy
component.


title:
The identification of an optical counterpart to the super-Eddington
  X-ray source, NGC 5204 X-1

abstract:
  We report the identification of a possible optical counterpart to the
super-Eddington X-ray source NGC 5204 X-1. New Chandra data shows that the
X-ray source is point-like, with a luminosity of 5.2 x 10^39 erg/s (0.5 - 8
keV). It displays medium- and long-term X-ray variability in observations
spanning a period of 20 years. The accurate Chandra position allows us to
identify a blue optical continuum source (m_v = 19.7) at the position of NGC
5204 X-1, using newly-obtained optical data from the INTEGRAL instrument on the
William Herschel Telescope. The X-ray and optical source properties are
consistent with the scenario in which we are observing the beamed X-ray
emission of a high-mass X-ray binary in NGC 5204, composed of an O star with
either a black hole or neutron star companion.


title: An XMM-Newton observation of the luminous Seyfert 1 galaxy Markarian 509

abstract:
  We present the spectral analysis of an early XMM-Newton observation of the
luminous Seyfert 1 galaxy Markarian 509. We find the hard (2-10 keV) continuum
slope, including reflection, to be somewhat flatter ($\Gamma=1.75$) than for a
typical BLS1. The most obvious feature in the hard X-ray spectrum is a narrow
emission line near 6.4 keV, with an equivalent width of 50 eV. The energy and
strength of this line is consistent with fluorescence from `neutral' iron in
the molecular torus, and we note the emerging ubiquity of this feature in
XMM-Newton and Chandra observations of Seyfert 1 galaxies over a wide
luminosity range. We also find evidence for a second emission line at 6.7-6.9
keV, which we attempt to model by reflection from a highly ionised disc. A
`soft excess', evident as an upward curvature in the continuum flux below 1.5
keV, cannot be explained solely by enhanced reflection from the ionised disc.
The RGS spectrum shows only weak discrete emission and absorption features in
the soft X-ray spectrum, supporting our conclusion that the soft excess
emission in Mkn 509 represents the high energy portion of optically thick,
thermal emission from the inner accretion disc.


title: Decays of supernova neutrinos

abstract:
  Supernova neutrinos could be well-suited for probing neutrino decay, since
decay may be observed even for very small decay rates or coupling constants. We
will introduce an effective operator framework for the combined description of
neutrino decay and neutrino oscillations for supernova neutrinos, which can
especially take into account two properties: One is the radially symmetric
neutrino flux, allowing a decay product to be re-directed towards the observer
even if the parent neutrino had a different original direction of propagation.
The other is decoherence because of the long baselines for coherently produced
neutrinos. We will demonstrate how to use this effective theory to calculate
the time-dependent fluxes at the detector. In addition, we will show the
implications of a Majoron-like decay model. As a result, we will demonstrate
that for certain parameter values one may observe some effects which could also
mimic signals similar to the ones expected from supernova models, making it in
general harder to separate neutrino and supernova properties.


title: Evidence for a Circumnuclear Disk in 1946+708

abstract:
  A number of extragalactic radio sources which exhibit symmetric jets on
parsec scales have now been found to have neutral hydrogen absorption at or
near the systemic velocities of their host galaxies. Understanding the spatial
distribution and kinematics of the HI detected toward the central parsecs of
these sources provides an important test of unified schemes for AGN.
  We present results of Global VLBI Network observations of the redshifted 21
cm HI line toward the Compact Symmetric Object 1946+708 (z=0.101). We find
significant structure in the gas on parsec scales. The peak column density of
the HI (N_HI~3x10^23 cm^-2(Ts/8000K)) occurs near the center of activity of the
source, as does the highest velocity dispersion (FWHM 350 to 400 km/s). There
is also good evidence for a torus of ionized gas with column density 7x10^22
cm^-2}. The jets in 1946+708 exhibit bi-directional motion measurable on
timescales of a few years. The resulting unique information about the geometry
of the continuum source greatly assists in the interpretation of the gas
distribution, which is strongly suggestive of a circumnuclear torus of neutral
atomic and ionized material with one or more additional compact clumps of gas
along the line of sight to the approaching jet.


title: A Flaring Megamaser in Mrk 348

abstract:
  We report new observations of the H2O megamaser in the Seyfert 2 galaxy Mrk
348. Following our initial detection in 2000 March using the Effelsberg 100 m
telescope, re-analysis of previous data on this source indicates that the maser
was present but only marginally detectable in late 1997. Monitoring through
late 2000 shows that the maser has again decreased to its original level. The
H2O line is redshifted by ~130 km/s with respect to the systemic velocity, is
extremely broad, with a FWHM of 130 km/s, and has no detectable high velocity
components within 1500 km/s on either side of the strong line. Followup VLBA
observations show that the maser emission emanates entirely from a region >0.25
pc in extent, toward the base of the radio jet.


title:
The Evolution of Galaxy Morphology for Galaxies in the Canada-France
  Redshift Survey

abstract:
  The images of 229 galaxies in the Canada France Redshift Survey have been
classified on the DDO system. These observations were combined with previous
classifications of galaxies with known redshifts in the Hubble Deep Field. The
combined sample provides homogeneous morphological classifications for 425
galaxies of known redshift. The fraction of all galaxies that are of types E +
S0 + E/S0 appears to remain approximately constant at ~17% over the redshift
range 0.25 < z < 1.2. Over the same range the fraction of irregular (Ir)
galaxies increases from ~5% to ~12%. Part of this increase may be due to mild
luminosity evolution of Ir galaxies. The frequency of mergers is found to rise
by a factor of two or three over the redshift range covered by the present
survey.


title: Magnetized Astrophysical Outflows: Cradle to Grave, Source to Effect

abstract:
  We discuss the propagation of radiative MHD jets and outflows focusing on
outflows driven by magneto-centrifugal rotators. Our goal is to link the
properties of the jets with the physics of the sources which produce them. We
find that density and magnetic field stratification (with radius) in jets from
magnetized rotators leads to new behavior including the development of a dense
inner jet core and a low density collar. We also report on more general studies
of ambipolar diffusion and field geometry in pulsed jets. Finally we describe a
new work designed to study the effects of magnetized winds on circumstellar
environments appropriate to YSOs and PNe.


title: Mid-Infrared Imaging of Candidate Vega-Like Systems

abstract:
  We have conducted deep mid-infrared imaging of a relatively nearby sample of
candidate Vega-like stars using the OSCIR instrument on the CTIO 4-meter and
Keck II 10-meter telescopes. Our discovery of a spatially-resolved disk around
HR 4796A has already been reported (Jayawardhana et al. 1998). Here we present
imaging observations of the other members of the sample, including the
discovery that only the primary in the HD 35187 binary system appears to harbor
a substantial circumstellar disk and the possible detection of extended disk
emission around 49 Ceti. We derive global properties of the dust disks, place
constraints on their sizes, and discuss several interesting cases in detail.
Although our targets are believed to be main sequence stars, we note that
several have large infrared excesses compared to prototype Vega-like systems,
and may therefore be somewhat younger. The disk size constraints we derive, in
many cases, imply emission from relatively large ($\gtrsim$ 10$\mu$m) particles
at mid-infrared wavelengths.


title: The LX-sigma Relation for Galaxies and Clusters of Galaxies

abstract:
  We demonstrate that individual elliptical galaxies and clusters of galaxies
form a continuous X-ray luminosity---velocity dispersion (LX-sigma) relation.
Our samples of 280 clusters and 57 galaxies have LX ~ sigma^4.4 and LX ~
sigma^10, respectively. This unified LX - sigma relation spans 8 orders of
magnitude in LX and is fully consistent with the observed and theoretical
luminosity---temperature scaling laws. Our results support the notion that
galaxies and clusters of galaxies are the luminous tracers of similar dark
matter halos.


title: Merger Histories in Warm Dark Matter Structure Formation Scenario

abstract:
  Observations on galactic scales seem to be in contradiction with recent high
resolution N-body simulations. This so-called cold dark matter (CDM) crisis has
been addressed in several ways, ranging from a change in fundamental physics by
introducing self-interacting cold dark matter particles to a tuning of complex
astrophysical processes such as global and/or local feedback. All these efforts
attempt to soften density profiles and reduce the abundance of satellites in
simulated galaxy halos. In this paper, we explore a somewhat different approach
which consists of filtering the dark matter power spectrum on small scales,
thereby altering the formation history of low mass objects. The physical
motivation for damping these fluctuations lies in the possibility that the dark
matter particles have a different nature i.e. are warm (WDM) rather than cold.
We show that this leads to some interesting new results in terms of the merger
history and large-scale distribution of low mass halos, as compared to the
standard CDM scenario. However, WDM does not appear to be the ultimate
solution, in the sense that it is not able to fully solve the CDM crisis, even
though one of the main drawbacks, namely the abundance of satellites, can be
remedied. Indeed, the cuspiness of the halo profiles still persists, at all
redshifts, and for all halos and sub-halos that we investigated. Despite the
persistence of the cuspiness problem of DM halos, WDM seems to be still worth
taking seriously, as it alleviates the problems of overabundant sub-structures
in galactic halos and possibly the lack of angular momentum of simulated disk
galaxies. WDM also lessens the need to invoke strong feedback to solve these
problems, and may provide a natural explanation of the clustering properties
and ages of dwarfs.


title: Testing the Cosmic Coincidence Problem and the Nature of Dark Energy

abstract:
  Dark energy models which alter the relative scaling behavior of dark energy
and matter could provide a natural solution to the cosmic coincidence problem -
why the densities of dark energy and dark matter are comparable today. A
generalized class of dark energy models is introduced which allows
non-canonical scaling of the ratio of dark matter and dark energy with the
Robertson-Walker scale factor a(t). Upcoming observations, such as a high
redshift supernova survey, application of the Alcock-Paczynski test to quasar
pairs, and cluster evolution, will strongly constrain the relative scaling of
dark matter and dark energy as well as the equation of state of the dark
energy. Thus, whether there actually is a coincidence problem, and the extent
of cosmic coincidence in the universe's recent past can be answered
observationally in the near future. Determining whether today is a special time
in the history of the universe will be a SNAP.


title: Spintessence! New Models for Dark Matter and Dark Energy

abstract:
  We investigate a class of models for dark matter and/or negative-pressure,
dynamical dark energy consisting of ``spintessence,'' a complex scalar field
$\phi$ spinning in a U(1)-symmetric potential $V(\phi)=V(|\phi|)$. As the
Universe expands, the field spirals slowly toward the origin. The internal
angular momentum plays an important role in the cosmic evolution and
fluctuation dynamics. We outline the constraints on a cosmic spintessence
field, describing the properties of the potential necessary to sustain a viable
dark energy model, making connections with quintessence and self-interacting
and fuzzy cold dark matter. Possible implications for the coincidence problem,
baryogenesis, and cosmological birefringence, and generalizations of
spintessence to models with higher global symmetry and models in which the
symmetry is not exact are also discussed.


title: Quiescent Thermal Emission from the Neutron Star in Aql X-1

abstract:
  We report on the quiescent spectrum measured with Chandra/ACIS-S of the
transient, type-I X-ray bursting neutron star Aql X-1, immediately following an
accretion outburst. The neutron star radius, assuming a pure hydrogen
atmosphere and hard power-law spectrum, is $R_\infty$=13.4{+5}{-4} (d/5 \kpc)
km. Based on the historical outburst record of RXTE/ASM, the quiescent
luminosity is consistent with that predicted by Brown, Bildsten and Rutledge
from deep crustal heating, lending support to this theory for providing a
minimum quiescent luminosity of transient neutron stars. While not required by
the data, the hard power-law component can account for 18+/-8% of the 0.5-10
keV thermal flux. Short-timescale intensity variability during this observation
is less than 15% rms (3 sigma; 0.0001-1 Hz, 0.2-8 keV). Comparison between the
Chandra spectrum and three X-ray spectral observations made between Oct 1992
and Oct 1996 find all spectra consistent with a pure H atmosphere, but with
temperatures ranging from 145--168 eV, spanning a factor of 1.87+/-0.21 in
observed flux. The source of variability in the quiescent luminosity on long
timescales (greater than years) remains a puzzle. If from accretion, then it
remains to be explained why the quiescent accretion rate provides a luminosity
so nearly equal to that from deep crustal heating.


title: Nonlinear redshift distortions: The two-point correlation function

abstract:
  We consider a situation where the density and peculiar velocities in real
space are linear, and we calculate \xi_s the two-point correlation function in
redshift space, incorporating all non-linear effects which arise as a
consequence of the map from real to redshift space. Our result is
non-perturbative and it includes the effects of possible multi-streaming in
redshift space. We find that the deviations from the predictions of the linear
redshift distortion analysis increase for the higher spherical harmonics of
\xi_s. While the deviations are insignificant for the monopole \xi_0, the
hexadecapole \xi_4 exhibits large deviations from the linear predictions. For a
COBE normalised \Gamma=0.25, h=0.5 CDM power spectrum our results for \xi_4
deviate from the linear predictions by a factor of two at the scales ~10 h^{-1}
Mpc. The deviations from the linear predictions depend separately on \Omega and
b. This holds the possibility of removing the degeneracy that exists between
these two parameters in the linear analysis of redshift surveys which yields
only \beta=\Omega^0.6/b. We also show that the commonly used phenomenological
model where the non-linear redshift two-point correlation function is
calculated by convolving the linear redshift correlation function with an
isotropic pair velocity distribution function is a limiting case of our result.


title:
Peak Luminosity-Spectral Lag Relation Caused by the Viewing Angle of the
  Collimated Gamma-Ray Bursts

abstract:
  We compute the kinematical dependence of the peak luminosity, the pulse width
and the spectral lag of the peak luminosity on the viewing angle $\theta_v$ of
a jet. For appropriate model parameters we obtain the peak luminosity-spectral
lag relation similar to the observed one including GRB980425. A bright (dim)
peak with short (long) spectral lag corresponds to a jet with small (large)
viewing angle. This suggests that the viewing angle of the jet might cause
various relations in GRBs such as the peak luminosity-variability relation and
the luminosity-width relation. Our model also suggests that X-ray rich GRBs (or
X-ray flushes or Fast X-ray transients) are typical GRBs observed from large
$\theta_v$ with large spectral lag and low variability.


title: Structure of Small-Scale Magnetic Fields in the Kinematic Dynamo Theory

abstract:
  A weak fluctuating magnetic field embedded into a turbulent conducting medium
grows exponentially while its characteristic scale decays. In the ISM and
protogalactic plasmas, the magnetic Pr is very large, so a broad spectrum of
growing magnetic fluctuations is excited at subviscous scales. We study the
statistical correlations that are set up in the field pattern and show that the
magnetic-field lines possess a folding structure, where most of the scale
decrease is due to rapid transverse field direction reversals, while the scale
of the field variation along itself stays approximately constant. Specifically,
we find that the field strength and the field-line curvature are
anticorrelated, and the curvature possesses a stationary limiting distribution
with the bulk located at the values of curvature comparable to the
characteristic wave number of the velocity field and a power tail extending to
large values of curvature. The regions of large curvature, therefore, occupy
only a small fraction of the total volume of the system. Our theoretical
results are corroborated by direct numerical simulations. The implication of
the folding effect is that the advent of the Lorentz back reaction occurs when
the magnetic energy approaches that of the smallest turbulent eddies. Our
results also directly apply to the problem of statistical geometry of the
material lines in a random flow.

categories: astro-ph cond-mat.soft nlin.CD physics.flu-dyn


title: Brillouin scattering and the CMB

abstract:
  Brillouin scattering of photons off the density fluctuations in a fluid is
potentially important for cosmology. We derive the Brillouin spectral
distortion of blackbody radiation, and discuss the possible implications for
the cosmic microwave background. The thermal Sunyaev-Zeldovich effect is
slightly modified by Brillouin distortion, but only at very long wavelengths.


title: The centrifugal force reversal and X-ray bursts

abstract:
  Heyl (2000) made an interesting suggestion that the observed shifts in QPO
frequency in type I X-ray bursts could be influenced by the same geometrical
effect of strong gravity as the one that causes centrifugal force reversal
discovered by Abramowicz and Lasota (1974). However, his main result contains a
sign error. Here we derive the correct formula and conclude that constraints on
the M(R) relation for neutron stars deduced from the rotational-modulation
model of QPO frequency shifts are of no practical interest because the correct
formula implies a weak condition R* > 1.3 Rs, where Rs is the Schwarzschild
radius. We also argue against the relevance of the rotational-modulation model
to the observed frequency modulations.


title: Origins of Quasars and Galaxy Clusters

abstract:
  The distribution on the sky of clusters of galaxies shows significant
association with relatively nearby, large, active galaxies. The pattern is that
of clusters paired equidistant across a central galaxy with the apparent
magnitudes and redshifts of their constituent galaxies being closely matched.
The clusters and the galaxies in them tend to be strong X-ray and radio
emitters and their redshifts occur at preferred redshift values. The central,
low redshift galaxies often show evidence of ejection in the direction of these
higher redshift clusters and the clusters often show elongation along these
lines. In most of these respects the clusters resemble quasars which have been
increasingly shown for the last 34 years to be similarly associated with active
parent galaxies. It is argued here that, empirically, the quasars are ejected
from active galaxies. They evolve to lower redshift with time, fragmenting at
the end of their development into clusters of low luminosity galaxies. The
cluster galaxies can be at the same distance as their lower redshift parents
because they still retain a component of their earlier, quasar intrinsic
redshift.


title:
The T Tauri star RXJ1608.6-3922 - not an eclipsing binary but a spotted
  single star

abstract:
  High-resolution spectroscopy and photometric monitoring of the pre-main
sequence star RXJ1608.6-3922 shows that it is not an eclipsing binary, as
previously claimed. Radial velocity measurements covering suitable time spans
in order to detect a spectroscopic binary with the claimed period of about 7
days have been performed. The scatter of the radial velocity does not exceed
2.4 km/s, defining an upper mass limit of 24 Jupiter masses for any eclipsing
companion orbiting this star with the claimed period. Photometric observations
of RXJ1608.6-3922 in 7 consecutive nights (i.e. as long as the claimed orbital
period) reveal brightness variations of the order of 0.2 mag with a period of
3.6 days. The shape of the detected light curve differs from a light curve of
the star recorded in 1996. The small variations of the radial velocity, the
variable shape of the light curve, as well as (B-V) color variations suggest
that the flux of RXJ 1608.6-3922 is modulated by spots on the stellar surface
with a rotational period of 3.6 days. The stellar activity of this star seems
to be highly variable, taking into account the variable shape of the light
curve, with an amplitude varying from 0.5 to 0.2 mag in a few years, as well as
hints for a variable H$_{\alpha}$ equivalent width.


title:
A turbulent MHD model for molecular clouds and a new method of accretion
  on to star-forming cores

abstract:
  We describe the results of a sequence of simulations of gravitational
collapse in a turbulent magnetized region. The parameters are chosen to be
representative of molecular cloud material. We find that several protostellar
cores and filamentary structures of higher than average density form. The
filaments inter-connect the high density cores. Furthermore, the magnetic field
strengths are found to correlate positively with the density, in agreement with
recent observations. We make synthetic channel maps of the simulations and show
that material accreting onto the cores is channelled along the magnetized
filamentary structures. This is compared with recent observations of S106, and
shown to be consistent with these data. We postulate that this mechanism of
accretion along filaments may provide a means for molecular cloud cores to grow
to the point where they become gravitationally unstable and collapse to form
stars.


title:
The formation of voids in a universe with cold dark matter and a
  cosmological constant

abstract:
  A spherical Lagrangian hydrodynamical code has been written to study the
formation of cosmological structures in the early Universe. In this code we
take into account the presence of collisionless non-baryonic cold dark matter
(CDM), the cosmological constant and a series of physical processes present
during and after the recombination era, such as photon drag resulting from the
cosmic background radiation and hydrogen molecular production. We follow the
evolution of the structure since the recombination era until the present epoch.
As an application of this code we study the formation of voids starting from
negative density perturbations which evolved during and after the recombination
era. We analyse a set of COBE-normalized models, using different spectra to see
their influence on the formation of voids. Our results show that large voids
with diameters ranging from 10h^{-1} Mpc up to 50h^{-1} Mpc can be formed in a
universe model dominated by the cosmological constant (\Omega_\Lambda ~ 0.8).
This particular scenario is capable of forming large and deep empty regions
(with density contrasts \delta < -0.6). Our results also show that the physical
processes acting on the baryonic matter produce a transition region where the
radius of the dark matter component is greater than the baryonic void radius.
The thickness of this transition region ranges from about tens of kiloparsecs
up to a few megaparsecs, depending on the spectrum considered. Putative objects
formed near voids and within the transition region would have a different
amount of baryonic/dark matter when compared with \Omega_b/\Omega_d. If one
were to use these galaxies to determine, by dynamical effects or other
techniques, the quantity of dark matter present in the Universe, the result
obtained would be only local and not representative of the Universe as a whole.


title: RXTE Observations of the Microquasar GRS 1915+105

abstract:
  We analyse a set of three RXTE Target of Opportunity observations of the
Galactic microquasar GRS 1915+105, observed in April 2000. We concentrate on
the timing properties of the source, and examine the properties of a low
frequency QPO, with its harmonic, in several energy ranges. The source was
found in two different states of the spectro/temporal classification of Belloni
et al. (2000), and exhibited in the three observations a strong, low frequency
QPO together with a strong harmonic. We discuss the properties of the QPO, of
its harmonic and of their spectral behaviour in the framework of the Accretion
Ejection Instability (AEI) (Tagger & Pellat, 1999; Varni\`ere, Rodriguez &
Tagger, 2001; Rodriguez et al., 2001).


title:
The Onsala blind 6.7 GHz survey of the galactic plane: new methanol
  masers in the northern hemisphere

abstract:
  We review the state of the Onsala blind survey of the galactic plane,
searching for new 6.7 GHz methanol masers. We also describe preliminary results
of millimeter follow-up observations of the new detections and high resolution
observations using the EVN. We conclude that blind surveys are important to
complement targeted searches done until now and give the possibility to detect
new classes of objects.


title:
Simultaneous X-Ray and TeV Gamma-Ray Observations of the TeV Blazar
  Markarian 421 during February and May 2000

abstract:
  In this paper we present the results of simultaneous observations of the TeV
blazar Markarian 421 (Mrk 421) at X-ray and TeV Gamma-ray energies with the
Rossi X-Ray Timing Explorer (RXTE) and the stereoscopic Cherenkov Telescope
system of the HEGRA (High Energy Gamma Ray Astronomy) experiment, respectively.
The source was monitored from February 2nd to February 16th and from May 3rd to
May 8th, 2000. We discuss in detail the temporal and spectral properties of the
source. Remarkably, the TeV observations of February 7th/8th showed
statistically significant evidence for substantial TeV flux variability on 30
min time scale. We show the results of modeling the data with a time dependent
homogeneous Synchrotron Self-Compton (SSC) model. The X-ray and TeV gamma-ray
emission strengths and energy spectra together with the rapid flux variability
strongly suggest that the emission volume is approaching the observer with a
Doppler factor of 50 or higher. The different flux variability time scales
observed at X-rays and TeV Gamma-rays indicate that a more detailed analysis
will require inhomogeneous models with several emission zones.


title: X-ray Spectroscopy of BAL and Mini-BAL QSOs

abstract:
  BAL QSOs are notoriously faint X-ray sources, presumably due to extreme
intrinsic absorption. However, several objects have begun to appear through the
obscuration with recent X-ray observations by Chandra and ASCA. Where enough
counts are present for X-ray spectroscopy, the signatures of absorption are
clear. The evidence is also mounting that the absorbers are more complicated
than previous simple models assumed; current absorber models need to be
extended to the high-luminosity, high-velocity, and high-ionization regime
appropriate for BAL QSOs.


title: The Halo White Dwarf WD 0346+246 Revisited

abstract:
  The extreme helium-rich atmospheric composition determined for the halo white
dwarf WD 0346+246 is reexamined. This solution is shown to be improbable from
an astrophysical point of view when accretion of hydrogen and metals from the
interstellar medium is taken into account. An alternate solution is proposed
where hydrogen and helium are present in the atmospheric regions in equal
amounts. The best fit at Teff=3780 K, log g=8.34, and N(He)/N(H)=1.3 is
achieved by including in the model calculations a bound-free opacity from the
Lyman edge associated with the so-called dissolved atomic levels of the
hydrogen atom, or pseudo-continuum opacity.


title:
Phase Calibration Sources in the Northern Sky at Galactic Latitudes
  $\mid b \mid < 2.5\arcdeg$

abstract:
  The Jodrell Bank - VLA Astrometric Survey (JVAS) of flat-spectrum sources
yielded a catalog of 2118 compact radio sources in the northern sky. Those
sources are being used as phase calibrators for many synthesis arrays. JVAS
suffers from a zone of avoidance because Galactic confusion prevented selection
of flat-spectrum sources at $\mid b \mid < 2.5\arcdeg$. This confusion problem
was overcome by selecting variable GB6 sources in the JVAS zone of avoidance. A
subset of these sources were observed at 8.5 GHz with the NRAO VLA in the JVAS
style, leading to 29 compact radio sources in the zone of avoidance whose
positions have been measured to an rms accuracy of about 10~mas or less. This
extension of the JVAS lists to lower Galactic latitudes (1) improves the
prospects for VLBA phase-referencing of Galactic targets and (2) strengthens
the lists' usefulness for studies of the Galactic interstellar medium.


title: The CHANDRA HETGS X-ray Grating Spectrum of Eta Car

abstract:
  Eta Car may be the most massive and luminous star in the Galaxy and is
suspected to be a massive, colliding wind binary system. The CHANDRA X-ray
observatory has obtained a calibrated, high-resolution X-ray spectrum of the
star uncontaminated by the nearby extended soft X-ray emisssion. Our 89 ksec
CHANDRA observation with the High Energy Transmission Grating Spectrometer
(HETGS) shows that the hot gas near the star is non-isothermal. The temperature
distribution may represent the emission on either side of the colliding wind
bow shock, effectively ``resolving'' the shock. If so, the pre-shock wind
velocities are ~ 700 and ~ 1800 km/s in our analysis, and these velocities may
be interpreted as the terminal velocities of the winds from Eta Car and from
the hidden companion star. The forbidden-to-intercombination (f/i) line ratios
for the He-like ions of S, Si and Fe are large, indicating that the line
forming region lies far from the stellar photosphere. The iron fluorescent line
at 1.93 Angstrom, first detected by ASCA, is clearly resolved from the thermal
iron line in the CHANDRA grating spectrum. The Fe fluorescent line is weaker in
our CHANDRA observation than in any of the ASCA spectra. The CHANDRA
observation also provides an uninterrupted high-time resolution lightcurve of
the stellar X-ray emission from Eta Car and suggests that there was no
significant, coherent variability during the CHANDRA observation. The Eta Car
CHANDRA grating spectrum is unlike recently published X-ray grating spectra of
single massive stars in significant ways and is generally consistent with
colliding wind emission in a massive binary.


title: Photometric observations of distant active comets

abstract:
  We present CCD VR_C observations of 6 distant comets located at heliocentric
distances of 3.4--7.2 AU. Time-series data were obtained on three nights in
July, 2000 covering 16 hours. Each comet was observed after the perihelion,
when a lower activity was expected. Contrary to expectation, we found
well-defined circular comae and extended tails visible out to a few (3-5)
arcminutes. We detected a quasi-cyclic light variation of C/1999 J2, while
C/1999 N4 showed some hints of a more complex variation. C/2000 K1 was constant
to +/-0.04 mag during the observing run. The standard V and R_C data were used
to estimate nuclear diameters, while the colour indices implied a slighly
reddish (V-R=0.68, C/2000 K1), a neutral (V-R=0.47, C/1999 N4) and a slightly
bluish (V-R=0.25, C/1999 J2) coma. Simple fits of the surface brightness
distributions are also presented enabling order of magnitude estimates of
nuclear radii. Beside the time-series observations, further single-shot
observations of three faint comets are briefly described.


title: High Resolution Spectra of Quasar AALs: 3C 191

abstract:
  We discuss new high-resolution (6.7 km/s) spectra of the associated
absorption lines (AALs) in the radio-loud quasar 3C 191. The measured AALs have
ionizations ranging from Mg I to N V, and multi-component profiles that are
blueshifted by ~400 to ~1400 km/s relative to the quasar's broad emission
lines. Excited-state absorption lines of Si II* and C II* imply volume
densities of ~300 cm^-3 and a nominal distance from the quasar of 28 kpc
(assuming photoionization). The total column density is N_H ~ 2 x 10^20 cm^-2.
Surprisingly, the absorber only partially covers the quasar emission source
along our line of sight. We propose a model for the absorber in which pockets
of dense neutral gas are surrounded by bigger clouds of generally lower density
and higher ionization. This outflowing material might be leftover from a
blowout associated with a nuclear starburst, the onset of quasar activity, or a
past broad absorption line (BAL) wind phase.


title:
Discovery of a Second High Frequency QPO from the Microquasar GRS
  1915+105

abstract:
  We report the discovery in archival Rossi X-ray Timing Explorer (RXTE) data
of a 40 Hz quasiperiodic oscillation (QPO) in the hard X-ray flux from the
galactic microquasar GRS 1915+105. The QPO is detected only in the hard X-ray
band above 13 keV and was discovered in observations in which the previously
known 67 Hz QPO is present. The 40 Hz QPO has a typical rms amplitude of 2 % in
the 13 - 27 keV band, and a width of about 8.5 Hz (FWHM). We show that the 67
and 40 Hz QPOs are detected in the same observations in July and November,
1997. However, the QPO is not detected in observations from April, May and
June, 1996 in which the 67 Hz QPO was first discovered. The frequency of the 67
Hz QPO is significantly higher in the 1997 observations by about 5 % compared
with the 1996 data. The identification of the 40 Hz QPO makes GRS 1915+105 the
second black hole binary to show a pair of simultaneous high frequency QPO (the
other being GRO J1655-40). The similarities between the properties of the 67 Hz
QPO in GRS 1915+105 and the recently discovered 450 Hz QPO in GRO J1655-40
suggest that the pairs of frequencies in these systems may be produced by the
same physical mechanism, with the frequency differences between the two being
likely due to different black hole masses in the two systems. We discuss the
implications of our result for the mass and spin of GRS 1915+105 as well as for
models of X-ray variability in black holes and neutron stars.


title:
Magellanic Cloud Structure from Near-IR Surveys I: The Viewing Angles of
  the LMC

abstract:
  We present a detailed study of the viewing angles of the LMC disk plane. We
find that our viewing direction differs considerably from the commonly accepted
values, which has important implications for the structure of the LMC. The
discussion is based on an analysis of spatial variations in the apparent
magnitude of features in the near-IR color-magnitude diagrams extracted from
the DENIS and 2MASS surveys. Sinusoidal brightness variations with a
peak-to-peak amplitude of approximately 0.25 mag are detected as function of
position angle, for both AGB and RGB stars. This is naturally interpreted as
the result of distance variations, due to one side of the LMC plane being
closer to us than the opposite side. The best fitting geometric model of an
inclined plane yields an inclination angle i = 34.7 +/- 6.2 degrees and
line-of-nodes position angle Theta = 122.5 +/- 8.3 degrees. There is tentative
evidence that the LMC disk plane may be warped. Traditional methods to estimate
the position angle of the line of nodes have used either the major axis
position angle Theta_maj of the spatial distribution of tracers on the sky, or
the position angle Theta_max of the line of maximum gradient in the velocity
field, given that for a circular disk Theta_maj = Theta_max = Theta. The
present study does not rely on the assumption of circular symmetry, and is
considerably more accurate than previous studies of its kind. We find that the
actual position angle of the line of nodes differs considerably from both
Theta_maj and Theta_max, for which measurements have fallen in the range
140-190 degrees. This indicates that the intrinsic shape of the LMC disk is not
circular, but elliptical, as discussed further in Paper II. [Abridged]


title:
Magellanic Cloud Structure from Near-IR Surveys II: Star Count Maps and
  the Intrinsic Elongation of the LMC

abstract:
  I construct a near-IR star count map of the LMC and demonstrate, using the
viewing angles derived in Paper I, that the LMC is intrinsically elongated. I
argue that this is due to the tidal force from the Milky Way. The near-IR data
from the 2MASS and DENIS surveys are used to create a star count map of RGB and
AGB stars, which is interpreted through ellipse fitting. The radial number
density profile is approximately exponential with a scale-length 1.3-1.5 kpc.
However, there is an excess density at large radii that may be due to the tidal
effect of the Milky Way. The position angle and ellipticity profile converge to
PA_maj = 189.3 +/- 1.4 degrees and epsilon = 0.199 +/- 0.008 for r > 5 deg. At
large radii there is a drift of the center of the star count contours towards
the near side of the plane, which can be undrestood as due to viewing
perspective. The fact that PA_maj differes from the line of nodes position
angle Theta = 122.5 +/- 8.3 (cf. Paper I) indicates that the LMC disk is not
circular, but has an intrinsic ellipticity of 0.31. The LMC is elongated in the
general direction of the Galactic center, and is elongated perpendicular to the
Magellanic Stream and the velocity vector of the LMC center of mass. This
suggests that the elongation of the LMC has been induced by the tidal force of
the Milky Way. The position angle of the line of nodes differs from the
position angle Theta_max of the line of maximum line of sight velocity
gradient: Theta_max - Theta = 20-60 degrees. This could be due to: (a)
streaming along non-circular orbits in the elongated disk; (b) uncertainties in
the transverse motion of the LMC center of mass; (c) precession and nutation of
the LMC disk as it orbits the Milky Way (expected on theoretical grounds).
[Abridged]


title: Detection of an X-ray Pulsar Wind Nebula and Tail in SNR N157B

abstract:
  We report Chandra X-ray observations of the supernova remnant N157B in the
Large Magellanic Cloud, which are presented together with an archival HST
optical image and a radio continuum map for comparison. This remnant contains
the recently discovered 16 ms X-ray pulsar PSR J0537-6910, the most rapidly
rotating young pulsar known.
  Using phase-resolved Chandra imaging, we pinpoint the location of the pulsar
to within an uncertainty of less than 1 arcsec. PSR J0537-6910 is not detected
in any other wavelength band. The X-ray observations resolve three distinct
features: the pulsar itself, a surrounding compact wind nebula which is
strongly elongated and a feature of large-scale diffuse emission trailing from
the pulsar. This latter comet tail-shaped feature coexists with enhanced radio
emission and is oriented nearly perpendicular to the major axis of the pulsar
wind nebula. We propose the following scenario to explain these features. The
bright, compact nebula is likely powered by a toroidal pulsar wind of
relativistic particles which is partially confined by the ram-pressure from the
supersonic motion of the pulsar. The particles, after being forced out from the
compact nebula (the head of the ``comet''), are eventually dumped into a bubble
(the tail), which is primarily responsible for the extended diffuse X-ray and
radio emission. The ram-pressure confinement also allows a natural explanation
for the observed X-ray luminosity of the compact nebula and for the unusually
small X-ray to spin-down luminosity ratio, compared to similarly energetic
pulsars. We estimate the pulsar wind Lorentz factor of N157B as about 4 times
10^6 (with an uncertainty of a factor about 2, consistent with that inferred
from the modeling of the Crab Nebula.


title: The Temperature Distribution in Turbulent Interstellar Gas

abstract:
  We discuss the temperature distribution in a two-dimensional, thermally
unstable numerical simulation of the warm and cold gas in the Galactic disk,
including the magnetic field, self-gravity, the Coriolis force, stellar energy
injection and a realistic cooling function. We find that ~50% of the turbulent
gas mass has temperatures in what would be the thermally unstable range if
thermal instability were to be considered alone. This appears to be a
consequence of there being many other forces at play than just thermal
pressure. We also point out that a bimodal temperature pdf is a simple
consequence of the form of the interstellar cooling function and is not
necessarily a signature of discontinuous phase transitions.


title:
Searching for the Physical Drivers of the Eigenvector 1 Correlation
  Space

abstract:
  [Latex markup removed] We recently discussed an Eigenvector 1 (E1) parameter
space that provides optimal discrimination between the principal classes of
broad-line Active Galactic Nuclei (AGN). In this paper we begin a search for
the most important physical parameters that are likely to govern correlations
and data point distribution in E1 space. We focus on the principal optical
parameter plane involving the width of the H-beta broad component and the
equivalent width ratio between the FeII blend at 4570 A and H-beta broad
component. We show that the observed correlation for radio-quiet sources can be
accounted for if it is primarily driven by the ratio of AGN luminosity to black
hole mass (L/M proportional to Eddington ratio) convolved with source
orientation. L/M apparently drives the radio-quiet correlation only for
FWHM(H-beta) < 4000 km/s which includes Narrow Line Seyfert 1 (NLSy1) galaxies
and can be said to define an AGN ``main sequence.'' Source orientation plays an
increasingly important role as FWHM(H-beta) increases. We also argue that AGN
lying outside the radio-quiet ``main sequence,'' and specifically those with
optical FeII much stronger than expected for a given FWHM(H-beta), may all be
Broad Absorption Line QSOs.


title: My Quarter Century with Jan van Paradijs

abstract:
  This is the final and somewhat expanded version. The quality of the color
photos of this version is poor as there is an upload limit on astro-ph. Much
higher quality versions are available at http://space.mit.edu/~lewin/jvp.html .
A bound hard copy with very high-resolution color photographs will be sent to
you at request. Please send e-mail with your complete mailing address to
lewin@mit.edu.


title:
The Imprint of Lithium Recombination on the Microwave Background
  Anisotropies

abstract:
  Following Loeb (2001), we explore the imprint of the resonant 6708A line
opacity of neutral lithium on the temperature and polarization anisotropies of
the cosmic microwave background (CMB) at observed wavelengths of 250-350 micron
(0.9-1.2 THz). We show that if lithium recombines in the redshift range of
z=400-500 as expected, then the standard CMB anisotropies would be
significantly modified in this wavelength band. The modified polarization
signal could be comparable to the expected polarization anisotropies of the
far-infrared background on sub-degree angular scales (l>100). Detection of the
predicted signal can be used to infer the primordial abundance of lithium, and
to probe structure in the Universe at z~500.


title:
A Continuum Description of Rarefied Gas Dynamics (I)--- Derivation From
  Kinetic Theory

abstract:
  We describe an asymptotic procedure for deriving continuum equations from the
kinetic theory of a simple gas. As in the works of Hilbert, of Chapman and of
Enskog, we expand in the mean flight time of the constituent particles of the
gas, but we do not adopt the Chapman-Enskog device of simplifying the formulae
at each order by using results from previous orders. In this way, we are able
to derive a new set of fluid dynamical equations from kinetic theory, as we
illustrate here for the relaxation model for monatomic gases. We obtain a
stress tensor that contains a dynamical pressure term (or bulk viscosity) that
is process-dependent and our heat current depends on the gradients of both
temperature and density. On account of these features, the equations apply to a
greater range of Knudsen number (the ratio of mean free path to macroscopic
scale) than do the Navier-Stokes equations, as we see in the accompanying
paper. In the limit of vanishing Knudsen number, our equations reduce to the
usual Navier-Stokes equations with no bulk viscosity.


title:
A Continuum Description of Rarefied Gas Dynamics (II)--- The Propagation
  of Ultrasound

abstract:
  The equations of fluid dynamics developed in paper I are applied to the study
of the propagation of ultrasound waves. There is good agreement between the
predicted propagation speed and experimental results for a wide range of
Knudsen numbers.


title:
A Continuum Description of Rarefied Gas Dynamics (III)--- The Structures
  of Shock Waves

abstract:
  We use the one-dimensional steady version of the equations derived in paper I
to compute the structure of shock waves. The agreement with experiment is good,
especially when we retain the experimental value of the Prandtl number adopted
in II.


title: The origin of angular momentum in dark matter halos

abstract:
  We propose a new explanation for the origin of angular momentum in galaxies
and their dark halos, in which the halos obtain their spin through the
cumulative acquisition of angular momentum from satellite accretion. In our
model, the build-up of angular momentum is a random walk process associated
with the mass assembly history of the halo's major progenitor. We assume no
correlation between the angular momenta of accreted objects. Using the extended
Press-Schechter approximation, we calculate the growth of mass, angular
momentum, and spin parameter $\lambda$ for many halos. Our random walk model
reproduces the key features of the angular momentum of halos found in N-body
simulations: a lognormal distribution in $\lambda$ with an average of
$<\lambda> \approx 0.04$, independent of mass and redshift. The evolution of
the spin parameter in individual halos in this model is quite different from
the steady increase with time of angular momentum in the tidal torque picture.
We find both in N-body simulations and in our random walk model that the value
of $\lambda$ changes significantly with time for a halo's major progenitor. It
typically has a sharp increase due to major mergers, and a steady decline
during periods of gradual accretion of small satellites. The model predicts
that on average the $\lambda$ of halos which had major mergers after redshift
$z=2$ should be substantially larger than the $\lambda$ of those which did not.
Perhaps surprisingly, this suggests that halos that host late-forming
elliptical galaxies should rotate faster than halos of spiral galaxies.


title: Second Parameter Effects in and between M3 and Palomar 3

abstract:
  We study the globular clusters M3 and Palomar 3 as a "second parameter (2ndP)
pair," showing that:
  i) M3 has a surprisingly strong *internal* 2ndP;
  ii) The dispersion in mass on the Pal 3 horizontal branch (HB) is
intrinsically very small, leading to the most apparent differences in HB
morphology between M3 and Pal 3;
  iii) Ignoring the difference in HB mass dispersion between M3 and Pal 3,
their relative HB types can be accounted for by a fairly small difference in
age, of order 0.5-1 Gyr.


title: Large Surveys in Cosmology: The Changing Sociology

abstract:
  Galaxy redshift surveys and Cosmic Microwave Background experiments are
undertaken with larger and larger teams, in a fashion reminiscent of particle
physics experiments and the human genome projects. We discuss the role of young
researchers, the issue of multiple authorship, and ways to communicate
effectively in teams of tens to hundreds of collaborators.


title: The Age of the Universe from Joint Analysis of Cosmological Probes

abstract:
  Analyses of various cosmological probes, including the latest Cosmic
Microwave Background anisotropies, the 2dF Galaxy Redshift Survey and
Cepheid-calibrated distance indicators suggest that the age of expansion is $13
\pm 3$ Gyr. We discuss some statistical aspects of this estimation, and we also
present results for joint analysis of the latest CMB and Cepheid data by
utilizing `Hyper-Parameters'. The deduced age of expansion might be
uncomfortably close to the age of the oldest Globular Clusters, in particular
if they formed relatively recently.


title: New Cosmological Data and the `Best-Fit' FRW Universe

abstract:
  Observational tests for the homogeneity of the Universe on large scales are
reviewed. Assuming the Cosmological Principle we then estimate cosmological
parameters by joint analysis of the Cosmic Microwave Background, Supernovae Ia,
peculiar velocities, cluster abundance and redshift surveys. Our results are
consistent with results obtained by other groups, suggesting a best-fit FRW
Universe with $\omegam = 1 -\lambda \approx 0.3$ and $H_0 \approx 75$
km/sec/Mpc. We point out some potential problems with this currently popular
model.


title: A Solvable Model for Nonlinear Mean Field Dynamo

abstract:
  We formulate a solvable model that describes generation and saturation of
mean magnetic field in a dynamo with kinetic helicity, in the limit of large
magnetic Prandtl number. This model is based on the assumption that the
stochastic part of the velocity field is Gaussian and white in time (the
Kazantsev-Kraichnan ensemble), while the regular part describing the back
reaction of the magnetic field is chosen from balancing the viscous and Lorentz
stresses in the MHD Navier-Stokes equation. The model provides an analytical
explanation for previously obtained numerical results.


title: Gravitational lensing of the farthest known supernova SN1997ff

abstract:
  We investigate the effects of gravitational lensing due to intervening
galaxies on the recently discovered Type Ia supernova at z=1.7, SN1997ff, in
the Hubble Deep Field North. We find that it is possible to obtain a wide range
of magnifications by varying the mass and/or the velocity dispersion
normalization of the lensing galaxies. In order to be able to use SN1997ff to
constrain the redshift-distance relation, very detailed modeling of the
galaxies to control the systematic effects from lensing is necessary. Thus we
argue, that based on our current limited knowledge of the lensing galaxies, it
is difficult to use SN1997ff to constrain the values of Omega_M and
Omega_Lambda, or even to place severe limits on grey dust obscuration or
luminosity evolution of Type Ia supernovae.


title: Supernova Remnants: acceleration of particles and gamma-ray emission

abstract:
  Particle acceleration in the dynamically evolving environment of Supernova
Remnants is discussed in the framework of a genuinely time-dependent nonlinear
theory, assuming spherical symmetry. As a consequence the dependence of
injection on the angle between shock normal and external magnetic field
direction requires a renormalisation of the calculated particle fluxes. The
recent observational results in TeV gamma-rays from such objects are discussed
and found to be consistent with theory. We conclude that for the present
instrumental sensitivities there are no reasons to draw premature negative
conclusions as to the possible origin of the Galactic Cosmic Rays below the
"knee" in Supernova Remnants. In addition, theoretical predictions and
observations are getting very close. Therefore the coming generation of
ground-based and space-borne detectors will decide this basic question of
astrophysics.


title: The Resolved Stellar Populations in NGC 1705

abstract:
  We present HST photometry of the resolved stellar population in the dwarf
irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and
NICMOS, and successful images have been obtained in the F555W, F814W, F110W and
F160W bands. The optical fields cover most of the galaxy, while the infrared
field (NIC2) maps only its central regions. The optical photometry provides
\~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in the
three WFCs. In the infrared we have been able to resolve ~ 2400 stars down to
m_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguously
measured in all the four bands. The corresponding color-magnitude diagrams
(CMDs) confirm the existence of an age gradient, showing that NGC 1705 hosts
both young (a few Myr old) and very old (up to 15 Gyr old) stars, with the
former strongly concentrated toward the galactic center and the latter present
everywhere, but much more easily visible in the external regions. The tip of
the red giant branch (TRGB) is clearly visible both in the optical and in the
infrared CMDs and allows us to derive the galaxy distance. Taking into account
the uncertainties related to both the photometry and the TRGB magnitude --
distance relation, we find that the distance modulus of NGC 1705 is
(m-M)_0=28.54 +- 0.26, corresponding to a distance D=5.1 +- 0.6 Mpc.


title:
The fluctuating gravitational field in inhomogeneous and clustered
  self-gravitating systems

abstract:
  In this paper I extend the results of Ahmad & Cohen (1973), regarding the
study of the probability distribution of the stochastic force in homogeneous
gravitational systems, to inhomogeneous gravitational ones. To this aim, I
study the stochastic force distribution using N-body realizations of Plummer's
spherically symmetric models. I find that the stochastic force distribution
obtained for the evolved system is in good agreement with Kandrup's (1980)
theory of stochastic force in inhomogeneous systems. Correlation effects that
arise during the evolution of the system of particles are well described by
Antonuccio-Delogu & Atrio-Barandela's (1992) theory.


title: Non radial motions and the large scale structure of the Universe

abstract:
  We study the effect of non-radial motions on the mass function, the VDF and
on the shape of clusters of galaxies using the model introduced in Del Popolo &
Gambera (1998a,b; 1999). The mass function of clusters, obtained using the
quoted model, is compared with the statistical data by Bahcall & Cen (1992) and
Girardi et al. (1998), while the VDF is compared with the CfA data by Zabludoff
et al. (1993) for local clusters. In both cases the model predictions are in
good agreement with the observational data showing once more how non-radial
motions can reduce many of the discrepancies between CDM model predictions and
observational data. Besides we study the effect of non-radial motions on the
intrinsic shape of clusters of galaxies showing that non-radial motions produce
clusters less elongated with respect to CDM model.


title:
Oxygen abundances in dwarf irregular galaxies and the metallicity -
  luminosity relationship

abstract:
  The low-luminosity dwarf irregular galaxies are considered. The oxygen
abundances in HII regions of dwarf irregular galaxies were recalculated from
published spectra through the recently suggested P - method. It has been found
that the metallicity of low-luminosity dwarf irregular galaxies, with a few
exceptions, correlates well with galaxy luminosity. The dispersion of oxygen
abundances around the metallicity - luminosity relationship increases with
decreasing of galaxy luminosity, as was found by Richer and McCall (1995). No
relationship between the oxygen abundance and the absolute magnitude in the
blue band for irregular galaxies obtained by Hidalgo-Gamez and Olofsson (1998)
can be explained by the large uncertainties in the oxygen abundances derived
through the Te - method, that in turn can be explained by the large
uncertainties in the measurements of the strengths of the weak oxygen line
[OIII]4363 used in the Te - method.


title: The lepton asymmetry: the last chance for a critical-density cosmology?

abstract:
  We use a wide range of observations to constrain cosmological models
possessing a significant asymmetry in the lepton sector, which offer perhaps
the best chance of reconciling a critical-density Universe with current
observations. The simplest case, with massless neutrinos, fails to fit many
experimental data and does not lead to an acceptable model. If the neutrinos
have mass of order one electron-volt (which is favoured by some neutrino
observations), then models can be implemented which prove a good fit to
microwave anisotropies and large-scale structure data. However, taking into
account the latest microwave anisotropy results, especially those from
Boomerang, we show that the model can no longer accommodate the observed baryon
fraction in clusters. Together with the observed acceleration of the present
Universe, this puts considerable pressure on such critical-density models.