Astrophysics

astro-ph new abstracts, Mon, 21 Aug 00 04:00:11 GMT
0008282 — 0008303 received

astro-ph/0008282 [abs, src, ps, other] :

Title: Halo Lensing or LMC Self-Lensing? Insights from the HST CMD of MACHO
Microlensing Source Stars
Authors:
C. Alcock,
R.A. Allsman,
D.R. Alves,
T.S. Axelrod,
A.C. Becker,
D.P. Bennett,
K.H. Cook,
A.J. Drake,
K.C. Freeman,
M. Geha,
K. Griest,
M.J. Lehner,
S.L. Marshall,
D. Minniti,
C.A. Nelson,
B.A. Peterson,
P. Popowski,
M.R. Pratt,
P.J. Quinn,
C.W. Stubbs,
W. Sutherland,
A.B. Tomaney,
T. Vandehei,
D. Welch (The MACHO Collaboration)
Comments: submitted to ApJ, 11 pages, 2 tables, 8 figures

The nature of Large Magellanic Cloud (LMC) microlensing is determined by the
location of the microlensing source stars. If the source stars are in the LMC
then LMC microlensing is predominantly halo-lensing, if the source stars are
located behind the LMC, then LMC microlensing is dominated by self-lensing. We
attempt to distinguish between source stars drawn from the average population
of the LMC and source stars drawn from a population behind the LMC by examining
the HST color-magnitude diagram (CMD) of microlensing source stars. We present
WFPC2 HST photometry of eight MACHO microlensing source stars and the
surrounding fields in the LMC. The microlensing source stars are identified by
deriving accurate centroids in the ground-based MACHO images using difference
image analysis (DIA) and then transforming the DIA coordinates to the HST
frame. We consider in detail a model for the background population of source
stars based on that presented by Zhao, Graff & Guhathakurta. In this model, the
source stars have an additional reddening <E(B-V)> = 0.13 mag and a slightly
larger distance modulus <Delta mu> ~ 0.3 mag than the average LMC population.
We also investigate a series of source star models, varying the relative
fraction of source stars drawn from the average and background populations and
the displacement of the background population from the LMC. The data suggest
that all of the MACHO microlensing source stars are not drawn from a background
population.
(191kb)

astro-ph/0008283 [abs, src, ps, other] :

Title: Lyman Continuum Emission from Galaxies at z~3.4
Authors:
C.C. Steidel (Caltech),
M. Pettini (IoA, Cambridge),
K.L Adelberger (Caltech)
Comments: Accepted for publication in ApJ, 15 pages, aastex

We report the detection of significant Lyman continuum flux in the composite
spectrum of 29 Lyman break galaxies (LBGs) with redshifts <z> = 3.40+/-0.09.
After correction for opacity due to intervening absorption using a new
composite QSO spectrum evaluated at the same redshift, the ratio of emergent
flux density at 1500 AA in the rest frame to that in the Lyman continuum is
L(1500)/L(900) = 4.6 +/- 1.0. If the relative intensity of the inferred
escaping Lyman continuum radiation is typical of LBGs at z ~ 3 (the galaxies in
this sample are drawn from the bluest quartile of LBG spectral energy
distributions due to known selection effects), then observed LBGs produce about
5 times more H-ionizing photons per unit co-moving volume than QSOs at z ~ 3.
The associated contribution to the metagalactic ionizing radiation field is
J_{
u}(912) = 1.2+/-0.3 x 10^{-21} ergs s^{-1} cm^{-2} Hz^{-1} sr^{-1} at z ~
3, very close to most estimates of the radiation field background based on the
“proximity effect”. A preliminary analysis of the density of faint QSOs in
our Lyman break galaxy survey indicates that the standard extrapolated QSO
luminosity function may slightly over-predict the QSO contribution to
J_{
u}(912) at z ~ 3. We briefly discuss the implications of a
galaxy-dominated UV background at high redshifts.
(34kb)

astro-ph/0008284 [abs, src, ps, other] :

Title: Construction and Testing of a Pixellated CZT Detector and Shield for a
Hard X-ray Astronomy Balloon Flight
Authors:
P. F. Bloser,
T. Narita,
J. A. Jenkins,
J. E. Grindlay (CfA)
Comments: 12 pages LaTeX, 11 figures, to appear in Proc. SPIE 4140, “X-Ray and
Gamma-Ray Instrumentation for Astronomy XI”

We report on the construction and laboratory testing of pixellated CZT
detectors mounted in a flip-chip, tiled fashion and read out by an ASIC, as
required for proposed hard X-ray astronomy missions. Two 10 mm x 10 mm x 5 mm
detectors were fabricated, one out of standard eV Products high-pressure
Bridgman CZT and one out of IMARAD horizontal Bridgman CZT. Each was fashioned
with a 4 x 4 array of gold pixels on a 2.5 mm pitch with a surrounding guard
ring. The detectors were mounted side by side on a carrier card, such that the
pixel pitch was preserved, and read out by a 32-channel VA-TA ASIC from IDE AS
Corp. controlled by a PC/104 single-board computer. A passive shield/collimator
surrounded by plastic scintillator encloses the detectors on five sides and
provides a ~40deg field of view. Thus this experiment tests key techniques
required for future hard X-ray survey instruments. The experiment was taken to
Ft Sumner, NM in May 2000 in preparation for a scientific balloon flight aboard
the joint Harvard-MSFC EXITE2/HERO payload. Although we did not receive a
flight opportunity, and are currently scheduled to fly in September 2000, we
present our calibration data in the flight configuration together with data
analysis techniques and simulations of the expected flight background spectrum.
(532kb)

astro-ph/0008285 [abs, src, ps, other] :

Title: Modern Supernova Search
Authors:
Myung Gyoon Lee (Astronomy Program, SEES, Seoul National University, Seoul, Korea)
Comments: 7 pages (LaTex), aipproc.sty, Presented at the Explosive Pheonomena
in Astrophysical Compact Objects meeting, KIAS, May 24-27, 2000

Supernovae play a critical role in observational cosmology as well as in
astrophysics of stars and galaxies. Recent era has seen dramatic progress in
the research of supernovae. Several programs to search systematically
supernovae in nearby to distant galaxies have been very successful. Recent
progresses in the modern supernova search are reviewed.
(224kb)

astro-ph/0008286 [abs, src, ps, other] :

Title: Morphological Number Counts of Galaxies in the Hubble Deep Field South
Authors:
Myung Gyoon Lee,
Narae Hwang (Astronomy Program, SEES, Seoul National University, Seoul, Korea)
Comments: 8 pages (LaTex), isasrep.sty, Presented at ISAS workshop, “Mid- and
Far-infrared Astronomy and Future Space Missions”, 17-18 April 2000, Japan

We present a study of photometric properties of the galaxies in the Hubble
Deep Field South (HDFS) based on the released WFPC2 images obtained with the
Hubble Space Telescope (HST). We have classified about 340 galaxies with $I<26$
mag in the HDFS as well as about 400 galaxies in the Hubble Deep Field North
(HDFN) using the visual classification supplemented by inspection of the
surface brightness profiles of the galaxies. Galaxy population statistics and
morphological number counts for the HDFS are found to be similar to be those
for the HDFN. We have also determined photometrically the redshifts of the
galaxies with $I<26$ mag in the HDFS and the HDFN using the empirical training
set method. Redshift distribution, color-redshift relation, and
magnitude-redshift for each type of galaxies are investigated.
(172kb)

astro-ph/0008287 [abs, src, ps, other] :

Title: Cosmology with Varying Constants
Authors:
C. J. A. P. Martins (DAMTP, CMS, University of Cambridge)
Comments: 6 pages; Based on talks given at the Cosmology2000 and X ENAA
workshops, July 2000

I motivate and discuss some recent work on theories with varying constants,
and consider some possible observational consequences and tests. Particular
emphasis is given to models which can (almost) exactly mimic the predictions of
standard inflationary models.
(11kb)

astro-ph/0008288 [abs, src, ps, other] :

Title: Small-scale structures in Compact Steep-Spectrum and GHz-Peaked-Spectrum
radio sources
Authors:
S. Jeyakumar,
D.J. Saikia,
A. Pramesh Rao,
V. Balasubramanian
Comments: Accepted for publication in Astronomy and Astrophysics; 16 Pages; 5
Figures; 6 Tables

We determine the small-scale structure for a large sample of Compact Steep
Spectrum (CSS) and Gigahertz Peaked Spectrum (GPS) sources and a sample of
larger radio sources at 327 MHz using the technique of Inter-Planetary
Scintillation (IPS) with the Ooty Radio Telescope. Our observations provide
structural information on angular scales ranging from about 50 to a few hundred
milliarcsec. We estimate the size and flux density of the scintillating
components from the IPS observations. The fraction of flux density of the
hotspots estimated from both the IPS observations as well as from
interferometric observations for larger sources from the literature exhibits no
significant dependence on either the radio luminosity or linear size for the
high-luminosity sources. We examine the collimation of radio jets using the
sizes of hotspots from both IPS and interferometric observations. The hotspot
sizes do not exhibit a significant dependence on the overall linear size for
the larger sources, although the CSS and GPS sources were earlier found to
evolve in a self-similar way. The IPS observations show evidence of
larger-scale structures compared to the known VLBI-scale structures in 8 CSS
and GPS radio sources. We discuss the origin of these structures. We also
examine the spectra of compact components in GPS sources using both IPS and
interferometric measurements, and attempt to distinguish between different
processes for explaining their low-frequency spectra.
(48kb)

astro-ph/0008289 [abs, src, ps, other] :

Title: On the degree of dust extinction in major mergers
Author:
Kenji Bekki,
Yasihiro Shioya
Comments: 11 pages 8 (2 jpg) figures accepted by A&A

We numerically investigate how fundamental properties of the spectral energy
distribution (SED) of a major gas-rich galaxy merger with dusty starburst are
determined by the initial orbital configuration of the merger. We found that an
infrared luminous galaxy with dusty starburst formed by a nearly
retrograde-retrograde merger suffers the most remarkable dust extinction of
stellar light and consequently shows very red colors. Considering that a
retrograde-retrograde merger does not produce strong and long tidal tails, this
result suggests that a luminous infrared galaxy without clear signs of
interaction and merging shows very large internal dust extinction and redder
colors.These numerical results furthermore imply that the morphology of a
luminous infrared galaxy can correlate with the degree of internal dust
extinction and thus with the shape of the SED, principally because the
morphology of a merger also depends strongly on the initial orbital
configuration of the merger.
(553kb)

astro-ph/0008290 [abs, src, ps, other] :

Title: The Origins of Disk Heating
Authors:
M.R. Merrifield (University of Nottingham),
J. Gerssen,
K. Kuijken (Kapteyn Institute)
Comments: 4 pages, 2 figures, using newpasp.sty (included) and epsf.sty.
Invited talk at “Galaxy Disks and Disk Galaxies,” Rome, June 2000. To be
published in the ASP Conference Series

By making spectral absorption-line observations of disk galaxies at
intermediate inclinations, we have been able to determine the amplitude of
their constituent stars’ random motions in three dimensions. This full measure
of the shape of the velocity ellipsoid is a useful diagnostic for determining
the “heating” mechanism responsible for creating the random motions. The
analysis implies that the main heating process varies with galaxy type, with
minor mergers dominating the heating in early-type disk galaxies, and spiral
density waves the most important mechanism in late-type systems.
(253kb)

astro-ph/0008291 [abs, src, ps, other] :

Title: The X-Ray Absorber in Broad Absorption Line Quasars
Authors:
T.G. Wang,
W. Brinkmann,
W. Yuan,
J.X. Wang,
Y.Y. Zhou
Comments: 11 pages, 6 Postscript figures
Journal-ref: ApJ vol 545

Recent observations of Broad Absorption Line (BAL) quasars demonstrated that
the soft X-ray emission of these objects is extremely weak and convincing
evidence for very strong absorption by a high column density (~ 10^23.5 cm^-2)
was obtained for PG 1411+442, even though it is one of the few BAL QSOs
strongly detected in soft X-rays. This paper examines the ionization status and
geometry of the X-ray absorber by combining the properties of the UV lines with
the X-ray continuum absorption. We show that the gas has to have large column
densities in ions of major UV absorption lines, such as CIV, NV, OVI and Ne
VIII, in order to have sufficient opacity around 0.2 to 0.35 keV. The UV
absorption lines have to be saturated if the X-ray absorber intersects the line
of sight to the UV continuum emission region. A uniformly covering UV and X-ray
absorption model can be constructed for PG 1411+442 but in some other soft
X-ray detected BAL QSOs, such as PG 1001+054, the observed line optical depth
is much lower than expected from the X-ray absorbing material. We propose a
scheme in which a substantial fraction of the line of sight to the continuum
source may be covered by either an optically thick flow or clouds in a narrow
velocity range, but in which the total covering factor of either the whole flow
or all clouds is close to unity.
(50kb)

astro-ph/0008292 [abs, src, ps, other] :

Title: Observational signatures of X-ray irradiated accretion disks
Authors:
C. Done,
S. Nayakshin
Comments: 13 pages, 5 figures, accepted for publication in ApJ

Reflection of X-rays from cool material around a black hole is one of the few
observational diagnostics of the accretion flow geometry. Models of this
reflected spectrum generally assume that the accretion disk can be
characterized by material in a single ionization state. However, several
authors have recently stressed the importance of the classic ionization
instability for X-ray irradiated gas in hydrostatic balance. This instability
leads to a discontinuous transition in the vertical structure of the disk,
resulting in a hot ionized skin above much cooler material. If the Compton
temperature of the skin is high then even iron is completely ionized, and the
skin does not produce any spectral features. These new models, where the
ionization structure of the disk is calculated self-consistently, require an
excessive amount of computing power and so are difficult to use in directly
fitting observed X-ray spectra. Instead, we invert the problem by simulating
X-ray spectra produced by the new reflection models, and then fit these with
the old, single zone reflection models, to assess the extent to which the
derived accretion geometry depends on the reflection model used. We find that
the single zone ionization models can severely underestimate the covering
fraction of the “cold” material as seen from the X-ray source if the optical
depth in the ionized skin is of order unity, and that this can produce an
apparent correlation between the covering fraction and the X-ray spectral index
similar in nature to that reported by Zdziarski, Lubinski and Smith (1999).
(56kb)

astro-ph/0008293 [abs, src, ps, other] :

Title: Spectroscopy of the candidate luminous blue variable at the center of
the ring nebula G79.29+0.46
Authors:
R. H. M. Voors (KNMI – Royal Dutch Meteorological Institute)
T. R. Geballe (Gemini Observatory),
L. B. F. M. Waters (Univ. Amsterdam),
F. Najarro (Instituto de Estructura de la Materia, CSIC, Madrid, Spain),
H.J.G.L.M. Lamers (Univ. Utrecht)
Comments: Accepted for publication in Astronomy and Astrophysics, 7 PostScript
figures, 5 tables

We report optical and near-infrared spectra of the central star of the radio
source G79.29+0.46, a candidate luminous blue variable. The spectra contain
numerous narrow (FWHM < 100 km/s emission lines of which the low-lying hydrogen
lines are the strongest, and resemble spectra of other LBVc’s and B[e]
supergiants. A few prominent infrared lines are unidentified. The terminal wind
speed is determined from H alpha to be 110 km/s. The strength of H alpha
implies the presence of a very dense wind. Extended emission from H alpha and
[N II] was detected but appears to be associated with the Cygnus X region
rather than the radio source. Both diffuse interstellar bands and interstellar
absorption lines are present in the optical spectrum of the central star,
suggesting that there are both diffuse and molecular cloud components to the
extinction and implying a minimum distance of 1 kpc and minimum luminosity of
~10^5 L(sun) for the star. The new spectra and their analysis indicate a low
excitation, dense, and slowly expanding wind and support the LBVc
classification.
(104kb)

astro-ph/0008294 [abs, src, ps, other] :

Title: On the Association of Gamma-ray Bursts with Massive Stars: Implications
for Number Counts and Lensing Statistics
Authors:
Cristiano Porciani,
Piero Madau
Comments: LaTeX, 22 pages, 7 figures, submitted to the ApJ

Recent evidence appears to link gamma-ray bursts (GRBs) to star-forming
regions in galaxies at cosmological distances. If short-lived massive stars are
the progenitors of GRBs, the rate of events per unit cosmological volume should
be an unbiased tracer (i.e. unaffected by dust obscuration and surface
brightness limits) of the cosmic history of star formation. Here we use
realistic estimates for the evolution of the stellar birthrate in galaxies to
model the number counts, redshift distribution, and time-delay factors of GRBs,
and indentify some of the remaining uncertainties. We find that present-day
data cannot fully discriminate between different plausible star formation
histories. The best-fitting parameters to the BATSE log N- log P relation imply
about 1-2 GRBs every one million Type II supernovae, and a characteristic
`isotropic-equivalent’ burst luminosity in the range 3-20 x 1e51 ergs/s (for
H_0=65 km/s/Mpc). We compute the rate of multiple imaging of background GRBs
due to foreground mass condensations in a Lambda-dominated cold dark matter
cosmology, assuming that dark halos approximate singular isothermal spheres on
galaxy scales and Navarro-Frenk-White profiles on group/cluster scales, and are
distributed in mass according to the Press-Schechter model. We show that the
expected sensivity increase of Swift relative to BATSE could result in a few
strongly lensed individual bursts detected down to a photon flux of 0.1
phot/s/cm^2 in a 3-year survey. Because of the partial sky coverage, however,
it is unlikely that the {it Swift} satellite will observe recurrent events
(lensed pairs).
(88kb)

astro-ph/0008295 [abs, src, ps, other] :

Title: RXTE observations of Seyfert-2 galaxies: evidence for spectral
variability
Authors:
I. Georgantopoulos,
I. Papadakis
Comments: 15 pages, submitted to the MNRAS

We present a series of RXTE observations of the nearby obscured Seyfert
galaxies ESO103-G35, IC5063, NGC4507 and NGC7172. The period of monitoring
ranges from seven days for NGC7172 up to about seven months for ESO103-G035.
The spectra of all galaxies are well fit with a highly obscured (N_H>10^23
cm-2) power-law and an Fe line at 6.4 keV. We find strong evidence for the
presence of a reflection component in ESO103-G35 and NGC4507. The observed flux
presents strong variability on day timescales in all objects. Spectral
variability is also detected in the sense that the spectrum steepens with
increasing flux similar to the behaviour witnessed in some Seyfert-1 galaxies.
(87kb)

astro-ph/0008296 [abs, src, ps, other] :

Title: Imaging the Universe in 3D with the VLT: The Next Generation Field
Spectrometer SPIFFI
Authors:
F. Eisenhauer,
M. Tecza,
S. Mengel,
N. Thatte,
C. Roehrle,
K. Bickert,
J. Schreiber
Comments: 9 pages, 6 figures (partially converted to bitmap), SPIE Proceedings
4008-29, Astronomical Telescopes and Instrumentation 2000, Munich, Germany

We present SPIFFI, the integral field spectrometer for the VLT. This
instrument allows simultaneous observation of infrared spectra in more than
1000 image points of a two dimensional field. With its set of four gratings and
a pixel scale that can be varied by a factor of ten, SPIFFI provides high
flexibility, and at the same time offers the unique possibility of diffraction
limited imaging spectroscopy at an 8m-class telescope, when fed by the adaptive
optics system MACAO. We outline the scientific drivers for building such an
instrument, the concept of image slicing, the optical design, and the
implementation of SPIFFI.
(360kb)

astro-ph/0008297 [abs, src, ps, other] :

Title: Electromagnetic radiation from superconducting string cusps
Authors:
J.J. Blanco-Pillado,
Ken D. Olum
Comments: 12 pages, LaTex, 2 figures

Cusps in superconducting cosmic strings produce strongly beamed
electromagnetic radiation. To calculate the energy emitted requires taking into
account the effect of the charge carriers on the string motion, which has
previously been done only heuristically. Here, we use the known exact solution
to the equations of motion for the case where the current is chiral to update
previous calculations for the total energy, spectrum and angular distribution
in that case. We analyze the dependence of the radiated energy on the cusp
parameters, and discuss which types of cusp dominate the total radiation
emitted from an ensemble.
(17kb)

astro-ph/0008298 [abs, src, ps, other] :

Title: Study of ammonia emission in the NGC 6334 region
Authors:
Anderson Caproni (IAG/USP),
Zulema Abraham (IAG/USP),
Jose W. S. Vilas-Boas (CRAAE/INPE/MCT)
Comments: 10 pages, 5 figures. Accepted for publication in Astronomy &
Astrophysics

The region centered in the NGC 6334 I(N) radio continuum source was surveyed
in an extension of 6 arcmin in right ascension and 12 arcmin in declination, in
the NH3(J,K) = (1,1) transition, using the Itapetinga radio telescope. The
spectra show non-LTE behavior, and gradients of velocity and line-width were
detected along the region. A detailed analysis of the spectra showed that the
surveyed region is composed of at least three overlapped sources related to
regions that are in different stages of star formation: NGC 6334 I, associated
with an already known molecular bipolar outflow, NGC 6334 I(N)w, the brightest
ammonia source, coincidental with the continuum source NGC 6334 I(N), and NGC
6334 I(N)e, weaker, more extended and probably less evolved than the others.
The physical parameters of the last two sources were calculated in non-LTE
conditions, assuming that their spectra are the superposition of the narrow
line spectra produced by small dense clumps. The H2 density, NH3 column
density, kinetic temperature, diameter and mass of the clumps were found to be
very similar in the two regions, but the density of clumps is lower in the
probably less evolved source NGC 6334 I(N)e. Differences between the physical
parameters derived assuming LTE and non-LTE conditions are also discussed in
this work.
(243kb)

astro-ph/0008299 [abs, src, ps, other] :

Title: Absolute dimensions of the unevolved B-type eclipsing binary GG Orionis
Authors:
G. Torres,
C. H. S. Lacy,
A. Claret,
J. A. Sabby
Comments: To appear in The Astronomical Journal, December 2000

We present photometric observations in B and V as well as spectroscopic
observations of the detached, eccentric 6.6-day double-lined eclipsing binary
GG Ori, a member of the Orion OB1 association. Absolute dimensions of the
components, which are virtually identical, are determined to high accuracy
(better than 1% in the masses and better than 2% in the radii) for the purpose
of testing various aspects of theoretical modeling. We obtain M(A) = 2.342 +/-
0.016 solar masses and R(A) = 1.852 +/- 0.025 solar radii for the primary, and
M(B) = 2.338 +/- 0.017 solar masses and R(B) = 1.830 +/- 0.025 solar radii for
the secondary. The effective temperature of both stars is 9950 +/- 200 K,
corresponding to a spectral type of B9.5. GG Ori is very close to the ZAMS, and
comparison with current stellar evolution models gives ages of 65-82 Myr or 7.7
Myr depending on whether the system is considered to be burning hydrogen on the
main sequence or still in the final stages of pre-main sequence contraction. We
have detected apsidal motion in the binary at a rate of dw/dt = 0.00061 +/-
0.00025 degrees per cycle, corresponding to an apsidal period of U = 10700 +/-
4500 yr. A substantial fraction of this (approximately 70%) is due to the
contribution from General Relativity.
(386kb)

astro-ph/0008300 [abs, src, ps, other] :

Title: The Cause of the Age Discrepancy in Pulsar B1757-24?
Authors:
D. Marsden (1),
R. E. Lingenfelter (2),
R. E. Rothschild (2) ((1) GSFC, (2) CASS/UCSD)
Comments: 10 terse pages with one incredibly busy figure. Submitted to ApJ
Letters

Radio pulsars are thought to spin-down primarily due to torque from magnetic
dipole radiation (MDR) emitted by the time-varying stellar magnetic field as
the star rotates. This assumption produces a `characteristic age’ for a pulsar
which has generally been assumed to be comparable to the actual age. Recent
observational limits on the proper motion of pulsar B1757-24, however, revealed
that the actual age (>39 kyr) of this pulsar is much greater than its MDR
characteristic age (16 kyr) – calling into question the assumption of pure MDR
spin-down for this and other pulsars. To explore the possible cause of this
discrepancy, we consider a scenario in which the pulsar acquired an accretion
disk from supernova ejecta, and the subsequent spin-down occurred under the
combined action of MDR and accretion torques. A simplified model of the
accretion torque involving a constant mass accretion rate can explain the age
and period derivative of the pulsar for reasonable values of the pulsar
magnetic field and accretion rate. We discuss testable predictions of this
model.
(22kb)

astro-ph/0008301 [abs, src, ps, other] :

Title: Optical Variability of the T Tauri Star HH30
Authors:
Kenneth Wood,
Scott Wolk,
K.Z. Stanek,
George Leussis (CfA),
Keivan Stassun (Wisconsin),
Michael Wolff,
Barbara Whitney (Space Science Institute)
Comments: 7 pages, 3 figures. Accepted for ApJ Letters

We report results of VRI photometric monitoring of the T Tauri star plus disk
system HH30. We find that HH30 is highly variable over timescales of a few days
with Delta V~1.5 mag, Delta I~1.1 mag. Furthermore we find hints of periodicity
with periodograms indicating possible periods of 11.6 and 19.8 days. The VRI
photometry is available through the anonymous ftp service. We model the
variability with Monte Carlo radiation transfer simulations for a spotted star
plus disk system and find that the large variability is best reproduced with a
single hot spot and circumstellar grains that are larger than typical
interstellar grains. The apparent existence of a single hot spot and the need
for larger, more forward throwing grains is consistent with previous modeling
of HST imagery.
(26kb)

astro-ph/0008302 [abs, src, ps, other] :

Title: A New Look at Simple Inhomogeneous Chemical Evolution
Authors:
M. S. Oey (STScI)
Comments: Accepted by ApJ Letters. 4 pages, 4 figures, uses emulateapj.sty.
Also available at this http URL

A rudimentary, one-zone, closed-box model for inhomogeneous chemical
evolution is offered as an alternative reference than the Simple model in the
limit of no mixing. The metallicity distribution functions (MDFs) of Galactic
halo and bulge stars can be matched by varying a single evolutionary parameter,
nQ. Q is the filling factor of contaminating regions and n is the number of
star-forming generations. Therefore, Q and n have equivalent roles, and
combinations of n and Q yield systems with different metallicities at any given
age. The model also revises interpretation of observed MDFs. Unevolved systems
probe the parent distribution of metal production f(z), for example, the
high-metallicity tail of the halo distribution agrees with a power-law f(z).
The Galactic disk G-dwarf Problem also improves.
(34kb)

astro-ph/0008303 [abs, src, ps, other] :

Title: Recent X-ray Observations of Disk Galaxies: Tracing the Dynamic
Interstellar Medium
Authors:
Q. Daniel Wang
Comments: 6 pages (including figures), an invited talk presented in the
conference, Galaxy Disks and Disk Galaxies

I review recent results from our deep ROSAT and Chandra observations of two
galaxies, M101 and NGC 4631, in fields of exceptionally low Galactic
extinction. Large amounts of X-ray-emitting gas are detected in these galaxies.
Such gas is produced primarily in massive star forming regions and have an
average characteristic temperature of a few times $10^6$ K. Cooler gas ($sim
10^6$ K) is found typically outside galactic disks and may represent outflows
from blown-out superbubbles. Propagation of star formation, driven by the
expansion of hot gas, appears to be operating in giant HII complexes. A
substantial fraction of photo-evaporated gas in such complexes may be
mass-loaded into hot gas, which explains their large X-ray luminosities. These
processes likely play an important role in determining the global properties of
the interstellar medium, especially the disk/halo interaction.
(493kb)

Replacements

astro-ph/0002274 [abs, src, ps, other] :

Title: Hidden source of high-energy neutrinos in collapsing galactic nucleus
Authors:
V.S.Berezinsky,
V.I.Dokuchaev
Comments: small changes, to be published in Astroparticle Physics
Note: replaced with revised version Fri, 18 Aug 2000 14:21:03 GMT (14kb)

astro-ph/0005288 [abs, src, ps, other] :

Title: MHD Stellar and Disk Winds: Application to Planetary Nebulae
Authors:
Eric G. Blackman,
Adam Frank,
Carl Welch (University of Rochester)
Comments: ApJ accepted version, incorporating some important revisions. 25
Pages, LaTex, + 5 figs
Note: replaced with revised version Fri, 18 Aug 2000 19:56:25 GMT (366kb)

astro-ph/0006271 [abs, src, ps, other] :

Title: The Earliest Luminous Sources and the Damping Wing of the Gunn-Peterson
Trough
Authors:
Piero Madau,
Martin J. Rees
Comments: LaTeX, 11 pages, 3 figures, revised version accepted for publication
in the ApJL
Note: replaced with revised version Fri, 18 Aug 2000 09:00:44 GMT (26kb)

astro-ph/0008234 [abs, src, ps, other] :

Title: Bi-metric Gravity and “Dark Matter”
Authors:
I. T. Drummond
Comments: 32 pages 2 figures ; references corrected
Note: replaced with revised version Fri, 18 Aug 2000 15:56:48 GMT (26kb)

gr-qc/0007010 [abs, src, ps, other] :

Title: Dynamical Systems Approach to Magnetised Cosmological Perturbations
Authors:
Stacey Hobbs,
Peter K. S. Dunsby
Comments: 15 pages RevTeX, 5 figures. Accepted for publication in Physical
Review D
Note: replaced with revised version Fri, 18 Aug 2000 14:59:39 GMT (253kb)

hep-ex/0001041 [abs, src, ps, other] :

Title: On the possibility of radar echo detection of ultra-high energy cosmic
ray- and neutrino-induced extensive air showers
Authors:
Peter. W. Gorham (JPL/Caltech)
Comments: 29 pages, 16 figures, uses aas2pp4.sty. Final version, to appear in
Astroparticle Physics. Contains new figs, better estimate of angular
precision possible
Subj-class: High Energy Physics – Experiment; Instrumentation and Detectors;
Space Physics
Note: replaced with revised version Fri, 18 Aug 2000 20:43:54 GMT (79kb)

hep-ph/0005104 [abs, src, ps, other] :

Title: Effects of neutrino mixing on high-energy cosmic neutrino flux
Authors:
H. Athar (1),
M. Jezabek (2),
O. Yasuda (1) ((1) Tokyo Metropolitan University, Japan, (2) Institute of Nuclear Physics, Poland)
Comments: 22 pages, Revtex, 5 figures included with epsfig, remarks added in
section II and section IV, some references updated, to appear in Phys. Rev. D
Note: replaced with revised version Fri, 18 Aug 2000 08:47:55 GMT (88kb)