Astrophysics

astro-ph new abstracts, Tue, 29 Aug 00 04:00:13 GMT
0008411 — 0008439 received

astro-ph/0008411 [abs, src, ps, other] :

Title: Exploring the Structure of Distant Galaxies with Adaptive Optics on the
Keck-II Telescope

Authors:
J. E. Larkin (UCLA),
T. M. Glassman (UCLA),
P. Wizinowich (KECK),
D. S. Acton (KECK),
O. Lai (KECK,CFHT),
A. V. Filippenko (UCB),
A. L. Coil (UCB),
T. Matheson (UCB)

Comments: 5 pages, Latex, 3 figures. Accepted for publication in P.A.S.P

We report on the first observation of cosmologically distant field galaxies
with an high order Adaptive Optics (AO) system on an 8-10 meter class
telescope. Two galaxies were observed at 1.6 microns at an angular resolution
as high as 50 milliarcsec using the AO system on the Keck-II telescope. Radial
profiles of both objects are consistent with those of local spiral galaxies and
are decomposed into a classic exponential disk and a central bulge. A
star-forming cluster or companion galaxy as well as a compact core are detected
in one of the galaxies at a redshift of 0.37+/-0.05. We discuss possible
explanations for the core including a small bulge, a nuclear starburst, or an
active nucleus. The same galaxy shows a peak disk surface brightness that is
brighter than local disks of comparable size. These observations demonstrate
the power of AO to reveal details of the morphology of distant faint galaxies
and to explore galaxy evolution.
(102kb)

astro-ph/0008412 [abs, src, ps, other] :

Title: Quintessence Model and Cosmic Microwave Background

Author:
Paul H. Frampton

Comments: 3 pages LaTeX. Write-up of talk at DPF2000, August 12, 2000

A particular kind of quintessence is considered, with equation of motion
$p_Q/
ho_Q = -1$, corresponding to a cosmological term with time-dependence
$Lambda(t) = Lambda(t_0) (R(t_0)/R(t))^{P}$ and we examine how values of
$Omega_m$ and $Omega_{Lambda}$ depend on $P$.
(6kb)

astro-ph/0008413 [abs, src, ps, other] :

Title: The state of two neutrinos moving in the opposite directions

Authors:
D.L. Khokhlov

Comments: 2 pages, LaTeX

It is considered the flux of neutrinos moving from the source in the opposite
directions. The state of two neutrinos moving in the opposite directions is a
superposition of the state $|psi_{12}>$ and the state $|psi_{21}>$ in which
the positions of neutrinos are interchanged. Neutrinos in the state
$|psi_{21}>$ are P-transformed with respect to neutrinos in the state
$|psi_{12}>$ therefore if one detects neutrinos in the state $|psi_{12}>$ it
is forbidden to detect neutrinos in the state $|psi_{21}>$. Under detection
the number of neutrinos decreases two times, and the energy of neutrinos
increases two times. Hence one can detect only one half of the solar neutrino
flux predicted by the SSM that may provide a solution for the solar neutrino
puzzle.
(2kb)

astro-ph/0008414 [abs, src, ps, other] :

Title: Detection of a Series of X-ray Dips Associated with a Radio Flare in GRS
1915+105

Authors:
S. Naik (1),
P. C. Agrawal (1),
A. R. Rao (1),
B. Paul (1,2),
S. Seetha (3),
K. Kasturirangan (3) ((1) Tata Institute of Fundamental Research, Mumbai, INDIA (2) Institute of Space and Astronautical Science 3-1-1 Yoshinodai, Sagamihara, Kanagawa, Japan (3) ISRO Satellite Center, Bangalore, INDIA)

Comments: 18 pages, 7 figures, Accepted for publication in ApJ

We report the detection of a series of X-ray dips in the Galactic black hole
candidate GRS 1915+105 during 1999 June 6-17 from observations carried out with
the Pointed Proportional Counters of the Indian X-ray Astronomy Experiment on
board the Indian satellite IRS-P3. The observations were made after the source
made a transition from a steady low-hard state to a chaotic state which
occuered within a few hours. Dips of about 20-160 seconds duration are observed
on most of the days. The X-ray emission outside the dips shows a QPO at ~ 4 Hz
which has characteristics similar to the ubiquitous 0.5 – 10 Hz QPO seen during
the low-hard state of the source. During the onset of dips this QPO is absent
and also the energy spectrum is soft and the variability is low compared to the
non-dip periods. These features gradually re-appear as the dip recovers. The
onset of the occurrence of a large number of such dips followed the start of a
huge radio flare of strength 0.48 Jy (at 2.25 GHz). We interpret these dips as
the cause for mass ejection due to the evacuation of matter from an accretion
disk around the black hole. We propose that a super-position of a large number
of such dip events produces a huge radio jet in GRS 1915+105.
(81kb)

astro-ph/0008415 [abs, src, ps, other] :

Title: Explorations in Hubble Space: A Quantitative Tuning Fork

Authors:
Roberto G. Abraham (1),
Michael R. Merrifield (2) ((1) Dept. of Astronomy, University of Toronto, (2) School of Physics & Astronomy, University of Nottingham)

Comments: Accepted for publication in The Astronomical Journal. Figure 2 is too
large to be embedded in the paper, and has been included as a JPEG image

In order to establish an objective framework for studying galaxy morphology,
we have developed a quantitative two-parameter description of galactic
structure that maps closely on to Hubble’s original tuning fork. Any galaxy can
be placed in this “Hubble space”, where the x-coordinate measures position
along the early-to-late sequence, while the y-coordinate measures in a
quantitative way the degree to which the galaxy is barred. The parameters
defining Hubble space are sufficiently robust to allow the formation of
Hubble’s tuning fork to be mapped out to high redshifts. In the present paper,
we describe a preliminary investigation of the distribution of local galaxies
in Hubble space, based on the CCD imaging atlas of Frei et al. (1996). We find
that barred, weakly-barred, and unbarred galaxies are remarkably well-separated
on this diagnostic diagram. The spiral sequence is clearly bimodal and indeed
approximates a tuning fork: strongly-barred and unbarred spirals do not simply
constitute the extrema of a smooth unimodal distribution of bar strength, but
rather populate two parallel sequences. Strongly barred galaxies lie on a
remarkably tight sequence, strongly suggesting the presence of an underlying
unifying physical process. Rather surprisingly, weakly barred systems do not
seem to correspond to objects bridging the parameter space between unbarred and
strongly barred galaxies, but instead form an extension of the regular spiral
sequence. This relation lends support to models in which the bulges of
late-type spirals originate from secular processes driven by bars.
(461kb)

astro-ph/0008416 [abs, src, ps, other] :

Title: A Magnetic Dynamo Origin For The Sub-mm Excess In Sgr A*

Authors:
Fulvio Melia,
Siming Liu,
Robert Coker

Comments: 38 pages, 9 figures, Submitted to ApJ

The sub-mm bump observed in the spectrum of Sgr A* appears to indicate the
existence of a compact emitting component within several Schwarzschild radii,
r_S, of the nucleus at the Galactic Center. This is interesting in view of the
predicted circularized flow within 5-10 r_S, based on detailed
multi-dimensional hydrodynamic simulations of Bondi-Hoyle accretion onto this
unusual object. In this paper, we examine the physics of magnetic field
generation by a Keplerian dynamo subject to the conditions pertaining to Sgr
A*, and show that the sub-mm bump can be produced by thermal synchrotron
emission in this inner region. This spectral feature may therefore be taken as
indirect evidence for the existence of this circularization. However, the
required accretion rate in the Keplerian flow is orders of magnitude smaller
than that predicted by the Bondi-Hoyle simulations. We speculate that rapid
evaporation, in the form of a wind, may ensue from the heating associated with
turbulent mixing of gas elements with large eccentricity as they settle down
into a more or less circular (i.e., low eccentricity) trajectory. The spectrum
of Sgr A* longward of 1-2 mm may be generated in the transition region outside
of the Keplerian flow.
(43kb)

astro-ph/0008417 [abs, src, ps, other] :

Title: The Nuclear and Circum-nuclear Stellar Population in Seyfert 2 Galaxies:
Implications for the Starburst-AGN Connection

Authors:
R. M. Gonzalez Delgado,
T. Heckman,
C. Leitherer

Comments: To be published in ApJ, 546, Jan 10, 2001

We report the results of a spectroscopic investigation of a sample of 20 of
the brightest type 2 Seyfert nuclei. Our goal is to search for the direct
spectroscopic signature of massive stars, and thereby probe the role of
circumnuclear starbursts in the Seyfert phenomenon. The method used is based on
the detection of the higher order Balmer lines and HeI lines in absorption and
the Wolf-Rayet feature at $sim$4680 AA in emission. These lines are strong
indicators of the presence of young (a few Myrs) and intermediate-age (a few
100 Myrs) stellar populations. In over half the sample, we have detected HeI
and/or strong stellar absorption features in the high-order (near-UV) Balmer
series together with relatively weak lines from an old stellar population. In
three others we detect a broad emission feature near 4680 AA that is most
plausibly ascribed to a population of Wolf-Rayet stars (the evolved descendants
of the most massive stars). We therefore conclude that the blue and near-UV
light of over half of the sample is dominated by young and/or intermediate age
stars. The “young” Seyfert 2’s have have larger far-IR luminosities, cooler
mid/far-IR colors, and smaller [OIII]/H$eta$ flux ratios than the “old”
ones. These differences are consistent with a starburst playing a significant
energetic role in the former class. We consider the possibility that there may
be two distinct sub-classes of Seyfert 2 nuclei (“starbursts” and “hidden
BLR”). However, the fact that hidden BLRs have been found in three of the
“young” nuclei argues against this, and suggests that nuclear starbursts may
be a more general part of the Seyfert phenomenon.
(489kb)

astro-ph/0008418 [abs, src, ps, other] :

Title: The peculiar motions of early-type galaxies in two distant regions –
VII. Peculiar velocities and bulk motions

Authors:
Matthew Colless,
R.P. Saglia,
D. Burstein,
R.L. Davies,
R.K. McMahan Jr,
Gary Wegner

Comments: to appear in MNRAS, 27 pages, EFAR paper 7

We present peculiar velocities for 84 clusters of galaxies in two large
volumes at distances between 6000 and 15000 km/s in the directions of
Hercules-Corona Borealis and Perseus-Pisces-Cetus. These velocities are based
on Fundamental Plane (FP) distance estimates for early-type galaxies in each
cluster. We fit the FP using a maximum likelihood algorithm which accounts for
both selection effects and measurement errors, and yields FP parameters with
smaller bias and variance than other fitting procedures. We find a best-fit FP
with coefficients consistent with the best existing determinations. We measure
the bulk motions of the sample volumes using the 50 clusters with the
best-determined peculiar velocities. We find the bulk motions in both regions
are small, and consistent with zero at about the 5% level. The EFAR results are
in agreement with the small bulk motions found by Dale et al. (1999) on similar
scales, but are inconsistent with pure dipole motions having the large
amplitudes found by Lauer & Postman (1994) and Hudson et al. (1999). The
alignment of the EFAR sample with the Lauer & Postman dipole produces a strong
rejection of a large-amplitude bulk motion in that direction, but the rejection
of the Hudson et al. result is less certain because their dipole lies at a
large angle to the main axis of the EFAR sample. We find the bulk motion of our
sample is consistent with most cosmological models that approximately reproduce
the shape and normalisation of the observed galaxy power spectrum. We conclude
that existing measurements of large-scale bulk motions provide no significant
evidence against standard models for the formation of structure.
(604kb)

astro-ph/0008419 [abs, src, ps, other] :

Title: Cosmological Microlensing

Authors:
Joachim Wambsganss (Universitaet Potsdam and MPI fuer Gravitationsphysik, Golm)

Comments: Invited review to appear in the proceedings of “Microlensing 2000: A
New Era of Microlensing Astrophysics”, Cape Town, South Africa (February
2000), ASP Conference Series, eds. J.W. Menzies and P.D. Sackett; 10 pages, 1
figure; uses newpasp.sty

Variability in gravitationally lensed quasars can be due to intrinsic
fluctuations of the quasar or due to “microlensing” by compact objects along
the line of sight. If disentangled from each other, microlens-induced
variability can be used to study two cosmological issues of great interest, the
size and brightness profile of quasars on one hand, and the distribution of
compact (dark) matter along the line of sight on the other. In particular,
multi-waveband observations are useful for this goal.

In this review recent theoretical progress as well as observational evidence
for quasar microlensing over the last few years will be summarized. Comparison
with numerical simulations will show where we stand. Particular emphasis will
be given to the questions microlensing can address regarding the search for
dark matter, both in the halos of lensing galaxies and in a cosmologically
distributed form. A discussion of desired observations and required theoretical
studies will be given as a conclusion/outlook.
(75kb)

astro-ph/0008420 [abs, src, ps, other] :

Title: Revealing deuterium Balmer lines in HII regions with VLT-UVES

Authors:
G. Hebrard (1),
D. Pequignot (2),
J. R. Walsh (3),
A. Vidal-Madjar (1),
R. Ferlet (1) ((1) IAP, (2) DAEC, (3) ESO)

Comments: 5 pages, latex, 7 figures, 2 tables, to be published in the special
Dec.1 issue of Astronomy & Astrophysics Letters “Early Science with the VLT:
The opening of Kueyen”

The search for deuterium Balmer lines with VLT-UVES is reported in HII
regions of the Galaxy and the Magellanic Clouds. The DI lines appear as faint,
narrow emission features in the blue wings of the HI Balmer lines and can be
distinguished from high-velocity HI emission. The previous identification to
deuterium is re-inforced beyond doubt. The detection of D-alpha and D-beta in
Orion (Hebrard et al. 2000) is confirmed and deuterium lines are now detected
up to at least D-eta. The UVES observations provide the first detection of
Balmer DI lines in four new HII regions (M 8, M 16, M 20, and DEM S 103 in
SMC), demonstrating that these lines are of common occurence.
(77kb)

astro-ph/0008421 [abs, pdf] :

Title: Photometric deprojection of edge-on galaxies

Authors:
Bizyaev D. (Sternberg Astronomical Institute, Moscow)

Comments: 6 pages including 3 figures. Have been published in Astronomy
Letters, v.26, p.219, 2000 (Translated from Pis’ma v Astronomicheskiy
Zhurnal, V.26, N.4, 2000, p.266.)

Photometric deprojection is used to determine the stellardisk and bulge
parameters for several edgeon galaxies from the FGC catalog. The assumption
that the galaxies of our sample belonging to the fourth (i.e., lowest)
surfacebrightness class in the FGC are edgeon, lowsurfacebrightness (LSB)
galaxies is considered.
(81kb)

astro-ph/0008422 [abs, src, ps, other] :

Title: Can dark matter see itself?

Authors:
Steen Hannestad

Comments: 6 pages, talk given at Dark2000 conference in Heidelberg, July 10-14
2000

Many independent high-resolution simulations of structure formation in cold
dark matter models show that galactic halos should have singular core profiles.
This is in stark contrast with observations of both low- and high-surface
brightness galaxies, which indicate that the dark matter has almost constant
density in the central parts of halos. Basically there are three possible
avenues to a solution to the problem, which we discuss in turn. Observations of
halo profiles could be more uncertain than previously thought, and higher
resolution observations could reveal that spirals do have a singular core
feature. The highest resolution simulations do not include a baryonic
component, and it is conceivable that violent star formation processes and
similar phenomena can destroy the singular dark matter core and lead to an
almost constant density core profile. Finally, we discuss in more detail the
intriguing possibility that the discrepancy hints at some new exotic physics of
the dark matter. Warm dark matter and self-interacting dark matter are two of
the most promising candidates.
(9kb)

astro-ph/0008423 [abs, src, ps, other] :

Title: s- and r-process elements in two very metal-poor stars

Authors:
Sean G. Ryan,
Wako Aoki,
Lisa A. J. Blake,
John E. Norris,
Timothy C. Beers,
Roberto Gallino,
Maurizio Busso,
Hiroyasu Ando

Comments: 10 pages, 3 figures. Proc. Torino-Melbourne-Pasadena “Wasserburg”
Workshop, U. Torino, 21-22 June 2000. For publication in Mem.S.A.It

New measurements of neutron-capture elements are presented for two very
metal-poor stars ([Fe/H] ~ -3). One (LP 625-44) has an s-process signature
believed to be due to mass transfer from a now-extinct metal-poor AGB
companion, and the second (CS 22897-008) is one of a number of very metal-poor
stars having high [Sr/Ba] ratios which is not expected from the r-process. In
the s-process star, many elements including lead have been detected, providing
strong constraints on the 13C pocket in the now-extinct AGB star. In the
Sr-rich star, Zn, Y, and Zr are also seen to be overabundant, and several
possible nucleosynthesis mechanisms are discussed.
(42kb)

astro-ph/0008424 [abs, src, ps, other] :

Title: Spectral Signatures of KiloHertz Quasi-Periodic Oscillations from
Accreting Neutron Stars

Authors:
Philip Kaaret (Harvard-Smithsonian CfA)

Comments: 10 pages, Invited talk to appear in the Proceedings of the Conference
X-ray Astronomy ‘999: Stellar Endpoints, AGNs and the Diffuse X-ray
Background

Correlations discovered between millisecond timing properties and spectral
properties in neutron star x-ray binaries are described and then interpreted in
relation to accretion flows in the systems. Use of joint timing and spectral
observations to test for the existence of the marginally stable orbit, a key
prediction of strong field general relativity, is described and observations of
the neutron star x-ray binary 4U1820-303 which suggest that the signature of
the marginally stable orbit has been detected are presented.
(26kb)

astro-ph/0008425 [abs, src, ps, other] :

Title: On the Absorption of X-rays in the Interstellar Medium

Authors:
J. Wilms (1),
A. Allen (2),
R. McCray (2) ((1) IAA Tuebingen, (2) JILA)

Comments: ApJ, in press, for associated software see
this http URL

We present an improved model for the absorption of X-rays in the ISM intended
for use with data from future X-ray missions with larger effective areas and
increased energy resolution such as Chandra and XMM, in the energy range above
100eV. Compared to previous work, our formalism includes recent updates to the
photoionization cross section and revised abundances of the interstellar
medium, as well as a treatment of interstellar grains and the H2molecule. We
review the theoretical and observational motivations behind these updates and
provide a subroutine for the X-ray spectral analysis program XSPEC that
incorporates our model.
(65kb)

astro-ph/0008426 [abs, src, ps, other] :

Title: Accelerated Electrons in Cassiopeia A: An Explanation for the Hard X-ray
Tail

Authors:
J. Martin Laming (US Naval Research Laboratory)

Comments: 20 pages, 3 figures, aasTeX502, accepted in ApJ

We propose a model for the hard X-ray (> 10 keV) emission observed from the
supernova remnant Cas A. Lower hybrid waves are generated in strong (mG)
magnetic fields, generally believed to reside in this remnant, by shocks
reflected from density inhomogeneities. These then accelerate electrons to
energies of several tens of keV. Around 4% of the x-ray emitting plasma
electrons need to be in this accelerated distribution, which extends up to
electron velocities of order the electron Alfven speed, and is directled along
magnetic field lines. Bremsstrahlung from these electrons produces the observed
hard x-ray emission. Such waves and accelerated electrons have been observed in
situ at Comet Halley, and we discuss the viability of the extrapolation from
this case to the parameters relevant to Cas A.
(30kb)

astro-ph/0008427 [abs, src, ps, other] :

Title: The Solar Helium Abundance in the Outer Corona Determined from
Observations with SUMER/SOHO

Authors:
J. M. Laming,
U. Feldman

Comments: 16 pages, 2 figures, accepted by ApJ

At altitudes of about 1.05 solar radii or more, the corona above quiet solar
regions becomes essentially isothermal. This obviates many of the difficulties
associated with the inverse problem of determining emission measure
distributions, and allows for fairly straightforward relative element abundance
measurements. We present new values for the He abundance. The first is based on
a reanalysis of the He/O ratio studied previously using data acquired by SUMER.
A more thorough evaluation of the atomic physics for He II, including a
detailed treatment of radiative recombination, increases the predicted emission
in the He II Balmer series compared to earlier analyses. We use a recently
revised value of the O abundance to derive an He/H abundance ratio of 0.038,
(mass fraction, Y=0.13), with an error of ~17% coming mainly from the O
abundance uncertainty. We demonstrate that this result may be affected by
gravitational settling of O relative to He. We also derive an abun