Astrophysics

astro-ph new abstracts, Wed, 30 Aug 00 04:00:12 GMT
0008440 — 0008467 received

astro-ph/0008440 [abs, src, ps, other] :

Title: The Supergiant Shell LMC2: II. Physical Properties of the 10^6 K Gas

Authors:
S. D. Points (1),
Y.-H. Chu (1),
S. L. Snowden (2),
L. Staveley-Smith (3) ((1) Univ. of Illinois at Urbana-Champaign, (2) NASA/Goddard Space Flight Center, (3) Australia Telescope National Facility)

Comments: 29 pages, 5 figures, to be published in ApJ

LMC2 has the highest X-ray surface brightness of all know supergiant shells
in the Large Magellanic Cloud (LMC). The X-ray emission peaks within the
ionized filaments that define the shell boundary, but also extends beyond the
southern border of LMC2 as an X-ray bright spur. ROSAT HRI images reveal the
X-ray emission from LMC2 and the spur to be truly diffuse, indicating a hot
plasma origin. We have obtained ROSAT PSPC and ASCA SIS spectra to study the
physical conditions of the hot gas interior to LMC2 and the spur. Raymond-Smith
thermal plasma model fits to the X-ray spectra, constrained by HI 21-cm
emission-line measurements of the column density, show the plasma temperature
of the hot gas interior of LMC2 to be kT = 0.1 – 0.7 keV and of the spur to be
kT = 0.1 – 0.5 keV. We have compared the physical conditions of the hot gas
interior to LMC2 with those of other supergiant shells, superbubbles, and
supernova remnants (SNRs) in the LMC. We find that our derived electron
densities for the hot gas inside LMC2 is higher than the value determined for
the supergiant shell LMC4, comparable to the value determined for the
superbubble N11, and lower than the values determined for the superbubble N44
and a number of SNRs.
(283kb)

astro-ph/0008441 [abs, src, ps, other] :

Title: N-Body Simulations of Compact Young Clusters near the Galactic Center

Authors:
Sungsoo S. Kim (UCLA),
Donald F. Figer (STScI),
Hyung Mok Lee (Seoul Univ.),
Mark Morris (UCLA)

Comments: 20 pages including 6 figures, To appear in ApJ, Dec 20 issue

We investigate the dynamical evolution of compact young star clusters (CYCs)
near the Galactic center (GC) using Aarseth’s Nbody6 codes. The relatively
small number of stars in the cluster (5,000-20,000) makes real-number N-body
simulations for these clusters feasible on current workstations. Using
Fokker-Planck (F-P) models, Kim, Morris, & Lee (1999) have made a survey of
cluster lifetimes for various initial conditions, and have found that clusters
with a mass <~ 2×10^4 Msun evaporate in ~10 Myr. These results were, however,
to be confirmed by N-body simulations because some extreme cluster conditions,
such as strong tidal forces and a large stellar mass range participating in the
dynamical evolution, might violate assumptions made in F-P models. Here we find
that, in most cases, the CYC lifetimes of previous F-P calculations are 5-30%
shorter than those from the present N-body simulations. The comparison of
projected number density profiles and stellar mass functions between N-body
simulations and HST/NICMOS observations by Figer et al. (1999) suggests that
the current tidal radius of the Arches cluster is ~1.0 pc, and the following
parameters for the initial conditions of that cluster: total mass of 2×10^4
Msun and mass function slope for intermediate-to-massive stars of 1.75 (the
Salpeter function has 2.35). We also find that the lower stellar mass limit,
the presence of primordial binaries, the amount of initial mass segregation,
and the choice of initial density profile (King or Plummer models) do not
significantly affect the dynamical evolution of CYCs.
(31kb)

astro-ph/0008442 [abs, src, ps, other] :

Title: VSOP observation of the quasar PKS 2215+020: a new laboratory for
core-jet physics at z=3.572

Authors:
A.P. Lobanov,
L.I. Gurvits,
S. Frey,
R.T. Schilizzi,
K.I. Kellermann,
N. Kawaguchi,
I.I.K. Pauliny-Toth

Comments: 15 pages, 6 figures, aastex macros; accepted for publication in The
Astrophysical Journal, V.546, N.2 *(January 10 2001)

We report results of a VSOP (VLBI Space Observatory Programme) observation of
a high redshift quasar PKS 2215+020 (z=3.572). The ~1 milliarcsecond resolution
image of the quasar reveals a prominent `core-jet’ structure on linear scales
from 5/h to 300/h pc ($H_0=100*h km/(s*Mpc). The brightness temperatures and
sizes of bright features identified in the jet are consistent with emission
from relativistic shocks dominated by adiabatic energy losses. The jet is
powered by the central black hole with estimated mass of ~4*10^9 solar masses.
Comparisons with VLA and ROSAT observations indicate a possible presence of an
extended radio/X–ray halo surrounding 2215+020.
(314kb)

astro-ph/0008443 [abs, src, ps, other] :

Title: Third-dredge-up oxygen in planetary nebulae

Authors:
D. Pequignot (Observatoire de Paris Meudon),
J. R. Walsh (ESO),
A. A. Zijlstra (UMIST),
G. Dudziak (Univ. Strathclyde)

Comments: To appear in Astron. Astrophys. Lett. (Latex, 5 pages, 1 postscript
figure)

The planetary nebulae He 2-436 and Wray 16-423 in the Sagittarius dwarf
galaxy appear to result from nearly twin stars, except that third-dredge-up
carbon is more abundant in He 2-436. A thorough photoionization-model analysis
implies that ratios Ne/O, S/O and Ar/O are significantly smaller in He 2-436,
indicative of third-dredge-up oxygen enrichment. The enrichment of oxygen with
respect to carbon is (7 +/- 4)%. Excess nitrogen in Wray 16-423 suggests third
dredge-up of late CN-cycle products even in these low-mass,
intermediate-metallicity stars.
(18kb)

astro-ph/0008444 [abs, src, ps, other] :

Title: The evolution of main sequence star + white dwarf binary systems towards
Type~Ia supernovae

Authors:
N. Langer,
A. Deutschmann,
S. Wellstein,
P. Hoeflich

Comments: 21 pages, accepted by A&A

Close binaries consisting of a main sequence star and a white dwarf are
considered as candidates for Type~Ia supernova progenitors. We present
selfconsistent calculations of the time dependence of the structure of the main
sequence star, the mass transfer rate, and the orbit by means of a binary
stellar evolution program. In contrast to results based on simple estimates of
the mass transfer rate in systems of the considered type, our results allow for
the possibility that even systems with rather small initial white dwarf masses
(~ 0.7 M_sun) may produce Type Ia supernovae. We present results for two
different metallicities, Z=0.02 and Z=0.001. We find that for systems with the
lower metallicity, the mass transfer rates are on average five times larger
than in comparable system at solar metallicity. This leads to a systematic
shift of the supernova Ia progenitor population. Firstly, while for Z=0.02
donor star initial masses in supernova progenitor systems are restricted to the
range 1.6…2.3 M_sun, they are in the interval 1.4…1.8 M_sun at low Z.
Secondly, the initial white dwarf masses need, on average, to be larger by 0.2
M_sun at low Z in order to obtain a Chandrasekhar mass white dwarf. This
metallicity dependences may be responsible for a drop of the Type Ia supernova
rate for low metallicity, and may introduce a Z-dependence in the properties of
supernovae. We also estimate the X-ray luminosities of the computed systems,
and investigate their donor star and orbital properties.
(142kb)

astro-ph/0008445 [abs, src, ps, other] :

Title: A Wide-Field CCD Survey for Centaurs and Kuiper Belt Objects

Authors:
S. Sheppard,
D. Jewitt,
C. Trujillo,
M. Brown,
M. Ashley

Comments: 13 pages, 5 figures, Accepted for Publication in AJ

A modified Baker-Nunn camera was used to conduct a wide-field survey of 1428
square degrees of sky near the ecliptic in search of bright Kuiper Belt objects
and Centaurs. This area is an order of magnitude larger than any previously
published CCD survey for Centaurs and Kuiper Belt Objects. No new objects
brighter than red magnitude m=18.8 and moving at a rate 1″/hr to 20″/hr were
discovered, although one previously discovered Centaur 1997 CU26 Chariklo was
serendipitously detected. The parameters of the survey were characterized using
both visual and automated techniques. From this survey the empirical projected
surface density of Centaurs was found to be SigmaCentaur(m<18.8)=7.8(+16.0
-6.6)x10^-4 per square degree and we found a projected surface density 3sigma
upper confidence limit for Kuiper Belt objects of SigmaKBO(m< 18.8)<4.1×10^-3
per square degree. We discuss the current state of the cumulative luminosity
functions of both Centaurs and Kuiper Belt objects. Through a Monte Carlo
simulation we show that the size distribution of Centaurs is consistent with a
q=4 differential power law, similar to the size distribution of the parent
Kuiper Belt Objects. The Centaur population is of order 10^7 (radius > 1 km)
assuming a geometric albedo of 0.04. About 100 Centaurs are larger than 50 km
in radius, of which only 4 are presently known. The current total mass of the
Centaurs is 10^-4 Earth Masses. No dust clouds were detected resulting from
Kuiper Belt object collisions, placing a 3sigma upper limit <600 collisionally
produced clouds of m<18.8 per year.
(290kb)

astro-ph/0008446 [abs, src, ps, other] :

Title: Looking for a varying $alpha$ in the Cosmic Microwave Background

Authors:
P.P. Avelino (CAUP and DF-FCUP, Porto),
C.J.A.P. Martins (DAMTP, Cambridge and CAUP),
G. Rocha (CAUP and Dep. Physics, Oxford),
P. Viana (CAUP and DMA-FCUP, Porto)

Comments: 15 pages; submitted to Phys. Rev. D

We perform a likelihood analysis of the recently released BOOMERanG and
MAXIMA data, allowing for the possibility of a time-varying fine-structure
constant. We find that in general this data prefers a value of $alpha$ that
was smaller in the past (which is in agreement with measurements of $alpha$
from quasar observations). However, there are some interesting degeneracies in
the problem which imply that strong statements about $alpha$ can not be made
using this method until independent accurate determinations of $Omega_b h^2$
and $H_0$ are available.

We also show that a preferred lower value of $alpha$ comes mainly from the
data points around the first Doppler peak, whereas the main effect of the
high-$ell$ data points is to increase the preferred value for $Omega_b h^2$
(while also tightening the constraints on $Omega_0$ and $H_0$). We comment on
some implications of our results.
(246kb)

astro-ph/0008447 [abs, src, ps, other] :

Title: Powerful jets from black hole X-ray binaries in Low/Hard X-ray states

Authors:
R. P. Fender (Amsterdam)

Comments: Accepted for publication in MNRAS

Four persistent (Cygnus X-1, GX 339-4, GRS 1758-258 and 1E 1740.7-2942) and
three transient (GS 2023+38, GRO J0422+32 and GS 1354-64) black hole X-ray
binary systems have been extensively observed at radio wavelengths during
extended periods in the Low/Hard X-ray state, which is characterised in X-rays
by a hard power-law spectrum and strong variability. All seven systems show a
persistent flat or inverted (in the sense that spectral index alpha >= 0) radio
spectrum in this state, markedly different from the optically thin radio
spectra exhibited by most X-ray transients within days of outburst.
Furthermore, in none of the systems is a high-frequency cut-off to this
spectral component detected, and there is evidence that it extends to
near-infrared or optical regimes. Luminous persistent hard X-ray states in the
black hole system GRS 1915+105 produce a comparable spectrum. This spectral
component is considered to arise in synchrotron emission from a conical,
partially self-absorbed jet, of the same genre as those originally considered
for Active Galactic Nuclei. Whatever the physical origin of the Low/Hard X-ray
states, these self-similar outflows are an ever-present feature. The power in
the jet component is likely to be a significant (> 5%) and approximately fixed
fraction of the total accretion luminosity. The correlation between hard X-ray
and synchrotron emission in all the sources implies that the jets are
intimately related to the Comptonisation process, and do not have very large
bulk Lorentz factors, unless the hard X-ray emission is also beamed by the same
factor.
(131kb)

astro-ph/0008448 [abs, src, ps, other] :

Title: The Impact of Adaptive Optics on Star Formation Research

Authors:
M. Feldt,
M. Kasper,
F. Eisenhauer,
S. Hippler

Comments: 10 pages, 8 figures (partially converted to bitmap), to appear in
SPIE Proceedings 4007, Astronomical Telescopes and Instrumentation 2000,
Munich, Germany

In this paper, we discuss the benefits of ground-based, adaptive optics (AO)
aided observations for star formation research. After outlining the general
advantages, we present results obtained during the ALFA science demonstration
programme in 1999. These results underline the absolute necessity of AO
assistance for almost any kind of observations regarding star formation
regions.
(243kb)

astro-ph/0008449 [abs, src, ps, other] :

Title: The complete S5 polar cap sample: en route to phase-delay global
astrometry

Authors:
E. Ros (MPIfR, Bonn, Germany),
J.M. Marcaide (U. Valencia, Spain),
J.C. Guirado (U. Valencia, Spain),
M.A. Perez-Torres (IRA/CNR, Bologna, Italy)

Comments: 2 pages, 1 figure, needs Cambridge University Press style file
(cupconf.sty), Proceedings of the 5th EVN Symposium, Eds. J. Conway, A.
Polatidis, R. Booth, Onsala Observatory, Sweden (June 2000)

We report on the present status of our S5 polar cap phase-connected
astrometry program. We observe 13 radio sources in the northernmost 20deg of
the sky at the wavelengths of 3.6cm and 2cm, and we plan to extend the program
to 0.7cm. We phase-connect jointly all our data successfully. We image the
radio sources and some of them show morphological changes, in which astrometric
registration is needed to determine the kinematics of the source components. We
aim at unprecedented astrometric accuracy and at a check of the jet standard
model at the 5-10 microarcsec/yr level.
(144kb)

astro-ph/0008450 [abs, src, ps, other] :

Title: Mapping the peculiar binary GP Com

Authors:
L. Morales-Rueda,
T. R. Marsh,
R. C. North (Univ. Southampton)

Comments: 6 pages, 3 postscript figures. To appear in Proceedings of
Astro-Tomography Workshop, Brussels, July 2000, Eds. H. Boffin, D. Steeghs,
Springer-Verlag Lecture Notes in Physics

We present high resolution spectra of the AM CVn helium binary GP Com at two
different wavelength ranges. The spectra show the same flaring behaviour
observed in previous UV and optical data. We find that the central spike
contributes to the flare spectra indicating that its origin is probably the
compact object. We also detect that the central spike moves with orbital phase
following an S-wave pattern. The radial velocity semiamplitude of the S-wave is
~10 km/s which indicates its origin is near the centre of mass of the system,
which in this case lies very close to the white dwarf. The Stark effect seems
to affect significantly the central spike of some of the lines suggesting that
it forms in a high electron density region. This again favours the idea that
the central spike originates in the white dwarf. We present Doppler maps
obtained for the emission lines which show three clear emission regions.
(138kb)

astro-ph/0008451 [abs, src, ps, other] :

Title: Massive warm dark matter

Authors:
Steen Hannestad

Comments: 5 pages, Revtex, 1 figure

Many independent high resolution simulations have indicated that the standard
collisionless cold dark matter model does not reproduce the structure of
observed present day galaxies well. Several possible solutions in the form of
modifications to the physics of the dark matter particles have been proposed.
One of the most promising is warm dark matter (WDM), particles with significant
thermal motion in the early universe. It is usually assumed that such particles
are relativistically decoupled particles with a mass of approximately 1 keV.
However, here we have investigated the possibility that much more massive
particles with highly non-thermal spectra could make up warm dark matter.
Several possible production mechanisms are reviewed and the only one found to
be viable is that the WDM is produced by the non-relativistic decay of some
massive species in the early universe. Such very massive warm dark matter could
possibly be detected in direct detection experiments, as opposed to standard
thermal warm dark matter.
(12kb)

astro-ph/0008452 [abs, src, ps, other] :

Title: On the coupling between different species during recombination

Authors:
Steen Hannestad

Comments: 15 pages, 2 figs

Measurements of fluctuations in the Cosmic Microwave Background Radiation
(CMBR) is one of the most promising methods for measuring the fundamental
cosmological parameters. However, in order to infer parameters from precision
measurements it is necessary to calculate the theoretical fluctuation spectrum
to at least the measurement accuracy. Standard treatments assume that
electrons, ions and neutral hydrogen are very tightly coupled during the entire
recombination history, and that the baryon-photon plasma can be treated as a
two-fluid system consisting of baryons and photons interacting via Thomson
scattering. We investigate the validity of this approximation by explicitly
writing down and solving the full set of Boltzmann equations for electrons,
ions, neutral hydrogen and photons. The main correction to the standard
treatment is from including Rayleigh scattering between photons and neutral
hydrogen, a change of less than 0.1% in the CMBR power spectrum. Our conclusion
is thus that the standard treatment of the baryon-photon system is a very good
approximation, better than any possible measurement accuracy.
(34kb)

astro-ph/0008453 [abs, src, ps, other] :

Title: The survival of subhaloes in galaxies and galaxy clusters

Authors:
Eelco van Kampen (IfA, Edinburgh)

Comments: 14 pages, 5 figures (of which one in colour), submitted to MNRAS, ask
evk@roe.ac.uk for a paper copy with the colour figure at high-resolution

This paper discusses physical and numerical disruption processes acting on
subhaloes in galaxy haloes and in galaxy cluster haloes, and compare the
effects of these processes on the subhalo abundance within both types of
haloes. N-body simulations with a resolution high enough not to suffer from
overmerging (subhalo disruption due to numerical processes) show a high
abundance of subhaloes in both galaxies and in galaxy clusters. However,
observations seem to show a high subhalo abundance in galaxy clusters only.
Thus, it appears that too many subhaloes survive in simulated galaxy haloes.
There are five main causes for this apparent galaxy subhalo problem. The most
radical one is a change to a cosmology in which structure formation is not
hierarchical below the galaxy halo mass scale. If this is unacceptable, four
causes remain, of which the most important one appears to be that dynamical
friction is not properly simulated yet, not even for the highest resolution
simulations to date, resulting in an ‘undermerging’ problem. The other causes
are (numerical) overmerging, differences in the timing of halo formation and
merging in hierarchical structure formation, and significant differences is
mass-to-light ratios. The net effect of these four causes is that galaxies have
a relatively low abundance of subhaloes, i.e. dwarfs, while at the same time a
large number of field dwarf galaxies can exist which are dark enough to be
missed observationally.
(309kb)

astro-ph/0008454 [abs, src, ps, other] :

Title: Density, Velocity, and Magnetic Field Structure in Turbulent Molecular
Cloud Models

Authors:
Eve C. Ostriker (1,2),
James M. Stone (1),
Charles F. Gammie (3) ((1) U. Maryland, (2) ITP, (3) U. Illinois)

Comments: 56 pages, 25 figures; Ap.J., accepted

We use 3D numerical MHD simulations to follow the evolution of cold,
turbulent, gaseous systems with parameters representing GMC conditions. We
study three cloud simulations with varying mean magnetic fields, but identical
initial velocity fields. We show that turbulent energy is reduced by a factor
two after 0.4-0.8 flow crossing times (2-4 Myr), and that the magnetically
supercritical cloud models collapse after ~6 Myr, while the subcritical cloud
does not collapse. We compare density, velocity, and magnetic field structure
in three sets of snapshots with matched Mach numbers. The volume and column
densities are both log-normally distributed, with mean volume density a factor
3-6 times the unperturbed value, but mean column density only a factor 1.1-1.4
times the unperturbed value. We use a binning algorithm to investigate the
dependence of kinetic quantities on spatial scale for regions of column density
contrast (ROCs). The average velocity dispersion for the ROCs is only weakly
correlated with scale, similar to the mean size-linewidth relation for clumps
within GMCs. ROCs are often superpositions of spatially unconnected regions
that cannot easily be separated using velocity information; the same difficulty
may affect observed GMC clumps. We analyze magnetic field structure, and show
that in the high density regime, total magnetic field strengths increase with
density with logarithmic slope 1/3 -2/3. Mean line-of-sight magnetic field
strengths vary widely across a projected cloud, and do not correlate with
column density. We compute simulated interstellar polarization maps at varying
orientations, and determine that the Chandrasekhar-Fermi formula multiplied by
a factor ~0.5 yields a good estimate of the plane-of sky magnetic field
strength provided the dispersion in polarization angles is < 25 degrees.
(590kb)

astro-ph/0008455 [abs, src, ps, other] :

Title: Long-term optical and X-ray observations of the old novae DI Lacertae
and V841 Ophiuchi

Authors:
D. W. Hoard,
Paula Szkody,
R. K. Honeycutt,
Jeff Robertson,
Vandana Desai,
T. Hillwig

Comments: 16 pages, 9 figures, 4 tables; accepted by PASP on 28 August 2000 for
the December 2000 issue

We present an analysis of ground-based optical photometry and spectroscopy,
and Rossi X-ray Timing Explorer X-ray observations of the old novae DI Lacertae
and V841 Ophiuchi. Our optical photometry data (obtained with the automated
photometry telescope RoboScope) comprise an almost decade-long light curve for
each star, while the contemporaneous spectroscopy and X-ray observations
repeatedly sampled each nova during separate intervals of ~45-55 d in length.
The long-term optical light curves of both novae reveal quasiperiodic
variability on typical time scales of ~30-50 d with amplitudes of dV ~ 0.4-0.8
mag. V841 Oph also displays a long-term, sinusoidal modulation of its optical
light on a time scale of 3.5-5 yr. The optical spectra of these novae display
quite different characteristics from each other, with DI Lac showing narrow
Balmer emission cores situated in broad absorption troughs while V841 Oph
exhibits strong single-peaked Balmer, He I and He II emission lines. We find
little change between spectra obtained during different optical brightness
states. The X-ray count rates for both novae were very low (< ~1.5 ct/s) and
there was no reliable correlation between X-ray and optical brightness. The
combined X-ray spectrum of DI Lac is best fit by a bremsstrahlung emission
model (with kT ~ 4 keV and N_H < 1.8×10^22 cm^-3); the X-ray spectrum of V841
Oph is too weak to allow model fitting. We discuss the possible origin of
variability in these old novae in terms of magnetic activity on the secondary
star, dwarf nova type disk instabilities, and the “hibernation” scenario for
cataclysmic variable stars.
(76kb)

astro-ph/0008456 [abs, src, ps, other] :

Title: High redshift CO line emission: perspectives

Authors:
F. Combes (Observatoire de Paris, DEMIRM)

Comments: 8 pages, 3 figures, in “Deep Millimeter Surveys: Implications for
Galaxy Formation and Evolution”, ed. J. Lowenthal and D. Hughes, World
Scientific Publishers

Although about a dozen high redshift (z larger than 2) starburst galaxies
have been recently detected in the CO lines, spectroscopic detections of
molecular gas of very young galaxies are still very difficult. The CO lines are
usually optically thick, which limits greatly the increase of emission with
redshift, as observed for the dust continuum. However the future instruments
(LMT, ALMA, etc..) will allow large progress in this domain, and perspectives
are discussed. Computations are based on a simple extrapolation of what is
known of starbursting galaxies at lower redshift.
(42kb)

astro-ph/0008457 [abs, src, ps, other] :

Title: An unresolved X-ray source inside the supernova remnant RCW 86

Authors:
Jacco Vink (1),
Fabrizio Bocchino (2,3),
Francesco Damiani (3),
Jelle S. Kaastra (4) ((1) AIP, Potsdam, Germany; (2) ESTEC, The Netherlands; (3) Osservatorio Astronomico di Palermo, Italy; (4) SRON, Utrecht, The Netherlands)

Comments: 4 pages, 3 figures. Accepted by Astronomy and Astrophyisics

We report on the discovery of an unresolved X-ray source inside the supernova
remnant G315.4-2.3 (RCW 86). The source is located 7′ to the Southwest of the
geometrical centre and may be close to the actual explosion centre of the
supernova, which makes this a candidate for the stellar remnant associated with
RCW 86. However, the presence of a possible optical counterpart with $V sim
14$ at 3″ from the X-ray position and evidence for long term variability means
that the source is probably an active star. A better X-ray position and better
X-ray spectroscopy along with an identification of the optical source are
needed to exclude the X-ray source as a neutron star candidate.
(67kb)

astro-ph/0008458 [abs, src, ps, other] :

Title: Kinematics of the Parsec-Scale Relativistic Jet in Quasar 3C 279: 1991 –
1997

Authors:
A. E. Wehrle,
B. G. Piner,
S. C. Unwin,
A. C. Zook,
W. Xu,
A. P. Marscher,
H. Terasranta,
E. Valtaoja

Comments: 30 pages, including 10 figures, emulateapj.sty, accepted to The
Astrophysical Journal Supplement

We present results of long-term high-frequency VLBI monitoring of the
relativistic jet in 3C279, consisting of 18 epochs at 22 GHz from 1991 to 1997
and 10 epochs at 43 GHz from 1995 to 1997. Three major results of this study
are: apparent speeds measured for six superluminal components range from 4.8 to
7.5 c (H_{0}=70 km s^{-1} Mpc^{-1}, q_{0}=0.1), variations in the total radio
flux are due primarily to changes in the VLBI core flux, and the uniform-sphere
brightness temperature of the VLBI core is about 1 x 10^{13} K at 22 GHz after
1995, one of the highest direct estimates of a brightness temperature. If the
variability brightness temperature measured for 3C279 by Lahteenmaki & Valtaoja
is an actual value and not a lower limit, then the rest-frame brightness
temperature of 3C279 is quite high and limited by inverse Compton effects
rather than equipartition. The parsec-scale morphology of 3C279 consists of a
bright, compact VLBI core, a jet component (C4) that moved from about 2 mas to
about 3.5 mas from the core during the course of our monitoring, and an inner
jet that extends from the core to a stationary component, C5, at about 1 mas
from the core. Components in the inner jet are relatively short-lived, and fade
by the time they reach about 1 mas from the core. The components have different
speeds and position angles from each other, but these differences do not match
the differences predicted by the precession model of Abraham & Carrara.
Although VLBI components were born about six months prior to each of the two
observed gamma-ray high states, the sparseness of the gamma-ray data prevents a
statistical analysis of possible correlations.
(329kb)

astro-ph/0008459 [abs, src, ps, other] :

Title: Low-luminosity AGN and Normal Galaxies

Authors:
A. Ptak

Comments: 10 pages, 7 figures. Invited review at Bologna X-ray Astronomy 1999.
To appear in Astrophysical Letters and Communications

Low-luminosity AGN (with X-ray luminosity < 1e42 ergs/s) far outnumber
ordinary AGN, and are therefore perhaps more relevant to our understanding of
AGN phenomena and the relationship between AGN and host galaxies. Many normal
galaxies harbor LINER and starburst nuclei, which, together with LLAGN, are a
class of “low-activity” galaxies that have a number of surprisingly similar
X-ray characteristics, despite their heterogenous optical classification. This
strongly supports the hypothesis of an AGN-starburst connection. Further, X-ray
observations of normal galaxies without starburst or AGN-like activity in their
nuclei offer opportunities to study populations of X-ray binaries, HII regions,
and warm or hot ISM under different conditions than is often the case in the
Milky Way. The results of recent X-ray observations of these types of galaxies
are reviewed, and what we hope to learn about both nearby and high redshift
galaxies of each type from observations with forthcoming and planned satellites
is discussed.
(314kb)

astro-ph/0008460 [abs, src, ps, other] :

Title: Embedded Star Clusters: The ISO View

Authors:
Anlaug Amanda Kaas (1,2),
Sylvain Bontemps (3) ((1) ESTEC, The Netherlands, (2) Nordic Optical Telescope, Spain, (3) Observatoire de Bordeaux, France)

Comments: 10 pages, 9 figures, To appear in the proceedings of the conference:
From Darkness to Light, Cargese, April 2000, eds. T. Montmerle and Ph. Andre.
See also this http URL

We summarize the main results of a mid-IR photometric survey with ISO for a
few nearby embedded clusters. The sensitivity and spatial resolution of ISOCAM
provided a wonderful tool for studies of the youngest stellar clusters, which
are still deeply embedded in their parent molecular clouds. Mid-IR photometry
is found to be extremely efficient in finding all the young stellar objects
(YSOs) with IR excesses, i.e. mainly T Tauri stars surrounded by circumstellar
disks and also some Class I sources. The spatial distribution of these sources
is highly clustered and even sub-clustered, with a tendency of stronger
concentration for the younger regions. The improved samples of IR-excess YSOs
appear complete down to 0.03 L_sun for the most nearby clusters. This gives
constraints on the mass functions well into the brown dwarf domain. The first
results show that the mass functions of these clusters are identical to the
field star IMF derived by Kroupa et al. (1993) with a flattening of the mass
function at M_star ~ 0.5 M_sun.
(409kb)

astro-ph/0008461 [abs, src, ps, other] :

Title: Snapshot Identification of Gamma Ray Burst Optical Afterglows

Authors:
James E. Rhoads

Comments: 9 pages; LaTeX manuscript + 5 postscript files comprising 4 figures.
Submitted to The Astrophysical Journal Letters

Gamma ray burst afterglows can be identified in single epoch observations
using three or more optical filters. This method relies on color measurements
to distinguish the power law spectrum of an afterglow from the curved spectra
of stars. Observations in a fourth filter will further distinguish between
afterglows and most galaxies up to redshifts z ~ 1. By allowing faster
identification of optical gamma ray burst afterglows, this new method will
increase the fraction of bursts for which optical spectroscopy and other
narrow-field observations can be obtained. The method is applicable for gamma
ray burst redshifts z < 2.5, after which intergalactic hydrogen absorption
redshifts into the optical bands. Because quasar colors can match those of
afterglows, the maximum error box size where an unambiguous identification can
be expected is set by the flux limit of the afterglow search and the quasar
number-flux relation. For currently typical error boxes (10 — 100 square
arcminutes), little contamination is expected at magnitudes R < 21.5 +- 0.5.
Archival data demonstrates that the afterglow of GRB 000301C could have been
identified using this method. In addition to finding gamma ray burst
counterparts, this method will have applications in “orphan afterglow” searches
used to constrain gamma ray burst collimation.
(50kb)

astro-ph/0008462 [abs, src, ps, other] :

Title: A Probabilistic Quantification of Galaxy Cluster Membership

Authors:
R.J. Brunner,
L.M. Lubin (Department of Astronomy, California Institute of Technology)

Comments: 21 Pages LaTex, 6 Figures, Accepted for publication in the November
issue of A.J

Clusters of galaxies are important laboratories for understanding both galaxy
evolution and constraining cosmological quantities. Any analysis of clusters,
however, is best done when one can reliably determine which galaxies are
members of the cluster. While this would ideally be done spectroscopically, the
difficulty in acquiring a complete sample of spectroscopic redshifts becomes
rather daunting, especially at high redshift where the background contamination
becomes increasingly larger. Traditionally, an alternative approach of applying
a statistical background correction has been utilized, which, while useful in a
global sense, does not provide information for specific galaxies. In this
paper, we develop a more robust technique which uses photometrically estimated
redshifts to determine cluster membership. This technique can either be used as
an improvement over the commonly used statistical correction method or it can
be used to determine cluster candidates on an individual galaxy basis. By
tuning the parameters of our algorithm, we can selectively maximize our
completeness or, alternatively, minimize our contamination. Furthermore, our
technique provides a statistical quantification of both our resulting
completeness and contamination from foreground and background galaxies.
(26kb)

astro-ph/0008463 [abs, src, ps, other] :

Title: Three-Dimensional Modeling of the Deflagration Stage of a Type Ia
Supernova Explosion

Authors:
A.M. Khokhlov (Naval Research Laboratory)

Comments: 15 pages, TeX, 15 figures, submitted to ApJ

The paper describes a physical model and numerical algorithm for modeling
Type Ia supernova (SNIa) explosions in three dimensions and presents first
results of modeling a deflagration explosion in a nonrotating,
Chandrasekhar-mass carbon-oxygen (CO) white dwarf. Simulations show that the
turbulent flame speed grows exponentially, reaches approximately 30% of the
speed of sound, and then declines as the large-scale turbulence is frozen by
expansion. The freezing of turbulent motions appears to be a crucial physical
mechanism regulating the rate of deflagration in SNIa. The energy of the
explosion is comparable to that of a typical SNIa. However, the presence of the
outer layer of unburned CO and the formation of intermediate mass elements and
pockets of unburned CO near the center pose problems for the modeling of SNIa
spectra. Delayed detonation is a way to alleviate these problems and to produce
consistent spectra.
(417kb)

astro-ph/0008464 [abs, src, ps, other] :

Title: Spherically symmetric model atmospheres for low mass pre-Main Sequence
stars with effective temperatures between 2000 and 6800~K

Authors:
France Allard,
Peter H. Hauschildt,
Andreas Schweitzer

Comments: ApJ, in press. Also available at
this ftp URL

We present a grid of spherically symmetric model atmospheres for young pre-MS
stars. This grid spans the parameter range $2000K leq Teff leq 6800K$ and
$2.0 leq logg leq 3.5$ for $M=0.1Msun$, appropriate for low mass stars and
brown dwarfs. A major improvement is the replacement of TiO and water line
lists with the newer line list calculated by the NASA-AMES group, for TiO
(about 175 million lines of 5 isotopes) and for water (about 350 million
lines in 2 isotopes). We provide the model structures, spectra and broad-band
colors in standard filters in electronic form.
(9kb)

astro-ph/0008465 [abs, src, ps, other] :

Title: TiO and H$_2$O absorption lines in cool stellar atmospheres

Authors:
France Allard,
Peter H. Hauschildt,
David Schwenke

Comments: ApJ, in press. Also available at
this ftp URL

We compare the structures of model atmospheres and synthetic spectra
calculated using different line lists for TiO and water vapor. We discuss the
effects of different line list combinations on the model strutures and spectra
for both dwarf and giant stars. It is shown that recent improvements result in
significantly improved spectra, in particular in the optical where TiO bands
are important. The water vapor dominated near-IR region remains problematic as
the current water line lists do not yet completely reproduce the shapes of the
observed spectra. We find that the AMES TiO list provides more opacity in most
bands and that the new, smaller oscillator strengths lead to systematically
cooler temperatures for early type M dwarfs than previous models. These effects
combine and will help to siginificantly improve the fits of models to
observations in the optical as well as result in improved synthetic photometry
of M stars. We show that the Davis 1986 fel-values for the $delta$ and
$varphi$ bands of TiO best reproduce the observed (V-I) color indices.
(17kb)

astro-ph/0008466 [abs, src, ps, other] :

Title: Stars and Planetary Nebulae in the Galactic Bulge

Authors:
F. Cuisinier,
J. Koppen,
A. Acker,
W.J. Maciel

Comments: to be published in the proceedings of the conference “Galaxy Disks
and Disk Galaxies”, Funes J.G. and Corsini E.M. eds., ASP Conf. Series

We compare the populations of Red Giant stars and Planetary Nebulae in the
Galactic Bulge, in the light of recent determinations of their abundances
patterns. We find both populations to be compatible. From the planetary
nebulae, we find evidences that the Bulge did not form stars recently. The
whole abundances pattern remains however puzzling, some elements favoring a
quick evolution of the Galactic Bulge (Mg and Ti), and others a much slower one
(He, O, Si, S, Ar and Ca).
(9kb)

astro-ph/0008467 [abs, src, ps, other] :

Title: Chandra X-ray Observations of Pictor A: High Energy Cosmic Rays in a
Radio Galaxy

Authors:
A. S. Wilson,
A. J. Young,
P. L. Shopbell

Comments: 42 pages, 9 figures (3 JPEG, 6 Postscript), accepted for publication
in the Astrophysical Journal

We report X-ray observations of the nearby, powerful radio galaxy Pictor A
with the Chandra Observatory and optical and near uv observations of its
western radio hot spot with the Hubble Space Telescope. X-ray emission is
detected from the nucleus, a 1.9 arcmin (110 kpc) long jet to the west of the
nucleus, the western radio hot spot some 4.2 arcmin (240 kpc) from the nucleus,
and the eastern radio lobe. The morphology of the western hot spot is
remarkably similar to that seen at radio and optical wavelengths, where the
emission is known to be synchrotron radiation. The X-ray spectrum of the hot
spot is well described by an absorbed power law with photon index Gamma = 2.07
(+/- 0.11). The X-ray jet coincides with a weak radio jet and is laterally
extended by simeq 2.0 arcsec (1.9 kpc). The observed jet is up to simeq 15
times brighter in X-rays than any counter jet, a difference ascribed to
relativistic boosting as the western radio lobe is probably the closer. The
jet’s spectrum is well modelled by an absorbed power law with Gamma = 1.94
(+0.43/-0.49) and poorly fitted by a Raymond-Smith thermal plasma model.

(Abstract truncated).
(279kb)

---

Replacements

---

astro-ph/9909370 [abs, src, ps, other] :

Title: Multiplicity Functions and X-ray emission of Clusters and Groups versus
Galaxies and Quasars

Authors:
P. Valageas,
R. Schaeffer

Comments: 23 pages, final version published in A&A. Improved modeling of the
temperature – X-ray luminosity relation for clusters. More detailed
discussion of the need to properly distinguish galactic halos from simple
“just-virialized” objects in order to draw meaningful conclusions for the
luminosity functions of galaxies and QSOs

Journal-ref: A&A (2000), 359, 821

Note: replaced with revised version Tue, 29 Aug 2000 22:33:03 GMT (104kb)

astro-ph/0001026 [abs, src, ps, other] :

Title: The Central X-Ray Point Source in Cassiopeia A

Authors:
Deepto Chakrabarty (MIT),
Michael J. Pivovaroff (MIT),
Lars E. Hernquist (CfA),
Jeremy S. Heyl (CfA/Caltech),
Ramesh Narayan (CfA)

Comments: 17 pages including 2 figs. To appear in ApJ, Vol. 546 (Jan 10, 2001).
Minor revisions per referee. Pulsation limits revised in light of HRC wiring
problem

Note: replaced with revised version Tue, 29 Aug 2000 22:35:54 GMT (192kb)

astro-ph/0002369 [abs, src, ps, other] :

Title: Breaking the degeneracy of cosmological parameters in galaxy redshift
surveys

Author:
Mikel Susperregi

Comments: 15 pages, 10 figures, uses mn.sty, one section added and minor
revisions, to appear ApJ Jan 01

Note: replaced with revised version Tue, 29 Aug 2000 13:52:01 GMT (632kb)

astro-ph/0003231 [abs, src, ps, other] :

Title: Topological defects: fossils of an anisotropic era?

Authors:
P.P. Avelino (CAUP, Porto),
C.J.A.P. Martins (DAMTP, CMS, Cambridge)

Comments: Fig.2 in jpg format (color version available for authors). Version to
appear in Phys. Rev. D

Note: replaced with revised version Tue, 29 Aug 2000 15:21:59 GMT (148kb)

astro-ph/0005210 [abs, src, ps, other] :

Title: Annihilating Cold Dark Matter

Authors:
M. Kaplinghat,
L. Knox,
M.S. Turner

Comments: Matches version accepted for publication in Physical Review Letters.
Included the effect of adiabatic expansion. Some changes in the abstract.
Conclusions unchanged

Note: replaced with revised version Tue, 29 Aug 2000 19:52:09 GMT (19kb)

astro-ph/0006027 [abs, src, ps, other] :

Title: High Energy Neutrinos from Gamma Ray Bursts: Event Rates in Neutrino
Telescopes

Authors:
J. Alvarez-Muniz,
F. Halzen,
D. W. Hooper

Comments: 19 pages, Latex2.09, uses epsfig.sty, 7 postscript figures. Version
to be published in Phys. Rev. D

Note: replaced with revised version Tue, 29 Aug 2000 18:54:36 GMT (114kb)

For subscribe options to combined physics archives,

e-mail To: physics@arXiv.org, Subject: subscribe

—————————————————————————–

For help on viewing and making submissions, see this http URL

For a list of archive mirror sites, see this http URL

—————————————————————————–

Third-party submissions cause excessive problems.

Author self-submissions are exceedingly preferred. Format if by e-mail:

\

Title: Recent Seminal Results

Authors: Author One and Author Two

Comments: 3 pages, latex, no figures, mpeg simulations available at

  ftp://myhost.mydomain/path/file.mpg or at http://myhost.mydomain/RSR/file.mpg

Report-no: EFI-2000-26

\

text of abstract (you do not need  Abstract: …)

\

body of paper (as mime attachments, uuencoded .tar.gz .zip, etc.)