Astrophysics astro-ph new abstracts 31 Aug 2000
Astrophysics
astro-ph new abstracts, Thu, 31 Aug 00 04:00:12 GMT
0008468 — 0008501 received
astro-ph/0008468 [abs, src, ps, other] :
Title: Stellar Populations in the Phoenix Dwarf (dIrr/dSph) Galaxy as Observed
by HST/WFPC2
Authors:
Jon A. Holtzman (NMSU),
Graeme H. Smith (UCO/Lick),
Carl Grillmair (SIRTF Science Center)
Comments: Accepted by AJ, 22 pages including 6 figures + 1 figure in JPEG
format
We present HST/WFPC2 photometry of the central regions of the Phoenix dwarf.
Accurate photometry allows us to: 1) confirm the existence of the horizontal
branch previously detected by ground-based observations, and use it to
determine a distance to Phoenix, 2) clearly detect the existence of multiple
ages in the stellar population of Phoenix, 3) determine a mean metallicity of
the old red giant branch stars in Phoenix, and suggest that Phoenix has evolved
chemically over its lifetime, 4) extract a rough star formation history for the
central regions which suggests that Phoenix has been forming stars roughly
continuously over its entire lifetime.
(211kb)
astro-ph/0008469 [abs, src, ps, other] :
Title: Secondary CMB Anisotropies from Cosmological Reionization
Authors:
Nickolay Y. Gnedin,
Andrew H. Jaffe
Comments: submitted to ApJ
We use numerical simulation of cosmological reionization to calculate the
secondary CMB anisotropies in a representative flat low density cosmological
model. We show that the kinetic Sunyaev-Zel’dovich effect (scattering off of
moving electrons in the ionized intergalactic medium) is dominated by the
nonlinear hydrodynamic and gravitational evolution of the density and velocity
fields, rather than the detailed distribution of the ionization fraction
(“patchy reionization”) on all angular scales. Combining our results with the
recent calculation of secondary CMB anisotropies by Springel et al., we are
able to accurately compute the power spectrum of the kinetic SZ effect on
almost all angular scales.
(296kb)
astro-ph/0008470 [abs, src, ps, other] :
Title: The Principal Axis of the Virgo Cluster
Authors:
Michael J. West (1),
John P. Blakeslee (2) ((1)University of Hawaii at Hilo, (2)University of Durham)
Comments: 8 pages, 4 figures, accepted for publication in ApJ Letters
Using accurate distances to individual Virgo cluster galaxies obtained by the
method of Surface Brightness Fluctuations, we show that Virgo’s brightest
ellipticals have a remarkably collinear arrangement in three dimensions. This
axis, which is inclined by 10 to 15 degrees from the line of sight, can be
traced to even larger scales where it appears to join a filamentary bridge of
galaxies connecting Virgo to the rich cluster Abell 1367. The orientations of
individual Virgo ellipticals also show some tendency to be aligned with the
cluster axis, as does the jet of the supergiant elliptical M87. These results
suggest that the formation of the Virgo cluster, and its brightest member
galaxies, have been driven by infall of material along the Virgo-A1367
filament.
(56kb)
astro-ph/0008471 [abs, src, ps, other] :
Title: Physical Diagnostics from a Narrow Fe-Kalpha Emission Line Detected by
Chandra in the Seyfert 1 Galaxy NGC 5548
Authors:
T. Yaqoob (1,2),
I. M. George (1,2),
K. Nandra (1,3),
T. J. Turner (1,2),
P. J. Serlemitsos (1),
Mushotzky,
R. F. (1) ((1) LHEA, NASA/GSFC (2) UMBC (3) USRA)
Comments: 35 pages including 3 figures. Latex file picks up figures. (Accepted
for Publication in the Astrophysical Journal)
We report the detection of a narrow Fe-K emission line in the Seyfert 1
galaxy NGC 5548 with the Chandra High-Energy Transmission Gratings. In the
galaxy frame we measure a center energy of 6.40 pm 0.03 keV, a FWHM of 4515
(+3525,-2645) km/s, and an equivalent width of 133 (+62,-54) eV. The line
energy is consistent with an origin in cold, neutral matter, but ionization
states up to ~Fe XVIII are not ruled out. Assuming Keplerian motion, the
velocity width is consistent with the line being produced in the outer
optical/UV broad-line region (BLR) at about a light-month from the central
X-ray source. We cannot rule out some contribution to the Fe-K line from a
putative, parsec-scale obscuring torus. The continuum intensity during the
Chandra observation was a factor ~2 less than typical historical levels. If the
X-ray continuum was > than a factor 2 higher in the recent past before the
Chandra observation and the narrow Fe-K line intensity had not yet responded,
then the predicted line equivalent width and intensity for an origin in the BLR
is within the 90% measurement errors. Retroactive spectral fitting of archival
ASCA data shows evidence that the narrow Fe-K has a variable intensity. It will
be important to measure the non-disk component of the Fe-K line in other AGN to
obtain a more complete understanding of the total Fe-K line emission.
Measurement of the narrow-line parameters provides powerful diagnostics of
conditions beyond the accretion disk and is necessary to properly deconvolve
the broad, relativistic Fe-K line.
(83kb)
astro-ph/0008472 [abs, src, ps, other] :
Title: ASTRO-F – The next generation of mir-infrared surveys
Authors:
Pearson C.,
Matsuhara H.,
Watarai H.,
Matsumoto T. (Institute of Space and Astronautical Sciences, Japan),
Onaka T. (University of Tokyo)
Comments: 18 pages, 17 figures, submitted to MNRAS
We present basic observational strategies for ASTRO-F (also known as the
Imaging Infra Red Surveyor(IRIS)) to be launched in 2003 by the Japanese
Institute of Space and Astronautical Science (ISAS). We examine 2 survey
scenarios, a deep ~1sq.deg. survey reaching sensitivities an order of magnitude
below than the deepest surveys performed by ISO in the mid-IR, and a shallow
~18sq.deg mid-infrared (7-25um in 6 bands) covering an area greater than the
entire area covered by all ISO mid-IR surveys. Using 2 cosmological models the
number of galaxies predicted for each survey is calculated. The first scenario
uses an enhancement of the pure luminosity evolution model of Pearson &
Rowan-Robinson and new models that incorporate a strongly evolving ULIG
component. For the deep survey, between 20,000-30,000 galaxies should be
detected in the shortest wavebands and ~5000 in the longest (25um) band. The
shallow survey would be expected to detect of the order of 100,000 – 150,000
sources.
(629kb)
astro-ph/0008473 [abs, src, ps, other] :
Title: Neutrino-electron processes in a strongly magnetized thermal plasma
Authors:
Stephen J. Hardy,
Markus H. Thoma
Comments: 17 pages, 8 figures
We present a new method of calculating the rate of neutrino-electron
interactions in a strong magnetic field based on finite temperature field
theory. Using this method, in which the effect of the magnetic field on the
electron states is taken into account exactly, we calculate the rates of all of
the lowest order neutrino-electron interactions in a plasma. As an example of
the use of this technique, we explicitly calculate the rate at which neutrinos
and antineutrinos annihilate in a highly magnetized plasma, and compare that to
the rate in an unmagnetized plasma. The most important channel for energy
deposition is the gyromagnetic absorption of a neutrino-antineutrino pair on an
electron or positron in the plasma ($
uar{
u} e^pmleftrightarrow e^pm$).
Our results show that the rate of annihilation increases with the magnetic
field strength once it reaches a certain critical value, which is dependent on
the incident neutrino energies and the ambient temperature of the plasma. It is
also shown that the annihilation rates are strongly dependent on the angle
between the incident particles and the direction of the magnetic field. If
sufficiently strong fields exist in the regions surrounding the core of a type
II supernovae or in the central engines of gamma ray bursts, these processes
will lead to more efficient plasma heating mechanism than in an unmagnetized
medium, and moreover, one which is intrinsically anisotropic.
(68kb)
astro-ph/0008474 [abs, src, ps, other] :
Title: A cannonball model of gamma-ray bursts: superluminal signatures
Authors:
Arnon Dar,
Alvaro De Rujula
Comments: 8 pages, 4 figs, latex2e
Recent observations suggest that the long-duration gamma ray bursts (GRBs)
and their afterglows are produced by highly relativistic jets emitted in
supernova explosions. We propose that the result of the event is not just a
compact object plus the ejecta: within a day, a fraction of the parent star
falls back to produce a thick accretion disk. The subsequent accretion
generates jets and constitutes the GRB “engine”, as in the observed ejection
of relativistic “cannonballs” of plasma by microquasars and active galactic
nuclei. The GRB is produced as the jetted cannonballs exit the supernova shell
reheated by the collision, re-emitting their own radiation and boosting the
light of the shell. They decelerate by sweeping up interstellar matter, which
is accelerated to cosmic-ray energies and emits synchrotron radiation: the
afterglow. We emphasize here a smoking-gun signature of this model of GRBs: the
superluminal motion of the afterglow, that can be searched for —the sooner
the better— in the particular case of GRB 980425.
(215kb)
astro-ph/0008475 [abs, src, ps, other] :
Title: A toroidal black hole for the AGN phenomenon
Authors:
Fulvio Pompilio (SISSA/ISAS, Italy),
S. M. Harun-or-Rashid (University of Helsinki, Finland),
Matts Roos (University of Helsinki,Finland)
Comments: 7 pages, 2 figures, accepted for pubblication in A&A
A new approach to the study of the AGN phenomenon is proposed, in which the
nucleus activity is related to the metric of the inner massive black hole. The
possibility of a Toroidal Black Hole (TBH), in contrast to the usual Spherical
Black Hole (SBH), is discussed as a powerful tool in understanding AGN related
phenomena, such as the energetics, the production of jets and the acceleration
of particles, the shape of the magnetic field and the lifetime of nucleus
activity.
(16kb)
astro-ph/0008476 [abs, src, ps, other] :
Title: A modification of the Monte Carlo method for simulation of radiative
transfer in molecular clouds
Authors:
Maxim A. Voronkov (Astro Space Center, Moscow, Russia)
Comments: 7 pages, 2 figures, 1 table
Journal-ref: Astronomy Letters, 1999, Vol. 25, p. 149
We propose a method of simulation that is based on the averaging of formal
solutions of the transfer equation by taking the integral by the Monte Carlo
method. This method is used to compute two models, which correspond to the
limiting cases of hot gas and cold background radiation and of hot background
radiation and cold gas, for E-methanol emission from a compact homogeneous
spherical region. We analyse model level populations by using rotational
diagrams in the limiting cases mentioned above. Model optical depths of the
lines with frequencies below 300 GHz up to J=11 inclusive are given.
(20kb)
astro-ph/0008477 [abs, src, ps, other] :
Title: Can a Very Low-Luminosity and Cold White Dwarf be a Self-gravitating
Bose Condensed System
Authors:
Nandini Nag,
Somenath Chakrabarty
Comments: REVTEX file no figure
An entirely new model for the structure as well as for the cooling mechanism
of white dwarfs has been proposed. We have argued that the massive part of the
constituents of white dwarfs- the positively charged ions are boson and under
the extreme physical condition (density and temperature) at the interior, it is
possible to have condensation of charged bose gas. We have tried to establish
that a cold white dwarf is a self gravitating charged bose condensed system.
(7kb)
astro-ph/0008478 [abs, src, ps, other] :
Title: Box- and peanut-shaped bulges: II. NIR observations
Authors:
R. Luetticke,
R.-J. Dettmar,
M. Pohlen (Astronomisches Institut, Ruhr-Universitaet Bochum)
Comments: LaTeX, 14 pages, 17 figures, accepted to be published in A&A
We have observed 60 edge-on galaxies in the NIR in order to study the stellar
distribution in galaxies with box/peanut-shaped bulges. The much smaller amount
of dust extinction at these wavelengths allows us to identify in almost all
target galaxies with box/peanut-shaped bulges an additional thin, central
component in cuts parallel to the major axis. This structure can be identified
with a bar. The length of this structure scaled by the length of the bulge
correlates with the morphologically classified shape of the bulge. This newly
established correlation is therefore mainly interpreted as the projection of
the bar at different aspect angles. Galaxies with peanut bulges have a bar seen
nearly edge-on and the ratio of bar length to thickness, 14 +/- 4, can be
directly measured for the first time. In addition, the correlation of the
boxiness of bulges with the bar strength indicates that the bar characteristic
could partly explain differences in the bulge shape. Furthermore, a new size
relation between the box/peanut structure and the central bulge is found. Our
observations are discussed in comparison to a N-body simulation for barred
galaxies (Pfenniger & Friedli 1991). We conclude that the inner region of
barred disk galaxies are build up by three distinct components: the spheroidal
bulge, a thin bar, and a b/p structure most likely representing the thick part
of the bar.
(354kb)
astro-ph/0008479 [abs, src, ps, other] :
Title: On the occurrence of dwarf nova outbursts in post novae
Authors:
M.R. Schriber,
B.T. Gaensicke,
J.K. Cannizzo
Comments: accepted for publication in Astronomy and Astrophysics
We show that irradiation of the accretion disc by the white dwarf limits the
occurrence of dwarf nova outbursts in post nova accretion discs. After the nova
explosion, the white dwarf has to cool for up to ~100 yr – depending on the
orbital period (i.e., disc size) and the temperature of the white dwarf after
the nova-eruption – before the disc can begin producing dwarf nova outbursts.
During this time the inner disc is maintained in a hot, ionised state. Using
these calculations, we interpret the long-term photometric variability of the
post nova V446 Her (Nova Her 1960) which shows today regular dwarf nova
outbursts. As the white dwarf in V446 Her continues to cool over the next
~10-20 yr, we predict an increase in the amplitude of outbursts and a decrease
of the outburst frequency, because the decreasing irradiation of the accretion
disc should allow an increasing annular extent of the accretion disc to
participate in limit cycle oscillations.
(41kb)
astro-ph/0008480 [abs, src, ps, other] :
Title: Modelling 20 years of synchrotron flaring in the jet of 3C 273
Authors:
M. Turler,
T. J.-L. Courvoisier,
S. Paltani
Comments: 13 pages, 6 figures, to be published in A&A
We present a phenomenological jet model which is able to reproduce well the
observed variations of the submillimetre-to-radio emission of the bright quasar
3C 273 during the last 20 years. It is a generalization of the original shock
model of Marscher & Gear (1985), which is now able to describe an accelerating
or decelerating shock wave, in a curved, non-conical and non-adiabatic jet. The
model defines the properties of a synchrotron outburst which is expected to be
emitted by the jet material in a small region just behind the shock front. By a
proper parameterization of the average outburst’s evolution and of the
peculiarities of individual outbursts, we are able to decompose simultaneously
thirteen long-term light-curves of 3C 273 in a series of seventeen distinct
outbursts. It is the first time that a model is so closely confronted to the
long-term multi-wavelength variability properties of a quasar. The ability of
the model to reproduce the very different shapes of the submillimetre-to-radio
light curves of 3C 273 gives strong support to the shock model of Marscher &
Gear (1985). Indirectly, it also reinforces the idea that the outbursts seen in
the light-curves are physically linked to the distinct features observed to
move along the jet with apparently superluminal velocities.
(159kb)
astro-ph/0008481 [abs, src, ps, other] :
Title: Gravitational Wave Astronomy: in Anticipation of First Sources to be
Detected
Authors:
L.P.Grishchuk,
V.M.Lipunov,
K.A.Postnov,
M.E.Prokhorov,
B.S.Sathyaprakash
Comments: latex, 100 pages, including 20 postscript figures. Accepted for
publication in Physics-Uspekhi
The first generation of long-baseline laser interferometric detectors of
gravitational waves will start collecting data in 2001-2003. We carefully
analyse their planned performance and compare it with the expected strengths of
astrophysical sources. The scientific importance of the anticipated discovery
of various gravitatinal wave signals and the reliability of theoretical
predictions are taken into account in our analysis. We try to be conservative
both in evaluating the theoretical uncertainties about a source and the
prospects of its detection. After having considered many possible sources, we
place our emphasis on (1) inspiraling binaries consisting of stellar mass black
holes and (2) relic gravitational waves. We draw the conclusion that
inspiraling binary black holes are likely to be detected first by the initial
ground-based interferometers. We estimate that the initial interferometers will
see 2-3 events per year from black hole binaries with component masses
10-15M_odot, with a signal-to-noise ratio of around 2-3, in each of a network
of detectors consisting of GEO, VIRGO and the two LIGOs. It appears that other
possible sources, including coalescing neutron stars, are unlikely to be
detected by the initial instruments. We also argue that relic gravitational
waves may be discovered by the space-based interferometers in the frequency
interval 2×10^{-3}-10^{-2} Hz, at the signal-to-noise ratio level around 3.
(637kb)
astro-ph/0008482 [abs, src, ps, other] :
Title: Blue Quasars and Blazar Unification Schemes
Authors:
Markos Georganopoulos
Comments: 11 pages, accepted for publication in ApJL
Blue quasars (BQs) are sources with strong broad emission lines and flat hard
X-ray spectra, properties that resemble classical flat spectrum radio quasars
(FSRQs), and high peak frequencies and steep soft X-ray spectra, properties
that resemble intermediate or high peak frequency BL Lacertae objects(IBLs and
HBLs respectively).
BQs challenge our understanding of blazar properties in terms of a luminosity
sequence, which makes their incorporation into current blazar unification
schemes problematic. In this work we show that this situation can be remedied
if, in addition to the intrinsic luminosity, the orientation of the blazar jet
is explicitly considered. We show, using published data, that the recently
studied BQs are relatively misaligned blazars, and we examine the predicted
aligned population. We examine both possible cases, sources with pure
synchrotron spectra and sources with an optical–UV thermal contribution, for
both constant velocity and accelerating jets. We show that the aligned sources
are similar to FSRQs, and we suggest ways to distinguish between constant
velocity and accelerating flows. We point out that IBLs are more aligned and
less powerful than BQs and we address the very different emission line
properties of these sources which display similar spectral energy
distributions.
(23kb)
astro-ph/0008483 [abs, src, ps, other] :
Title: Cluster galaxies: contribution to the arc statistics
Authors:
Massimo Meneghetti,
Micol Bolzonella,
Matthias Bartelmann,
Lauro Moscardini,
Giuseppe Tormen
Comments: 4 pages, Latex using moriond2000.sty. To be published in Proc. of the
XXXVth Rencontres de Moriond–XXth Moriond Astrophysics Meeting on “L2K
Cosmological Physics with Gravitational Lensing”, J.P. Kneib, Y. Mellier, M.
Moniez, J. Tran Thanh Van eds., Editions Frontieres
We present the results of a set of numerical simulations aiming at evaluating
the effects of cluster galaxies on the arc statistics. At this goal we use nine
different galaxy clusters obtained from N-body simulations. We mimic the
presence of a population of galaxies inside each cluster, trying to reproduce
the observed luminosity function and the spatial distribution. We compare the
statistical distributions of the properties of the gravitational arcs produced
by our clusters with and without galaxies. We find that the cluster galaxies do
not introduce perturbations strong enough to significantly change the number of
arcs and the distributions of the properties of long arcs.
(255kb)
astro-ph/0008484 [abs, src, ps, other] :
Title: Prospects for future far-infrared/submillimeter studies of the
high-redshift Universe
Authors:
Andrew W. Blain (Cambridge)
Comments: 4 pages, no figures, uses newpasp.sty. To appear in Proc. JD9, IAU
Manchester 2000, `Cold Gas and Dust at High Redshift’ ed. D.J. Wilner,
Highlights of Astronomy Vol. 12. PASP
Observations made using COBE, SCUBA, ISO and MAMBO have provided a reasonable
working knowledge of both the intensity of the submm and far-infrared
background radiation and the source counts of luminous high-redshift dusty
galaxies. However, because there are uncertainties in the background intensity
determinations, the samples of detected galaxies are small, and most
importantly, their redshift distributions are very incomplete, details of the
evolution of dusty galaxies remain unresolved. The next steps forward in the
field will be the launches of SIRTF and ASTRO-F, the commissioning of SOFIA and
new, more capable ground-based mm/submm-wave cameras – BOLOCAM, SHARC-II and
SCUBA-II – the use of ultra-long duration balloon experiments, such as BLAST,
the construction of ALMA and the arrival of FIRST, and ultimately the advent of
space-borne far-infrared interferometers, such as SPECS. There are also
exciting prospects for direct mm/submm-wave CO-line redshift surveys using
wide-band spectrographs. Using these new facilities, the number of
high-redshift dusty galaxies known will be increased dramatically. Spectroscopy
using SIRTF, SOFIA and FIRST will probe the astrophysical processes within
these sources in detail, hopefully addressing the open question of the fraction
of the counts and background radiation that is generated by the formation of
high-mass stars and by active galactic nuclei (AGNs). The spatial and spectral
structure of distant dusty galaxies will finally be resolved in detail using
ALMA and SPECS.
(8kb)
astro-ph/0008485 [abs, src, ps, other] :
Title: Isolated neutron stars discovered by ROSAT
Authors:
C. Motch
Comments: 10 pages 2 figures, invited paper at Bologna X-ray Astronomy 1999. To
appear in Astrophysical Letters and Communications
ROSAT has discovered a new group of isolated neutron stars characterized by
soft black-body like spectra (kT ~ 50-120 eV), apparent absence of radio
emission and no association with supernovae remnants. So far only six such
sources are known. A small fraction of these stars exhibit X-ray pulsations
with relatively long periods of the order of 10 sec. Two very different
mechanisms may be envisaged to explain their properties. The neutron stars may
be old and re-heated by accretion from the ISM in which case their population
properties could provide information on past stellar formation and secular
magnetic field decay. Alternatively, this group may at least partly be made of
relatively young cooling neutron stars possibly descendant from magnetars. We
review the last observational results and show how they can shed light on the
evolutionary path of these new objects within the whole class of isolated
neutron stars.
(69kb)
astro-ph/0008486 [abs, src, ps, other] :
Title: Multiwaveband studies of the hard ROSAT SMC transient 1WGA J0053.8-7226:
a new X-ray pulsar
Authors:
D.A.H. Buckley (SAAO),
M.J. Coe (Southampton),
J.B. Stephens (Southampton),
K. van der Heyden (SAAO),
L. Angelini (NASA/GSFC),
N. White (NASA/GSFC),
P. Giommi (NASA/GSFC)
Comments: Accepted for publication in MNRAS
We report on two optical candidates for the counterpart to an X-ray source in
the SMC, 1WGA J0053.8-7226, identified as a serendipitous X-ray source from the
ROSAT PSPC archive, and also observed by the Einstein IPC. Its X-ray
properties, namely the hard X-ray spectrum, flux variability and column density
indicate a hard, transient source, with a luminosity of 3.8 x 10E35 ergs/sec.
XTE and ASCA observations have confirmed the source to be an X-ray pulsar, with
a 92 s spin period. Our optical observations reveal two possible candidates
within the error circle. Both exhibit strong H alpha and weaker H beta
emission. The optical colours indicate that both objects are Be-type stars. The
Be nature of the stars implies that the counterpart is most likely a Be/X-ray
binary system. Subsequent IR photometry JHK of one of the objects shows the
source varies by at least 0.5 mag, while the (J-K) measured nearly
simultaneously with the UBVRI and spectroscopic observations indicate an
IR-excess of approximately 0.3 magnitudes.
(287kb)
astro-ph/0008487 [abs, src, ps, other] :
Title: Diffraction-limited near-IR imaging at Keck reveals asymmetric,
time-variable nebula around carbon star CIT 6
Authors:
J. D. Monnier (1),
P. G. Tuthill (2),
W. C. Danchi (3) ((1) Harvard-Smithsonian Center for Astrophysics (2) University of Sydney (3) NASA-GSFC)
Comments: 18 pages, 5 figures (one in color), to appear in the Astrophysical
Journal
We present multi-epoch, diffraction-limited images of the nebula around the
carbon star CIT 6 at 2.2 microns and 3.1 microns from aperture masking on the
Keck-I telescope. The near-IR nebula is resolved into two main components, an
elongated, bright feature showing time-variable asymmetry and a fainter
component about 60 milliarcseconds away with a cooler color temperature. These
images were precisely registered (~35 milliarcseconds) with respect to recent
visible images from the Hubble Space Telescope (Trammell et al. 2000), which
showed a bipolar structure in scattered light. The dominant near-IR feature is
associated with the northern lobe of this scattering nebula, and the
multi-wavelength dataset can be understood in terms of a bipolar dust shell
around CIT 6. Variability of the near-IR morphology is qualitatively consistent
with previously observed changes in red polarization, caused by varying
illumination geometry due to non-uniform dust production. The blue emission
morphology and polarization properties can not be explained by the above model
alone, but require the presence of a wide binary companion in the vicinity of
the southern polar lobe. The physical mechanisms responsible for the breaking
of spherical symmetry around extreme carbon stars, such as CIT 6 and IRC+10216,
remain uncertain.
(154kb)
astro-ph/0008488 [abs, src, ps, other] :
Title: Raman-Scattering Wings of H alpha in Symbiotic Stars
Authors:
Hee-Won Lee
Comments: 6 pages including 2 figures, to appear in the Astrophysical Journal
Letters
Nussbaumer et al. (1989) proposed that broad H$alpha$ wings can be formed
through Raman scattering of Ly$eta$ photons, and in this Letter we argue that
the H$alpha$ wings prevalently seen in symbiotic stars may be indeed formed in
this way. Assuming a flat incident UV radiation around Ly$eta$, we generate
template wing profiles around H$alpha$ that are formed through Raman
scattering in a plane-parallel H~I region. We perform profile fitting analyses
to show that the template wing profiles are in excellent agreement with the
observed ones that are provided by van Winckel et al. (1993) and Ivison et al.
(1994). The wing flux is determined by the scattering H~I column density and
the incident Ly$eta$ flux strength and profile. From our profile analysis it
is proposed that the Raman scattering component may be identified with the
neutral envelope with a column density ranging $10^{18-20} {
m cm^{-2}}$ that
surrounds the binary system. We briefly discuss alternative candidates for the
wing formation mechanism and observational implications of Raman scattering in
symbiotic stars and in other astronomical objects including planetary nebulae,
post AGB stars and active galactic nuclei.
(102kb)
astro-ph/0008489 [abs, src, ps, other] :
Title: The Theory of Steady State Super-Eddington Winds and its Application to
Novae
Authors:
Nir J. Shaviv
Comments: 13 Pages, 8 figures, submitted to MNRAS (uses mn.sty)
We present a model for steady state winds of systems with super-Eddington
luminosities. These radiatively driven winds are expected to be optically thick
and clumpy as they arise from an instability driven porous atmosphere. The
model is then applied to derive the mass loss observed in bright classical
novae. The main results are:
1) A general relation between the mass loss rate and the total luminosity in
super-Eddington systems.
2) A quantitative agreement between the observed luminosity evolution which
is used to predict both the mass loss and temperature evolution, and their
observations.
3) An agreement between the predicted average integrated mass loss of novae
as a function of WD mass and its observations.
4) A natural explanation for the `transition phase’ of novae.
5) Agreement with eta Carinae which was used to double check the theory. The
prediction for the mass shed in the star’s great eruption agrees with
observations to within the measurement error.
(97kb)
astro-ph/0008490 [abs, src, ps, other] :
Title: How many young star clusters exist in the Galactic center?
Authors:
Simon Portegies Zwart,
Junichiro Makino,
Stephen McMillan,
Piet Hut
Comments: 16 pages, submitted to ApJ Letters
We study the evolution and observability of young compact star clusters
within about 200pc of the Galactic center. Calculations are performed using
direct N-body integration on the GRAPE-4, including the effects of both stellar
and binary evolution and the external influence of the Galaxy. The results of
these detailed calculations are used to calibrate a simplified model applicable
over a wider range of cluster initial conditions. We find that clusters within
200 pc from the Galactic center dissolve within about 70 Myr. However, their
projected densities drop below the background density in the direction of the
Galactic center within 20 Myr, effectively making these clusters undetectable
after that time. Clusters farther from the Galactic center but at the same
projected distance are more strongly affected by this selection effect, and may
go undetected for their entire lifetimes. Based on these findings, we conclude
that the region within 200 pc of the Galactic center could easily harbor some
50 clusters with properties similar to those of the Arches or the Quintuplet
systems.
(67kb)
astro-ph/0008491 [abs, src, ps, other] :
Title: Cosmic Rays and the Structure of Space-Time
Authors:
Aurelio F. Grillo,
Roberto Aloisio
Comments: 8 pages, 1 figure, talk presented at the Vulcano Workshop 2000
Even a fundamental symmetry like Lorentz Invariance is an experimental fact
and must be experimentally verified. We show that the study of the interactions
of Cosmic Rays with universal diffuse background radiation can provide very
stringent tests of this symmetry. The interactions we consider are the ones
characterized by well defined energy thresholds whose energy position can be
predicted on the basis of Special Relativity. We argue that the experimental
verification of these thresholds can address the physics of supra-Planckian
scales.
(14kb)
astro-ph/0008492 [abs, src, ps, other] :
Title: Far-Ultraviolet Spectra of AGN: First Results from FUSE
Authors:
Gerard A. Kriss
Comments: 17 pages, 11 figures, to appear in the proceedings of the GSFC
workshop on “Probing the Physics of AGN by Multiwavelength Monitoring”
Composite spectra derived from HST observations of moderate redshift (z~1)
quasars show that the peak of the spectral energy distribution is at ~1000 A,
and that the ionizing continuum falls off smoothly to shorter wavelengths as
$f_
u sim
u^{-1.8}$. HUT and FUSE observations of nearby AGN (e.g., 3C 273)
show similar results. FUSE spectra reveal a wealth of detail in the 912-1200 A
spectral range, including multi-component intrinsic absorption in the O VI
doublet and Lyb, and surprisingly strong narrow-line emission from O VI
1032,1038. Monitoring observations of AGN below 1200 A are feasible, and such
experiments could help to unravel the structure of intrinsic absorbers in AGN
as well as the highest ionization portions of the broad-line region.
(294kb)
astro-ph/0008493 [abs, src, ps, other] :
Title: How young are early-type cluster galaxies ? Quantifying the young
stellar component in a rich cluster at z=0.41
Authors:
I. Ferreras,
J. Silk (Oxford)
Comments: Accepted for publication in ApJ Letters. Uses emulateapj.sty. 5 pages
with 3 embedded EPS figures
We present a new method of quantifying the mass fraction of young stars in
galaxies by analyzing near-ultraviolet (NUV)-optical colors. We focus our
attention on early-type cluster galaxies, whose star formation history is at
present undetermined. Rest-frame NUV (F300W) and optical (F702W) images of
cluster Abell 851 (z=0.41) using HST/WFPC2 allow us to determine a NUV-optical
color-magnitude relation, whose slope is incompatible with a monolithic
scenario for star formation at high redshift. A degeneracy between a young
stellar component and its fractional mass contribution to the galaxy is found,
and a photometric analysis comparing the data with the predictions for a simple
two-stage star formation history is presented. The analysis shows that some of
the early-type galaxies may have fractions higher than 10% of the total mass
content in stars formed at z~0.5. An increased scatter is found in the
color-magnitude relation at the faint end, resulting in a significant fraction
of faint blue early-type systems. This would imply that less massive galaxies
undergo more recent episodes of star formation, and this can be explained in
terms of a positive correlation between star formation efficiency and
luminosity.
(57kb)
astro-ph/0008494 [abs, src, ps, other] :
Title: Neutrinos and the Supernova Origin of the Elements
Authors:
Yong-Zhong Qian (UMN)
Comments: 6 pages including 2 figures, to appear in the Proceedings of Neutrino
2000
Intense fluxes of neutrinos are emitted by the hot neutron star produced in a
supernova. The electron neutrino and antineutrino capture reactions on neutrons
and protons, respectively, provide heating to drive a wind from the hot neutron
star. The same reactions also determine the neutron-richness of the wind
material. Nucleosynthesis via rapid neutron capture, the r-process, may occur
in the wind material as it expands away from the neutron star. The
neutron-richness of the wind material, and hence, the r-process nucleosynthesis
therein, are sensitive to mixing between the muon (or tauon)
neutrino/antineutrino and the electron (or sterile) neutrino/antineutrino.
Indirect arguments and direct tests for the supernova origin of the r-process
elements are discussed with a goal to establish supernova r-process
nucleosynthesis as an important probe for neutrino mixing.
(22kb)
astro-ph/0008495 [abs, src, ps, other] :
Title: What Is The BBN Prediction for the Baryon Density and How Reliable Is
It?
Authors:
Scott Burles,
Kenneth M. Nollett,
Michael S. Turner
Comments: 6 pages, 3 figures, submitted to Phys. Rev. D
Together, the standard theory of big-bang nucleosynthesis (BBN) and the
primeval deuterium abundance very precisely peg the baryon density, Omega_B h^2
= 0.0190pm 0.0018 (95% cl). The uncertainty is due to that in the deuterium
abundance and input nuclear data, in the ratio of about 2 to 1. We discuss
critically the reliability of the BBN baryon density, and conclude that within
the standard cosmology and standard theory of BBN a baryon density Omega_B h^2
= 0.030 (the central value implied by recent CMB anisotropy measurements)
simply cannot be accommodated.
(37kb)
astro-ph/0008496 [abs, src, ps, other] :
Title: A moving cold front in the intergalactic medium of A3667
Authors:
A. Vikhlinin,
M. Markevitch,
S. S. Murray
Comments: Submitted to ApJ. 9 pages with embedded color figures, uses
emulateapj5. Postscript with higher quality figures is available at
this http URL
We present results from a Chandra observation of the central region of the
galaxy cluster A3667, with emphasis on the prominent sharp X-ray brightness
edge spanning 0.5 Mpc near the cluster core. Our temperature map shows
large-scale nonuniformities characteristic of the ongoing merger, in agreement
with earlier ASCA results. The brightness edge turns out to be a boundary of a
large cool gas cloud moving through the hot ambient gas, very similar to the
“cold fronts” discovered by Chandra in A2142. The higher quality of the A3667
data allows the direct determination of the cloud velocity. At the leading edge
of the cloud, the gas density abruptly increases by a factor of 3.9+-0.8, while
the temperature decreases by a factor of 1.9+-0.2 (from 7.7 keV to 4.1 keV).
The ratio of the gas pressures inside and outside the front shows that the
cloud moves through the ambient gas at near-sonic velocity, M=1+-0.2 or
v=1400+-300 km/s. In front of the cloud, we observe the compression of the
ambient gas with an amplitude expected for such a velocity. A smaller surface
brightness discontinuity is observed further ahead, ~350 kpc in front of the
cloud. We suggest that it corresponds to a weak bow shock, implying that the
cloud velocity may be slightly supersonic. Given all the evidence, the cold
front appears to delineate the remnant of a cool subcluster that recently has
merged with A3667. The cold front is remarkably sharp. The upper limit on its
width, 3.5 arcsec or 5 kpc, is several times smaller than the Coulomb mean free
path. This is a direct observation of suppression of the transport processes in
the intergalactic medium, most likely by magnetic fields.
(503kb)
astro-ph/0008497 [abs, src, ps, other] :
Title: Dynamics of Black-Hole Nuclei
Authors:
D. Merritt,
F. Cruz,
M. Milosavljevic (Rutgers University)
Comments: 12 pages, 5 figures, uses newpasp.sty, Invited review to appear in
“Dynamics of Star Clusters and the Milky Way,” eds. S. Deiters et al
Space-based observations are beginning to yield detailed information about
the stellar kinematics at the very centers of galaxies, within the sphere of
gravitational influence of the black hole. The structure and dynamics of these
regions is probably determined in part by the infall and coalescence of black
holes during galaxy mergers. A goal of N-body simulations is to reproduce the
kinematics near the black holes as well as the relations that exist between the
nuclear and global properties of galaxies. However, the problem is
computationally difficult due to the wide range of length and time scales, and
no single N-body code can efficiently follow the evolution from kiloparsec to
sub-parsec scales. We review existing N-body work on this problem and present
the first, fully self-consistent merger simulations of galaxies containing
dense stellar cusps and black holes.
(269kb)
astro-ph/0008498 [abs, src, ps, other] :
Title: Scattering of energetic particles by anisotropic magnetohydrodynamic
turbulence with a Goldreich-Sridhar power spectrum in the quasilinear
approximation
Authors:
Benjamin D. G. Chandran
Comments: accepted for publication in Physical Review Letters
Scattering rates for a Goldreich-Sridhar (GS) spectrum of anisotropic,
incompressible, magnetohydrodynamic turbulence are calculated in the
quasilinear approximation. Because the small-scale fluctuations are constrained
to have wave vectors nearly perpendicular to the background magnetic field,
scattering is too weak to provide either the mean free paths commonly used in
Galactic cosmic-ray propagation models or the mean free paths required for
acceleration of cosmic rays at quasi-parallel shocks. Where strong pitch-angle
scattering occurs, it is due to fluctuations not described by the GS spectrum,
such as fluctuations generated by streaming cosmic rays.
(16kb)
astro-ph/0008499 [abs, src, ps, other] :
Title: Chandra estimate of the magnetic field strength near the cold front in
A3667
Authors:
A. Vikhlinin,
M. Markevitch,
S. S. Murray
Comments: Submitted to ApJ Letters. 4 pages with embedded color figures, uses
emulateapj5. Postscript with higher quality figures is available at
this http URL
We use the Chandra observation of the cold front in the intracluster gas of
A3667 to estimate the magnetic field strength near the front. The front is seen
in the Chandra data as a sharp discontinuity in the gas density which
delineates a large body of dense cool gas moving with the near-sonic velocity
through the less dense, hotter gas. Without a magnetic field, the front should
be quickly disturbed by the Kelvin-Helmholtz instability arising from
tangential motion of gas layers. However, Chandra image shows that the front is
stable within a +-30deg sector in the direction of the cloud motion, beyond
which it gradually disappears. We suggest that the Kelvin-Helmholtz instability
within the +-30deg sector is suppressed by surface tension of the magnetic
field whose field lines are parallel to the front. The required field strength
is B ~ 10 muG. Magnetic field near the front is expected to be stronger and
have very different structure compared to the bulk of the intergalactic medium,
because the field lines are stretched by the tangential gas motions. Such a
magnetic configuration, once formed, would effectively stop the plasma
diffusion and heat conduction across the front, and may inhibit gas mixing
during the subcluster merger. We note that even the increased magnetic field
near the front contributes only 10-20% to the total gas pressure, and therefore
magnetic pressure is unimportant for hydrostatic cluster mass estimates.
(221kb)
astro-ph/0008500 [abs, src, ps, other] :
Title: omega Centauri – a former nucleus of a dissolved dwarf galaxy? New
evidence from Stroemgren photometry
Authors:
Michael Hilker (1),
Tom Richtler (2) ((1) P. Univ. Catolica, Santiago, Chile, (2) Univ. de Concepcion, Chile)
Comments: 16 pages, 15 figures, accepted for publication in Astronomy &
Astrophysics
CCD vby Stroemgren photometry of a statistically complete sample of red
giants and stars in the main sequence turn-off region in omega Centauri has
been used to analyse the apparently complex star formation history of this
cluster. From the location of stars in the (b-y),m_1 diagram metallicities have
been determined. These have been used to estimate ages of different
sub-populations in the color-magnitude diagram. The metal-poor population
around -1.7 dex is the oldest population found. More metal-rich stars between
[Fe/H]=-1.5 and -1.0 dex tend to be 1-3 Gyr younger. These stars are more
concentrated towards the center than the metal-poor ones. The most-metal rich
stars (-0.7 dex) might be up to 6 Gyr younger than the oldest population. They
are asymmetrically distributed around the center with an excess of stars
towards the South. We argue that the Stroemgren metallicity in terms of element
abundances has another meaning than in other globular clusters. From a
comparison with spectroscopic element abundances, we find the best correlation
with the sum C+N. The high Stroemgren metallicities, if interpreted by strong
CN-bands, result from progressively higher N and perhaps C abundances in
comparison to iron. We see an enrichment already among the metal-poor
population, which is difficult to explain by self-enrichment alone. An
attractive speculation is that omega Cen was the nucleus of a dwarf galaxy. We
propose a scenario in which omega Cen experienced mass inflow over a long
period of time, until the gas content of its host galaxy was so low that star
formation in omega Cen stopped, or alternatively the gas was stripped off
during its infall in the Milky Way potential. A mass inflow occuringcn a clumpy
and discontinuous manner could explain several of our findings.
(430kb)
astro-ph/0008501 [abs, src, ps, other] :
Title: The Effects of Rotation on the Evolution of Rising Omega-loops in a
Stratified Model Convection Zone
Authors:
W. P. Abbett,
G. H. Fisher,
Y. Fan
Comments: 25 pages, 8 figures, ApJ in press
We present three-dimensional MHD simulations of buoyant magnetic flux tubes
that rise through a stratified model convection zone in the presence of solar
rotation. The equations of MHD are solved in the anelastic approximation, and
the results are used to determine the effects of solar rotation on the dynamic
evolution an Omega-loop. We find that the Coriolis force significantly
suppresses the degree of fragmentation at the apex of the loop during its
ascent toward the photosphere. If the initial axial field strength of the tube
is reduced, then, in the absence of forces due to convective motions, the
degree of apex fragmentation is also reduced. We show that the Coriolis force
slows the rise of the tube, and induces a retrograde flow in both the
magnetized and unmagnetized plasma of an emerging active region.
Observationally, we predict that this flow will appear to originate at the
leading polarity, and will terminate at the trailing polarity.
(299kb)
Replacements
astro-ph/0005543 [abs, src,
