13,613 research outputs found
H_2 and CO Emission from Disks around T Tauri and Herbig Ae Pre-Main-Sequence Stars and from Debris Disks around Young Stars: Warm and Cold Circumstellar Gas
We present ISO Short-Wavelength Spectrometer observations of H_2 pure-rotational line emission from the disks around low- and intermediate-mass pre-main-sequence stars as well as from young stars thought to be surrounded by debris disks. The pre-main-sequence sources have been selected to be isolated from molecular clouds and to have circumstellar disks revealed by millimeter interferometry. We detect "warm" (T â100-200 K) H_2 gas around many sources, including tentatively the debris-disk objects. The mass of this warm gas ranges from ~ 10^(-4) M_â up to 8 x 10^(-3) and can constitute a nonnegligible fraction of the total disk mass. Complementary single-dish ^(12)CO 3-2/^(13)CO 3-2, and ^(12)CO 6-5 observations have been obtained as well. These transitions probe cooler gas at T â 20-80 K. Most objects show a double-peaked CO emission profile characteristic of a disk in Keplerian rotation, consistent with interferometer data on the lower J lines. The ratios of the ^(12)CO 3-2/^(13)CO 3-2 integrated fluxes indicate that ^(12)CO 3-2 is optically thick but that ^(13)CO 3-2 is optically thin or at most moderately thick. The ^(13)CO 3-2 lines have been used to estimate the cold gas mass. If a H_2/CO conversion factor of 1 x 10^(-4) is adopted, the derived cold gas masses are factors of 10-200 lower than those deduced from 1.3 millimeter dust emission assuming a gas/dust ratio of 100, in accordance with previous studies. These findings confirm that CO is not a good tracer of the total gas content in disks since it can be photodissociated in the outer layers and frozen onto grains in the cold dense part of disks, but that it is a robust tracer of the disk velocity field. In contrast, H_2 can shield itself from photodissociation even in low-mass "optically thin" debris disks and can therefore survive longer. The warm gas is typically 1%-10% of the total mass deduced from millimeter continuum emission, but it can increase up to 100% or more for the debris-disk objects. Thus, residual molecular gas may persist into the debris-disk phase. No significant evolution in the H_2 CO, or dust masses is found for stars with ages in the range of 10^6-10^7 yr, although a decrease is found for the older debris-disk star ÎČ Pictoris. The large amount of warm gas derived from H_2 raises the question of the heating mechanism(s). Radiation from the central star as well as the general interstellar radiation field heat an extended surface layer of the disk, but existing models fail to explain the amount of warm gas quantitatively. The existence of a gap in the disk can increase the area of material influenced by radiation. Prospects for future observations with ground- and space-borne observations are discussed
Far-Ultraviolet Radiation from Elliptical Galaxies
Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in
elliptical galaxies and early-type spiral bulges. It is the most variable
photometric feature associated with old stellar populations. Recent
observational and theoretical evidence shows that it is produced mainly by
low-mass, small-envelope, helium-burning stars in extreme horizontal branch and
subsequent phases of evolution. These are probably descendents of the dominant,
metal rich population of the galaxies. Their lifetime UV outputs are remarkably
sensitive to their physical properties and hence to the age and the helium and
metal abundances of their parents. UV spectra are therefore exceptionally
promising diagnostics of old stellar populations, although their calibration
requires a much improved understanding of giant branch mass loss, helium
enrichment, and atmospheric diffusion.Comment: 46 pages; includes LaTeX text file, 9 PS figures, 1 JPG figure, 2
style files. Full resolution figures and PS version available at
http://www.astro.virginia.edu/~rwo/araa99/. Article to appear in Annual
Reviews of Astronomy & Astrophysics, 199
UV Properties of Galactic Globular Clusters with GALEX I. The Color-Magnitude Diagrams
We present GALEX data for 44 Galactic globular clusters obtained during 3
GALEX observing cycles between 2004 and 2008. This is the largest homogeneous
data set on the UV photometric properties of Galactic globular clusters ever
collected. The sample selection and photometric analysis are discussed, and
color-magnitude diagrams are presented. The blue and intermediate-blue
horizontal branch is the dominant feature of the UV color-magnitude diagrams of
old Galactic globular clusters. Our sample is large enough to display the
remarkable variety of horizontal branch shapes found in old stellar
populations. Other stellar types that are obviously detected are blue
stragglers and post core-He burning stars. The main features of UV
color-magnitude diagrams of Galactic globular clusters are briefly discussed.
We establish the locus of post-core He burning stars in the UV color-magnitude
diagram and present a catalog of candidate AGB-manqu \'e, post early-AGB, and
post-AGB stars within our cluster sample.Comment: Accepted for publication by The Astronomical Journal. 46 pages,
including 21 Figures and 3 tables. All data will be made publicly available
by the time the article is published. In the meantime, please contact the
authors for data requests. Revised version fixed error with figure numbers
and caption
The Formation and Gravitational-Wave Detection of Massive Stellar Black-Hole Binaries
If binaries consisting of two 100 Msun black holes exist they would serve as
extraordinarily powerful gravitational-wave sources, detectable to redshifts of
z=2 with the advanced LIGO/Virgo ground-based detectors. Large uncertainties
about the evolution of massive stars preclude definitive rate predictions for
mergers of these massive black holes. We show that rates as high as hundreds of
detections per year, or as low as no detections whatsoever, are both possible.
It was thought that the only way to produce these massive binaries was via
dynamical interactions in dense stellar systems. This view has been challenged
by the recent discovery of several stars with mass above 150 Msun in the R136
region of the Large Magellanic Cloud. Current models predict that when stars of
this mass leave the main sequence, their expansion is insufficient to allow
common envelope evolution to efficiently reduce the orbital separation. The
resulting black-hole--black-hole binary remains too wide to be able to coalesce
within a Hubble time. If this assessment is correct, isolated very massive
binaries do not evolve to be gravitational-wave sources. However, other
formation channels exist. For example, the high multiplicity of massive stars,
and their common formation in relatively dense stellar associations, opens up
dynamical channels for massive black hole mergers (e.g., via Kozai cycles or
repeated binary-single interactions). We identify key physical factors that
shape the population of very massive black-hole--black-hole binaries. Advanced
gravitational-wave detectors will provide important constraints on the
formation and evolution of very massive stars.Comment: ApJ accepted, extended description of modelin
Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO
Gamma-ray bursts are believed to originate in core-collapse of massive stars.
This produces an active nucleus containing a rapidly rotating Kerr black hole
surrounded by a uniformly magnetized torus represented by two counter-oriented
current rings. We quantify black hole spin-interactions with the torus and
charged particles along open magnetic flux-tubes subtended by the event
horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of
frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with
GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii)
aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich
et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al.
2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating
LIGO/Virgo detectors enables searches for nearby events and their spectral
closure density 6e-9 around 250Hz in the stochastic background radiation in
gravitational waves. At current sensitivity, LIGO-Hanford may place an upper
bound around 150MSolar in GRB030329. Detection of Egw thus provides a method
for identifying Kerr black holes by calorimetry.Comment: to appear in PRD, 49
On the possible sources of gravitational wave bursts detectable today
We discuss the possibility that galactic gravitational wave sources might
give burst signals at a rate of several events per year, detectable by
state-of-the-art detectors. We are stimulated by the results of the data
collected by the EXPLORER and NAUTILUS bar detectors in the 2001 run, which
suggest an excess of coincidences between the two detectors, when the resonant
bars are orthogonal to the galactic plane. Signals due to the coalescence of
galactic compact binaries fulfill the energy requirements but are problematic
for lack of known candidates with the necessary merging rate. We examine the
limits imposed by galactic dynamics on the mass loss of the Galaxy due to GW
emission, and we use them to put constraints also on the GW radiation from
exotic objects, like binaries made of primordial black holes. We discuss the
possibility that the events are due to GW bursts coming repeatedly from a
single or a few compact sources. We examine different possible realizations of
this idea, such as accreting neutron stars, strange quark stars, and the highly
magnetized neutron stars (``magnetars'') introduced to explain Soft Gamma
Repeaters. Various possibilities are excluded or appear very unlikely, while
others at present cannot be excluded.Comment: 24 pages, 20 figure
Gamma-Ray Bursts: Progress, Problems & Prospects
The cosmological gamma-ray burst (GRB) phenomenon is reviewed. The broad
observational facts and empirical phenomenological relations of the GRB prompt
emission and afterglow are outlined. A well-tested, successful fireball shock
model is introduced in a pedagogical manner. Several important uncertainties in
the current understanding of the phenomenon are reviewed, and prospects of how
future experiments and extensive observational and theoretical efforts may
address these problems are discussed.Comment: 86 pages, 17 figures, 566 references, an invited review for
International Journal of Modern Physics A, in pres
Binary compact object coalescence rates: The role of elliptical galaxies
We estimate binary compact object merger detection rates for LIGO, including
the binaries formed in ellipticals long ago. Specifically, we convolve hundreds
of model realizations of elliptical- and spiral-galaxy population syntheses
with a model for elliptical- and spiral-galaxy star formation history as a
function of redshift. Our results favor local merger rate densities of 4\times
10^{-3} {Mpc}^{-3}{Myr}^{-1} for binary black holes (BH), 3\times 10^{-2}
{Mpc}^{-3}{Myr}^{-1} for binary neutron stars (NS), and 10^{-2}
{Mpc}^{-3}{Myr}^{-1} for BH-NS binaries. Mergers in elliptical galaxies are a
significant fraction of our total estimate for BH-BH and BH-NS detection rates;
NS-NS detection rates are dominated by the contribution from spiral galaxies.
Using only models that reproduce current observations of Galactic NS-NS
binaries, we find slightly higher rates for NS-NS and largely similar ranges
for BH-NS and BH-BH binaries. Assuming a detection signal-to-noise ratio
threshold of 8 for a single detector (as part of a network), corresponding to
radii \Cv of the effective volume inside of which a single LIGO detector could
observe the inspiral of two 1.4 M_\sun neutron stars of 14 Mpc and 197 Mpc, for
initial and advanced LIGO, we find event rates of any merger type of 2.9*
10^{-2} -- 0.46 and 25-400 per year (at 90% confidence level), respectively. We
also find that the probability P_{detect} of detecting one or more mergers with
this single detector can be approximated by (i) P_{detect}\simeq 0.4+0.5\log
(T/0.01{yr}), assuming \Cv=197 {Mpc} and it operates for T years, for T between
2 days and 0.1 {yr}); or by (ii) P_{detect}\simeq 0.5 + 1.5 \log \Cv/32{Mpc},
for one year of operation and for \Cv between 20 and 70 Mpc. [ABRIDGED]Comment: 22 pages, 11 figures. Accepted by ApJ. v2 adds several figures, an
electronic-only table of all intermediate binary evolution simulations
(tab1.txt here), and new subsections outlining broader significance (e.g.,
5.4; 4.6; 6.1
Open Questions in GRB Physics
Open questions in GRB physics are summarized as of 2011, including
classification, progenitor, central engine, ejecta composition, energy
dissipation and particle acceleration mechanism, radiation mechanism, long term
engine activity, external shock afterglow physics, origin of high energy
emission, and cosmological setting. Prospects of addressing some of these
problems with the upcoming Chinese-French GRB mission, SVOM, are outlined.Comment: 27 pages. To appear in a special issue of Comptes Rendus Physique
"GRB studies in the SVOM era", Eds. F. Daigne, G. Dubu
Resonant-plane locking and spin alignment in stellar-mass black-hole binaries: a diagnostic of compact-binary formation
We study the influence of astrophysical formation scenarios on the
precessional dynamics of spinning black-hole binaries by the time they enter
the observational window of second- and third-generation gravitational-wave
detectors, such as Advanced LIGO/Virgo, LIGO-India, KAGRA and the Einstein
Telescope. Under the plausible assumption that tidal interactions are efficient
at aligning the spins of few-solar mass black-hole progenitors with the orbital
angular momentum, we find that black-hole spins should be expected to
preferentially lie in a plane when they become detectable by gravitational-wave
interferometers. This "resonant plane" is identified by the conditions
\Delta\Phi=0{\deg} or \Delta\Phi=+/-180{\deg}, where \Delta\Phi is the angle
between the components of the black-hole spins in the plane orthogonal to the
orbital angular momentum. If the angles \Delta \Phi can be accurately measured
for a large sample of gravitational-wave detections, their distribution will
constrain models of compact binary formation. In particular, it will tell us
whether tidal interactions are efficient and whether a mechanism such as mass
transfer, stellar winds, or supernovae can induce a mass-ratio reversal (so
that the heavier black hole is produced by the initially lighter stellar
progenitor). Therefore our model offers a concrete observational link between
gravitational-wave measurements and astrophysics. We also hope that it will
stimulate further studies of precessional dynamics, gravitational-wave template
placement and parameter estimation for binaries locked in the resonant plane.Comment: 26 pages, 11 figures, 3 tables, accepted in Physical Review D. 4
movies illustrating resonance locking are available online: for links, see
footnote 8 of the pape
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