2,054 research outputs found
A star disrupted by a stellar black hole as the origin of the cloud falling toward the Galactic center
We propose that the cloud moving on a highly eccentric orbit near the central
black hole in our Galaxy, reported by Gillessen et al., is formed by a
photoevaporation wind originating in a disk around a star that is tidally
perturbed and shocked at every peribothron passage. The disk is proposed to
have formed when a stellar black hole flew by the star, tidally disrupted its
envelope, and placed the star on its present orbit with some of the tidal
debris forming a disk. A disrupting encounter at the location of the observed
cloud is most likely to be caused by a stellar black hole because of the
expected dynamical mass segregation; the rate of these disk-forming encounters
may be as high as per year. The star should also be spun up by
the encounter, so the disk may subsequently expand by absorbing angular
momentum from the star. Once the disk expands up to the tidal truncation
radius, the tidal perturbation of the outer disk edge at every peribothron may
place gas streams on larger orbits which can give rise to a photoevaporation
wind that forms the cloud at every orbit. This model predicts that, after the
cloud is disrupted at the next peribothron passage in 2013, a smaller
unresolved cloud will gradually grow around the star on the same present orbit.
An increased infrared luminosity from the disk may also be detectable when the
peribothron is reached. We also note that this model revives the encounter
theory for planet formation.Comment: To be published in Ap
Soft X-ray Absorption by High-Redshift Intergalactic Helium
The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has
been measured with HST in four quasars over the last few years, over the
redshift range 2.4 < z < 3.2. These observations have indicated that the HeII
reionization may not have been completed until z\simeq 2.8, and that large
fluctuations in the intensity of the HeII-ionizing background were present
before this epoch. The detailed history of HeII reionization at higher
redshifts is, however, model-dependent and difficult to determine from these
observations, because the IGM can be completely optically thick to Lya photons
when only a small fraction of the helium remains as HeII. In addition, finding
quasars in which the HeII Lya absorption can be observed becomes increasingly
difficult at higher redshift, owing to the large abundance of hydrogen Lyman
limit systems.
It is pointed out here that HeII in the IGM should also cause detectable
continuum absorption in the soft X-rays. The spectrum of a high-redshift source
seen behind the IGM when most of the helium was HeII should recover from the
HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic
absorption will generally be stronger, but not by a large factor; the
intergalactic HeII absorption can be detected as an excess over the expected
Galactic absorption from the 21cm HI column density. In principle, this method
allows a direct determination of the fraction of helium that was singly ionized
as a function of redshift, if the measurement is done on a large sample of
high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter
`First Light' in the Universe; What Ended the "Dark Age"?
The universe would have been completely dark between the epoch of
recombination and the development of the first non-linear structure. But at
redshifts beyond 5 -- perhaps even beyond 20 -- stars formed within
`subgalaxies' and created the first heavy elements; these same systems
(together perhaps with `miniquasars') generated the UV radiation that ionized
the IGM, and maybe also the first significant magnetic fields. Although we can
already probe back to , these very first objects may be so faint
that their detection must await next-generation optical and infrared
telescopes. Observations in other wavebands may offer indirect clues to when
reionization occurred. Despite the rapid improvements in numerical simulations,
the processes of star formation and feedback are likely to remain a challenge
for the next decade.Comment: For ``Physics Reports'' special issue in memory of D.N. Schram
Gravitational Lensing in Clusters of Galaxies: New Clues Regarding the Dynamics of Intracluster Gas
Long arcs in clusters of galaxies, produced by gravitational lensing, can be
used to estimate the mass interior to the arcs and therefore, constrain the
cluster mass distribution. The radial density distribution of the intracluster
gas (ICM) can be extracted from the X-ray surface brightness observations. If
the gas temperature is also known, it is then possible to probe the dynamical
state of the gas and test whether the ICM is in hydro- static equilibrium
within the gravitational potential of the cluster as a result of thermal
pressure support. We analyze three clusters that exhibit large arcs, whose
X-ray surface brightness profiles have been observed, and whose gas
temperatures have been determined. In two of the clusters, A2218 and A1689, the
central mass implied by lensing is a factor of -- too large for the gas
at the observed temperature to be in hydrostatic equilibrium solely due to
thermal pressure support. In other words, if we accept the mass estimate
derived from the lensing analysis and demand that the X-ray surface brightness
profile be consistent with the observations, the required gas temperature is a
factor of -- higher than observed. The results for the third cluster,
A2163 (the most luminous and the hottest cluster known), are more ambiguous.
The discrepancy between the X-ray and the lensing mass estimates arise because
the presence of arcs imply a highly concentrated cluster mass distribution
whereas the observed X-ray profiles imply a more extended mass distribution.
The large X-ray core radii are not the result of the limited resolution of the
X-ray detectors. We consider various possibilities that could account for the
discrepancy.Comment: 20 pages, uuencoded compressed postscript, CITA/93/3
Images of Bursting Sources of High-Energy Cosmic Rays. I: Effects of Magnetic Fields
It has recently been shown that the highest energy cosmic rays (CRs) may
originate in the same cosmological objects producing -ray bursts. This
model requires the presence of intergalactic magnetic fields (IGMF) to delay
the arrival times of eV CRs by 50 years or longer relative to
the -rays, of an amplitude that is consistent with other observational
constraints. Sources of CRs coming from individual bursts should be resolved
with the planned ``Auger'' experiment, with as many as hundreds of CRs for the
brightest sources. We analyze here the apparent angular and energy distribution
of CRs from bright sources below the pion production threshold (in the energy
range ) expected in this model.
This observable distribution depends on the structure of the IGMF: the apparent
spectral width is small, , if the
intergalactic field correlation length is much larger than , and large, , in the opposite limit . The apparent angular size is also larger for smaller . If the
sources of CRs we predict are found, they will corroborate the bursting model
and they will provide us with a technique to investigate the structure of the
IGMF.Comment: Submitted to the ApJL; 10 pages AASTeX, including 2 PostScript
figure
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