136 research outputs found
The structure and evolution of quasi-stars
The existence of bright quasars at high redshifts implies that supermassive
black holes were able to form in the early Universe. Though a number of
mechanisms to achieve this have been proposed, none yet stands out. A recent
suggestion is the formation of quasi-stars, initially stellar-mass black holes
accreting from hydrostatic giant-like envelopes of gas, formed from the
monolithic collapse of pre-galactic gas clouds. In this work, we modify the
Cambridge STARS stellar evolution package to construct detailed models of the
evolution of these objects. We find that, in all of our models, the black hole
inside the envelope is able to reach slightly more than one-tenth of the total
mass of the system before hydrostatic equilibrium breaks down. This breakdown
occurs after a few million years of evolution. We show that the mechanism which
causes the hydrostatic evolution to end is present in polytropic models. We
also show that the solutions are highly sensitive to the size of the inner
boundary radius and that no physical solutions exist if the inner boundary is
chosen to be less than about 0.3 of the Bondi radius.Comment: 12 pages, 11 figures. Published in MNRAS. Very belatedly updated to
(more closely) match published versio
String Theoretic Bounds on Lorentz-Violating Warped Compactification
We consider warped compactifications that solve the 10 dimensional
supergravity equations of motion at a point, stabilize the position of a
D3-brane world, and admit a warp factor that violates Lorentz invariance along
the brane. This gives a string embedding of ``asymmetrically warped'' models
which we use to calculate stringy (\alpha') corrections to standard model
dispersion relations, paying attention to the maximum speeds for different
particles. We find, from the dispersion relations, limits on gravitational
Lorentz violation in these models, improving on current limits on the speed of
graviton propagation, including those derived from field theoretic loops. We
comment on the viability of models that use asymmetric warping for self-tuning
of the brane cosmological constant.Comment: 20pg, JHEP3; v2 additional references, slight change to intro; v3.
added referenc
The Effects of Accretion Luminosity upon Fragmentation in the Early Universe
We introduce a prescription for the luminosity from accreting protostars into
smoothed particle hydrodynamics simulation, and apply the method to simulations
of five primordial minihalos generated from cosmological initial conditions. We
find that accretion luminosity delays fragmentation within the halos, but does
not prevent it. In halos that slowly form a low number of protostars, the
accretion luminosity can reduce the number of fragments that are formed before
the protostars start ionising their surroundings. However, halos that rapidly
form many protostars become dominated by dynamical processes, and the effect of
accretion luminosity becomes negligible. Generally the fragmentation found in
the halos is highly dependent on the initial conditions. Accretion luminosity
does not substantially affect the accretion rates experienced by the
protostars, and is far less important than dynamical interactions, which can
lead to ejections that effectively terminate the accretion. We find that the
accretion rates onto the inner regions of the disks (20 AU) around the
protostars are highly variable, in contrast to the constant or smoothly
decreasing accretion rates currently used in models of the pre-main sequence
evolution of Population III stars.Comment: 12 pages, 10 figures and 3 tables. Accepted by MNRA
Aplastic Anemia Complicating Orthotopic Liver Transplantation for Non-A, Non-B Hepatitis
Aplastic anemia developed in 9 of 32 patients (28 percent) undergoing orthotopic liver transplantation for acute non-A, non-B hepatitis, at one to seven weeks after the procedure. No patient previously had evidence of hematologic dysfunction or conditions known to be associated with aplastic anemia. No other cases of aplastic anemia were identified among 1463 patients undergoing liver transplantation for all other indications at the four centers participating in the study (chi-square = 415, P<0.001; 95 percent confidence interval for the incidence of aplastic anemia after transplantation for non-A, non-B hepatitis, 13 to 44 percent, vs. 0.00 to 0.13 percent for all other indications). The operative and postoperative treatment of these patients was not otherwise different, indicating that the aplastic anemia was a complication of the hepatitis, not of the transplantation procedure. Four of the nine patients died of complications due to infections. Three of the surviving patients have been followed for less than six months, one for one year, and one for two years. The two patients followed the longest have recovered marrow function to an appreciable degree, and two of the others have evidence of early recovery. We conclude that patients undergoing orthotopic liver transplantation for non-A, non-B hepatitis are at a high risk for the development of aplastic anemia. (N Engl J Med 1988; 319:393–6.) © 1988, Massachusetts Medical Society. All rights reserved
Dark Matter Candidates: A Ten-Point Test
An extraordinarily rich zoo of non-baryonic Dark Matter candidates has been
proposed over the last three decades. Here we present a 10-point test that a
new particle has to pass, in order to be considered a viable DM candidate: I.)
Does it match the appropriate relic density? II.) Is it {\it cold}? III.) Is it
neutral? IV.) Is it consistent with BBN? V.) Does it leave stellar evolution
unchanged? VI.) Is it compatible with constraints on self-interactions? VII.)
Is it consistent with {\it direct} DM searches? VIII.) Is it compatible with
gamma-ray constraints? IX.) Is it compatible with other astrophysical bounds?
X.) Can it be probed experimentally?Comment: 29 pages, 12 figure
Does changing contexts affect linkages throughout the mergers and acquisition process? A multiphasic investigation of motives, pre‐ and post‐acquisition and performance
Mergers and Acquisitions (M&A) is an important organizational change phenomena that has attracted significant research focus to many of its aspects but only recently has attention begun to investigate its complex multi-phasic nature and its embeddedness in multiple contexts. So far the influence of the macro context in M&A deals has been little studied and its interaction with M&A’s multiphasic nature has been ignored. This article aims to address this limitation by investigating whether different macro-economic wave and non-wave periods of M&A activity influence different phases of M&A in terms of motives, pre- and post-acquisition processes and acquisition performances. Drawing on a dataset of Greek M&As, an under researched M&A context, findings suggest that acquisitions in non-wave periods are driven by different motives from those in wave periods. Interestingly though, we found no statistically significant changes in pre- and post-acquisition processes and M&A performance between wave and non-wave periods. We suggest this organizational change finding maybe due to the early stage nature of Greek M&A and may also apply to other countries with similar levels of M&A experience. Further implications for theory and practice of these findings are discusse
Formation of Supermassive Black Holes
Evidence shows that massive black holes reside in most local galaxies.
Studies have also established a number of relations between the MBH mass and
properties of the host galaxy such as bulge mass and velocity dispersion. These
results suggest that central MBHs, while much less massive than the host (~
0.1%), are linked to the evolution of galactic structure. In hierarchical
cosmologies, a single big galaxy today can be traced back to the stage when it
was split up in hundreds of smaller components. Did MBH seeds form with the
same efficiency in small proto-galaxies, or did their formation had to await
the buildup of substantial galaxies with deeper potential wells? I briefly
review here some of the physical processes that are conducive to the evolution
of the massive black hole population. I will discuss black hole formation
processes for `seed' black holes that are likely to place at early cosmic
epochs, and possible observational tests of these scenarios.Comment: To appear in The Astronomy and Astrophysics Review. The final
publication is available at http://www.springerlink.co
The Growth of the Stellar Seeds of Supermassive Black Holes
The collapse of baryons into extremely massive stars with masses exceeding
10^4 M_Sun in a small fraction of protogalaxies at z > 10 is a promising
candidate for the origin of supermassive black holes, some of which grow to a
billion solar masses by z ~ 7. We determine the maximum masses such stars can
attain by accreting primordial gas. We find that at relatively low accretion
rates the strong ionizing radiation of these stars limits their masses to M_* ~
10^3 M_Sun (dM_acc/dt / 10^-3 M_Sun yr^-1)^8/7, where dM_acc/dt is the rate at
which the star gains mass. However, at the higher central infall rates usually
found in numerical simulations of protogalactic collapse (>~ 0.1 M_Sun yr^-1),
the lifetime of the star instead limits its final mass to >~ 10^6 M_Sun.
Furthermore, for the spherical accretion rates at which the star can grow, its
ionizing radiation is confined deep within the protogalaxy, so the evolution of
the star is decoupled from that of its host galaxy. Lyman alpha emission from
the surrounding H II region is trapped in these heavy accretion flows and
likely reprocessed into strong Balmer series emission, which may be observable
by the James Webb Space Telescope. This, along with strong He II 1640 Angstrom
and continuum emission, are likely to be the key observational signatures of
the progenitors of supermassive black holes at high redshift.Comment: 14 pages, 4 figures; now accepted for publication in Ap
Resource limitation drives spatial organization in microbial groups.
Dense microbial groups such as bacterial biofilms commonly contain a diversity of cell types that define their functioning. However, we have a limited understanding of what maintains, or purges, this diversity. Theory suggests that resource levels are key to understanding diversity and the spatial arrangement of genotypes in microbial groups, but we need empirical tests. Here we use theory and experiments to study the effects of nutrient level on spatio-genetic structuring and diversity in bacterial colonies. Well-fed colonies maintain larger well-mixed areas, but they also expand more rapidly compared with poorly-fed ones. Given enough space to expand, therefore, well-fed colonies lose diversity and separate in space over a similar timescale to poorly fed ones. In sum, as long as there is some degree of nutrient limitation, we observe the emergence of structured communities. We conclude that resource-driven structuring is central to understanding both pattern and process in diverse microbial communities
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