oalib
Search Results: 1 - 10 of 100 matches for " "
All listed articles are free for downloading (OA Articles)
Page 1 /100
Display every page Item
Gas depletion in primordial globular clusters due to accretion onto stellar-mass black holes  [PDF]
Nathan W. C. Leigh,Torsten Boker,Thomas J. Maccarone,Hagai B. Perets
Physics , 2012, DOI: 10.1093/mnras/sts554
Abstract: (abridged) We consider the effect of stellar remnants on the interstellar medium of a massive star cluster following the initial burst of star formation. We argue that accretion onto stellar-mass black holes (BHs) is an effective mechanism for rapid gas depletion in clusters of all masses, as long as they contain progenitor stars more massive than \gtrsim 50\msun. This scenario is attractive for the progenitor systems of present-day massive globular clusters (GCs) which likely had masses M \gtrsim 10^7\msun. In such clusters, supernovae and stellar winds cannot provide a plausible explanation for the sudden removal of the primordial gas reservoir that is required to explain their complex chemical enrichment history. In order to consider different regimes in the gas accretion rate onto stellar-mass BHs, we consider both the Bondi-Hoyle and Eddington approximations. For either model, our results show that the gas can be significantly depleted within only a few tens of Myrs. This process will affect the distribution of BH masses, and may accelerate the dynamical decoupling of the BH population and, ultimately, their dynamical ejection. Moreover, the timescales for gas depletion are sufficiently short that the accreting BHs could significantly affect the chemistry of subsequent star formation episodes. The gas depletion times and final mass in BHs are sensitive to the assumed model for the accretion rate, and to the initial mass of the most massive BH which, in turn, is determined by the upper mass cut-off of the stellar IMF. Our results imply that the remnant accretion history can have an important bearing on the observed present-day cluster mass-to-light ratio. In particular, we show that an increase of the upper mass cut-off with decreasing metallicity could contribute to the observed anti-correlation between the mass-to-light ratio and the metallicity of GCs.
Numerical estimates of the accretion rate onto intermediate-mass black holes  [PDF]
Carolina Pepe,Leonardo J. Pellizza
Physics , 2012, DOI: 10.1093/mnras/stt080
Abstract: The existence of intermediate-mass ($\sim 10^3 M_\odot$) black holes in the center of globular clusters has been suggested by different observations. The X-ray sources observed in NGC 6388 and in G1 in M31 could be interpreted as being powered by the accretion of matter onto such objects. In this work we explore a scenario in which the black hole accretes from the cluster interstellar medium, which is generated by the mass loss of the red giants in the cluster. We estimate the accretion rate onto the black hole and compare it to the values obtained via the traditional Bondi-Hoyle model. Our results show that the accretion rate is no longer solely defined by the black hole mass and the ambient parameters but also by the host cluster itself. Furthermore, we find that the more massive globular clusters with large stellar velocity dispersion are the best candidates in which accretion onto IMBHs could be detected.
Inner region accretion flows onto black holes  [PDF]
Menas Kafatos,Prasad Subramanian
Physics , 1999,
Abstract: We examine here the inner region accretion flows onto black holes. A variety of models are presented. We also discuss viscosity mechanisms under a variety of circumstances, for standard accretion disks onto galactic black holes and supermassive black holes and hot accretion disks. Relevant work is presented here on unified aspects of disk accretion onto supermassive black holes and the possible coupling of thick disks to beams in the inner regions. We also explore other accretion flow scenarios. We conclude that a variety of scenarios yield high temperatures in the inner flows and that viscosity is likely not higher than alpha $\sim$ 0.01.
Hydrodynamics of Accretion onto Black Holes  [PDF]
Myeong-Gu Park,Jeremiah P. Ostriker
Physics , 1998, DOI: 10.1016/S0273-1177(98)00127-6
Abstract: Spherical and axisymmetric accretion onto black holes is discussed. Physical processes in various families of solutions are explained and their characteristics are summarized. Recently discovered solutions of axisymmetric flow provide us with various radiation efficiency and spectrum, which may successfully model diverse accretion systems. Possible role of preheating is also speculated. The various families of solutions can be plotted as trajectories on the plane of (l,e) [or (l,dot m)] where l is the luminosity in units of the Eddington luminosity, dot m is the similarly defined mass accretion rate and e is the efficiency defined by e equiv L/dot M c^2 = l/dot m. We discuss the domains on these planes where solutions are known or expected to be unstable to either spherical or on-spherical perturbations. Preliminary analysis indicates that a preheating instability will occur along the polar direction of the advection dominated flow for e \gsim 10^{-2}.
Bondi accretion onto cosmological black holes  [PDF]
Janusz Karkowski,Edward Malec
Physics , 2012, DOI: 10.1103/PhysRevD.87.044007
Abstract: In this paper we investigate a steady accretion within the Einstein-Straus vacuole, in the presence of the cosmological constant. The dark energy damps the mass accretion rate and --- above certain limit --- completely stops the steady accretion onto black holes, which in particular is prohibited in the inflation era and after (roughly) $10^{12}$ years from Big Bang (assuming the presently known value of the cosmological constant). Steady accretion would not exist in the late phases of the Penrose's scenario - known as the Weyl curvature hypothesis - of the evolution of the Universe.
Dark matter and dark energy accretion onto intermediate-mass black holes  [PDF]
C. Pepe,L. J. Pellizza,G. E. Romero
Physics , 2011, DOI: 10.1111/j.1365-2966.2011.20252.x
Abstract: In this work we investigate the accretion of cosmological fluids onto an intermediate-mass black hole at the centre of a globular cluster, focusing on the influence of the parent stellar system on the accretion flow. We show that the accretion of cosmic background radiation and the so-called dark energy onto an intermediate-mass black hole is negligible. On the other hand, if cold dark matter has a nonvanishing pressure, the accretion of dark matter is large enough to increase the black hole mass well beyond the present observed upper limits. We conclude that either intermediate-mass black holes do not exist, or dark matter does not exist, or it is not strictly collisionless. In the latter case, we set a lower limit for the parameter of the cold dark matter equation of state.
Swift J164449.3+573451 event: generation in the collapsing star cluster?  [PDF]
V. I. Dokuchaev,Yu. N. Eroshenko
Physics , 2011, DOI: 10.1134/S0202289312040044
Abstract: We discuss the multiband energy release in a model of a collapsing galactic nucleus, and we try to interpret the unique super-long cosmic gamma-ray event Swift J164449.3+573451 (GRB 110328A by early classification) in this scenario. Neutron stars and stellar-mass black holes can form evolutionary a compact self-gravitating subsystem in the galactic center. Collisions and merges of these stellar remnants during an avalanche contraction and collapse of the cluster core can produce powerful events in different bands due to several mechanisms. Collisions of neutron stars and stellar-mass black holes can generate gamma-ray bursts (GRBs) similar to the ordinary models of short GRB origin. The bright peaks during the first two days may also be a consequence of multiple matter supply (due to matter release in the collisions) and accretion onto the forming supermassive black hole. Numerous smaller peaks and later quasi-steady radiation can arise from gravitational lensing, late accretion of gas onto the supermassive black hole, and from particle acceleration by shock waves. Even if this model will not reproduce exactly all the Swift J164449.3+573451 properties in future observations, such collapses of galactic nuclei can be available for detection in other events.
Accretion onto black holes formed by direct collapse  [PDF]
Jarrett L. Johnson,Sadegh Khochfar,Thomas H. Greif,Fabrice Durier
Physics , 2010, DOI: 10.1111/j.1365-2966.2010.17491.x
Abstract: One possible scenario for the formation of massive black holes (BHs) in the early Universe is from the direct collapse of primordial gas in atomic-cooling dark matter haloes in which the gas is unable to cool efficiently via molecular transitions. We study the formation of such BHs, as well as the accretion of gas onto these objects and the high energy radiation emitted in the accretion process, by carrying out cosmological radiation hydrodynamics simulations. In the absence of radiative feedback, we find an upper limit to the accretion rate onto the central object which forms from the initial collapse of hot (~ 10^4 K) gas of the order of 0.1 MSun per year. This is high enough for the formation of a supermassive star, the immediate precursor of a BH, with a mass of the order of 10^5 MSun. Assuming that a fraction of this mass goes into a BH, we track the subsequent accretion of gas onto the BH self-consistently with the high energy radiation emitted from the accretion disk. Using a ray-tracing algorithm to follow the propagation of ionizing radiation, we model in detail the evolution of the photoionized region which forms around the accreting BH. We find that BHs with masses of the order of 10^4 MSun initially accrete at close to the Eddington limit, but that the accretion rate drops to of order 1 percent of the Eddington limit after ~ 10^6 yr, due to the expansion of the gas near the BH in response to strong photoheating and radiation pressure. One signature of the accretion of gas onto BHs formed by direct collapse, as opposed to massive Pop III star formation, is an extremely high ratio of the luminosity emitted in He II 1640 to that emitted in H_alpha (or Ly_alpha); this could be detected by the James Webb Space Telescope. Finally, we briefly discuss implications for the coevolution of BHs and their host galaxies.
Spherical accretion onto neutron stars and black holes  [PDF]
L. Titarchuk,A. Mastichiadis,N. D. Kylafis
Physics , 1996,
Abstract: Spectral formation in steady state, spherical accretion onto neutron stars and black holes is examined by solving numerically and analytically the equation of radiative transfer. The photons escape diffusively and their energy gains come from their scattering off thermal electrons in the converging flow of the accreting gas. We show that the bulk motion of the flow is more efficient in upscattering photons than thermal Comptonization in the range of non-relativistic electron temperatures. The spectrum observed at infinity is a power law with an exponential turnover at energies of order the electron rest mass. Especially in the case of accretion into a black hole, the spectral energy power-law index is distributed around 1.5. Because bulk motion near the horizon (1-5 Schwarzschild radii) is most likely a necessary characteristic of accretion into a black hole, we claim that observations of an extended power law up to about the electron rest mass, formed as a result of bulk motion Comptonization, is a real observational evidence for the existence of an underlying black hole.
Time-dependent Hypercritical Accretion onto Black Holes  [PDF]
Luca Zampieri
Physics , 1996,
Abstract: Results are presented from a time-dependent, numerical investigation of super-Eddington spherical accretion onto black holes with different initial conditions. We have studied the stability of stationary solutions, the non-linear evolution of shocked models and the time-dependent accretion from an expanding medium.
Page 1 /100
Display every page Item


Home
Copyright © 2008-2017 Open Access Library. All rights reserved.