Catálogo de publicaciones - libros

We propose a model of accretion onto a black hole at the center of our Galaxy. This model describes a situation in which Sgr A* is accreting a hot stellar wind predominantly from star complex IRS13E. We further assume an existence of a cold, optically thick, geometrically thin disk that surrounds a black hole. A fraction of matter inflowing into area described by our model settles onto a disk. The rest of it is falling under horizon.

We estimate a fraction of matter that settles onto a disk. We assume that all kinetic energy is lost during the impact, and then re-radiated from the disk surface. Our main goal was to compare the modeled flux emitted by the disk with flux obtained from observations. Our calculations show that existence of such disk is rather excluded.

High resolution observations (NTT, VLT and KECK) showed that the super-massive black hole (SBH) at the Galactic Centre is located at the centre of a compact stellar cluster [2,5]. However, the observed stellar sources may represent only a fraction of the total mass implying high M/L(2m). Stars like S2, with orbital periods as short as 15 yrs [4,7] play a key role in determining the potential in the vicinity of the SBH. We show that it is possible, by fitting the available data on S2, to estimate an upper limit on the amount of the extended dark mass and thus on the M/L(2m) that could be present at the very centre of the Galaxy.

The velocity dispersion of stars in the Milky Way’s central stellar cluster traces the gravitational potential of a point mass of 3–4×10 M from distances of a light year down to a few light days from the compact, non-thermal radio source Sgr A* (e.g., [3,7,4]).

While supermassive black holes probably gained most of their mass via accretion of gas, the galactic nuclei in which they are currently situated are dominated by stars. This article reviews recent theoretical work on the interaction between black holes and their stellar environment, and highlights ways in which the observed structure of galactic nuclei can be used to constrain the formation history of black holes. Nuclei may also contain dark matter, and the possibility of detecting supersymmetric particles via annihilation radiation from the Galactic center has generated some interest [26]. The evolution of the dark matter distribution in the presence of a black hole in a stellar nucleus is also discussed.

The black holes (BHs) at the centers of present-day bright early-type galaxies might represent the most massive endpoints for BH growth in the Universe. The majority of these galaxies contains detectable amounts of emission-line gas at their centers which indicate that their black holes are still growing, but typically at very modest rates. The gas kinematics potentially form a valuable diagnostic of both this growth process and the gravitational potential well of the BH. Here we focus on the nuclear gas velocity dispersion which often exceeds the stellar velocity dispersion. This could be due to either the gravitational potential of the black hole or turbulence associated with the accretion process. We try to discriminate between these two scenarios.

We discuss a clumpy model of obscuring dusty tori around AGN. Cloud-cloud collisions lead to an effective viscosity and a geometrically thick accretion disk, which has the required properties of a torus. Accretion in the combined gravitational potential of central black hole and stellar cluster generates free energy, which is dissipated in collisions, and maintains the thickness of the torus. A quantitative treatment for the torus in the prototypical Seyfert 2 nucleus of NGC 1068 together with a radiative transfer calculation for NIR re-emission from the torus is presented.

The outer parts ( > 1000 ) of Active Galactic Nuclei (AGN) disks are expected to become self-gravitating and form stars [6]. Indeed, in our own Galactic Centre a few dozens of young hot stars are confined to two well defined rings of truly nuclear scale (~ 0.1 pc) [3]. This lends support to theoretical predictions of star formation in accretion disks, but also creates a problem: how do luminous quasars transfer the gas to the super-massive black hole (SMBH) avoiding turning all of this gas into stars [4]? Here we attempt to answer whether the newly born stars can leave the disk midplane, cutting off their further growth by accretion.

We explore the structure of a stellar cluster dominated by gravitation of a central mass (a supermassive black hole, BH) and by a dissipative interaction of orbiting stars with an accretion disc. We show that the cluster properties are determined predominantly by the radial profile of the disc characteristics (i.e. surface density, geometrical thickness, viscosity, etc). We develop a simple steady-state model of the cluster structure and we estimate the rate at which stars migrate to the centre.

Is there an upper limit to the accretion rate , preventing astrophysical black holes to swallow all the matter infalling into them? Many astrophysicists are convinced that the answer to this fundamental question should be: ,

Now that the magneto-rotational instability mechanism is well-established as the basis of angular momentum transport in accretion disks, it is possible to do large-scale numerical simulations with reasonable confidence that they are describing actual physical processes in disks. Recently several groups have extended the methods of MHD simulation to incorporate fully general relativistic dynamics. Rapidly-rotating black holes are found to generate strong outward electromagnetic stresses. One consequence is that it may be very difficult to spin up black holes by accretion to greater than some critical / that is likely to be rather less than the value, 0.998, suggested many years ago on the basis of photon capture.