We investigate Hoyle–Lyttleton accretion for the case where the central source is a luminous accretion disk. In classical Hoyle-Lyttleton accretion onto a “spherical” source, accretion takes place in an axially symmetric manner around a so-called accretion axis. In the spherical case the accretion rate Ṁ is given as ṀHL(1− Γ), where ṀHL is the accretion rate of the classical Hoyle–Lyttleton acc...

We investigate the stability of black hole accretion disks in a primordial environment (POP III disks for short), by solving the vertical structure of optically thick disks, including convective energy transport, and by employing a one-zone model for optically thin isothermal disks. Because of the absence of metals in POP III disks, we find significant differences in stability associated with i...

We find a new two-temperature hot branch of equilibrium solutions for stationary accretion disks around black holes. In units of Eddington accretion rate defined as 10LEdd/c , the accretion rates to which these solutions correspond are within the range ṁ1 <∼ ṁ <∼ 1, here ṁ1 is the critical rate of advection-dominated accretion flow (ADAF). In these solutions, the energy loss rate of the ions by...

We examine the nucleosynthesis products that are produced in the outflow from rapidly accreting disks. We find that the type of element synthesis varies dramatically with the degree of neutrino trapping in the disk and therefore the accretion rate of the disk. Disks with relatively high accretion rates such as Ṁ = 10M⊙/s can produce very neutron rich nuclei that are found in the r process. Disk...

We investigate the evolution of hydromagnetic perturbations in a small section of accretion disks. It is known that molecular viscosity is negligible in accretion disks. Hence, it has been argued that a mechanism, known as magnetorotational instability (MRI), is responsible for transporting matter in the presence of a weak magnetic field. However, there are some shortcomings, which question the...

I briefly review the theoretical models of radiatively efficient, geometrically thin and optically thick accretion disk spectra that currently exist for AGN. I then discuss three recent observational developments that have real potential to teach us about the physics of these flows. Finally, I present results on the most recent, thermodynamically consistent simulations of magnetorotational turb...

1. Abstract Recent high resolution near infrared (HST-NICMOS) and mm-interferometric imaging have revealed dense gas and dust accretion disks in nearby ultra-luminous galactic nuclei. In the best studied ultraluminous IR galaxy, Arp 220, the 2µm imaging shows dust disks in both of the merging galactic nuclei and mm-CO line imaging indicates molecular gas masses ∼10 9 M ⊙ for each disk. The two ...

Accretion rates of order 10 M⊙yr −1 are observed in young pre–main– sequence (PMS) stars of approximately a solar mass with evidence of circumstellar disks. The accretion rate is significantly lower for PMS stars of smaller mass, approximately proportional to the second power of the stellar mass, Ṁaccr ∝ M . The traditional view is that the observed accretion is the consequence of the angular m...

We present a detailed temporal analysis of a set of hydrodynamic and magnetohydrodynamic (MHD) simulations of geometrically-thin (h/r ∼ 0.05) black hole accretion disks. The black hole potential is approximated by the Paczynski-Wiita pseudo-Newtonian potential. In particular, we use our simulations to critically assess two widely discussed models for high-frequency quasi-periodic oscillations, ...

We examine the evolution and influence of viscosity-induced diskoseismic modes in simulated black hole accretion disks. Understanding the origin and behavior of such oscillations will help us to evaluate their potential role in producing astronomically observed high-frequency quasi-periodic oscillations in accreting black hole binary systems. Our simulated disks are geometrically-thin with a co...