arXiv subjects

M. M. Romanova

Publications and source records attributed to M. M. Romanova.

74 records · Page 5Linked to original sources

arXiv1998

Hydrodynamic Simulations of Counterrotating Accretion Disks

Hydrodynamic simulations have been used to study accretion disks consisting of counterrotating components with an intervening shear layer(s). Configurations of this type can arise from the accretion of newly supplied counterrotating matter onto an existing corotating disk. The grid-dependent numerical viscosity of our hydro code is used to simulate the influence of a turbulent viscosity of the disk. Firstly, we consider the case where the gas well above the disk midplane rotates with angular rate +Ω(r) and that well below has the same properties but rotates with rate -Ω(r). We find that there is angular momentum annihilation in a narrow equatorial boundary layer in which matter accretes supersonically with a velocity which approaches the free-fall velocity and the average accretion speed of the disk can be enormously larger than that for a conventional α-disk rotating in one direction. Secondly, we consider the case of a corotating accretion disk for r r_t. In this case we observed, that matter from the annihilation layer lost its stability and propagated inward pushing matter of inner regions of the disk to accrete. Thirdly, we investigated the case where counterrotating matter inflowing from large radial distances encounters an existing corotating disk. Friction between the inflowing matter and the existing disk is found to lead to fast boundary layer accretion along the disk surfaces and to enhanced accretion in the main disk. These models are pertinent to the formation of counterrotating disks in galaxies and possibly in Active Galactic Nuclei and in X-ray pulsars in binary systems.

astro-ph↗

arXiv1994

Spin-Up/Spin-Down of Magnetized Stars with Accretion Disks and Outflows

An investigation is made of disk accretion of matter onto a rotating star with an aligned dipole magnetic field. A new aspect of this work is that when the angular velocity of the star and disk differ substantially we argue that the $\bf B$ field linking the star and disk rapidly inflates to give regions of open field lines extending from the polar caps of the star and from the disk. The open field line region of the disk leads to the possibility of magnetically driven outflows. An analysis is made of the outflows and their back affect on the disk structure assuming an ``$\ap$" turbulent viscosity model for the disk and a magnetic diffusivity comparable to this viscosity. The outflows are found to extend over a range of radial distances inward to a distance close to $r_{to}$, which is the distance of the maximum of the angular rotation rate of the disk. We find that $r_{to}$ depends on the star's magnetic moment, the accretion rate, and the disk's magnetic diffusivity. The outflow regime is accompanied in general by a spin-up of the rotation rate of the star. When $r_{to}$ exceeds the star's corotation radius $r_{cr} = (GM/\om_*^2)^{1\ov 3}$, we argue that outflow solutions do not occur, but instead that ``magnetic braking" of the star by the disk due to field-line twisting occurs in the vicinity of $r_{cr}$. The magnetic braking solutions can give spin-up or spin-down (or no spin change) of the star depending mainly on the star's magnetic moment and the mass accretion rate. For a system with $r_{to}$ comparable to $r_{cr}$, bimodal behavior is possible where extraneous perturbations cause the system to flip between spin-up and spin-down.

astro-ph↗