Comparison of theoretical radiation-driven winds from stars and discs

We compare models of line-driven winds from accretion discs and single spherical stars. We look at the problem of scaling mass-loss rates and velocities of stellar and disc winds with model parameters. We find that stellar and disc winds driven by radiation, within the CAK framework, are very similar as far as mass-loss rates and velocities are concerned. Thus we can use analytic results for stellar winds to rescale, in a first order approximation, numerical results for disc winds. We also show how the CAK stellar solutions change when we take into account effects of very low luminosities or line-driving force.Comment: LaTeX, 13 pages, including three tables, 4 Postscript files, requires mn.sty, to appear in MNRA

On the diversity and complexity of absorption line profiles produced by outflows in Active Galactic Nuclei

Understanding the origin of AGN absorption line profiles and their diversity could help to explain the physical structure of the accretion flow, and also to assess the impact of accretion on the evolution of the AGN host galaxies. Here we present our first attempt to systematically address the issue of the origin of the complexities observed in absorption profiles. Using a simple method, we compute absorption line profiles against a continuum point source for several simulations of accretion disk winds. We investigate the geometrical, ionization, and dynamical effects on the absorption line shapes. We find that significant complexity and diversity of the absorption line profile shapes can be produced by the non-monotonic distribution of the wind velocity, density, and ionization state. Non-monotonic distributions of such quantities are present even in steady-state, smooth disk winds, and naturally lead to the formation of multiple and detached absorption troughs. These results demonstrate that the part of a wind where an absorption line is formed is not representative of the entire wind. Thus, the information contained in the absorption line is incomplete if not even insufficient to well estimate gross properties of the wind such as the total mass and energy fluxes. In addition, the highly dynamical nature of certain portions of disk winds can have important effects on the estimates of the wind properties. For example, the mass outflow rates can be off up to two orders of magnitude with respect to estimates based on a spherically symmetric, homogeneous, constant velocity wind.Comment: 10 pages, 10 figures, to appear in Ap

Time Evolution of the 3-D Accretion Flows: Effects of the Adiabatic Index and Outer Boundary Condition

We study a slightly rotating accretion flow onto a black hole, using the fully three dimensional (3-D)numerical simulations. We consider hydrodynamics of an inviscid flow, assuming a spherically symmetric density distribution at the outer boundary and a small, latitude-dependent angular momentum. We investigate the role of the adiabatic index and gas temperature, and the flow behaviour due to non-axisymmetric effects. Our 3-D simulations confirm axisymmetric results: the material that has too much angular momentum to be accreted forms a thick torus near the equator and the mass accretion rate is lower than the Bondi rate. In our previous study of the 3-D accretion flows, for gamma=5/3, we found that the inner torus precessed, even for axisymmetric conditions at large radii. The present study shows that the inner torus precesses also for other values of the adiabatic index: gamma=4/3, 1.2 and 1.01. However, the time for the precession to set increases with decreasing gamma. In particular, for gamma=1.01 we find that depending on the outer boundary conditions, the torus may shrink substantially due to the strong inflow of the non-rotating matter and the precession will have insufficient time to develop. On the other hand, if the torus is supplied by the continuous inflow of the rotating material from the outer radii, its inner parts will eventually tilt and precess, as it was for the larger gamma's.Comment: 19 pages, 19 figures; accepted to ApJ; version with full resolution figures may be downloaded from http://users.camk.edu.pl/agnes/publ_en.htm

Three-Dimensional Simulations of Dynamics of Accretion Flows Irradiated by a Quasar

We study the axisymmetric and non-axisymmetric, time-dependent hydrodynamics of gas that is under the influence of the gravity of a super massive black hole (SMBH) and the radiation force produced by a radiatively efficient flow accreting onto the SMBH. We have considered two cases: (1) the formation of an outflow from the accretion of the ambient gas without rotation and (2) that with weak rotation. The main goals of this study are: (1) to examine if there is a significant difference between the models with identical initial and boundary conditions but in different dimensionality (2-D and 3-D), and (2) to understand the gas dynamics in AGN. Our 3-D simulations of a non-rotating gas show small yet noticeable non-axisymmetric small-scale features inside the outflow. The outflow as a whole and the inflow do not seem to suffer from any large-scale instability. In the rotating case, the non-axisymmetric features are very prominent, especially in the outflow which consists of many cold dense clouds entrained in a smoother hot flow. The 3-D outflow is non-axisymmetric due to the shear and thermal instabilities. In both 2-D and 3-D simulations, gas rotation increases the outflow thermal energy flux, but reduces the outflow mass and kinetic energy fluxes. Rotation also leads to time variability and fragmentation of the outflow in the radial and latitudinal directions. The collimation of the outflow is reduced in the models with gas rotation. The time variability in the mass and energy fluxes is reduced in the 3-D case because of the outflow fragmentation in the azimuthal direction. The virial mass estimated from the kinematics of the dense cold clouds found in our 3-D simulations of rotating gas underestimates the actual mass used in the simulations by about 40 %. (Abbreviated)Comment: 19 pages, 14 figures. Accepted by ApJ. Movies and a full resolution version of the preprint can be downloaded at "http://www.physics.unlv.edu/~rk/research/agn_3d_rot.html