I'm currently associated Prof. at Ferdowsi University of Mashhad. I'm interests in theoretical and computational astrophysics, including:
Shahram Abbassi
Department of Physics, School of Sciences, Ferdowsi University of Mashhad
Mashhad, PO Box 91775-1436, Iran
+98 511 8796983
abbassi@ipm.ir
• last update: Nov 2018
See my complete publication list
Accretion onto black holes has been known as a powerful source of energy in the universe. Accreting gas with sufficiently high angular momentum tends to form a disclike structure around the central object. In accreting processes, viscosity causes angular momentum transport outward and also it releases gravitational energy. According to the standard accretion disc model, the released energy is converted into radiation and escapes from the disc in the same place of its generation. The modern standard theory was formulated in Shakura (1972), Novikov & Thorne (1973) and Shakura & Sunyaev (1973). It provided remarkably successful contributions to understanding quasars, X-ray binaries and active galactic nuclei. One
Finding a reliable description of star formation has been one of the pivotal challenges of astrophysics for decades. Having such a model for star formation which involves many diverse physical phenomena would provide invaluable information to understand the evolution of galactic structures, star clusters, binary stars and even the formation of planets. It has been long established that stars form from collapsed dense molecular cloud (MC) cores. These cores are substantially denser than their parents so they collapse faster. Nevertheless, once these fragments turn into stars they start to affect the surrounding (by radiation, winds or supernova explosions) preventing it from collapsing and forming stars.
The gravitational instability of a filamentary molecular cloud in non-ideal magnetohydrodynamics is investigated. The filament is assumed to be in hydrostatic equilibrium. We add the effect of ambipolar diffusion to the filament which is threaded by an initial uniform axial magnetic field along its axis. We write down the fluid equations in cylindrical coordinates and perform linear perturbation analysis. We integrate the resultant differential equations and then derive the numerical dispersion relation. We find that, a more efficient ambipolar diffusion leads to an enhancement of the growth of the most unstable mode, and to increase of the fragmentation scale of the filament.
Collaborators: Mohammad Hosseinirad, Mahmood Roshan, Kazem NaficyWe obtain two-dimensional exact analytic solutions for the structure of the hot accretion flows without wind. We assume that the only non-zero component of the stress tensor is Trφ. Furthermore we assume that the value of viscosity coefficient α varies with θ. We find radially self-similar solutions and compare them with the numerical and the analytical solutions already studied in the literature. The no-wind solution obtained in this paper may be applied to the nuclei of some cool-core clusters.
Collaborators: Asiyeh Habibi, Mohsen ShadmehriThe stability of the interface between HII region and molecular clouds in the presence of the radiation pressure, has been studied using the linear perturbation analysis for the certain range of the wavelengths. The linear analysis show that consideration of the radiation pressure intensifies the growth rate of KH modes and consequently de- creases the e-fold time-scale of the instability. On the other hand the domain of the instability is extended and includes the more wavelengths, consisting of smaller ones rather than the case when the effect of the radiation pressure is not considered.
Collaborators: Akram Yaghouti, Mohsen Nejad-AsgharThis paper explores the effects of large-scale magnetic fields in hot accretion flows for asymmetric configurations with respect to the equatorial plane. The solutions that we have found show that the large-scale asymmetric magnetic field can significantly affect the dynamics of the flow and also cause notable outflows in the outer parts. Previously, we treated a viscous resistive accreting disc in the presence of an odd symmetric B- field about the equatorial plane. Now we extend our earlier work by taking into account another configuration of large-scale magnetic field which is no longer symmetric. We provide asymmetric field structures with small deviations from even and odd symmetric B-field. Our results show that the disc's dynamics and appearance become different above and below the equatorial plane. The set of solutions also predicts that even a small deviation in a symmetric field causes the disc to compress on one side and expand on the other. In some cases, our solution represents a very strong outflow from just one side of the disc. Therefore, the solution may potentially explain the origin of one-sided jets in radio galaxies.
Collaborators: Maryam Samadi, Richard V. E. LovelaceProin gravida nibh vel velit auctor aliquet. Aenean sollicitudin, lorem quis bibendum auctor, nisi elit consequat ipsum, nec sagittis sem nibh id elit.
Collaborators:Proin gravida nibh vel velit auctor aliquet. Aenean sollicitudin, lorem quis bibendum auctor, nisi elit consequat ipsum, nec sagittis sem nibh id elit.
Collaborators:Proin gravida nibh vel velit auctor aliquet. Aenean sollicitudin, lorem quis bibendum auctor, nisi elit consequat ipsum, nec sagittis sem nibh id elit.
Collaborators:The meetings are on Mondays at 9:30 in my room unless explicitly indicated. See the schedule.
Shahram Abbassi
Department of Physics, School of Sciences, Ferdowsi University of Mashhad
Mashhad, PO Box 91775-1436, Iran
+98 511 8796983