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Shahram Abbassi

Publications and source records attributed to Shahram Abbassi.

50 records · Page 3Linked to original sources
arXiv2016

The influence of outflow in supercritical accretion flows

We solve the radiation-hydrodynamic (RHD) equations of supercritical accretion flows in the presence of radiation force and outflow by using self similar solutions. Compare with the pioneer works, in this paper we consider power-law function for mass inflow rate as $ \dot{M} \propto r^{s} $. We found that $ s = 1 $ when the radiative cooling term is included in the energy equation. Correspondingly, the effective temperature profile with respect to the radius was obtained as $ T_{\text{eff}} \varpropto r^{-1/2} $. In addition, we investigated the influence of the outflow on the dynamics of the accretion flow. We also calculated the continuum spectrum emitted from the disk surface as well as the bolometric luminosity of the accretion flow. Furthermore, our results show that the advection parameter, $ f $, strongly depends on mass inflow rate.

astro-ph.HE↗
arXiv2015

The Effect of Large Scale Magnetic Field on Outflow in ADAFs: an Odd Symmetry Configuration

We construct self-similar inflow-outflow solutions for a hot viscous-resistive accretion flow with large scale magnetic fields that have odd symmetry with respect to the equatorial plane in $B_θ$, and even symmetry in $B_r$ and $B_ϕ$. Following previous authors, we also assume that the polar velocity $v_θ$ is nonzero. We focus on four parameters: $β_{r0}$, $β_{\phi0}$ (the plasma beta parameters for associated with magnetic field components at the equatorial plane), the magnetic resistivity $η_0$, and the density index $n=-d\lnρ/d\ln r$. The resulting flow solutions are divided into two parts consisting of an inflow region with a negative radial velocity ($v_r<0$) and an outflow region with $v_r>0$. Our results show that stronger outflows emerge for smaller $β_{r0}$ ($\le10^{-2}$ for $n>1$) and larger values of $β_{\phi0}$, $η_0$ and $n$.

astro-ph.HE↗
arXiv2015

Hydrodynamical wind on vertically self-gravitating ADAFs in the presence of toroidal magnetic field

We present the effect of a hydrodynamical wind on the structure and the surface temperature of a vertically self-gravitating magnetized ADAFs using self-similar solutions. Also a model for an axisymmetric, steady-state, vertically self-gravitating hot accretion flow threaded by a toroidal magnetic field has been formulated. The model is based on $α-$prescription for turbulence viscosity. It is found that the thickness and radial velocity of the disc are reduced significantly as wind gets stronger. In particular, the solutions indicated that the wind and advection have the same effects on the structure of the disc. We also find that in the optically thin ADAF becomes hotter by including the wind parameter and the self-gravity parameter.

astro-ph.HE↗
arXiv2015

Vertical structure of Advection dominated Accretion Flows

We solve the set of hydrodynamic (HD) equations for optically thin Advection Dominated Accretion Flows (ADAFs) by assuming radially self-similar in spherical coordinate system $ (r, θ, ϕ) $. The disk is considered to be steady state and axi-symmetric. We define the boundary conditions at the pole and the equator of the disk and to avoid singularity at the rotation axis, the disk is taken to be symmetric with respect to this axis. Moreover, only the $ τ_{r ϕ} $ component of viscous stress tensor is assumed and we have set $ v_θ = 0 $. The main purpose of this study is to investigate the variation of dynamical quantities of the flow in the vertical direction by finding an analytical solution. As a consequence, we found that the advection parameter, $ f^{adv} $, varies along the $ θ$ direction and reaches to its maximum near the rotation axis. Our results also show that, in terms of no-outflow solution, thermal equilibrium still exists and consequently advection cooling can balance viscous heating.

astro-ph.HE↗
arXiv2015

Local stability of self-gravitating disks in $f(R)$ gravity

In the framework of metric $f(R)$ gravity, we find the dispersion relation for the propagation of tightly wound spiral density waves in the surface of rotating, self-gravitating disks. Also, new Toomre-like stability criteria for differentially rotating disks has been derived for both fluid and stellar disks.

astro-ph.GA↗
arXiv2015

On the stability of a galactic disk in modified gravity

We find the dispersion relation for tightly wound spiral density waves in the surface of rotating, self-gravitating disks in the framework of Modified Gravity (MOG). Also, the Toomre-like stability criterion for differentially rotating disks has been derived for both fluid and stellar disks. More specifically, the stability criterion can be expressed in terms of a matter density threshold over which the instability occurs. In other words the local stability criterion can be written as $Σ_0<Σ_{\text{crit}}(v_s,κ,α,μ_0)$, where $Σ_{\text{crit}}$ is a function of $v_s$ (sound speed), $κ$ (epicycle frequency) and $α$ and $μ_0$ are the free parameters of the theory. In the case of a stellar disk the radial velocity dispersion $σ_r$ appears in $Σ_{\text{crit}}$ instead of $v_s$. We find the exact form of the function $Σ_{\text{crit}}$ for both stellar and fluid self-gravitating disks. Also, we use a sub-sample of THINGS catalog of spiral galaxies in order to compare the local stability criteria. In this perspective, we have compared MOG with Newtonian gravity and investigated the possible and detectable differences between these theories.

astro-ph.GA↗
arXiv2014

Double field domain walls with explicit symmetry breaking

We study the dynamics of domain walls in a double-field model in which the U(1) symmetry is broken both spontaneously and explicitly. The global U(1) symmetry of the system is restored when the symmetry breaking parameter $ε$ is set to zero. Two pairs of degenerate kinks exist in the model with are related to each other by a $Z_2$ transformation. We first calculate the single domain wall solutions and then investigate collision processes. These include simple scattering, pair annihilation, pair capture, and other interesting processes. The possibility of the domain wall being punctured by a string is also investigated.

math-ph↗
arXiv2014

Jeans analysis in modified gravity

MOdified Gravity (MOG) is a covariant modification of Einstein's general relativity. This theory is one of the current alternatives to dark matter models. We describe dynamics of collisionless self-gravitating systems in the context of MOG. By studying the weak field approximation of this theory, we derive the equations governing the dynamics of the self-gravitating systems. More specifically, we consider the Jeans instability for self-gravitating fluid and stellar systems, and derive new Jeans mass limit $\tilde{M}_J$ and wave-number $\tilde{k}_J$. Furthermore, considering the gravitational instability in star forming regions, we show that MOG has not a significant difference with general relativity on this astrophysical scale. However, at larger scales such as intergalactic space MOG may lead to different galaxy and structure formation processes.

astro-ph.GA↗
arXiv2014

Dynamics of clumps embedded in a hot accretion flow with toroidal magnetic field

Dynamics of clumps within a magnetized advection dominated accretion flow is investigated by solving the collisionless Boltzmann equation and considering the drag force due to the relative velocity between the clumps and the gas. Toroidal component of the magnetic field is assumed to be dominant. Dynamical properties of the hot gaseous component such as the radial and the rotational velocities are affected by the magnetic effects, and so, drag force varies depending on the strength of magnetic field and the velocity dispersion of the clumps is then modified significantly. We show that when magnetic pressure is less than the gas pressure, the root of the averaged radial velocity square of the clumps decreases at the inner parts of the hot flow and increases slightly at its outer edge.

astro-ph.GA↗
arXiv2013

Structure of Advection-Dominated Accretion Disks with Outflows: Role of Toroidal Magnetic Field

The main aim of this paper is studying the effect of toroidal magnetic field on the structure of Advection-Dominated Accretion Flows (ADAF) in the presence of the turbulence viscosity and diffusivity due to viscosity and magnetic field respectively. We use self-similar assumption in radial direction to solve the magnetohydrodynamic (MHD) equations for hot accretion disk. We use spherical coordinate $ (r, θ, φ) $ to solve our equation. The toroidal component of magnetic field is considered and all three components of the velocity field $ \mathbf{v}\equiv (v_{r}, v_θ, v_φ) $ are present in our work. We reduce the equations to a set of differential equations about $ θ$ and apply the symmetric boundary condition at the equatorial plane of the disk. Our results indicate that the outflow region, where the redial velocity becomes positive in a certain inclination angle $ θ_{0} $, always exist. The results represent that the stronger the magnetic field, the smaller the inclination angle, $ θ_{0} $ becomes. It means that a magnetized disk is thinner compared to a non-magnetized disk. According to the work by \citealt{jw}, we can define three regions. The first one is called inflow region, which starts from the disk midplane to a certain inclination $ θ_0 $ where $ v_{r}(θ_{0}) = 0 $. In this region, the velocity has a negative value and the accretion material moves toward the central object. The outflow region, where $ v_{r}(θ) > 0 $, is placed between $ θ_{0} $ and surface of the disk, $ θ_{0} < θ< θ_{s} $. In this area, the accretion flow moves away from the central object. The third region, which is located between the surface of the disk and the polar axis, is called wind region.

astro-ph.HE↗
arXiv2013

The Effect of Toroidal Magnetic Field on Thickness of a Viscose-Resistive Hot Accreting Flow

By taking into account the effect of toroidal magnetic field and its correspond heating, we determine the thickness of advection-dominated accretion flows. We consider an axisymetric, rotating, steady viscous-resistive, magnetized accretion flow under an advection dominated stage. The dominant mechanism of energy dissipation is assumed to be turbulence viscosity and magnetic diffusivity. We adopt a self-similar assumption in the radial direction to obtain the dynamical quantities, that is, radial, azimuthal, sound and Alfv\' en velocities. Our results show the vertical component of magnetic force acts in the opposite direction of gravity and compresses the disc, thus compared with the non-magnetic case, in general the disc half-thikness, $Δθ$, significantly is reduced. On the other hand, two parameters, appearing due to action of magnetic field and reaction of the flow, affect the disc thickness. The first one, $β_0$, showing the magnetic field strength at the equatorial plane, decreases $Δθ$. The other one, $η_0$ is the magnetic resistivity parameter and when it increases, $Δθ$ increases, too.

astro-ph.HE↗
arXiv2012

Viscous accretion of a polytropic self-gravitating disk in the presence of wind

Self-similar and semi-analytical solutions are found for the height-averaged equations govern the dynamical behavior of a polytropic, self-gravitating disk under the effects of winds, around the nascent object. In order to describe time evolution of the system, we adopt a radius dependent mass loss rate, then highlight its importance on both the traditional $α$ and innovative $β$ models of viscosity prescription. In agreement with some other studies, our solutions represent that Toomre parameter is less than one in most regions on the $β$-disk which indicate that in such disks gravitational instabilities can occur in various distances from the central accretor and so the $β$-disk model might provide a good explanation of how the planetary systems form. The purpose of the present work is twofold. First, examining the structure of disk with wind in comparison to no-wind solution; and second, to see if the adopted viscosity prescription affects significantly the dynamical behavior of the disk-wind system. We also considered the temperature distribution in our disk by a polytropic condition. The solutions imply that, under our boundary conditions, the radial velocity is larger for $α$-disks and increases as wind becomes stronger in both viscosity models. Also, we noticed that the disk thickness increases by amplifying the wind or adopting larger values for polytropic exponent $γ$. It also may globally decrease if one prescribe $β$-model for the viscosity. Moreover, in both viscosity models, surface density and mass accretion rate reduce as wind gets stronger or $γ$ increases.

astro-ph.SR↗
arXiv2006

Self-Similar Evolutionary Solutions of Self-Gravitating, Polytropic $β$-Viscous Disks

We carry out the effect of $β$-prescription for viscosity which introduced by Duschel et al. 2000 & Hure, Richard & Zhan 2001, in a standard self-gravitating thin disks. We were predicted in a self-gravitating thin disk the $β$-model will have different dynamical behavior compare the well known $α$-prescriptions. We used self-similar methods for solving the integrated equations which govern the dynamical behavior of the thin disks. We present the results of self-similar solutions of the time evolution of axisymmetric, polytropic, self-gravitating viscous disks around a new born central object. We apply a $β$-viscosity prescription which has been derived from rotating shear flow experiments ($ν=βr^2Ω$). Using reduced equations in a slow accretion limit, we demonstrate inside-out self-similar solutions after core formation in the center. Some physical quantities for $β$-disks are determined numerically.We have compared our results with $α$-disks under the same initial conditions. It has been found that the accretion rate onto the central object for $β$-disks more than $α$-disks at least in the outer regions where $β$-disks are more efficient. Our results show that Toomre instability parameter is less than one everywhere on the $β$-disk which means that in such disks gravitational instabilities can be occurred, so the $β$-disk model can be a good candidate for the origin of planetary systems. Our results show that the $β$-disks will decouple in the outer part of the disk where the self-gravity plays an important role which is in agreement with Duschl predictions.

astro-ph↗