arXiv subjects

Mohsen Shadmehri

Publications and source records attributed to Mohsen Shadmehri.

66 records · Page 4Linked to original sources

arXiv2005

Self-similar structure of the magnetized radiation-dominated accretion disks

We investigate the effects of a large-scale magnetic field with open field lines on the steady-state structure of a radiation-dominated accretion disk, using self-similarity technique. The disk is supposed to be turbulent and possesses an effective viscosity and an effective magnetic diffusivity. We consider the extreme case in which the generated energy due to viscous and magnetic dissipation is balanced by the advection cooling. While the magnetic field outside of the disk is treated in a phenomenological way, the internal field is determined self-consistently. Magnetized and nonmagnetized solutions have the same radial dependence, irrespective of the values of the input parameters. Generally, our self-similar solutions are very sensitive to the viscosity or diffusivity coefficients. For example, the density and the rotation velocity increase when the viscosity coefficient decreases. The gas rotates with sub-Keplerian angular velocity with a factor less than unity which depends on the magnetic field configuration. Magnetic field significantly reduce disk thickness, however, tends to increase the radial velocity comparing to the nonmagnetic self-similar solutions.

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arXiv2004

Gravitational collapse of polytropic, magnetized, filamentary clouds

When the gas of a magnetized filamentary cloud obeys a polytropic equation of state, gravitational collapse of the cloud is studied using a simplified model. We concentrate on the radial distribution and restrict ourselves to the purely toroidal magnetic field. If the axial motions and poloidal magnetic fields are sufficiently weak, we could reasonably expect our solutions to be a good approximation. We show that while the filament experiences gravitational condensation and the density at the center increases, the toroidal flux-to-mass ratio remains constant. A series of spatial profiles of density, velocity and magnetic field for several values of the toroidal flux-to-mass ratio and the polytropic index, is obtained numerically and discussed.

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arXiv2004

The Lifetimes and Evolution of Molecular Cloud Cores

We discuss the lifetimes and evolution of clumps and cores formed as turbulent density fluctuations in nearly isothermal molecular clouds. In the non-magnetic case, clumps are unlikely to reach a hydrostatic state, and instead are expected to either proceed directly to collapse, or else ``rebound'' towards the mean pressure and density of the parent cloud. Rebounding clumps are delayed in their re-expansion by their self-gravity. From a simple virial calculation, we find re-expansion times of a few free-fall times. In the magnetic case, we present a series of driven-turbulence, ideal-MHD isothermal numerical simulations in which we follow the evolution of clumps and cores in relation to the magnetic criticality of their ``parent clouds'' (the numerical boxes). In subcritical boxes, magnetostatic clumps do not form. A few moderately-gravitationally bound clumps form which however are dispersed by the turbulence in < 1.3 Myr. An estimate of the ambipolar diffusion (AD) time scale t_AD in these cores gives t_AD > 1.3 Myr, only slightly longer than the dynamical times. In supercritical boxes, some cores become locally supercritical and collapse in typical times ~ 1 Myr. We also observe longer-lived supercritical cores that however do not collapse because they are smaller than the local Jeans length. Fewer clumps and cores form in these simulations than in their non-magnetic counterpart. Our results suggest that a) A fraction of the cores may not form stars, and may correspond to some of the observed starless cores. b) Cores may be out-of-equilibrium structures, rather than quasi-magnetostatic ones. c) The magnetic field may help reduce the star formation efficiency by reducing the probability of core formation, rather than by significantly delaying the collapse of individual cores.

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arXiv2004

Variability in the stellar initial mass function at high mass: coalescence models for starburst clusters

A coalescence model using the observed properties of pre-stellar condensations (PSCs) shows how an initially steep IMF that might be characteristic of primordial cloud fragmentation can change into a Salpeter IMF or shallower IMF in a cluster of normal density after one dynamical time, even if the PSCs are collapsing on their own dynamical time. The model suggests that top-heavy IMFs in some starburst clusters originate with PSC coalescence.

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arXiv2004

Time-dependent evolution of quasi-spherical, self-gravitating accretion flow

A self-similar solution for time evolution of quasi-spherical, self-gravitating accretion flows is obtained under the assumption that the generated heat by viscosity is retained in the flow. The solutions are parameterized by the ratio of the mass of the accreting gas to the central object mass and the viscosity coefficient. While the density and the pressure are obtained simply by solving a set of ordinary differential equations, the radial and the rotational velocities are presented analytically. Profiles of the density and the rotational velocities show two distinct features. Low density outer accreting flow with relatively flat rotation velocity, surrounds an inner high density region. In the inner part, the rotational velocity increases from the center to a transition radius where separates the inner and outer portions. We show that the behaviour of the solutions in the inner region depends on the ratio of the heat capacities, $γ$, and the viscosity coefficient, $α$.

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arXiv2004

A Model for Quasi-Spherical Magnetized Accretion Flow

A model for axisymmetric magnetized accretion flow is proposed. The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence in the accretion flow. In analogy to the advection-dominated accretion flow (ADAF) solutions, a constant fraction of the resistively dissipated energy is stored in the accreting gas and the rest is radiated. The general self-similar solutions which describe a resistive and {\it nonrotating} flow with purely poloidal magnetic field is first introduced. The radial dependence of physical quantities is identical to viscous ADAF solutions. Although the main focus of this study is nonrotating magnetized accretion flow, for rotating flow with both poloidal and toroidal components of magnetic field we find the radial scaling of solutions similar to the nonrotating case. We show that the accretion and the rotation velocities are both below the Keplerian rate, irrespective of the amount of cooling. We show that the set of the equations is reduced to a second order differential equation for a nonrotating flow. The geometrical shape of the disk changes depending on the fraction of the resistively dissipated energy which is stored in the accreting gas. However, there is a hot low-density gas above the disk in almost all cases. The net accretion rate is calculated for a set of illustrative parameters.

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arXiv2003

Can Hydrostatic Cores Form Within Isothermal Molecular Clouds?

Under the assumptions that molecular clouds are nearly spatially and temporally isothermal and that the density peaks (``cores'') within them are formed by turbulent fluctuations, we argue that cores cannot reach a hydrostatic (or magneto-static) state as a consequence of their formation process. In the non-magnetic case, stabilization requires a Bonnor-Ebert truncation at a finite radius, which is not feasible for a single-temperature flow, unless it amounts to a shock, which is clearly a dynamical feature. Instead, in this case, cores must be dynamical entities that can either be pushed into collapse, or else ``rebound'' towards the mean pressure and density as the parent cloud. Nevertheless, rebounding cores are delayed in their re-expansion by their own self-gravity. We give a crude estimate for the re-expansion time as a function of the closeness of the final compression state to the threshold of instability, finding typical values $\sim 1$ Myr, i.e., of the order of a few free-fall times. Our results support the notion that not all cores observed in molecular clouds need to be on route to forming stars, but that instead a class of ``failed cores'' should exist, which must eventually re-expand and disperse, and which can be identified with observed starless cores. In the magnetic case, recent observational and theoretical work suggests that all cores are critical or supercritical, and are thus qualitatively equivalent to the non-magnetic case. Our results support the notion that the entire star formation process is dynamical, with no intermediate hydrostatic stages.

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arXiv2002

Constraints on Star Formation from the Close Packing of Protostars in Clusters

The mm-wave continuum sources (MCS) in Ophiuchus have mutual collision rates less than their collapse rates by a factor of 10 to 100, suggesting most will form stars without further interactions. However, this ratio of rates would have exceeded unity in the past if the MCS were 2.5 times larger than they are now. Such a high previous ratio was not likely, suggesting that the MCS grew from smaller sizes at a constant high density, possible behind turbulent shocks. Proximity constraints are even more important in massive clusters, where massive stars with the same or greater space density are more strongly interacting than the Ophiuchus MCS. As a result, the density contrast for MCS must be larger in massive clusters than it is in Ophiuchus, or significant coalescence will occur in the protostellar phase, possible forming a massive black hole. A proportionality to the second power of the Mach number allows the MCS cores to collapse independently. These results suggest that stars in dense clusters generally form on a dynamical time by the continuous collection and rapid collapse of turbulence-shocked gas. Implications of proximity constraints on the initial stellar mass function are also discussed.

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arXiv2002

Cooling Flows of Self-Gravitating, Rotating, Viscous Systems

We obtain self-similar solutions that describe the dynamics of a self-gravitating, rotating, viscous system. We use simplifying assumptions; but explicitly include viscosity and the cooling due to the dissipation of energy. By assuming that the turbulent dissipation of energy is as power law of the density and the speed v_{rms} and for a power-law dependence of viscosity on the density, pressure, and rotational velocity, we investigate turbulent cooling flows. It has been shown that for the cylindrically and the spherically cooling flows the similarity indices are the same, and they depend only on the exponents of the dissipation rate and the viscosity model. Depending on the values of the exponents, which the mechanisms of the dissipation and viscosity determine them, we may have solutions with different general physical properties. The conservation of the total mass and the angular momentum of the system strongly depends on the mechanisms of energy dissipation and the viscosity model.

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arXiv2001

Virial theorem analysis of 3D numerical simulations of MHD self-gravitating turbulence

We discuss the virial balance of all members of a cloud ensemble in numerical simulations of self-gravitating MHD turbulence. We first discuss the choice of reference frame for evaluating the terms entering the virial theorem (VT), concluding that the balance of each cloud should be measured in its own reference frame. We then report preliminary results suggesting that a) the clouds are far from virial equilibrium, with the ``geometric'' (time derivative) terms dominating the VT. b) The surface terms in the VT are as important as the volume ones, and tend to decrease the action of the latter. c) This implies that gravitational binding should be considered including the surface terms in the overall balance.

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arXiv2001

Self-Similar Equilibria of Self-Gravitating, Magnetized, Rotating, Isothermal Systems

The self-similar equilibrium models of self-gravitating, rotating, isothermal systems are investigated analytically. In these models the rotation velocity is constant and the density varies as $\frac{f(θ, ϕ)}{r^2}$, where $r$ and $θ$ are the spherical radius and the co-latitude, respectively. The nonaxisymmetric solutions contain three free parameters, one of the parameters depends on the rotation velocity. These parameters determine the overall shape of the density distribution. By assuming that the dominant component of the magnetic field is purely toroidal and the ratio of the purely toroidal magnetic pressure to the gas pressure, $α$, is spatially constant, the axisymmetric solutions generalized so as the effect of magnetic field could be studied. We find that the equilibria of axially symmetric systems yield ellipsoids or spheres only when the ratio of rotation velocity to the sound speed is taken to be $\sqrt{2α}$.

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arXiv2001

Radiative Cooling Flows of Self-Gravitating Filamentary Clouds

We study the dynamics of a self-gravitating cooling filamentary cloud using a simplified model. We concentrate on the radial distribution and restrict ourselves to quasi-hydrostatic, cylindrically symmetric cooling flows. For a power-law dependence of cooling function on the temperature, self-similar solutions which describe quasi-hydrostatic cooling flows are derived. We consider obtically thin filaments with a constant mass per unit length and the solutions are parameterized by their line masses. There is no polytropic relation between the density and the pressure. The filament experiences radiative condensation, irrespective of the γ, the gas specific heat ratio. So, the filament becomes denser due to the quasi-hydrostatic flows and the density at the center increases in proportion to (t_{0}-t)^{-1}, where t denotes the time. The term, t_{0}, denotes an epoch at which the central density increases infinitely. We also found that the radius of the filament decreases in proportion to (t_{0}-t)^{0.5}.

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