arXiv subjects

Mohsen Nejad-Asghar

Publications and source records attributed to Mohsen Nejad-Asghar.

18 recordsLinked to original sources

arXiv2025

The effect of non-uniformity density structure on the molecular cloud-cores magnetic braking in the ideal MHD framework

The phenomenon of magnetic braking is one of the significant physical effects of the magnetic field in rotating molecular clouds. The physical characteristics of the core can affect on the core rotation rate. and one of the important parameter is the core density structure. According to observation, by regarding the power-law density distribution, $r^{-p}$, for molecular cloud cores, using smoothed particle hydrodynamics simulation, the results show that the increasing of density steepness (i.e., larger $p$) leads to the intensity of the toroidal components of the magnetic field and as a result larger $B_ϕ$-components lead to more transfer of angular momentum to the outward. Thus, results show that the magnetic braking being stronger with increasing density slope in non-uniform molecular core. For example, the rotation of the system can approximately decrease by fifty percent from $p=0.2$ to $p=1.8$ for a non-uniform system.

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arXiv2024

Global Thermal Instability in the Spherical Interstellar Clouds

Thermal instability (TI) is a trigger mechanism, which can explain the formation of small condensations through some regions of the interstellar clouds. The instability criterion for flat geometry approximations has been investigated in previous works. Here, we focus on spherical perturbations in the spherical clouds. Our goal here is to examine the conditions for the occurrence of TI through the thermally dominated (i.e., gravitationally stable) quasi-static spherical interstellar clouds. First, we obtain the profiles of density, temperature, pressure, and enclosed mass of a symmetric spherical cloud. Then, we use the perturbation method to investigate the linear regime of instability and find its growth rate. Considering spherical perturbations on the quasi-static spherical cloud, instead of a thermal and dynamical equilibrium flat cloud, changes the instability criterion so that we can conclude that sphericalness can increase the occurrence of TI. The results show that in the spherical clouds, perturbations with shorter wavelengths have more chance to grow via TI (i.e., greater growth rates).

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arXiv2022

Some Aspects of Rotation and Magnetic Field Morphology in the Infrared Dark Cloud G34.43+00.24

The infrared dark clouds (IRDCs) are molecular clouds with relatively greater values in their magnetic field strengths. For example, the IRDC G34.43+00.24 (G34) has magnetic field strength of the order of a few hundred micro-Gauss. In this study, we investigate if the dynamic motions of charged particles in an IRDC such as G34 can produce this magnetic field strength inside it. The observations show that the line-of-sight velocity of G34 has global gradient. We assume that the measured global velocity gradient can correspond to the cloud rotation. We attribute a large-scale current density to this rotating cloud by considering a constant value for the incompleteness of charge neutrality and the velocity differences between the positive and negative particles with very low ionization fractions. We use the numerical package FISHPACK to obtain the magnetic field strength and its morphology on the plane-of-sky within G34. The results show that the magnetic field strengths are of the order of several hundred micro-Gauss, and its morphology in the plane-of-sky is somewhat consistent with the observational results. We also obtain the relationship between magnetic field strength and density in G34. The results show that with increasing density, the magnetic field strength increases approximately as a power-law function. The amount of power is approximately equal to 0.45, which is suitable for molecular clouds with strong magnetic fields. Therefore, we can conclude that the dynamical motion of IRDCs, and especially their rotations, can amplify the magnetic field strengths within them.

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arXiv2019

Thermal instability through the outer half of quasi-static spherically symmetric molecular clumps and cores

Thermal instability (TI) is a trigger mechanism, which can explain formation of condensations through some regions of the interstellar clouds. Our goal here is to investigate some conditions for occurrence of TI and formation of pre-condensations through the outer half a quasi-static spherical molecular clump or core. The inner half is nearly singular and ambiguous so out of scope of this research. We consider a spherically symmetric molecular cloud in quasi-static and thermally equilibrium state, and we use the linear perturbation method to investigate occurrence of TI through its outer half. The origin of perturbations are assumed to be as Inside-Rush-Perturbation (IRP) with outward perturbed velocity at inner region of the cloud, and Outside-Rush-Perturbation (ORP) with inward perturbed velocity originated at the outer parts of the cloud. The local thermal balance at the outer half of the molecular cloud leads to a local loosely constrained power-law relation between the pressure and density as $p \propto ρ^{1+χ}$, where $-0.4\lesssim χ\lesssim 0.05$ depends on the functional form of the net cooling function. Physically, the value of $χ$ depends on the power of dependence of magnetic field to the density, $η$, and also on the value of magnetic field gradient, $ζ$. For strong magnetic field (smaller $η$) and/or large field gradient (greater $ζ$), the value of $χ$ decreases, and vice versa. The results show that increasing of the value of $χ$ leads to form a flatter density profiles at the thermally equilibrium outer half of the molecular clump or core, and to occur more thermally unstable IRP and ORP with smaller growth time-scales, and vice versa.

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arXiv2018

The effect of magnetic field morphology on the structure of massive IRDC clumps

Infrared dark clouds (IRDCs) have dense elongated clumps and filaments with the favorable viewing condition of being on the near-side of a bright mid-infrared background. The clumps usually have multiple cores around the center. In this work, we study the effect of magnetic field morphology on the structure of massive IRDC clumps. To achieve this goal, we consider an axisymmetric isothermal oblate IRDC clump, embedded into a constant external magnetic field. We assume a polynomial function for the magnetic field morphology inside the clump. We use the numerical iterative methods to solve the equations: the successive over-relaxation method to find the magnetic and gravitational fluxes, and then the bicongugate gradient method to find the optimized values of mass and current densities. The results show that the IRDC clump will be very elongated along the perpendicular direction of the external magnetic field lines. Also, the assumption of choosing of a polynomial function for the magnetic field morphology leads to the formation of dense regions around the center. The greater the density of the central region, the larger the density of these dense regions and the closer to the center. The presence of these dense regions can lead to the formation of cores at these points.

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arXiv2017

The Kelvin-Helmholtz instability in the Orion nebula: The effect of radiation pressure

The recent observations of rippled structures on the surface of the Orion molecular cloud (Berné et al. 2010), have been attributed to the Kelvin-Helmholtz (KH) instability. The wavelike structures which have mainly seen near star-forming regions taking place at the interface between the hot diffuse gas, which is ionized by massive stars, and the cold dense molecular clouds. The radiation pressure of massive stars and stellar clusters is one of the important issues that has been considered frequently in the dynamics of clouds. Here, we investigate the influence of radiation pressure, from well-known Trapezium cluster in the Orion nebula, on the evolution of KH instability. The 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 decreases 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. Our results shows that for $λ_{\rm KH}>0.15\rm pc$, the growth rate of KH instability dose not depend to the radiation pressure. Based on our results, the radiation pressure is a triggering mechanism in development of the KH instability and subsequently formation of turbulent sub-structures in the molecular clouds near massive stars. The role of magnetic fields in the presence of the radiation pressure is also investigated and it is resulted that the magnetic field suppresses the effects induced by the radiation pressure.

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arXiv2016

Occurrence of instability through the protostellar accretion disks by landing of low-mass condensations

Low-mass condensations (LMCs) are observed inside the envelope of the collapsing molecular cloud cores. In this research, we investigate the effects of landing LMCs for occurrence of instability through the protostellar accretion disks. We consider some regions of the disk where duration of infalling and landing of the LMCs are shorter than the orbital period. In this way, we can consider the landing LMCs as density bumps and grooves in the azimuthal direction of an initial thin axisymmetric steady state self-gravitating protostellar accretion disk (nearly Keplerian). Using the linear effects of the bump quantities, we obtain a characteristic equation for growth/decay rate of bumps; we numerically solve it to find occurrence of instability. We also evaluate the minimum-growth-time-scale (MGTS) and the enhanced mass accretion rate. The results show that infalling and landing of the LMCs in the inner regions of the protostellar accretion disks can cause faster unstable modes and less enhanced accretion rates relative to the outer regions. Also, more fragmentation of landed LMCs in the azimuthal direction have less chance for instability, and then can produce more values of enhanced mass accretion rate.

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arXiv2014

Rossby-wave instability in viscous discs

The Rossby wave instability (RWI), which depends on the density bumps and extremum in the vortensities in the differentially rotating discs, plays an important role in the evolution of the protoplanetary discs. In this article, we investigate the effect of viscosity on the non-axisymmetric RWI in the self-graviting accretion discs. For this purpose, we add the viscosity to the work of Lovelace and Hohlfeld (2013). Consideration of viscosity complicates the problem so that we use the numerical method to investigate the stable and unstable modes. We consider three ranges of viscosities: high viscosity in the ranges $0.1\leq α\leq 0.4$, moderate viscosity in the ranges $0.01\leq α< 0.1$, and low viscosity in the ranges $α< 0.01$. The results show that the occurrence of the RWI is related to the value of viscosity so that the effect of high viscosity is important, while the low viscosity is negligible. These results may be applied for the study of the RWI role in planet formation and angular momentum transport for different kinds of the protoplanetary discs with different viscosities.

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arXiv2011

Matter Infall in Collapsing Molecular Cloud Cores with an Axial Magnetic Field

The magnetic fields affect collapse of molecular cloud cores. Here, we consider a collapsing core with an axial magnetic field and investigate its effect on infall of matter and formation of accretion disk. For this purpose, the equations of motion of ions and neutral infalling particles are numerically solved to obtain the streamlines of trajectories. The results show that in non-steady state of ionization and ion-neutral coupling, which is not unexpected in the case of infall, the radius of accretion disk will be larger as a consequence of axial magnetic field.

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arXiv2011

Relaxation of protostellar accretion shocks using the smoothed particle hydrodynamics

It is believed that protostellar accretion disks to be formed from nearly ballistic infall of the molecular matters in rotating core collapse. Collisions of these infalling matters lead to formation of strong supersonic shocks, which if they cool rapidly, result in accumulation of that material in a thin structure in the equatorial plane. Here, we investigate the relaxation time of the protostellar accretion post-shock gas using the smoothed particle hydrodynamics (SPH). For this purpose, a one-dimensional head-on collision of two molecular sheets is considered, and the time evolution of the temperature and density of the post-shock region simulated. The results show that in strong supersonic shocks, the temperature of the post-shock gas quickly increases proportional to square of the Mach number, and then gradually decreases according to the cooling processes. Using a suitable cooling function shows that in appropriate time-scale, the center of the collision, which is at the equatorial plane of the core, is converted to a thin dense molecular disk, together with atomic and ionized gases around it. This structure for accretion disks may justify the suitable conditions for grain growth and formation of proto-planetary entities.

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arXiv2011

Formation of low-mass condensations in the molecular cloud cores via thermal instability

The low-mass condensations (LMCs) have been observed within the molecular cloud cores. In this research, we investigate the effect of isobaric thermal instability (TI) applied for forming these LMCs. For this purpose, at first we investigate the occurrence of TI in the molecular clouds. Then, for studying the significance of linear isobaric TI, we use a contracting axisymmetric cylindrical core with axial magnetic field. Consideration to cooling and heating mechanisms in the molecular clouds shows that including the heating due to ambipolar diffusion can lead to the occurrence of TI in a time-scale smaller than dynamical time-scale. Application of linear perturbation analysis shows that isobaric TI can take place in outer region of the molecular cloud cores. Furthermore, the results showthat perturbations with wavelengths greater than few astronomical units are protected from destabilization property of thermal conduction, so they can grow to form LMCs. Thus, the results show that the mechanism of TI can be used to explain the formation of LMCs as the progenitors of collapsing proto-stellar entities, brown dwarfs, or proto-planets.

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arXiv2010

Non-similar collapse of singular isothermal spherical molecular cloud cores with nonzero initial velocities

Theoretically, stars have been formed from the collapse of cores in the molecular clouds. Historically, the core had been assumed as an singular isothermal sphere (SIS), and the collapse had been investigated by a self-similar manner. This is while the rotation and magnetic field lead to non-symmetric collapse so that a spheroid shape may be occurred. Here, the resultant of the centrifugal force and magnetic field gradient is assumed to be in the normal direction of the rotational axis, and its components are supposed to be a fraction $β$ of the local gravitational force. In this research, a collapsing SIS core is considered to find the importance of the parameter $β$ for oblateness of the mass shells which are above the head of the expansion wave. We apply the Adomian decomposition method to solve the system of nonlinear partial differential equations because the collapse does not occur in a spherical symmetry with self-similar behavior. In this way, we obtain a semi-analytical relation for the mass infall rate $\dot{M}$ of the shells at the envelope. Near the rotational axis, the $\dot{M}$ decreases with increasing of the non-dimensional radius $ξ$, while a direct relation is observed between $\dot{M}$ and $ξ$ in the equatorial regions. Also, the values of $\dot{M}$ in the polar regions are greater than the equatorial values, and this difference is more often at smaller values of $ξ$. Overall, the results show that before reaching the head of expansion wave, the visible shape of the molecular cloud cores can evolve to oblate spheroids. The ratio of major to minor axes of oblate cores increases with increasing the parameter $β$, and its value can approach to the apparently observed elongated shapes of cores in the maps of molecular clouds such as Taurus and Perseus.

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arXiv2009

Thermal instability in ionized plasma

We study magnetothermal instability in the ionized plasmas including the effects of Ohmic, ambipolar and Hall diffusion. Magnetic field in the single fluid approximation does not allow transverse thermal condensations, however, non-ideal effects highly diminish the stabilizing role of the magnetic field in thermally unstable plasmas. Therefore, enhanced growth rate of thermal condensation modes in the presence of the diffusion mechanisms speed up the rate of structure formation.

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arXiv2008

Investigating thermal evolution of the self-gravitating one dimensional molecular cloud by smoothed particle hydrodynamics

The heating of the ion-neutral (or ambipolar) diffusion may affect the thermal phases of the molecular clouds. We present an investigation on the effect of this heating mechanism in the thermal instability of the molecular clouds. A weakly ionized one dimensional slab geometry, which is allowed for self-gravity and ambipolar diffusion, is chosen to study its thermal phases. We use the thermodynamic evolution of the slab to obtain the regions where slab cloud becomes thermally unstable. We investigate this evolution using the model of ambipolar diffusion with two-fluid smoothed particle hydrodynamics, as outlined by Hosking & Whitworth. Firstly, some parts of the technique are improved to test the pioneer works on behavior of the ambipolar diffusion in an isothermal self-gravitating slab. Afterwards, the improved two-fluid technique is used for thermal evolution of the slab. The results show that the thermal instability may persist inhomogeneities with a large density contrast at the intermediate parts of the cloud. We suggest that this feature may be responsible for the planet formation in the intermediate regions of a collapsing molecular cloud and/or may also be relevant to the formation of star forming dense cores in the clumps.

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arXiv2006

Formation of Small-Scale Condensations in the Molecular Clouds via Thermal Instability

A systematic study of the linear thermal instability of a self-gravitating magnetic molecular cloud is carried out for the case when the unperturbed background is subject to local expansion or contraction. We consider the ambipolar diffusion, or ion-neutral friction on the perturbed states. In this way, we obtain a non-dimensional characteristic equation that reduces to the prior characteristic equation in the non-gravitating stationary background. By parametric manipulation of this characteristic equation, we conclude that there are, not only oblate condensation forming solutions, but also prolate solutions according to local expansion or contraction of the background. We obtain the conditions for existence of the Field lengths that thermal instability in the molecular clouds can occur. If these conditions establish, small-scale condensations in the form of spherical, oblate, or prolate may be produced via thermal instability.

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arXiv2006

Simulation of Shock Waves by Smoothed Particle Hydrodynamics

Isothermal and adiabatic shocks, which are produced from fast expansion of the gas, is simulated with smoothed particle hydrodynamics (SPH). The results are compared with the analytic solutions. The algorithm of the program is explained and the package, which is written in Fortran, is presented in the appendix of this paper. It is possible to change (to complete) the program for a wide variety of applications ranging from astrophysics to fluid mechanics.

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arXiv2005

Linear Thermal Instability and Fluctuations in Molecular Clouds

Evidence of small-scale condensations in the magnetic molecular clouds has been accumulating over the past decades through radio and optical/ultraviolet observations. The origin and shape of these small-scale condensations is a disputable issue. Nejad-Asghar & Ghanbari (2004 hereafter NG) have recently studied the effect of the linear thermal instability on the formation of fluctuations in molecular clouds. The authors inferred that under certain conditions (e.g., depending on expansion or contraction of the background) thermal instability and ambipolar diffusion can produce spherical, oblate, or prolate condensations.

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arXiv2003

Linear Thermal Instability and Formation of Clumpy Gas Clouds Including the Ambipolar Diffusion

Thermal instability is one of the most important processes in the formation of clumpy substructure in magnetic molecular clouds. On the other hand, ambipolar diffusion, or ion-neutral friction, has long been thought to be an important energy dissipation mechanism in these clouds. Thus, we would interested to investigate the effect of ambipolar diffusion on the thermal instability and formation of clumps in the magnetic molecular clouds. For this purpose, in the first step, we turn our attention to the linear perturbation stage. In this way, we obtain a non-dimensional characteristic equation which reduces to the prior characteristic equation in the absence of the magnetic field and ambipolar diffusion. With numerical manipulation of this characteristic equation, we conclude that there are solutions where the thermal instability allows compression along the magnetic field but not perpendicular to it. We infer that this aspect might be an evidence in formation of observed disc-like (oblate) clumps in magnetic molecular clouds.

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