Dwarf novae are a class of cataclysmic variables that consist of a white dwarf star (as primary star) and a low-mass donor star (as secondary star). The white dwarf and the secondary star orbit each other once every few hours. When the low-mass secondary star fills its Roche-lobe, it becomes tidally distorted due to its vicinity to the white dwarf. Thus, the secondary star matter is transferred into the white dwarf Roche-lobe. The matter being transferred has high angular momentum in respect to the white dwarf and it forms an accretion disk around the white dwarf. The angular momentum of the accretion disk is transferred by viscous torques from the inner regions of the disk to the outer regions. The viscosity is also responsible for heat generation in the accretion disk. The energy generated by viscous dissipation inside the disk is radiated away from the surface of it. That is why the accretion disk has been found to be the source of the luminosity in the dwarf novae. The light curve of a dwarf nova shows the suddenly increase of brightness which is known as outburst. The SU UMa stars, which are a subclass of dwarf novae, exhibit two distinct modes of outburst, normal outburst and superoutburst. The normal outbursts have amplitude of approximately equal to three magnitude and last typically from one to four days. However, the superoutbursts are approximately one magnitude brighter than normal outbursts and last as long as a couple of few weeks rather than just a few days. During superoutburst, the periodic humps with the name of superhump appear in the light curve of SU UMa stars. The superhump is an additional variation of the brightness which has a period that is a few percent longer than the orbital period of binary star system. The superhumps are seen in systems with the mass ratio smaller than 0.3, with being the mass ratio as the ratio of masses of secondary star to primary star. In such systems, the disk grows to a size a where and a tidal instability that induces the accretion disc to become eccentric and starts to process in the corotating frame. The superhumps are appeared in the light curve, because of the tidal effects of donor star on the disk and also the viscous dissipation is large when the bulk of the eccentric disc passes the donor star. The observations of OT J002656.6+284933 have confirmed that this object should be a dwarf nova of SU UMa type which the superhumps appear in its light curve in during superoutburst. The recent observations imply that dwarf nova OT J002656.6+284933 has the orbital period 0.13d, the secondary star with mass 0.2 Msun and the superhump period 0.13225d. The analysis of observational data has not been successful to calculate an exact value for the mass ratio (which is the ratio of masses of secondary star to primary star); their estimation for the mass ratio has the uncertainty between 0.1 and 0.15. To solve the problem regarding mass ratio estimation and to simulate this object, we have simulated this dwarf nova in a two dimensional approach using the smoothed particle hydrodynamics method. As mentioned before, the superhumps are the distinctive humps on the light curve and its period can be estimated accurately. Thus, we applied the different values of the mass ratio in the simulations that for which one of them, we could reach to the observational superhump period value. In the simulations, we assumed that the observational orbital period is 0.13d and applied the mass ratio between 0.14 and 0.18. The simulations imply that the superhump period 0.13225d can be obtained approximately for the mass ratio 0.145. © 2018 Institute of Geophysics. All right reserved.

In this paper,we study the dynamics of a geometrically thin, steady and axisymmetric accretion disc surrounding a rotating and magnetized star. The magnetic field lines of star penetrate inside the accretion disc and are twisted due to the differential rotation between the magnetized star and the disc. We apply the Hall diffusion effect in the accreting plasma, because of the Hall diffusion plays an important role in both fully ionized plasma and weakly ionized medium. In the current research, we show that the Hall diffusion is also an important mechanism in accreting plasma around neutron stars. For the typical system parameter values associated with the accreting X-ray binary pulsar, the angular velocity of the inner regions of disc departs outstandingly from Keplerian angular velocity, due to coupling between the magnetic field of neutron star and the rotating plasma of disc. We found that the Hall diffusion is very important in inner disc and increases the coupling between the magnetic field of neutron star and accreting plasma. On the other word, the rotational velocity of inner disc significantly decreases in the presence of the Hall diffusion. Moreover, the solutions imply that the fastness parameter decreases and the angular velocity transition zone becomes broad for the accreting plasma including the Hall diffusion. © 2016 The Authors.

The purpose of this paper is to explore the influences of cooling timescale on fragmentation of self-gravitating protoplanetary disks. We assume the cooling timescale, expressed in terms of the dynamical timescale Ω t cool, has a power-law dependence on temperature and density, Ω tcool ∝ Σ-aT-b, where a and b are constants. We use this cooling timescale in a simple prescription for the cooling rate, du/dt = -u/tcool, where u is the internal energy. We perform our simulations using the smoothed particle hydrodynamics method. The simulations demonstrate that the disk is very sensitive to the cooling timescale, which depends on density and temperature. Under such a cooling timescale, the disk becomes gravitationally unstable and clumps form in the disk. This property even occurs for cooling timescales which are much longer than the critical cooling timescale, Ω tcool ≳ 7. We show that by adding the dependence of a cooling timescale on temperature and density, the number of clumps increases and the clumps can also form at smaller radii. The simulations imply that the sensitivity of a cooling timescale to density is more than to temperature, because even for a small dependence of the cooling timescale on density, clumps can still form in the disk. However, when the cooling timescale has a large dependence on temperature, clumps form in the disk. We also consider the effects of artificial viscosity parameters on fragmentation conditions. This consideration is performed in two cases, where Ω tcool is a constant and Ω tcool is a function of density and temperature. The simulations consider both cases, and results show the artificial viscosity parameters have rather similar effects. For example, using too small of values for linear and quadratic terms in artificial viscosity can suppress the gravitational instability and consequently the efficiency of the clump formation process decreases. This property is consistent with recent simulations of self-gravitating disks. We perform simulations with and without the Balsara form of artificial viscosity. We find that in the cooling and self-gravitating disks without the Balsara switch, the clumps can form more easily than those with the Balsara switch. Moreover, in both cases where the Balsara switch is present or absent, the simulations show that the cooling timescale strongly depends on density and temperature. © 2014 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.

In this paper, we explore the radial structure of radiatively inefficient accretion flows (RIAFs) in the presence of an ordered magnetic field and convection. We assume the magnetic field has the toroidal and vertical components. We apply the influences of convection on equations of angular momentum and energy. The convective instability can transport the angular momentum inward or outward. We establish two cases for consideration of the effects of convection parameter on magnetized RIAFs. In the first case, we assume the convection parameter as a free parameter and in the other case we calculate convection parameter through use of mixing length theory. In both cases, the solutions show that a magnetized RIAF is very sensitive to the convection parameter and transport direction of angular momentum due to convection. Moreover, we show that the convection strength strongly depends on magnetic field and viscosity. © 2014 Springer Science+Business Media Dordrecht.

The models of radiatively inefficient accretion flow (RIAF) are used to explain the observations of the hot gas that surrounds Sgr A* and the supermassive black holes of elliptical galaxies. The direct numerical simulations of RIAFs have shown that convection is a general feature in such accretion flows. In this paper, we analytically investigate time evolution of RIAFs in the presence of convection. The convection turbulent will be responsible to transport of angular momentum and energy. We consider the cases of inward and outward transports of angular momentum by convection. To estimate strength of convection diffusion, we use mixing length theory and introduce the convection parameter αc in analogy to α-prescription of standard theory of accretion discs. We apply a wide range of viscosity parameter, α = 0.001-0.3, to study the influences of viscosity on the strength of convection parameter. We found that the results critically depend on the parametrized magnitude of viscosity and the transport direction of angular momentum by convection. As, with magnitude of viscosity, the gas density decreased and the radial infall velocity increased. The unlike to these physical variables, the convection parameter does not show a regular behaviour with respect to different ranges of viscosity parameter. But, in the both cases of inward and outward angular momentum transfer, the convection becomes important in accreting gas if the viscosity parameter is sufficiently small, α ≲ 0.1, else the convection becomes weaker than viscosity throughout the accreting gas. © 2014 The Author Published by Oxford University Press on behalf of the Royal Astronomical Society.

The importance of cooling for the structure and evolution of self-gravitating accretion discs has been confirmed through the use of direct numerical simulations. In this paper, we present a two-dimensional study for self-gravitating accretion discs, to investigate the influence of the cooling rate on the latitudinal structure of such accretion discs. The disc is cooled using a simple parametrization for the cooling function, (de/dt)cool=−e/tcoolwith e as the internal energy and tcoolas the cooling timescale. The cooling timescale is in units of the dynamical timescale, tdyn[=Ω−1], is Ωtcool=β, where β is a free parameter. The mechanism of energy dissipation is assumed to be turbulent viscosity in the disc and an α-prescription is applied for the kinematic coefficient of viscosity. To study the gravitational stability of the self-gravitating disc, we use the Toomre parameter. We obtain the radial dependence of the physical variables through the use of a self-similar method and we numerically solve the equations to obtain the latitudinal dependence of the physical variables. The solutions show that the radial velocity is smaller than the Keplerian rotational velocity; however, the disc, dependent on the values of parameters α and β and only near the zone close to the equatorial plane, can rotate in a super-Keplerian manner. With the magnitude of both parameters α and β, the disc thickness increases due to the increase of the vertical pressure gradient. The dependence of the gas density on the parameters α and β indicates two zones in the accretion disc. In the first zone near the equatorial plane, the mass density decreases by increasing these parameters. However, in the second zone, the regions with higher latitude, the mass density increases with the magnitude of parameters α and β. © 2014, Springer Science+Business Media Dordrecht.

The purpose of this paper is to explore the effect of magnetic fields on the dynamics of magnetized filamentary molecular clouds. We suppose there is a filament with cylindrical symmetry and two components of axial and toroidal magnetic fields. In comparison to previous works, the novelty in the present work involves a similarity solution that does not define a function of the magnetic fields or density. We consider the effect of the magnetic field on the collapse of the filament in both axial and toroidal directions and show that the magnetic field has a braking effect, which means that the increasing intensity of the magnetic field reduces the velocity of collapse. This is consistent with other studies. We find that the magnetic field in the central region tends to be aligned with the filament axis. Also, the magnitude and the direction of the magnetic field depend on the magnitude and direction of the initial magnetic field in the outer region. Moreover, we show that more energy dissipation from the filament causes a rise in the infall velocity. © 2014 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.

We explore the time evolution of radiatively-inefficient accretion flows. Since these types of accretion flows are convectively unstable, we also study the effects of convection in the present model. The effects of convection are applied to equations describing angular momentum and energy. In analogy to the traditional α-prescription, we introduce the convection parameter αc to study the influences of convection on physical quantities. The model is studied in two cases: the transport of angular momentum due to convection inward and outward. We found the physical variables are sensitive to the parameter αc and are also dependent on the direction of angular momentum that is transported by convection. As for angular momentum transfer inward, the accretion flow can be convectively dominated and radial infall velocity becomes zero. Moreover, we found the radial dependence of the density and radial velocity takes an intermediate place between steady state radiatively-inefficient accretion flow and steady state advection-dominated accretion flow. This property is in accord with direct numerical simulation of radiatively-inefficient accretion flows. © 2013 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd..

The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A* and a few other nearby galactic nuclei. On the other hand, the existence of outflow in accretion flows is confirmed by observations and magnetohydrodynamic (MHD) simulations. In this research, we study the influence of both thermal conduction and outflow on hot accretion flows with ordered magnetic field. Since the inner regions of hot accretion flows are, in many cases, collisionless with an electron mean free path due to Coulomb collision larger than the radius, we use a saturated form of thermal conduction, as is appropriate for weakly collisional systems. We also consider the influence of outflow on accretion flow as a sink for mass, and the radial and the angular momentum, and energy taken away from or deposited into the inflow by outflow. The magnetic field is assumed to have a toroidal component and a vertical component as well as a stochastic component. We use a radially self-similar method to solve the integrated equations that govern the behavior of such accretion flows. The solutions show that with an ordered magnetic field, both the surface density and the sound speed decrease, while the radial and angular velocities increase. We found that a hot accretion flow with thermal conduction rotates more quickly and accretes more slowly than that without thermal conduction. Moreover, thermal conduction reduces the influences of the ordered magnetic field on the angular velocities and the sound speed. The study of this model with the magnitude of outflow parameters implies that the gas temperature decreases due to mass, angular momentum, and energy loss. This property of outflow decreases for high thermal conduction. © 2013 Springer Science+Business Media Dordrecht.

We investigate the effects of the cooling function in the formation of clumps of protoplanetary disks using two-dimensional smoothed particle hydrodynamic simulations. We use a simple prescription for the cooling rate of the flow, du/dt = -u/τcool, where u and τcool are the internal energy and cooling timescale, respectively. We assume the ratio of local cooling to dynamical timescale, Ωτcool = β, to be a constant and also a function of the local temperature. We found that for the constant β and γ = 5/3, fragmentation occurs only for β ≲ 7. However, in the case of β having temperature dependence and γ = 5/3, fragmentation can also occur for largervalues of β. By increasing the temperature dependence of the cooling timescale, the mass accretion rate decreases, the population of clumps/fragments increases, and the clumps/fragments can also form in the smaller radii. Moreover, we found that the clumps can form even in a low mass accretion rate, ≲ 10-7M ȯ yr-1, in the case of temperature-dependent β. However, clumps form with a larger mass accretion rate, > 10 -7Mȯ yr-1, in the case of constant β. © 2013 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd..

We considered the effects of convection on the radiatively inefficient accretion flows (RIAF) in the presence of resistivity and toroidal magnetic field. We discussed the effects of convection on transports of angular momentum and energy. We established two cases for the resistive and magnetized RIAFs with convection: assuming the convection parameter as a free parameter and using mixing-length theory to calculate convection parameter. A self-similar method is used to solve the integrated equations that govern the behavior of the presented model. The solutions show that the accretion and rotational velocities decrease by adding the convection parameter, while the sound speed increases. Moreover, by using mixing-length theory to calculate convection parameter, we found that the convection can be important in RIAFs with magnetic field and resistivity. © 2012 Springer Science+Business Media B.V.

The existence of outflows in accretion flows has been confirmed by observations and by magnetohydrodynamics simulations. In this paper, we study the outflows of advection-dominated accretion flows (ADAFs) in the presence of resistivity and a toroidal magnetic field. The mechanism of energy dissipation in the flow is assumed to be the viscosity and the magnetic diffusivity as a result of turbulence in the accretion flow. It is also assumed that the magnetic diffusivity and the kinematic viscosity are not constant and that they vary by position, and the α-prescription is used for these. The influence of outflows emanating from an accretion disc is considered as a sink for mass, angular momentum and energy. The self-similar method is used to solve the integrated equations that govern the behaviour of the accretion flow in the presence of outflows. The solutions represent the disc that rotates faster and becomes cooler for stronger outflows. Moreover, by adding magnetic diffusivity, the surface density and rotational velocity decrease, while the radial velocity and temperature increase. A study of the present model with the magnitude of a magnetic field implies that the disc rotates and accretes faster and becomes hotter, while the surface density decreases. The thickness of the disc increases when adding a magnetic field or resistivity, while the disc becomes thinner for more mass and energy losses resulting from the outflows. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

We have studied the effects of thermal conduction on the structure of viscous and resistive advection-dominated accretion flows (ADAFs). The importance of thermal conduction on hot accretion flow is confirmed by observations of hot gas that surrounds Sgr A * and a few other nearby galactic nuclei. In this research, thermal conduction is studied by a saturated form of it, as is appropriated for weakly-collisional systems. It is assumed the viscosity and the magnetic diffusivity are due to turbulence and dissipation in the flow. The viscosity also is due to angular momentum transport. Here, the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and α-prescription is used for them. The govern equations on system have been solved by the steady self-similar method. The solutions show the radial velocity is highly subsonic and the rotational velocity behaves sub-Keplerian. The rotational velocity for a specific value of the thermal conduction coefficient becomes zero. This amount of conductivity strongly depends on magnetic pressure fraction, magnetic Prandtl number, and viscosity parameter. Comparison of energy transport by thermal conduction with the other energy mechanisms implies that thermal conduction can be a significant energy mechanism in resistive and magnetized ADAFs. This property is confirmed by non-ideal magnetohydrodynamics (MHD) simulations. © 2011 Springer Science+Business Media B.V.

In this paper, self-similar solutions of resistive advection dominated accretion flows (ADAF) in the presence of a pure azimuthal magnetic field are investigated. The mechanism of energy dissipation is assumed to be the viscosity and the magnetic diffusivity due to turbulence in the accretion flow. It is assumed that the magnetic diffusivity and the kinematic viscosity are not constant and vary by position and α-prescription is used for them. In order to solve the integrated equations that govern the behavior of the accretion flow, a self-similar method is used. The solutions show that the structure of accretion flow depends on the magnetic field and the magnetic diffusivity. As the radial infall velocity and the temperature of the flow increase by magnetic diffusivity, the rotational velocity decreases. Also, the rotational velocity for all selected values of magnetic diffusivity and magnetic field is sub-Keplerian. The solutions show that there is a certain amount of magnetic field for which rotational velocity of the flow becomes zero. This amount of the magnetic field depends upon the gas properties of the disc, such as adiabatic index and viscosity, magnetic diffusivity, and advection parameters. The mass accretion rate increases by adding the magnetic diffusivity and the solutions show that in high magnetic pressure, the ratio of the mass accretion rate to the Bondi accretion rate is reduced with an increase in magnetic pressure. Also, the study of Lundquist and magnetic Reynolds numbers based on resistivity indicates that the linear growth of magnetorotational instability (MRI) of the flow reduces by resistivity. This property is qualitatively consistent with resistive magnetohydrodynamics (MHD) simulations. © 2012 Indian Academy of Sciences.

The effect of self-gravity on protoplanetary disks is investigated. The mechanisms of angular momentum transport and energy dissipation are assumed to be the viscosity due to turbulence in the accretion disk. The energy equation is considered in a situation where the released energy by viscosity dissipation is balanced with cooling processes. The viscosity is obtained by equality of dissipation and cooling functions, and is used to derive the angular momentum equation. The cooling rate of the flow is calculated by a prescription, du/dt = -u/τ cool, where u and τ cool are the internal energy and cooling timescale, respectively. The ratio of local cooling to dynamical timescales Ωτ cool is assumed to be a constant and also a function of the local temperature. The solutions for protoplanetary disks show that in the case of Ωτ cool = constant, the disk does not exhibit any gravitational instability over small radii for a typical mass accretion rate, = 10 -6 M ⊙ yr -1, but when choosing Ωτ cool to be a function of temperature, gravitational instability can occur for this value of mass accretion rate or even less in small radii. Also, by studying the viscosity parameter α, we find that the strength of turbulence in the inner part of self-gravitating protoplanetary disks is very low. These results are qualitatively consistent with direct numerical simulations of protoplanetary disks. Also, in the case of cooling with temperature dependence, the effect of physical parameters on the structure of the disk is investigated. These solutions demonstrate that disk thickness and the Toomre parameter decrease by adding the ratio of disk mass to central object mass. However, the disk thickness and the Toomre parameter increase by adding mass accretion rate. Furthermore, for typical input parameters such as mass accretion rate 10 -6M ⊙ yr -1, the ratio of the specific heat γ = 5/3 and the ratio of disk mass to central object mass q = 0.1, gravitational instability can occur over the whole radius of the disk excluding the region very near the central object. © 2012 National Astronomical Observatories of Chinese Academy of Sciences and IOP Publishing Ltd.

The purpose of this paper is to explore the dynamical behaviour of hot accretion flows with thermal conduction. The importance of thermal conduction in hot accretion flows is confirmed by observations of the hot gas that surrounds SgrA* and a few other nearby galactic nuclei. In this research, the effect of thermal conduction is studied through a saturated form, as is appropriate for weakly collisional systems. The angular momentum transport is assumed to be a result of viscous turbulence and the α-prescription is used for the kinematic coefficient of viscosity. The equations of accretion flow are solved in a simplified one-dimensional model that neglects the latitudinal dependence of the flow. To solve the integrated equations that govern the dynamical behaviour of the accretion flow, we have used an unsteady self-similar solution. The solution provides some insights into the dynamics of quasi-spherical accretion flow and avoids the limits of the steady self-similar solution. In comparison with accretion flows without thermal conduction, the disc generally becomes cooler and denser. These properties are qualitatively consistent with simulations performed in hot accretion flows. Moreover, the angular velocity increases with the magnitude of conduction, while the radial infall velocity decreases. The mass accretion rate on to the central object is reduced in the presence of thermal conduction. We found that viscosity and thermal conduction have opposite effects on the physical variables. Furthermore, the flow represents a transonic point that displaces inward with the magnitude of conduction or viscosity. © 2011 The Author Monthly Notices of the Royal Astronomical Society © 2011 RAS.
AUTHOR KEYWORDS: Accretion, accretion discs; Conduction; Hydrodynamics