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 conse