Journal of Astrophysics http://www.hindawi.com The latest articles from Hindawi Publishing Corporation © 2014 , Hindawi Publishing Corporation . All rights reserved. Numerical Experiments for Nuclear Flashes toward Superbursts in an Accreting Neutron Star Wed, 23 Jul 2014 12:18:18 +0000 http://www.hindawi.com/journals/jas/2014/817986/ We show that the superburst would be originated from thermonuclear burning ignited by accumulated fuels in the deep layers compared to normal X-ray bursts. Two cases are investigated for models related to superbursts by following thermal evolution of a realistic neutron star: helium flash and carbon flash accompanied with many normal bursts. For a helium flash, the burst shows the long duration when the accretion rate is low compared with the observation. The flash could become a superburst if the burning develops to the deflagration and/or detonation. For a carbon flash accompanied with many normal bursts, after successive 2786 normal bursts during 1.81 × 109 s, the temperature reaches the deflagration temperature. This is due to the produced carbon which amount reaches to ≈0.1 in the mass fraction. The flash will develop to dynamical phenomena of the deflagration and/or detonation, which may lead to a superburst. Masa-aki Hashimoto, Reiko Kuromizu, Masaomi Ono, Tsuneo Noda, and Masayuki Y. Fujimoto Copyright © 2014 Masa-aki Hashimoto et al. All rights reserved. Modulation Instability of Ion-Acoustic Waves in Plasma with Nonthermal Electrons Wed, 16 Jul 2014 12:03:52 +0000 http://www.hindawi.com/journals/jas/2014/785670/ Modulational instability of ion-acoustic waves has been theoretically investigated in an unmagnetized collisionless plasma with nonthermal electrons, Boltzmann positrons, and warm positive ions. To describe the nonlinear evolution of the wave amplitude a nonlinear Schrödinger (NLS) equation has been derived by using multiple scale perturbation technique. The nonthermal parameter, positron concentration, and ion temperature are shown to play significant role in the modulational instability of ion-acoustic waves and the formation of envelope solitons. Basudev Ghosh and Sreyasi Banerjee Copyright © 2014 Basudev Ghosh and Sreyasi Banerjee. All rights reserved. Delta : Polynomial Approximation of Time Period 1620–2013 Wed, 09 Jul 2014 10:32:53 +0000 http://www.hindawi.com/journals/jas/2014/480964/ The difference between the Uniform Dynamical Time and Universal Time is referred to as (delta ). Delta is used in numerous astronomical calculations, that is, eclipses,and length of day. It is additionally required to reduce quantified positions of minor planets to a uniform timescale for the purpose of orbital determination. Since Universal Time is established on the basis of the variable rotation of planet Earth, the quantity mirrors the unevenness of that rotation, and so it changes slowly, but rather irregularly, as time passes. We have worked on empirical formulae for estimating and have discovered a set of polynomials of the 4th order with nine intervals which is accurate within the range of ±0.6 seconds for the duration of years 1620–2013. M. Khalid, Mariam Sultana, and Faheem Zaidi Copyright © 2014 M. Khalid et al. All rights reserved. Studies on Longer Wavelength Type II Radio Bursts Associated with Flares and CMEs during the Rise and Decay Phase of 23rd Solar Cycle Tue, 03 Jun 2014 11:51:36 +0000 http://www.hindawi.com/journals/jas/2014/168718/ A statistical study on the properties of CMEs and flares associated with DH-type II bursts in the 23rd solar cycle during the period 1997–2008 is carried out. A sample of 229 events from our recent work is used for the present study (Vasanth and Umapathy, 2013). The collected events are divided into two groups as (i) solar cycle rise phase events and (ii) solar cycle decay phase events. The properties of CMEs in the two groups were compared and the results are presented. It is noted that there is no difference in the properties of type II burst like start frequency and end frequency between the solar cycle rise phase events and decay phase events. The mean CME speed of solar cycle decay phase events (1373 km s−1) is slightly higher than the solar cycle rise phase events (1058 km s−1). The mean CME acceleration of solar cycle decay phase events (−15.18 m s−2) is found to be higher than that of the solar cycle rise phase events (−1.32 m s−2). There exists good correlation between (i) CME speed and width and (ii) CME speed and acceleration for solar cycle decay phase events (, ) compared to solar cycle rise phase events (, ). These results indicate that the type II bursts parameters do not depend upon the time of appearance in the solar cycle. V. Vasanth and S. Umapathy Copyright © 2014 V. Vasanth and S. Umapathy. All rights reserved. Analytical Results Connecting Stellar Structure Parameters and Extended Reaction Rates Mon, 12 May 2014 08:11:59 +0000 http://www.hindawi.com/journals/jas/2014/656784/ Possible modification in the velocity distribution in the nonresonant reaction rates leads to an extended reaction rate probability integral. The closed form representation for these thermonuclear functions is used to obtain the stellar luminosity and neutrino emission rates. The composite parameter that determines the standard nuclear reaction rate through the Maxwell-Boltzmann energy distribution is extended to by the extended reaction rates through a more general distribution than the Maxwell-Boltzmann distribution. The new distribution is obtained by the pathway model introduced by Mathai (2005). Simple analytic models considered by various authors are utilized for evaluating stellar luminosity and neutrino emission rates and are obtained in generalized special functions such as Meijer's G-function and Fox's H-function. The standard and extended nonresonant thermonuclear functions are compared by plotting them. Behaviour of the new energy distribution, which is more general than the Maxwell-Boltzmann, is also studied. Hans J. Haubold and Dilip Kumar Copyright © 2014 Hans J. Haubold and Dilip Kumar. All rights reserved. Chaos and Intermittency in the DNLS Equation Describing the Parallel Alfvén Wave Propagation Mon, 14 Apr 2014 08:36:53 +0000 http://www.hindawi.com/journals/jas/2014/812052/ When the Hall effect is included in the magnetohydrodynamics equations (Hall-MHD model) the wave propagation modes become coupled, but for propagation parallel to the ambient magnetic field the Alfvén mode decouples from the magnetosonic ones, resulting in circularly polarized waves that are described by the derivative nonlinear Schrödinger (DNLS) equation. In this paper, the DNLS equation is numerically solved using spectral methods for the spatial derivatives and a fourth order Runge-Kutta scheme for time integration. Firstly, the nondiffusive DNLS equation is considered to test the validity of the method by verifying the analytical condition of modulational stability. Later, diffusive and excitatory effects are incorporated to compare the numerical results with those obtained by a three-wave truncation model. The results show that different types of attractors can exist depending on the diffusion level: for relatively large damping, there are fixed points for which the truncation model is a good approximation; for low damping, chaotic solutions appear and the three-wave truncation model fails due to the emergence of new nonnegligible modes. Gustavo Krause, Sergio Elaskar, and Andrea Costa Copyright © 2014 Gustavo Krause et al. All rights reserved. Invariant Imbedding and the Radiation Transfer in a Plane-Parallel Inhomogeneous Atmosphere Thu, 30 Jan 2014 09:40:16 +0000 http://www.hindawi.com/journals/jas/2014/193059/ The invariant imbedding technique is applied to the problems of radiation transfer in a plane-parallel inhomogeneous atmosphere. All the parameters which describe the elementary event of scattering and the distribution of the energy sources are allowed to vary with depth. Mathematically, the considered standard problems of the theory are reduced to initial-value problems which are better adapted to capabilities of the modern high speed computers. The reflectance of an atmosphere is shown to play a prominent role in describing the diffusion process since all the other characteristics of the radiation field are expressed through it. Three transfer problems frequently encountered in astrophysical applications are discussed: the radiation diffusion in the source-free medium, in a medium with arbitrarily distributed energy sources, as well as the problem of finding the statistical mean quantities, characteristics of the multiple scattering in the atmosphere. Arthur G. Nikoghossian Copyright © 2014 Arthur G. Nikoghossian. All rights reserved. -Process Nucleosynthesis in MHD Jet Explosions of Core-Collapse Supernovae Tue, 22 Oct 2013 15:05:44 +0000 http://www.hindawi.com/journals/jas/2013/506146/ We investigate the -process nucleosynthesis during the magnetohydrodynamical (MHD) explosion of a supernova in a helium star of 3.3 , where effects of neutrinos are taken into account using the leakage scheme in the two-dimensional (2D) hydrodynamic code. Jet-like explosion due to the combined effects of differential rotation and magnetic field is able to erode the lower electron fraction matter from the inner layers. We find that the ejected material of low electron fraction responsible for the -process comes out from just outside the neutrino sphere deep inside the Fe-core. It is found that heavy element nucleosynthesis depends on the initial conditions of rotational and magnetic fields. In particular, the third peak of the distribution is significantly overproduced relative to the solar system abundances, which would indicate a possible -process site owing to MHD jets in supernovae. Motoaki Saruwatari, Masa-aki Hashimoto, Ryohei Fukuda, and Shin-ichiro Fujimoto Copyright © 2013 Motoaki Saruwatari et al. All rights reserved. On Robe's Circular Restricted Problem of Three Variable Mass Bodies Tue, 22 Oct 2013 09:57:09 +0000 http://www.hindawi.com/journals/jas/2013/898794/ This paper investigates the motion of a test particle around the equilibrium points under the setup of the Robe’s circular restricted three-body problem in which the masses of the three bodies vary arbitrarily with time at the same rate. The first primary is assumed to be a fluid in the shape of a sphere whose density also varies with time. The nonautonomous equations are derived and transformed to the autonomized form. Two collinear equilibrium points exist, with one positioned at the center of the fluid while the other exists for the mass ratio and density parameter provided the density parameter assumes value greater than one. Further, circular equilibrium points exist and pairs of out-of-plane equilibrium points forming triangles with the centers of the primaries are found. The out-of-plane points depend on the arbitrary constant , of the motion of the primaries, density ratio, and mass parameter. The linear stability of the equilibrium points is studied and it is seen that the circular and out-of-plane equilibrium points are unstable while the collinear equilibrium points are stable under some conditions. A numerical example regarding out-of-plane points is given in the case of the Earth, Moon, and submarine system. This study may be useful in the investigations of dynamic problem of the “ocean planets” Kepler-62e and Kepler-62f orbiting the star Kepler-62. Jagadish Singh and Oni Leke Copyright © 2013 Jagadish Singh and Oni Leke. All rights reserved. Field Independent Cosmic Evolution Mon, 21 Oct 2013 15:03:01 +0000 http://www.hindawi.com/journals/jas/2013/590171/ It has been shown earlier that Noether symmetry does not admit a form of corresponding to an action in which is coupled to scalar-tensor theory of gravity or even for pure theory of gravity taking anisotropic model into account. Here, we prove that theory of gravity does not admit Noether symmetry even if it is coupled to tachyonic field and considering a gauge in addition. To handle such a theory, a general conserved current has been constructed under a condition which decouples higher-order curvature part from the field part. This condition, in principle, solves for the scale-factor independently. Thus, cosmological evolution remains independent of the form of the chosen field, whether it is a scalar or a tachyon. Nayem Sk and Abhik Kumar Sanyal Copyright © 2013 Nayem Sk and Abhik Kumar Sanyal. All rights reserved. New Classes of Charged Spheroidal Models Mon, 30 Sep 2013 11:44:54 +0000 http://www.hindawi.com/journals/jas/2013/539847/ New classes of exact solutions to the Einstein-Maxwell system is found in closed form by assuming that the hypersurface is spheroidal. This is achieved by choosing a particular form for the electric field intensity. A class of solution is found for all positive spheroidal parameter for a specific form of electric field intensity. In general, the condition of pressure isotropy reduces to a difference equation with variable, rational coefficients that can be solved. Consequently, an explicit solution in series form is found. By placing restrictions on the parameters, it is shown that the series terminates and there exist two classes of solutions in terms of elementary functions. These solutions contain the models found previously in the limit of vanishing charge. Solutions found are directly relating the spheroidal parameter and electric field intensity. Masses obtained are consistent with the previously reported experimental and theoretical studies describing strange stars. A physical analysis indicates that these models may be used to describe a charged sphere. S. Thirukkanesh Copyright © 2013 S. Thirukkanesh. All rights reserved. Constraint on Heavy Element Production in Inhomogeneous Big-Bang Nucleosynthesis from the Light Element Observations Sun, 01 Sep 2013 13:19:44 +0000 http://www.hindawi.com/journals/jas/2013/587294/ We investigate the observational constraints on the inhomogeneous big-bang nucleosynthesis that Matsuura et al. (2005) suggested that states the possibility of the heavy element production beyond 7Li in the early universe. From the observational constraints on light elements of 4He and D, possible regions are found on the plane of the volume fraction of the high-density region against the ratio between high- and low-density regions. In these allowed regions, we have confirmed that the heavy elements beyond Ni can be produced appreciably, where p- and/or r-process elements are produced well simultaneously. Riou Nakamura, Masa-aki Hashimoto, Shin-ichiro Fujimoto, and Katsuhiko Sato Copyright © 2013 Riou Nakamura et al. All rights reserved. The Total Solar Irradiance, UV Emission and Magnetic Flux during the Last Solar Cycle Minimum Mon, 22 Jul 2013 12:57:16 +0000 http://www.hindawi.com/journals/jas/2013/368380/ We have analyzed the total solar irradiance (TSI) and the spectral solar irradiance as ultraviolet emission (UV) in the wavelength range 115–180 nm, observed with the instruments TIM and SOLSTICE within the framework of SORCE (the solar radiation and climate experiment) during the long solar minimum between the 23rd and 24th cycles. The wavelet analysis reveals an increase in the magnetic flux in the latitudinal zone of the sunspot activity, accompanied with an increase in the TSI and UV on the surface rotation timescales of solar activity complexes. In-phase coherent structures between the midlatitude magnetic flux and TSI/UV appear when the long-lived complexes of the solar activity are present. These complexes, which are related to long-lived sources of magnetic fields under the photosphere, are maintained by magnetic fluxes reappearing in the same longitudinal regions. During the deep solar minimum (the period of the absence of sunspots), a coherent structure has been found, in which the phase between the integrated midlatitude magnetic flux is ahead of the total solar irradiance on the timescales of the surface rotation. E. E. Benevolenskaya and I. G. Kostuchenko Copyright © 2013 E. E. Benevolenskaya and I. G. Kostuchenko. All rights reserved. Ion-Acoustic Instabilities in a Multi-Ion Plasma Tue, 16 Jul 2013 11:24:31 +0000 http://www.hindawi.com/journals/jas/2013/838534/ We have, in this paper, studied the stability of the ion-acoustic wave in a plasma composed of hydrogen, positively and negatively charged oxygen ions, and electrons, which approximates very well the plasma environment around a comet. Modelling each cometary component (, , and ) by a ring distribution, we find that ion-acoustic waves can be generated at frequencies comparable to the hydrogen ion plasma frequency. The dispersion relation has been solved both analytically and numerically. We find that the ratio of the ring speed () to the thermal spread (ts) modifies the dispersion characteristics of the ion-acoustic wave. The contrasting behaviour of the phase velocity of the ion-acoustic wave in the presence of ions for ts (and vice versa) can be used to detect the presence of negatively charged oxygen ions and also their thermalization. Noble P. Abraham, Sijo Sebastian, G. Sreekala, R. Jayapal, C. P. Anilkumar, and Venugopal Chandu Copyright © 2013 Noble P. Abraham et al. All rights reserved. Dark Energy from the Gas of Wormholes Thu, 20 Jun 2013 13:17:53 +0000 http://www.hindawi.com/journals/jas/2013/543717/ We assume the space-time foam picture in which the vacuum is filled with a gas of virtual wormholes. It is shown that virtual wormholes form a finite (of the Planckian order) value of the energy density of zero-point fluctuations. However such a huge value is compensated by the contribution of virtual wormholes to the mean curvature and the observed value of the cosmological constant is close to zero. A nonvanishing value appears due to the polarization of vacuum in external classical fields. In the early Universe some virtual wormholes may form actual ones. We show that in the case of actual wormholes vacuum polarization effects are negligible while their contribution to the mean curvature is apt to form the observed dark energy phenomenon. Using the contribution of wormholes to dark matter and dark energy we find estimates for characteristic parameters of the gas of wormholes. A. A. Kirillov and E. P. Savelova Copyright © 2013 A. A. Kirillov and E. P. Savelova. All rights reserved.