Cosmological Evolution of String Effective Gravitation with a Dilaton Potential and Higher-Loop Corrections. I

Cosmological evolution is investigated within the framework of low-energy string gravitation with higher-loop corrections to the dilaton coupling functions in the presence of a dilaton potential and a nongravitational source. It is shown that for homogeneous and isotropic models with a flat space, the cosmological system of equations reduces to an autonomous, third-order, dynamical system. Subclasses of models with a constant dilaton, which provide the basis for various cosmological mechanisms of dilaton stabilization, are considered. A class of solutions is distinguished with asymptotic scaling behavior of the energy density of the dilaton field.     

String Cosmology in a Stationary Cylindrically Symmetric Space-Time

We have studied string-dust cosmological model in a stationary cylindrically symmetric space-time. A one parameter family of solutions is obtained for the field equations of the system.     

String Cosmology With Magnetic Field in Cylindrically Symmetric Space-Time

In this paper, the Einstein field equations have been solved for inhomogeneous cylindrically symmetric space with string source in the presence of magnetic field. Certain physical and geometrical properties of the models have been examined. Also, the string solutions in which magnetic field are absent are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Anisotropic Bianchi Type-II Cosmological Models in Self-Creation Cosmology

Totally anisotropic Bianchi type-II cosmological solutions are presented in Barber's second self-creation theory of gravitation both in vacuum and in the presence of stiff-matter. The corresponding cosmological models have no finite singularity. The stiff-matter model gives essentially an empty universe for large time. Some physical and kinematical properties of the models are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dilaton stabilization in string cosmology. I

A new mechanism is proposed for stabilization of the scalar dilaton field within the framework of lowenergy string gravitation with loop corrections to the dilaton coupling functions. It is based on the assumption that the loop corrections generate a kinetic dilaton function, which is singular for some finite value of the dilaton field. For a nongravitational source of the barotropic type, the system of equations describing the evolution of homogeneous and isotropic cosmological models is represented in the form of a thirdorder, autonomous, dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems. It is shown that there is a class of solutions, different from solutions of the general theory of relativity, with a constant dilaton. The conditions under which these solutions are an attractor for a general solution with a variable dilaton are determined. Translated from Astrofizika, Vol. 43, No. 1, pp. 123-136, January–March, 2000.     

Dilaton stabilization in string cosmology. II

A mechanism for stabilization of the dilaton field within the framework of low-energy string gravitation with loop corrections to the dilaton coupling function was proposed in the first part of this paper. The mechanism is based on the assumption that loop corrections generate a singular dilaton kinetic function for a certain value of the dilaton field. For a nortgravitational source with a constant barotropic index, the system of cosmological equations reduces to an autonomous, third-order dynamical system. The behavior of the general solution in the vicinity of singularities of the dilaton coupling function is investigated by methods of the qualitative theory of dynamical systems for different values of the singularity index. The conditions under which solutions with a constant dilaton are attractors for a general solution with a variable dilaton are determined. The evolution of models is considered, the corresponding phase diagrams are constructed, and the question of the effectiveness of dilaton stabilization is investigated. Translated from Astrofizika, Vol. 43, No. 2, pp. 313–324, April–June, 2000.     

Bianchi Type I String Cosmological Models with Bulk Viscosity and Magnetic Field

Some locally rotationally symmetric (LRS) Bianchi type I cosmological models for a cloud string with bulk viscosity and magnetic field are presented. Where an equation of state ρ = kλ and a relation between metric potential R = AS ⁿ are considered. The solution describes a shearing and nonrotating model with a big bang start. In the absence of magnetic field it reduces to a string model with bulk viscosity, where the relation between the coefficient of bulk viscosity and energy density is ζ ∝ ρ^1/2. After choosing k = , it further reduces to a string model without viscosity and magnetic field. This revised version was published online in July 2006 with corrections to the Cover Date.     

Some Exact Solutions of String Cosmological Models in Bianchi Type-V Space-Time

The Bianchi type-V cosmological solutions of massive strings in the presence and absence of the magnetic field are investigated. The physical behavior of these models are discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Testing the Models for Jet Generation with Hubble Space Telescope Observations

Many magneto-hydrodynamic (MHD) models have been developed to describe the acceleration and collimation of stellar jets, in the framework of an infall/outflow process. Thanks to high angular resolution instrumentation, such as the one on-board the Hubble Space Telescope (HST), we are finally able to test observationally the proposed ideas. We present the results obtained by us from the first 0”.1 resolution spectra of the initial portion (within 100–200 AU from the source) of the outflows from visible T Tauri stars, taken with the Space Telescope Imaging Spectrograph (STIS). We obtain the jet morphology, kinematics and excitation in different velocity intervals, and we derive the jet mass and momentum fluxes. These results confirm the predictions of magneto-centrifugal models for the jet launch. Recently we have also found indications for rotation in the peripheral regions of several flows. The derived rotational motions appear to be in agreement with the expected extraction of angular momentum from the star/disk system caused by the jet, which in turn allows the star to accrete up to its final mass. Improvements to resolution are expected from observations with STIS in the ultraviolet, and with the forthcoming AMBER spectrometer to be mounted at the VLTI.     

String cosmology with magnetized bulk viscous fluid in Bianchi I universe

The objective of the present paper is to study an anisotropic Bianchi-I cosmological model filled with bulk viscous fluid and magnetic field in string cosmology. The magnetic field is due to an electric current produced along the x-axis. The expansion in the model is considered to be proportional to one of the components of the shear tensor. We obtain two different quadrature forms of volume scale factor by considering two different relations between bulk viscosity and expansion scalar. We discuss the behavior of the classical potential with respect to the volume scale factor in the presence or absence of magnetic field and bulk viscosity in each case. We observe the role of bulk viscosity on the classical potential and also on the choices of bulk viscous pressure. By introduction of magnetic field or bulk viscosity or both into the model it results in changes in the potential as well as in volume scale factors. The physical and geometrical aspects of the solutions are discussed in detail.     

Bianchi Type-III String Cosmological Model with Bulk Viscous Fluid in General Relativity

A Bianchi type-III string cosmological model with bulk viscous fluid for massive string is investigated. To get a determinate solution, a supplementary condition B=C ⁿ, between metric potentials, is used whereB and C are function of time alone. The behaviour of the model in presence and absence of bulk viscosity, is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

HiRISE observations of gas sublimation-driven activity in Mars’ southern polar regions: III. Models of processes involving translucent ice

Enigmatic surface features, known as ‘spiders’, found at high southern martian latitudes, are probably caused by sublimation-driven erosion under the seasonal carbon dioxide ice cap. The Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) has imaged this terrain in unprecedented details throughout southern spring. It has been postulated [Kieffer, H.H., Titus, T.N., Mullins, K.F., Christensen, P.R., 2000. J. Geophys. Res. 105, 9653-9700] that translucent CO₂ slab ice traps gas sublimating at the ice surface boundary. Wherever the pressure is released the escaping gas jet entrains loose surface material and carries it to the top of the ice where it is carried downslope and/or downwind and deposited in a fan shape. Here we model two stages of this scenario: first, the cleaning of CO₂ slab ice from dust, and then, the breaking of the slab ice plate under the pressure built below it by subliming ice. Our modeling results and analysis of HiRISE images support the gas jet hypothesis and show that outbursts happen very early in spring.     

Gravitating configurations with magnetic field. III. Dedekind and Riemann ellipsoids

Kiev Polytechnic Institute. Translated from Astrofizika, Vol. 30, No. 1, pp. 208–219, January–February, 1989.