Gravitational decoupling of anisotropic stars in the Brans-Dicke theory

Anisotropic spherically symmetric solutions are found in the Brans-Dicke theory by using the technique of the gravitational decoupling through a minimal geometric transformation that effectively splits the Einstein field equations into two distinct systems, leading to the deformation of the radial metric component. The first set includes the effects of the seed source obtained from the metric functions of the isotropic Tolman IV solution, while the anisotropic source is subjected to two constraints to solve the second set. With matching conditions to assess the unknown constants at the stellar boundary, the internal structure of stellar systems is investigated in detail by exploring the decoupling parameter, the Brans-Dicke parameters and a scalar field on the structural characteristics of anisotropic spherically symmetric spacetimes under the strong energy conditions.Comment: 13 page

Dynamical Instability of Spherical Anisotropic Sources in f(R,T,RμνTμν)f(R,T,R_{\mu\nu}T^{\mu\nu}) Gravity

In this paper, we study the effects of modification of gravity on the problem of dynamical instability of the spherical relativistic anisotropic interiors. We have considered non-zero influence of expansion scalar throughout during the evolutionary phases of spherical geometry that led to the use of fluid stiffness parameter. The modified hydrostatic equation for the stellar anisotropic matter distributions is constructed and then solved by using radial perturbation scheme. Such a differential equation can be further used to obtain instability constraints at both weak field and post-Newtonian approximations after considering a particular Harrison-Wheeler equation of state. This approach allows us to deal with the effects of usual and effective matter variables on the stability exotic stellar of self-gravitating structures.Comment: 24 pages, no figure, version accepted for publication in the European Physical Journal

Dynamics of self-gravitating systems in non-linearly magnetized chameleonic Brans-Dicke gravity

We study the effects of magnetic fields of non-linear electrodynamics in chameleonic Brans-Dicke theory under the existence of anisotropic spherical fluid. In particular, we explore dissipative and non-dissipative self-gravitating systems in the quasi-homologous regime with the minimal complexity constraint. As a result, under the aforementioned circumstances, several analytic solutions are found. Furthermore, by analyzing the dynamics of a dissipative fluid, it is demonstrated that a void covering the center can satisfy the Darmois criteria. The temperature of the self gravitating systems is also investigated.Comment: 26 pages, version accepted for publication in General Relativity and Gravitatio

Collapsing dynamics of relativistic fluid in modified gravity admitting a conformal Killing vector

The collapsing dynamics of relativistic fluid are explored in $f(R)$ gravity in a detailed systematic manner for the non-static spherically symmetric spacetime satisfying the equation of the conformal Killing vector. With quasi-homologous condition and diminishing complexity factor condition, exact solutions for dissipative as well as for non-dissipative systems are found and the astrophysical applications of these exact solutions are discussed. Furthermore, it is demonstrated that $f(R)=R$, which is the extensive restriction of $f(R)$ gravity, prior solutions of the collapsing fluid in general relativity, can be retrieved.Comment: 26 pages, version accepted for publication in the European Physical Journal

On the Structure and Scale of Cosmic Ray Modified Shocks

Strong astrophysical shocks, diffusively accelerating cosmic rays (CR) ought to develop CR precursors. The length of such precursor $L_{p}$ is believed to be set by the ratio of the CR mean free path $\lambda$ to the shock speed, i.e., $L_{p}\sim c\lambda/V_{sh}\sim cr_{g}/V_{sh}$, which is formally independent of the CR pressure $P_{c}$. However, the X-ray observations of supernova remnant shocks suggest that the precursor scale may be significantly shorter than $L_{p}$ which would question the above estimate unless the magnetic field is strongly amplified and the gyroradius $r_{g}$ is strongly reduced over a short (unresolved) spatial scale. We argue that while the CR pressure builds up ahead of the shock, the acceleration enters into a strongly nonlinear phase in which an acoustic instability, driven by the CR pressure gradient, dominates other instabilities (at least in the case of low $\beta$ plasma). In this regime the precursor steepens into a strongly nonlinear front whose size scales with \emph{the CR pressure}as $L_{f}\sim L_{p}\cdot(L_{s}/L_{p})^{2}(P_{c}/P_{g})^{2}$, where $L_{s}$ is the scale of the developed acoustic turbulence, and $P_{c}/P_{g}$ is the ratio of CR to gas pressure. Since $L_{s}\ll L_{p}$, the precursor scale reduction may be strong in the case of even a moderate gas heating by the CRs through the acoustic and (possibly also) the other instabilities driven by the CRs.Comment: EPS 2010 paper, to appear in PPC