Gravitational Instability in Presence of Bulk Viscosity: the Jeans Mass and the Quasi-Isotropic Solution

This paper focuses on the analysis of the gravitational instability in presence of bulk viscosity both in Newtonian regime and in the fully-relativistic approach. The standard Jeans Mechanism and the Quasi-Isotropic Solution are treated expressing the bulk-viscosity coefficient $\zeta$ as a power-law of the fluid energy density $\rho$, i.e., \zeta=\zo\rho^{s}. In the Newtonian regime, the perturbation evolution is founded to be damped by viscosity and the top-down mechanism of structure fragmentation is suppressed. The value of the Jeans Mass remains unchanged also in presence of viscosity. In the relativistic approach, we get a power-law solution existing only in correspondence to a restricted domain of \zo.Comment: 3 pages, Proceedings of The XII Marcel Grossmann Meetin

Coexistence of Magneto-Rotational and Jeans Instabilities in an Axisymmetric Nebula

We analyze the magneto-rotational instability (MRI) effects on gravitational collapse and its influence on the instability critical scale. In particular, we study an axisymmetric nonstratified differentially rotating cloud, embedded in a small magnetic field, and we perform a local linear stability analysis, including the self gravity of the system. We demonstrate that the linear evolution of the perturbations is characterized by the emergence of an anisotropy degree of the perturbed mass densities. Starting with spherical growing overdensities, we see that they naturally acquire an anisotropy of order unity in their shape. Despite the linear character of our analysis, we infer that such a seed of anisotropy can rapidly grow in a nonlinear regime, leading to the formation of filament-like structures. However, we show how such an anisotropy is essentially an intrinsic feature of the Jean instability, and how MRI only plays a significant role in fixing the critical scale of the mode spectrum. We then provide a characterization of the present analysis in terms of the cosmological setting, in order to provide an outlook of how the present results could concern the formation of large-scale structures across the Universe.Comment: 8 pages, 4 figure

Quasi-linear model for the beam-plasma instability: analysis of the self-consistent evolution

We re-analyze the quasi-linear self consistent dynamics for the beam-plasma instability, by comparing the theory predictions to numerical simulations of the corresponding Hamiltonian system. While the diffusive features of the asymptotic dynamics are reliably predicted, the early temporal mesoscale transport appears less efficient in reproducing the convective feature of the self-consistent scenario. As a result, we identify the origin of the observed discrepancy in the underlying quasi-linear model assumption that the distribution function is quasi-stationary. Furthermore, we provide a correction to the instantaneous quasi-linear growth rate based on a linear expansion of the distribution function time dependence, and we successfully test this revised formulation for the spectral evolution during the temporal mesoscale.Comment: 12 pages, 5 figure