arXiv subjects

Raoul D. Viollier

Publications and source records attributed to Raoul D. Viollier.

31 records · Page 2Linked to original sources

arXiv2001

Does Dark Matter at the Center and in the Halo of the Galaxy Consist of the Same Particles?

After a discussion of the properties od degenerate fermion balls, we analize the orbits of the star S0-1, which has the smallest projected distance to Sgr A*, in the supermassive black hole as well as in the fermion ball scenarios of the Galactic center. It is shown that both scenarios are consistent with the data,as measured during the last six yiers by Genzel ae al. and Ghez et al.. We then consider a self-gravitating ideal fermion gas at nonzero temperature as a model for the Galactic halo. The Galactic halo of mass ~2 x 10^12 Msol enclosed within a radius of ~200 kpc implies the existence of a supermassive compact dark object at the Galactic center that is in hydrostatic and thermal equilibrium with the halo. The central object has a maximal mass of ~2.3 x 10^6 Msol within a minimal radius of ~18 mpc for fermion masses ~15 keV. We thus conclude that both the supermassive compact dark object and the halo could be made of the same weakly interacting ~15 keV particle.

astro-ph↗

arXiv2001

The Dynamics of the Formation of Degenerate Heavy Neutrino Stars

Through reformulating the cold, self-gravitating fermion gas as a Bose condensate by identifying their mutual Thomas-Fermi limits, the dissipationless formation of a heavy neutrino star in gravitational collapse is numerically demonstrated. Such stars offer an alternative to supermassive black holes for the compact dark objects at the centers of galaxies.

astro-ph↗

arXiv2001

On the Formation of Degenerate Heavy Neutrino Stars

The dynamics of a self-gravitating cold Fermi gas is described using the analogy with an interacting self-gravitating Bose condensate having the same Thomas-Fermi limit. The dissipationless formation of a heavy neutrino star through gravitational collapse and ejection of matter is demonstrated numerically. Such neutrino stars offer an alternative to black holes for the supermassive compact dark objects at the centers of galaxies.

astro-ph↗

arXiv2000

Fermion stars as gravitational lenses

We study in detail gravitational lensing caused by a supermassive fermion star and compare it with lensing by a black hole of the same mass. It is argued that lensing effects, being very distinct, may shed some light on the yet unexplained nature of the compact dark massive object at the Galactic center.

astro-ph↗

arXiv1999

Emission spectrum of Sagittarius A$^*$ and the neutrino ball scenario

The emission spectrum of the supermassive compact dark object at the Galactic center is calculated in the framework of standard thin accretion disk theory assuming that the compact object is a neutrino ball of $2.6 \times 10^{6}M_{\odot}$ instead of a supermassive black hole. The neutrino ball scenario could explain the observed radio and infrared emission spectrum of the Galactic center for wavelengths between $λ=0.3 {\rm cm}$ and $λ=10^{-3} {\rm cm}$, if the neutrino mass and the accretion rate fulfil some constraints.

astro-ph↗

arXiv1999

Dynamics of the star S0-1 and the nature of the compact dark object at the Galactic center

It has been recently shown (Munyaneza, Tsiklauri and Viollier 1998) that the analysis of the orbits of the fast moving stars close to Sgr A$^{*}$ (Eckart and Genzel 1997) provides a valuable tool to probe the gravitational potential near the Galactic center. As an example, we present here the results on a calculation of possible orbits of the star S0-1 in both, the black hole and degenerate neutrino ball scenarios of the central mass, based on the recent measurements of stellar proper motions at the Galactic center by Ghez et al. 1998. Taking into account the error bars of their analysis, it is shown that within a few years time, the orbit of S0-1 may indeed reveal the nature of the supermassive compact dark object at the Galactic center.

astro-ph↗

arXiv1999

General-Relativistic Thomas-Fermi model

A system of self-gravitating massive fermions is studied in the framework of the general-relativistic Thomas-Fermi model. We study the properties of the free energy functional and its relation to Einstein's field equations. A self-gravitating fermion gas we then describe by a set of Thomas-Fermi type self-consistency equations.

gr-qc↗

arXiv1998

Thermodynamic instability of self-gravitating heavy neutrino matter

It is shown, in the framework of the Thomas-Fermi model at finite temperature, that a cooling non-degenerate gas of massive neutrinos will, at a certain temperature, become unstable and undergo a first-order phase transition in which quasi-degenerate supermassive neutrino stars are formed through gravitational collapse. For neutrinos in the mass range of 10 to 25 keV/c$^{2}$, these compact dark objects could mimic the role of supermassive black holes that are reported to exist at the centres of galaxies and quasi-stellar objects.

astro-ph↗

arXiv1998

Gravitational Phase Transition of Fermionic Matter in a General-Relativistic Framework

The Thomas-Fermi model at finite temperature is extended to describe a system of self-gravitating weakly interacting massive fermions in a general-relativistic framework. By cooling a nondegenerate gas of weakly interacting massive fermions below some critical temperature, a condensed phase emerges, consisting of quasidegenerate fermion stars. For fermion masses of 10 to 25 keV, these fermion stars may very well provide an alternative explanation for the supermassive compact dark objects that are observed at galactic centers.

hep-ph↗

arXiv1998

Heavy neutrino ball as a possible solution to the "blackness problem" of the Galactic center

It has been recently shown (Tsiklauri & Viollier, 1998a) that the matter concentration inferred from observed stellar motion at the galactic center (Eckart & Genzel, 1997, MNRAS, 284, 576 and Genzel et al., 1996, ApJ, 472, 153) is consistent with a supermassive object of $2.5 \times 10^6$ solar masses, composed of self-gravitating, degenerate heavy neutrinos. It has been furthermore suggested (Tsiklauri & Viollier, 1998a) that the neutrino ball scenario may have an advantage that it could possibly explain the so-called "blackness problem" of the galactic center. Here, we present a quantitative investigation of this statement, by calculating the emitted spectrum of Sgr A$^*$ in the framework of standard accretion disk theory.

astro-ph↗

arXiv1998

Dark matter concentration in the galactic center

It is shown that the matter concentration observed through stellar motion at the galactic center (Eckart & Genzel, 1997, MNRAS, 284, 576 and Genzel et al., 1996, ApJ, 472, 153) is consistent with a supermassive object of $2.5 \times 10^6$ solar masses composed of self-gravitating, degenerate heavy neutrinos, as an alternative to the black hole interpretation. According to the observational data, the lower bounds on possible neutrino masses are $m_ν\geq 12.0$ keV$/c^2$ for $g=2$ or $m_ν\geq 14.3$ keV$/c^2$ for $g=1$, where $g$ is the spin degeneracy factor. The advantage of this scenario is that it could naturally explain the low X-ray and gamma ray activity of Sgr A$^*$, i.e. the so called "blackness problem" of the galactic center.

astro-ph↗

arXiv1998

Stars and Halos of Degenerate Relativistic Heavy-Neutrino and Neutralino Matter

Heavy-neutrino (or neutralino) stars are studied using the general relativistic equations of hydrostatic equilibrium and the relativistic equation of state for degenerate fermionic matter. The Tolman-Oppenheimer-Volkoff equations are then generalized to include a system of degenerate neutrino and neutralino matter that is gravitationally coupled. The properties and implications of such an interacting astrophysical system are discussed in detail.

astro-ph↗

arXiv1996

Gravitational Phase Transition of Heavy Neutrino Matter

We study the phase transition of a system of self-gravitating neutrinos in the presence of a large radiation density background in the framework of the Thomas-Fermi model. We show that, by cooling a non-degenerate gas of massive neutrinos below some critical temperature, a condensed phase emerges, consisting of quasi-degenerate supermassive neutrino stars. These compact dark objects could play an important role in structure formation in this universe, as they might in fact provide the seeds for galactic nuclei and quasi-stellar objects.

astro-ph↗