------------------------------------------------------------------------
 belmont06.tex A&A, Mar 2006, in press
Date: Wed, 26 Apr 2006 16:22:24 -0700
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%astro-ph/0603499

\documentclass{article}
\begin{document}

\title{A Viscous Heating Mechanism for the Hot Plasma in the Galactic  
Center Region}
\author{R. Belmont\thanks{\email{belmont@cea.fr}} \and M. Tagger}
\institute{CEA Service d'Astrophysique, UMR "AstroParticlues et  
Cosmologie", Orme de merisiers, 91191 Gif-sur-Yvette, France }
\date{Accepted 14 March 2006}

\abstract{
In addition to lines originating in a soft phase at $\sim$ 0.8 keV  
and to cold molecular clouds, the X-ray spectra from the Galactic  
center region also exhibit properties similar to those of a diffuse,  
thin, very hot plasma at 8 keV on a scale of hundreds of parsecs.  
This phase is surprising for more than one reason. First, such a hot  
plasma should not be bound to the Galactic plane and the power needed  
to sustain the escaping matter would be higher then any known source.  
Second, there is no known mechanism able to heat the plasma to more  
than a few keV. Recently we have suggested that, hydrogen having  
escaped, the hot plasma could be a helium plasma, heavy enough to be  
gravitationally confined. In this case, the required power is much  
more reasonable.
We present here a possible heating mechanism which taps the  
gravitational energy of the molecular clouds. We note that the 8 keV  
plasma is highly viscous and we show how viscous friction of  
molecular clouds flowing within the hot phase can dissipate energy in  
the gas and heat it.
We detail the MHD wake of a spherical cloud by considering the  
different MHD waves the cloud can excite.
We find that most of the energy is dissipated by the damping of Alfv 
\'enic perturbations in two possible manners, namely by non-linear  
effects and by a large scale curvature of the field lines. We find  
that the total dissipation rate depends on the field strength. For  
fields B~$\lesssim 200\mu$G both mechanisms produce power comparable  
to or higher than the radiative losses; for strong fields B~$\gtrsim 
$~1 mG, only the curvature damping can balance the X-ray emission and  
requires a radius of curvature $R_\mathrm{c}\lesssim$ 100 pc; whereas  
for intermediate fields, the total dissipation is more than one order  
of magnitude smaller, requiring a higher accretion rate. We note that  
the plasma parameters may be optimal to make the dissipation most  
efficient, suggesting a self-regulation mechanism. The loss of  
kinetic and gravitational energy also causes accretion of the clouds  
and may have significant action on the gas dynamics in this region  
between the large scale, bar dominated flow and the central accretion  
to the massive black hole.
\keywords{Galaxy: center -- X-rays: ISM -- ISM: clouds  -- ISM:  
magnetic fields -- Plasma -- ISM: kinematics and dynamics} }

\end{document}