Accretion of stellar winds onto Sgr A*

Details Accretion of stellar winds onto Sgr A* Accretion of stellar winds onto Sgr A*

Apr 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007iaus..238..191c&link_type=abstract

Black Holes from Stars to Galaxies -- Across the Range of Masses. Edited by V. Karas and G. Matt. Proceedings of IAU Symposium #

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Located at the centre of our Galaxy is Sgr A*, a 3 million Msun super-massive black hole (SMBH). Sgr A* is hundreds of times closer than any other known SMBH, and as such it is an ideal object to study accretion physics. Recent observations have revealed the presence of dozens of young mass-losing stars. The winds from these stars provide in principle enough material to make Sgr A* radiate as an AGN. However, its observed luminosity is orders of magnitude smaller than what is expected from simple accretion theories. This seems to be caused, at least partially, by a low accretion rate onto the SMBH. Here we report a 3-dimensional numerical study of the accretion of stellar winds onto Sgr A*. Compared with previous investigations, we allow the stars to be on realistic orbits, include the recently discovered slow wind sources and allow for optically thin radiative cooling. Our study highlights the importance of source distribution geometry and net angular momentum in the problem. We find that for wind sources distributed in a disc, the accretion rate is a few times 10-6 Msun/yr, an order of magnitude lower than the one reported by previous studies, and much more in line with Chandra observations. In addition, introducing the slow winds, we find these to shock and rapidly cool, forming cold gas blobs and filaments. The accretion rate in this case consists of two components: the hot quasi steady-state one, and the cold one that is highly time variable in time-scales of tens to hundreds of years. Because of the non-linear nature of the accretion processes, the variability in the accretion rate would result in an even more variable luminosity. This result suggests that, on time-scales of hundreds of years, Sgr A* may be a much more important energy source for the inner Galaxy than what is inferred from its current low luminosity.

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