Astronomy and Astrophysics – Astrophysics
Scientific paper
2006-04-25
Mon.Not.Roy.Astron.Soc.370:219,2006
Astronomy and Astrophysics
Astrophysics
10 pages,7 figures, accepted to MNRAS, references added to section 3.5 and to bibliography
Scientific paper
10.1111/j.1365-2966.2006.10470.x
The material accreting onto Sgr A* most probably comes from the nearby stars. We analyze the pattern of this flow at distances of a fraction of a parsec and we argue that the net angular momentum of this material is low but non-negligible, and the initially supersonic disk accretion changes into subsonic flow with constant angular momentum. Next we estimate the flow parameters at a distance $R_{BHL}$ from the black hole and we argue that for the plausible parameter range the accretion flow is non-stationary. The inflow becomes supersonic at distance of $\sim 10^4 R_g$ but the solution does not continue below the horizon and the material piles up forming a torus, or a ring, at a distance of a few up to tens of Schwarzchild radii. Such a torus is known to be unstable and may explain strong variability of the flow in Sgr A*. Our considerations show that the temporary formation of such a torus seems to be unavoidable. Our best fitting model predicts a rather large accretion rate of around $4 \cdot 10^{-6} M_{\odot}/yr$ directly on Sgr A*. We argue that magnetic fields in the flow are tangled and this allows our model to overcome the disagreement with the Faraday rotation limits.
Czerny Bozena
Das Tapas K.
Moscibrodzka Monika
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