Hydrodynamic simulations of the inner accretion flow of Sagittarius A* fuelled by stellar winds

Ressler, S. M.; Quataert, E.; Stone, J. M.

We present ATHENA++ grid-based, hydrodynamic simulations of accretion on to Sagittarius A* via the stellar winds of the ̃30 Wolf-Rayet stars within the central parsec of the galactic centre. These simulations span ̃4 orders of magnitude in radius, reaching all the way down to 300 gravitational radii of the black hole, ̃32 times further than in previous work. We reproduce reasonably well the diffuse thermal X-ray emission observed by Chandra in the central parsec. The resulting accretion flow at small radii is a superposition of two components: (1) a moderately unbound, sub-Keplerian, thick, pressure-supported disc that is at most (but not all) times aligned with the clockwise stellar disc, and (2) a bound, low-angular momentum inflow that proceeds primarily along the southern pole of the disc. We interpret this structure as a natural consequence of a few of the innermost stellar winds dominating accretion, which produces a flow with a broad distribution of angular momentum. Including the star S2 in the simulation has a negligible effect on the flow structure. Extrapolating our results from simulations with different inner radii, we find an accretion rate of approximately a few ×10^-8 M_☉ yr^-1 at the horizon scale, consistent with constraints based on modelling the observed emission of Sgr A*. The flow structure found here can be used as more realistic initial conditions for horizon scale simulations of Sgr A*.