Multi-scale Simulations of Accretion Onto Sgr A* via Stellar Winds

Sagittarius A* (Sgr A*), the supermassive black hole at the center of our galaxy, presents a unique laboratory to study low-luminosity black hole accretion. This is because the source of accretion is believed to be known, namely, the winds of ~30 Wolf-Rayet stars orbiting within about a parsec from the black hole. Since the wind speeds, mass-loss rates, and orbits of these stars (in addition to the mass and distance to Sgr A*) are well-constrained observationally, one could imagine performing a first principles simulation of the entire dynamic range of accretion, tracking the gas provided by the winds all the way down to the event horizon of the black hole. Such a simulation would be highly predictive, with little free parameters or assumptions, and would shed much light on many of the unsolved problems surrounding the emission from the galactic center. Unfortunately, in practice, such a simulation is computationally infeasible with the current generation of computers. Instead, seek to do the next best thing: simulate the large scale accretion sourced by the Wolf-Rayet stars as far in as we can go and then use the results of that simulations to provide boundary/initial conditions for general relativistic magneto-hydrodynamic simulations of the regions closest to the event horizon.


Non-Magnetized Winds

rho_stars rho_stars
2D-slices in density and temperature of a three dimensional hydrodynamic simulation of accretion onto Sgr A* as provided by the winds of the ∼ 30 Wolf-Rayet stars. The stars in our simulation act as sources of mass, momentum, and energy. Their cold, dense winds interact and quickly shock-heat to high temperatures that light up in X-rays. Most of the material is unbound and outflowing; only a small fraction accretes on the central black hole. The latter material we track all the way down to ∼ 300 gravitational radii.
stream disk
At much smaller radii, we find that the flow is comprised of two distinct components: 1) A low angular momentum, bound, Bondi-type flow that flows radially inwards along the poles and 2) a nearly Keplerian, unbound, thick "disk" of material that is slowly outflowing. This structure differs from the most commonly used initial conditions for GRMHD simulations used to model Sgr A* and could have interesting consequences.

Magnetized Winds