Galactic Centre stellar winds and Sgr A* accretion

We present a detailed discussion of our new 3D numerical models for the accretion of stellar winds on to Sgr A*. In our most sophisticated models, we put stellar wind sources on realistic orbits around Sgr A*, we include recently discovered ‘slow’ winds (vw∼ 300 km s−1), and we account for optically thin radiative cooling. We test our approach by first modelling only one-phase ‘fast’ stellar winds (vw∼ 1000 km s−1). For stellar wind sources fixed in space, the accretion rate is of the order of Graphic, fluctuates by ≲10 per cent, and is in good agreement with previous models. In contrast, Graphic decreases by an order of magnitude for wind sources following circular orbits, and fluctuates by ∼50 per cent. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively after being shocked, forming cold clumps and filaments immersed into the X-ray-emitting gas. We investigate two orbital configurations for the stars in this scenario, an isotropic distribution and two rotating discs with perpendicular orientation. The morphology of cold gas is quite sensitive to the orbital distribution of the stars. In both cases, however, most of the accreted gas is hot, producing a quasi-steady ‘floor’ in the accretion rate, of the order of ∼3 × 10−6 M⊙ yr−1, consistent with the values deduced from Chandra observations. The cold gas accretes in intermittent, short but powerful accretion episodes, which may give rise to large-amplitude variability in the luminosity of Sgr A* on time-scales of tens to hundreds of years. The circularization radii for the flows are about 103 and 104 Schwarzschild radii, for the one- and two-phase wind simulations, respectively, never forming the quasi-spherical accretion flows suggested in some previous work. Our work suggests that, averaged over time-scales of hundreds to thousands of years, the radiative and mechanical luminosity of Sgr A* may be substantially higher than it is in its current state. Further improvements of the wind accretion modelling of Sgr A* will rely on improved observational constraints for the wind velocities, mass-loss rates and stellar orbits.