Galactic Centre stellar winds and Sgr A* accretion

Abstract

(ABRIDGED) We present in detail our new 3D numerical models for the accretion of stellar winds on to Sgr A*. In our most sophisticated models, we put stars on realistic orbits around Sgr A*, include `slow' winds (300 km/s), and account for radiative cooling. We first model only one phase `fast' stellar winds (1000 km/s). For wind sources fixed in space, the accretion rate is Mdot ~ 1e-5 Msun/yr, fluctuates by < 10%, and is in a good agreement with previous models. In contrast, Mdot decreases by an order of magnitude for stars following circular orbits, and fluctuates by ~ 50%. Then we allow a fraction of stars to produce slow winds. Much of these winds cool radiatively, forming cold clumps immersed into the X-ray emitting gas. We test 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 orbits. In both cases, however, most of the accreted gas is hot, with an almost constant Mdot ~ 3e-6 Msun/yr, consistent with Chandra observations. The cold gas accretes in intermittent, short but powerful episodes which may give rise to large amplitude variability in the luminosity of Sgr A* on time scales of 10s to 100s of years. The circularisation radii for the flows are ~ 1e3 and 1e4 Rsch, 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 100s to 1000s 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 properties and stellar orbits.Comment: 16 pages, 18 colour figures. Accepted by MNRAS. Full resolution paper and movies available at http://www.mpa-garching.mpg.de/~jcuadra/Winds/ . (v2: minor changes

SGR A* and Its Environment: Low-Mass Star Formation, the Origin of X-Ray Gas and Collimated Outflow

“…The estimated mass accretion rate onto Sgr A * is several orders of magnitude smaller than the rate at which young, windy stars in the innermost 0.5 pc supply mass to the Bondi radius of Sgr A * (Coker & Melia 1997;Cuadra et al 2006Cuadra et al , 2008. Chandra observations have characterized the X-ray emission surrounding SgrA * is being spatially extended with a radius of ∼1 5 (Baganoff et al 2003;Wang et al 2013).…”

“…This gas is generally thought to be supplied by the combined winds of the mass-losing young stars in the central parsec of the Galaxy, which are estimated to supply material to Sgr A * at a rate of a few times 10 −6 - M yr 1 (Coker & Melia 1997;Rockefeller et al 2004;Cuadra et al 2006Cuadra et al , 2008. However, the discovery of populations of young stars within the S-cluster which consists of ∼16 B dwarfs and 3 O stars on highly eccentric orbits within 1″, begs the question whether mass loss from these stars plays a dominant role in supplying gas to the vicinity of Sgr A * .…”

Evolution of binary black holes in self gravitating discs

“…We simulate a self-gravitating gaseous disc of mass M d = 0.2 M around two BHs of combined mass M = M 1 + M 2 , mass ratio q = M 2 /M 1 = 1/3, eccentricity e and semi-major axis a, using the SPH-code Gadget-2 (Springel 2005) in a modified version that includes sink-particles which model accretion on to the BHs Cuadra et al 2006). Moreover, the orbit of the BHB is followed very accurately by using a fixed small time-step and summing up directly the gravitational force from every other particle in the simulation (Cuadra et al 2009).…”

Clump formation through colliding stellar winds in the Galactic Centre

“…They interpreted this source as a bright knot of a larger gas streamer that includes a G2-type object called G1 in a similar orbit, but preceding it by 13 yr. G1 and G2 could be explained as the result of the partial tidal disruption of a star (Guillochon et al 2014) or as one of many gas clumps created by the collision of stellar winds from the young stars in the Galactic centre Schartmann et al 2012). Such dense, cold clumps are copiously produced in the Smoothed-Particle Hydrodynamics (hereafter SPH) simulations of the Galactic centre gas dynamics performed by Cuadra et al (2005Cuadra et al ( , 2006Cuadra et al ( , 2008Cuadra et al ( , 2015 (see also Lützgendorf et al 2015 § 3.3), and could survive pericentre passage if magnetised (McCourt et al 2015). Moreover, G2's orbit lies on the plane of the 'clockwise disc', defined by the orbits of many young stars Yelda et al 2014), and its apocentre coincides with the inner rim of that disc.…”