Accretion of Stellar Winds in the Galactic Centre

Publications

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 (~ 300 km/s), and we account for optically thin radiative cooling. We test our approach by first modelling only one phase `fast' stellar winds (1000 km/s). For stellar wind sources fixed in space, the accretion rate is of the order of Mdot ~ 10^-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 wind sources following circular orbits, and fluctuates by ~ 50%. 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 ~ 3x10^-6 Msun/yr, 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 circularisation radii for the flows are about 10^3 and 10^4 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 orders of magnitude 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.

Accretion of cool stellar winds on to Sgr A*: another puzzle of the Galactic Centre?

Sgr A* is currently being fed by winds from a cluster of gravitationally bound young mass-losing stars. Using observational constraints on the orbits, mass-loss rates and wind velocities of these stars, we numerically model the distribution of gas in the ~ 0.1-10 arcsec region around Sgr A*. We find that radiative cooling of recently discovered slow winds leads to the formation of many cool filaments and blobs, and to a thin and rather light accretion disc on a scale of about an arcsecond. The disc, however, does not extend all the way to our inner boundary. Instead, hot X-ray-emitting gas dominates the inner arcsecond. In our simulations, cool streams of gas frequently enter this region on low angular momentum orbits, and are then disrupted and heated up to the ambient hot gas temperature. The accreting gas around Sgr A* is thus two-phase, with a hot component, observable at X-ray wavelengths, and a cool component, which may be responsible for the majority of the time variability of Sgr A* emission on time-scales of 100-1000 yr. We obtain an accretion rate of a few ×10^-6M_solaryr^-1, consistent with Chandra estimates, but variable on time-scales even shorter than 100 yr. These results strongly depend on the chosen stellar orbits and wind parameters. Further observational input is thus key to a better modelling of the Sgr A* wind accretion.

Movies

Click on the images to watch movies taken from our simulations. (On Apple or Windows computers, you may have to install the free divx-codec to play the movie.)

	This movie corresponds to Fig. 7 of our paper "Galactic Centre stellar winds and Sgr A* accretion". The field is 14 arcsec across and it's centered in Sgr A*, the super-massive black hole in the centre of our Galaxy. The dots represent stars with a high mass-loss rate that are orbiting the central black hole. Some of the stars produce slow winds, which suffer radiative cooling after colliding with the winds from other stars. The cooled gas form clumps (shown in yellow) that are immersed in the hot X-ray emitting gas. While most of the gas escapes from the computational domain, some clumps are directed inward, where they are sheared by Sgr A* potential, and form filaments. In this particular simulation we attemp to reproduce the stellar distribution reported by Paumard et al (2001) and Genzel et al (2003). We locate most of the stars with slow winds close to the black hole and rotating in the same plane. As a result, most of the clumps settle on coplanar circular orbits, and form a disc.
	This movie corresponds to Fig. 12 of our paper "Galactic Centre stellar winds and Sgr A* accretion". Since the stellar distribution is still known only roughly, we run a test with the same parameters, but with orbits oriented isotropically. In this case there is no strongly preferred direction. Instead of a relatively large disc, only a tiny ring is formed in this simulation. This result shows the strong dependence of the cold gas morphology on the stellar orbits. On the other hand, we still find a two-phase medium in the inner region, where cold clumps are immersed in the hot gas. This is a robust result, depending only on the well determined stellar wind velocities.
	This movie corresponds to Fig. 1 of our paper "Accretion of cool stellar winds on Sgr A*: another puzzle of the Galactic Center?".

Posters

Click on the image to download a PDF version of a poster.

Poster presented at the conference "The Paradoxes of Massive Black Holes: A Case Study in the Milky Way", Santa Barbara, USA, April 2005.
Poster presented during the evaluation of the IMPRS in Garching, April 2005. It shows different projects related to the Galactic centre stars, including the winds' accretion.

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Links

• Slides of a contributed talk at the Meeting on "Modelling Dense Stellar Systems", Edinburgh, UK, Dec. 2004.
• Talk by Sergei Nayakshin at the conference "The Paradoxes of Massive Black Holes: A Case Study in the Milky Way", Santa Barbara, USA, April 2005. (From minute 20 / slide 26 is about this project.)
• Abstract of the letter, Czech translation(!)
• Slides of a contributed talk at the Conference on "High Energy Phenomena in the Galactic Center", Paris, France, June 2005.