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STELLAR WIND NEAR MASSIVE BLACK HOLES

Image: X-rays from Chandra in blue and infrared emission from the Hubble Space Telescope in red and yellow. The inset shows a close-up view of Sgr A* in X-rays only, covering a region half a light year wide. The diffuse X-ray emission is from hot gas captured by the black hole and being pulled inwards. This hot gas originates from winds produced by a disk-shaped distribution of young massive stars observed in infrared observations. Credit: X-ray: NASA/UMass/D.Wang et al., IR: NASA/STScI

Within the central tenth of a parsec in the middle of our galaxy there is a concentration of young stars (S-stars) that interact with a supermassive black hole Sagittarius A* (Sgr A*).


The S-stars in the galactic center are thought to be massive, early B-type stars and therefore should exhibit hot stellar winds. These winds provide gaseous material that can be accreted by the black hole which is thought to be the source of X-ray emission close to Sgr A*.

Image: Morgan-Keenan-Kellman spectral classification of main-sequence stars (Sun - type G).
Credit: LucasVB / Wikimedia.
In contrast to gas, the orbits of the stars are governed by gravitation only and therefore provide an excellent tracer for the gravitational potential in our Galactic center. This unique setup provides the best measurement of the mass of a black hole to date and unambiguously confirms the existence of a supermassive black hole in the center of our galaxy.

In a recent paper (Lutzgendorf et al 2016), the authors simulate the gravitational physics, stellar evolution and hydrodynamics of the S-stars orbiting the supermassive black hole, and they use this framework to determine the amount of gas that is accreted onto the black hole.

Image: snapshots at 6 different times in the simulation. The images are centered on the black hole (white cross). The initial position of the stars are marked in the first panel with green circles. Credit: Lutzgendorf et al 2016.

They found that the accretion rate is sensitive to the wind properties of the S-stars, and that the simulations are consistent with the observed accretion rate of Sgr A* (~10^{-6} solar masses/year) only if the stars exhibit high wind massloss rates that are comparable with those of evolved 7-10 Myr old stars with masses of M=19-25 solar masses. This result is in contrast with observations that have shown that these stars are rather young, main-sequence B-stars.
The authors conclude that the S-stars in their present stage are not the main contributors to the accretion rate of Sgr A* and the inflow of gas from the massive O-stars (located farther from Sgr A*) is needed.

(Animation of the simulation - Lutzgendorf et al 2016)

The paper (Lutzgendorf et al 2016) is available online and is published in the MNRAS. >>
http://arxiv.org/abs/1512.03304
http://mnras.oxfordjournals.org/content/456/4/3645.abstract
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