Interactions of stars with their environment

Dense star clusters surround nuclei of galaxies, including the centre of our own Milky Way. Studying the rapid motion of stars within the central arcsecond and their interactions with surrounding environment provides an essential tool to determine the mass of the central supermassive black hole in the Galaxy.

The structure of compact nuclear clusters are of interest reflects the presence of a central supermasive black hole. However, direct effect of the black hole on stellar motion can be expected only within its sphere of gravitational influence, which can be directly resolved only in the case of our own Galactic centre, known as Sagittarius A*.

We proposed that Kozai's phenomenon is responsible for the long-term evolution of stellar orbits near supermassive black holes in galactic cores. Nuclei of active galaxies are surrounded also by rather dense gaseous environment that affects stellar motions and causes the gradual orbital decay. In the case of Sgr A*, we pursue the idea that this process may be driven by a fossil accretion disc in the centre of our Galaxy, while setting some stars on highly elliptic trajectories. We thus investigate orbits that undergo repetitive transitions across the disc, typically over the period of ten million years. The disc mass has to be small compared to the central black hole, and its gravitational field comparatively weak, yet non-zero

In our research we show how a model trajectory decays and circularizes, but at some point the mean eccentricity is substantially increased by Kozai's resonance. In consequence the orbital decay of highly eccentric orbits is accelerated. A combination of an axially symmetric gravitational field and dissipative environment can provide a mechanism explaining the origin of stars on highly eccentric orbits tightly bound to the central black hole. In the context of other S-stars, we can conclude that an acceptable mass of the disc in Sagittarius A* is compatible with their surprisingly young age and small pericentre distances, provided these stars were formed at ~10^5 gravitational radii.


  • Subr L., Kroupa P., Baumgardt H. (2008), "A new method to create initially mass segregated star clusters in virial equilibrium", Monthly Notices of the Royal Astronomical Society, 385, 1673-1680
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  • Karas V., Subr L. (2007), "Enhanced activity of massive black holes by stellar capture assisted by a self-gravitating accretion disc", Astronomy and Astrophysics, 470, 11-19
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