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 interestreflects the presence of a central supermasive black hole.However, direct effect of the black hole on stellar motion can beexpected only within its sphere of gravitational influence,which can be directly resolved only in the case of our ownGalactic centre, known as Sagittarius A*.

We proposed that Kozai's phenomenon is responsible for the long-termevolution of stellar orbits near supermassive black holesin galactic cores. Nuclei of active galaxies are surroundedalso by rather dense gaseous environment that affects stellarmotions and causes the gradual orbital decay. In the case of SgrA*, we pursuethe idea that this process may be driven by a fossil accretiondisc inthe centre of our Galaxy, while setting some stars on highlyelliptic trajectories.We thus investigate orbits that undergo repetitive transitionsacross the disc, typically over the period of ten million years.The disc mass has to be small compared to the central black hole,and itsgravitational field comparatively weak, yet non-zero

In our research we show howa model trajectory decays and circularizes, but at some point themeaneccentricity is substantially increased by Kozai's resonance. Inconsequence the orbital decay of highly eccentric orbits isaccelerated. A combination of an axially symmetric gravitationalfieldand dissipative environment can provide a mechanism explaining theorigin of stars on highly eccentric orbits tightly bound to thecentralblack hole. In the context of other S-stars, we can concludethat an acceptable mass of the disc in Sagittarius A* iscompatible with their surprisingly youngage and small pericentre distances, provided thesestars were formed at ~10^5 gravitational radii.