Stars in the Galactic Centre
b'In the centre of our Galaxy lies Sgr~A*, a $3.5 \\\\times 10^6$ solar mass black\\nhole, immersed in a star cluster with dozens of massive stars. The very low\\nluminosity of Sgr~A*, and the presence of young stars in the close proximity\\nof a super-massive black hole, make the Galactic centre a very interesting\\nregion on its own. Moreover, its proximity allows the study of the physics of\\ngalactic nuclei with a level of detail unattainable in any other system.\\n\\nIn this thesis, we first show that the interaction of massive stars\\nwith an accretion disc would appear as strongly variable emission in\\nthe near infra-red. Since observations have not shown this\\nvariability, we strongly constrain the current existence of such a\\ndisc in the Galactic centre. We argue however that a massive gaseous\\ndisc existed around Sgr~A* only a few million years ago. The evidence\\nfor this idea comes from the presence of young massive stars in two\\nstellar discs. We estimate the properties of the gaseous disc that\\ngave rise to the massive stars, and we analyse the stellar orbits to\\nconstrain this scenario. A related but separate topic presented here\\nis the role of the stellar winds expelled by the same stars in feeding\\nSgr~A* and shaping its immediate gaseous environment. We find that a\\nfraction of these stellar winds form cold clumps that coexist with the\\nX-ray emitting gas, forming a two-phase medium. Only a small fraction\\nof the gas is captured by Sgr~A*, with an accretion rate strongly\\nvariable on time-scales of hundreds of years. This variability\\nsuggests that the time-averaged energy output of Sgr~A* may be much\\nlarger than what is currently observed.'