Stars in the Galactic Centre

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