On the Nature of the Fast-Moving Star S2 in the Galactic Center

We analyze the properties of the star S2 orbiting the supermassive black hole at the center of the Galaxy. A high-quality SINFONI H - and K -band spectrum obtained from co-adding 23.5 hr of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-B2.5 V). Using model atmospheres, we constrain its stellar and wind properties. We show that S2 is a genuine massive star and not the core of a stripped giant star, as sometimes speculated to resolve the problem of star formation so close to the supermassive black hole. We give an upper limit on its mass-loss rate and show that it is He enriched, possibly because of the presence of a magnetic field.