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

Abstract

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. Based on observations collected at the ESO Very Large Telescope (programs 075.B-0547, 076.B-0259, 077.B-0503, 078.B-0520, and 179.B-0261).