On the nature of the fast moving star S2 in the Galactic Center

TL;DR

This paper characterizes the star S2 near the Galactic Center, confirming it as a massive B dwarf with specific stellar properties, and discusses implications for star formation near supermassive black holes.

Contribution

It provides detailed spectroscopic analysis of S2, establishing its nature as a genuine massive star and ruling out alternative hypotheses like a stripped giant core.

Findings

S2 is an early B dwarf star.

S2 is He enriched, possibly due to magnetic fields.

S2's mass loss rate is constrained to be low.

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 coadding 23.5 hours of observation between 2004 and 2007 reveals that S2 is an early B dwarf (B0-2.5V). 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.