The Statistics of S Stars and Their Correlation with Hypervelocity Stars

Abstract

Central, or “circumnuclear,” stars of the Galaxy, known as S stars, are studied. These stars can be used to investigate the gravitational potential near the Galactic center, which hosts a supermassive black hole (SMBH), as well as to impose constraints on estimates of the mass and position of this SMBH. The evolutionary relationship between S stars and hypervelocity stars is analyzed in the classical Hills scenario, which considers the dynamical capture of a binary star in the field of a SMBH. Numerical simulations in the three-body and N-body problems are used to obtain consistent statistics for the populations of S stars and high-velocity stars, and to estimate their lifetimes, including the time scale for the capture of an S star in the neighborhood of the central SMBH. The mass spectrum of the S stars is also analyzed, and the distribution of the semi-major axes for their orbits around the SMBH is derived. A comparison of the computed and observed distributions is used to place constraints on the time scale and cross section for the capture of binaries in the vicinity of the SMBH. The genetic relationship between high-velocity stars and S stars as former components of binary-star systems is confirmed.