Lobe Overflow as the Likely Cause of Pericenter Outburst in an SMBH Orbiter

Abstract

A very large lobe overflow event is suggested to explain the $0.^m4$ brightening observed in K band at pericenter passage of the star known as S2 that orbits the Galaxy's supermassive black hole (SMBH). Known observed properties of S2 that contribute to lobe filling are 1) the enormous mass ratio, $M_{SMBH}/M_{S2}$, 2) S2's fast rotation, and 3) S2's large orbital eccentricity. Published estimates have given limiting lobe sizes of order 100 to 300 $R_\odot$ but, with S2's fast rotation taken into account, the computed lobe size is much smaller, being compatible with either a main sequence OB star or a stripped evolved star. An important evolutionary consideration that predicts very large pericenter overflows is envelope expansion following mass loss that is characteristic of highly evolved stars. Material removed by lobe overflow at pericenter is replenished by envelope expansion as an evolved star awaits its next pericenter passage. An observational signature of lobe overflow for upcoming pericenter passages would be appearance of emission lines as the ejected gas expands and becomes optically thin.