Revealing the Population of Isolated, Massive Stars in the Central Molecular Zone

Abstract

We report on the current census of isolated massive stars in the Galactic center region, including recently discovered objects. The latest discoveries were selected for their hard X-ray counterparts, detected with the Chandra X-ray Observatory; or for their Paschen-α emission-line excess, detected in narrowband images with HST/NICMOS. The confirmed stars span a wide range of massive star evolutionary stages: OIa, Of, Ofpe, LBV, WNh, WN, WNE, WC, and WCd. We suspect that the majority of the hard X-ray-emitting, massive stars we have identified are colliding-wind binaries, although some may be HMXBs. Most of the confirmed massive stars have no obvious association with the known, young stellar clusters, but some may have escaped from them. Extrapolation of their numbers suggests the existence of a massive star population, comparable in size to that contained within the clusters collectively, which could account for the integrated far-IR luminosity emerging from the central half-kiloparsec. This additional massive-star population may have been supplemented by the tidal disruption of stellar clusters, or suggests an alternate mode of isolated massive star formation operating in the Galactic center region. Future experiments will constrain their kinematics, binary characteristics, and mode of formation.