Origins of massive field stars in the Galactic Centre: a spectroscopic study

Outside of the known star clusters in the Galactic Center, a large number of evolved massive stars have been detected; but their origins remain uncertain. We present a spectroscopic study of eight such stars, based on new Gemini GNIRS and NIFS near-infrared observations. This work has led to the discovery of a new O If+ star. We compare the reddening-corrected J-K vs K diagram for our stars with the massive ones in the Arches cluster and use stellar evolutionary tracks to constrain their ages and masses. The radial velocities of both the stars and their nearby H II regions are also reported. All of the stars are blueshifted relative to the Arches cluster by > 50 km/s. We find that our source P35 has a velocity consistent with that of the surrounding molecular gas. The velocity gradient of nearby ionized gas along the Gemini GNIRS long slit, relative to P35 and the adjacent -30-0 km/s molecular cloud, can best be explained by a pressure-driven flow model. Thus, P35 most likely formed in situ. Three more of our stars have radial velocities different from their adjacent molecular gas, indicating that they are interlopers. The four stars closest to the Arches cluster have similar spectra, ages and masses to known cluster members, suggesting that they were likely ejected from the cluster via three-body interactions. Therefore, we find that the relatively isolated stars partly formed in situ and partly were ejected from the known star clusters in the Galactic Center.