Constraining the population of isolated massive stars within the Central Molecular Zone

Abstract

Context. Many galaxies host pronounced circumnuclear starbursts, fuelled by infalling gas. Such activity is expected to drive the secular evolution of the nucleus and generate super winds, while the intense radiation fields and extreme gas and cosmic ray densities present may act to modify the outcome of star formation with respect to more quiescent galactic regions. Aims. The centre of the Milky Way is the only example of this phenomenon where, by virtue of its proximity, individual stars may be resolved. Previous studies reveal that it hosts a rich population of very massive stars; these are located within three clusters, with an additional cohort dispersed throughout the Central Molecular Zone. In this paper we investigate the size and composition of the latter contingent. Methods. We utilised the VLT+KMOS to obtain homogeneous, high signal-to-noise ratio observations of known and candidate massive stars suitable for spectral classification and quantitative analysis. Results. Including previously identified examples, we find a total of 83 isolated massive stars within the Galactic Centre, which are strongly biased towards objects supporting powerful stellar winds and/or extensive circumstellar envelopes. No further stellar clusters, or their tidally stripped remnants, were identified, although an apparent stellar overdensity is found to be coincident with the Sgr B1 star forming region. Conclusions. The cohort of isolated massive stars within the Central Molecular Zone is directly comparable in size to that of the known clusters and, due to observational biases, is likely highly incomplete at this time. Combining both populations yields 320 spectroscopically classified stars that would be expected to undergo core collapse within the next ∼ 20Myr. Given that this is almost certainly a substantial underestimate of the true number, the population of massive stars associated with the Galactic Centre appears unprecedented amongst star formation complexes within the Milky Way, and it appears probable that they play a substantial role in the energetics and evolution of the nuclear region.