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 which enrich the interstellar and intergalactic medium. Moreover, given the intense radiation fields and extreme gas and cosmic ray densities present within such regions, one might question whether star formation proceeds in a different manner to that occurring in more quiescent regions of the galactic disc, potentially leading to a dependence of the stellar initial mass function on the local environment. Aims. To address the physics of circumnuclear starbursts, we are driven to observe the centre of our own Galaxy, which is the only example where individual stars may be resolved. Previous studies have revealed a rich population of very massive stars, found in three young massive clusters as well as being distributed, in apparent isolation, throughout the inner ∼500 pc of the Galaxy. In this paper we investigate the size and composition of the latter cohort in order to constrain its origin and role in the lifecycle of the Galactic Centre. Methods. To accomplish this, we utilised the Very Large Telescope + K-band Multi-Object Spectrograph to obtain homogeneous, high signal-to-noise ratio observations of known and candidate massive stars suitable for spectral classification and quantitative analysis. Results. We identified 17 new isolated massive stars and reclassified a further 19 known examples, leading to a total of at least 83 within the Galactic Centre. Due to the selection criteria employed, these were strongly biased towards stars with powerful stellar winds and/or extensive circumstellar envelopes; as such, we suspect the resultant census to be incomplete. No further stellar clusters, or their tidally stripped remnants, were identified, although an apparent overdensity of very young and massive stars is found to be coincident with the Sgr B1 star forming region. Conclusions. Despite the limitations of the current dataset, the size of the cohort of outlying massive stars within the Galactic Centre is directly comparable to that of the known clusters and, assuming a comparable mass function, is expected to exceed this number. Combining both cluster and isolated populations yields ≳320 spectroscopically classified stars within the Galactic Centre that are sufficiently massive that they might be anticipated to undergo core collapse within the next ∼20 Myr. Given this is almost certainly a substantial underestimate of the true number, the population of massive stars associated with the Galactic Centre appears unprecedented within the Milky Way, and it appears unavoidable that they play a substantial role in the energetics and evolution of this region.