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.

A wind-blown bubble in the Central Molecular Zone cloud G0.253+0.016

“…One hypothesis that would be consistent with the quiescent picture of G0.253+0.016, is that the arc represents a shell swept up by the wind of an interloper star. High-mass stars possess powerful winds and the CMZ is unique in our Galaxy in that there is a rich population of 'field' high-mass stars distributed throughout (Mauerhan et al 2010;Dong et al 2011;Clark et al 2021). The origin of this population is unclear.…”

“…Reconciliation may be possible if: i) our assumed mass loss rate and wind velocity are underestimated; ii) both ρ 0 and v * are overestimated. Regarding the former scenario, Some of the 'field' high-mass stars located within the Galactic Centre are more evolved Wolf-Rayet (WR) stars (Mauerhan et al 2010;Dong et al 2011;Clark et al 2021). WR stars have powerful stellar winds, with mass loss rates that can be 100× that of O stars.…”

“…High-mass stars rarely (if at all) form in isolation (de Wit et al 2004Wit et al , 2005. Though isolated high-mass stars have been identified throughout the Galactic Centre (Mauerhan et al 2010;Dong et al 2011;Clark et al 2021), the cluster formation efficiency in CMZ clouds may be as high as ∼ 30-40% (Ginsburg & Kruijssen 2018).…”

Grids of stellar models with rotation VII: Models from 0.8 to 300 M$_\odot$ at super-solar metallicity (Z = 0.020)

“…The best studied massive young star cluster in the inner Galactic disk is Westerlund 1 (Wd1), at a distance of ∼4 kpc (Beasor et al 2021), while there are also several older massive clusters at the end of the Galactic Bar which are rich in red supergiants (Davies et al 2009). Within the Galactic Centre, at a distance of 8.2 kpc (GM SHOULD BE PC NOT KPC) (Gravity Collaboration et al 2019), there are several young high mass ( 10 4 M ) clusters including the Arches, Quintuplet and Galactic Centre clusters, plus a rich massive star population within the Central Molecular Zone (Clark et al 2021). We compare the present models to these clusters in Sect.…”

Understanding the missing clusters problem at the Galactic center. Dissolved cluster(s) detection in the Sgr B1 region.

“…In particular the presence of 3 Wolf Rayet stars can be interpreted as an age estimator of the detected dissolved cluster(s) that agree with the estimated age of between 5 and 10 Myr. Furthermore, recent KMOS observations have detected evidence of an apparent over density of mid-O super-/hypergiants and WNLha-type stars spatially coincident with the Sgr B1 complex 29 . On the other hand, SOFIA FIFI-LS observations have shown that the ionized gas in the Sgr B1 region must be due to stars widely spread throughout the field, and pointed towards the presence of a young stellar population that were not formed in situ 7 .…”