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

Abstract

G0.253+0.016, commonly referred to as "the Brick" and located within the Central Molecular Zone, is one of the densest (≈ 10 3−4 cm −3 ) molecular clouds in the Galaxy to lack signatures of widespread star formation. We set out to constrain the origins of an arc-shaped molecular line emission feature located within the cloud. We determine that the arc, centred on {l 0 , b 0 } = {0. • 248, 0. • 018}, has a radius of 1.3 pc and kinematics indicative of the presence of a shell expanding at 5.2 +2.7 −1.9 km s −1 . Extended radio continuum emission fills the arc cavity and recombination line emission peaks at a similar velocity to the arc, implying that the molecular and ionised gas are physically related. The inferred Lyman continuum photon rate is N LyC = 10 46.0 -10 47.9 photons s −1 , consistent with a star of spectral type B1-O8.5, corresponding to a mass of ≈ 12-20 M . We explore two scenarios for the origin of the arc: i) a partial shell swept up by the wind of an interloper high-mass star; ii) a partial shell swept up by stellar feedback resulting from in-situ star formation. We favour the latter scenario, finding reasonable (factor of a few) agreement between its morphology, dynamics, and energetics and those predicted for an expanding bubble driven by the wind from a high-mass star. The immediate implication is that G0.253+0.016 may not be as quiescent as is commonly accepted. We speculate that the cloud may have produced a 10 3 M star cluster 0.4 Myr ago, and demonstrate that the high-extinction and stellar crowding observed towards G0.253+0.016 may help to obscure such a star cluster from detection.

FEEDBACK from the NGC7538 HII region

“…To estimate expected expansion velocities, for the thermal expansion we follow the Spitzer solution (Spitzer 1998), whereas for the wind-driven expansion, the approach by Weaver et al (1977) and its adaption to include radiative cooling by Mac Low & McCray (1988) is adopted. A comprehensive summary of these two approaches is given in Henshaw et al (2021). The expansion velocities depending on time for the thermal expansion v th (t) and the wind-driven expansion v w (t) are:…”