CAREER: Mass Measurements in the Galaxy

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This award is funded in whole or in part under the American Rescue Plan Act of 2021 (Public Law 117- 2). Massive stars, i.e., those more than ten times the Sun’s mass, are comparatively rare in local galaxies due to their short lifetimes. They nevertheless play an outsized role in galaxy evolution. Their energy output, for example, largely determines the thermodynamics of the interstellar medium and helps set the timescale for star formation in molecular clouds. Massive stars are also responsible for the creation and dispersal of a significant fraction of heavy elements (necessary for rocky planets and life) in galaxies through core-collapse supernovae and mergers of exotic binary systems comprised of neutron stars and black holes - themselves end-points of massive star evolution. The first generation of massive stars may also have ionized the intergalactic medium soon after the Big Bang. The proposed research will use new and archival data from the Atacama Large Sub/Millimeter Array (ALMA) to help determine the dominant mechanism(s) involved in the formation of massive stars and the role played by their environments, both incompletely understood. This project will also support research activities by a postdoctoral researcher and a graduate student, as well as efforts by the principal investigator to develop and distribute python based optical/radio data analysis tools and tutorials.The research team will derive accurate masses for a sample of High Mass Young Stellar Objects (HMYSOs) - the predecessors of massive stars - as well as lower mass protostars within the same cluster using ALMA observations in the Milky Way’s disk and center. While many HMYSOs appear to grow by accreting gas from their associated disks, there exists evidence that massive stars can also grow through mergers or inflow-driven instabilities. To distinguish between these possibilities, they will first measure dynamical masses of HMYSOs through the analysis of disk kinematics as traced by salt (NaCl and KCl) emission lines. This represents a novel approach to measure masses at the start of massive stars’ lives as these lines appear to be commonly detected for HMYSOs in the ALMA archive. The relative importance of stellar interactions, turbulent inflow, and disk-mediated accretion requires determining properties of lower-mass proto-stellar populations in the vicinity of the HMYSOs. The same ALMA observations will enable such censuses. Combining dust emission maps with temperature measurements using NH3 or CH3CN lines, the team will measure the luminosity function and the spatial distribution of protostars surrounding HMYSOs.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.