Where Do Stars Explode in the ISM?

The properties of interstellar medium (ISM) where supernovae (SNe) occur is a major factor determining their ability to drive turbulence and outflows, produce hot gas and disperse molecular clouds, all of which coordinate the star-forming properties of the ISM. However, detailed observations of SN ISM environments have been challenging due to the low SN-rate in the nearby universe, and the poor spatial resolution of ISM observations in distant galaxies. In this talk I will present a novel strategy to measure the ISM distribution around SNe — by directly observing the cold molecular/atomic gas around evolved massive SN progenitors, i.e. red supergiants and Wolf-Rayet stars, in the Local Group galaxies. These stars will likely explode within 0.1-1 Myr (and therefore relatively close to their current location), so our strategy provides a much larger sample of “SN locations” than possible with current SN surveys. This is a novel, potentially powerful view of SN feedback, but it also requires the best, sharpest view of atomic gas to make measurements of the atomic gas column at the relevant physical scales. This will be routine with the ngVLA and SKA but is currently only possible with the highest resolution 21-cm maps of the nearest galaxies. In this talk I will show first results applying this novel technique to new observations in M33 at <50 pc resolution from the “Extra Large” VLA Local Group L-Band Survey (LGLBS), folding in parallel observations of molecular gas from ALMA also taken by our team. We find strong evidence that more massive stars explode in denser, molecular-dominated gas, but a substantial fraction of stars (~44-72%) also explode in the lower-density atomic ISM, and at least 1 WR was found inside a molecular cavity. I will discuss the implications for SN feedback models, and how ngVLA/SKA-era atomic and molecular gas maps will enable such studies to galaxies beyond the Local Group, to a wider range of star-forming environments.