Dust around massive stars is agnostic to galactic environment: New insights from PHAT/BEAST

Abstract: Resolving the environments of massive stars is crucial for understanding their formation mechanisms and their impact on galaxy evolution. An important open question is whether massive stars found in diffuse regions outside spiral arms formed in-situ or migrated there after forming in denser environments. To address this question, we use multi-resolution measurements of extinction in the Andromeda Galaxy (M31) to probe the ISM surrounding massive stars across galactic environments. We construct a catalog of 42,107 main-sequence massive star candidates ($M \geq 8 M_{\odot}$) using resolved stellar photometry from the Panchromatic Hubble Andromeda Treasury (PHAT) program, plus stellar and dust model fits from the Bayesian Extinction and Stellar Tool (BEAST). We quantify galactic environments by computing surrounding stellar densities of massive stars using Kernel Density Estimation. We then compare high-resolution line-of-sight extinction estimates from the BEAST with 25-pc resolution dust maps from PHAT, measuring the total column density distribution of extinction. Our key finding is that, although the average total column density of dust increases with the density of massive stars, the average line-of-sight extinction towards massive stars remains constant across all environments. This suggests that massive stars have a uniform amount of dust in their immediate environment, regardless of their location in the galaxy. One possible explanation for these findings is that small molecular clouds are still capable of forming massive stars, even if they are not resolvable at 25-pc. These results indicate that massive stars are forming in the sparse regions of M31, as opposed to migrating there.