Constraining the Evolution of Massive Stars

Massive stars play a crucial role in the universe. Yet, our understanding of massive stars remains incomplete due to their rarity, short lifetimes, complexity of binary interactions, and imprecise Galactic distances. An important challenge is to understand the physics and relative importance of steady and eruptive mass loss in the most massive stars. For example, the luminous blue variable (LBV) is one such poorly constrained class of eruptive stars. LBVs are the brightest blue irregular variable stars in any large star-forming galaxy. They can achieve the highest mass-loss rates of any known types of stars, and they exhibit a wide diversity of irregular and eruptive variability. In the single-star scenario, the hypothesis is that most stars above ~30 solar mass pass through an LBV phase. However, the relative isolation of LBVs from O stars challenges this interpretation, and another hypothesis is emerging that the LBV phenomenon is the product of binary evolution. To test these hypotheses, we modeled the dissolution of young clusters and the separation between O stars and LBVs. We find that the single-star scenario is inconsistent with the observed LBV environments. If LBVs are single stars, then the lifetimes inferred from their luminosity and mass are far too short to be consistent with their isolation from O stars. This implies that LBVs are likely products of binary evolution. To further constrain these hypotheses, we must first infer the fundamental properties of LBVs such as luminosity, mass, and age. Ultimately, these depend upon accurate Galactic distances. Using Gaia parallaxes, we find that nearly half of the Galactic LBVs are significantly closer than previous literature estimates; these new distances lower their luminosities and their initial masses. We also infer a closer distance to the massive cluster, Westerlund 1, which hosts an LBV, 24 Wolf- Rayet stars, 6 yellow hypergiants, and a magnetar. Together these Gaia-based distances are more accurate (at least a factor of ten) and have consequences for the late-stage evolution of massive stars.