Dr. Athira Menon

The majority of massive red and blue supergiants, core helium-burning massive stars and progenitors of Type II supernovae, are observed to be single in several galaxies. Most massive stars are, however, born in binary systems with companions- how did their evolved populations end up being single? In my work, I show that the most likely answer is stellar mergers of binary systems. 'Evolved' stellar mergers, beyond the main sequence, are expected to occur for about ~30% of massive binary stars. Yet, their outcomes are largely neglected in current stellar population models which form the basis of galaxy and cosmological feedback mechanisms. I will present the first detailed synthetic stellar populations that explicitly incorporate models of post-MS mergers and compare them with observations of the Small and Large Magellanic Clouds, which are excellent laboratories to study metal-poor massive stars. We find that these merger products live twice as long as all other post-main-sequence massive stars, and make up between 50-75% of this population. Moreover, they robustly produce blue supergiants during core helium-burning, a subgroup of massive stars that had long evaded standard stellar models. This unprecedented dominance of merger products and their distinct chemical signatures make them an extremely important evolutionary channel that cannot be neglected, from their contributions to the nitrogen-rich high-redshift galaxies observed by JWST, to the broader landscape of stellar populations and core-collapse supernovae across cosmic time.