Using detailed single star and binary evolution models to probe the large observed luminosity spread of red supergiants in young open star clusters

TL;DR

This study uses detailed binary and single star evolution models to explain the wide luminosity spread of red supergiants in young clusters, highlighting binary mergers as a key factor but also noting other influences.

Contribution

It provides a comprehensive analysis of binary evolution effects on RSG luminosities, showing mergers can explain observed ranges, but binary effects alone are insufficient for the brightest RSGs.

Findings

Binary mergers can produce a tenfold luminosity range in RSGs.

Mass transfer effects modestly increase RSG luminosities.

Binary effects alone cannot explain the most luminous RSGs.

Abstract

Red supergiants (RSGs) represent a late evolutionary stage of massive stars. Recent observations reveal that the observed luminosity range of RSGs in young open clusters is wider than expected from single star evolution models. Binary evolution effects have been suggested as a possible explanation. Here, we analyse 3670 detailed binary-evolution models, as well as corresponding single-star models, to probe the contribution of binary mass transfer and binary mergers on the luminosity distribution of RSGs in star clusters with ages up to 100 Myr. We confirm that the expected luminosity range of RSGs in a coeval population can span a factor of ten, as a consequence of mergers between two main-sequence stars, which reproduces the observed red supergiant luminosity ranges in rich clusters well. While the luminosity increase as consequence of mass transfer is more limited, it may help to increase the number of overluminous RSGs. However, our results also demonstrate that binary effects alone are insufficient to account for the number of RSGs found with luminosities of up to three times those predicted by current single-star models. We discuss observational accuracy, rotational mixing, age spread, and intrinsic RSG variability as possible explanations. Further observations of RSGs in young open clusters, in particular studies of their intrinsic brightness variability, appear crucial for disentangling these effects.