Contributed Talk - Splinter Stars

Tuesday, 14 September 2021, 10:20   (virtual Stars)

Superadiabaticity and the metallicity independence of the Humphreys-Davidson limit

Gautham Sabhahit, Jorick Vink, Erin Higgins, Andreas Sander
Armagh Observatory and Planetarium

The Humphreys-Davidson (HD) limit sets the boundary for the different evolutionary channels of massive stars. Stars above this limit that evolve as hotter blue supergiants and Wolf-Rayet stars are dominant contributors of ionising photons and furthermore contribute to the mechanical feedback owing to their fast winds. Stars below this limit evolve as red supergiants and produce no ionising photons. Thus, knowing the location of the 'tipping point' in luminosity as a function of metallicity Z and whether stellar models are in agreement with the empirical results are paramount for our theoretical understanding of post-main sequence evolution of massive stars in the upper HRD. These stars evolve close to their radiative Eddington limit and are predicted to undergo substantial envelope inflation, however such inflation effects could be an artefact of 1D modelling. Convection in stellar envelopes still remains an unsolved problem, however progress might be made through the comparison of stellar models with the observed upper-luminosity limit. We produce grids of MESA star models at three initial metallicities: Galactic, LMC and SMC, with two different overshooting mixing efficiencies above the convective core and allow for efficient mixing in the envelopes, thus choosing to suppress the effects of inflation. We use these models to systematically study the effects of post-main sequence mass loss and envelope mixing on the luminosity limit of the red supergiants. We find that the excess mixing in the superadiabatic layers is stronger at lower metallicities, as it depends on the opacities in the hydrogen bump at log(Teff/K) = 4, which become more pronounced at lower metallicity. This excess mixing has an effect on the RSG luminosity limit shifting it to lower values at lower metallicities, thus balancing the first-order direct effect of mass loss. The opposing effects of mass loss and excess envelope mixing during post-main sequence evolution with higher overshooting potentially results in a metallicity-independent upper luminosity limit.