What limits the luminosity of red supergiants?

Sammanfattning: The observed upper limit of the luminosity of red supergiants, often called the Humphreys-Davidson limit, has recently been shown to be independent of metallicity and to not be explainable by single star evolution – findings that both contradict established ideas of the origins of the limit. It is now known that the vast majority of stars with sufficient mass to possibly evolve into red supergiants above the limit interact with companion stars at some point during their lives. In this thesis, the possibility of binary star interactions causing the Humphreys-Davidson limit is investigated. This is done numerically with the use of synthesized populations at M31, LMC, and SMC metallicities, after confirming that the simulations work as intended by producing a number of single star results. It is found that while binary star interactions can not wholly reproduce the limit, such interactions are sufficient to reproduce the limit for any red supergiant that remains in a binary and does not actively transfer mass to its companion in the red supergiant phase. For such systems upper luminosity limits of log_10(L/L_⊙) = 5.43+0.06/-0.09 for M31, log_10(L/L_⊙) = 5.48+0.07/-0.09 for the LMC, and log_10(L/L_⊙) = 5.53+0.07/-0.11 for the SMC are found, which are consistent with recent estimates of the Humphreys-Davidson limit. However, stars that are the result of a binary merging are found to be able to surpass the limit. This may be due to the physics of mergers not being modelled appropriately. If such stars genuinely do exist, interactions with a tertiary star may be what stops them from surpassing the limit.