The luminosities of cool supergiants in the Magellanic Clouds, and the Humphreys-Davidson limit revisited

Abstract

The empirical upper luminosity boundary Lmax of cool supergiants (SGs), often referred to as the Humphreys-Davidson limit, is thought to encode information on the general massloss behaviour of massive stars. Further, it delineates the boundary at which single stars will end their lives stripped of their hydrogen-rich envelope, which in turn is a key factor in the relative rates of Type-II to Type-Ibc supernovae from single star channels. In this paper we have revisited the issue of Lmax by studying the luminosity distributions of cool SGs in the Large and Small Magellanic Clouds (LMC/SMC). We assemble samples of cool SGs in each galaxy which are highly complete above logL/L⊙= 5.0, and determine their spectral energy distributions from the optical to the mid-infrared using modern multiwavelength survey data. We show that in both cases Lmax appears to be lower than previously quoted, and is in the region of log L/L⊙ = 5.5. There is no evidence for Lmax being higher in the SMC than in the LMC, as would be expected if metallicity-dependent winds were the dominant factor in the stripping of stellar envelopes. We also show that Lmax aligns with the lowest luminosity of single nitrogen-rich Wolf-Rayet stars, indicating of a change in evolutionary sequence for stars above a critical mass. From population synthesis analysis we show that the Geneva evolutionary models greatly overpredict the numbers of cool SGs in the SMC. We also argue that the trend of earlier average spectral types of cool SGs in lower metallicity environments represents a genuine shift to hotter temperatures. Finally, we use our newbolometric luminosity measurements to provide updated bolometric corrections for cool SGs.