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

TL;DR

This study revises the upper luminosity boundary of cool supergiants in the Magellanic Clouds, finding it lower than previously thought and exploring implications for stellar evolution and supernova progenitors.

Contribution

It provides new measurements of the luminosity limit of cool supergiants in the Magellanic Clouds, challenging previous estimates and analyzing their evolutionary implications.

Findings

$L_{max}$ is around $oxed{5.5}$ in $oxed{ ext{log } L/L_ ext{sun}}$ in both Clouds.

No significant metallicity dependence of $L_{max}$ was observed.

Geneva models over-predict cool supergiant numbers in the SMC.

Abstract

The empirical upper luminosity boundary $L_{\rm max}$ of cool supergiants, often referred to as the Humphreys-Davidson limit, is thought to encode information on the general mass-loss 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 $L_{\rm max}$ by studying the luminosity distributions of cool supergiants (SGs) in the Large and Small Magellanic Clouds (LMC/SMC). We assemble samples of cool SGs in each galaxy which are highly-complete above $\log L/L_{\odot}$=5.0, and determine their spectral energy distributions from the optical to the mid-infrared using modern multi-wavelength survey data. We show that in both cases $L_{\rm max}$ appears to be lower than previously quoted, and is in the region of $\log L/L_{\odot}$=5.5. There is no evidence for $L_{\rm max}$ 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 $L_{\rm max}$ 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 over-predict 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 new bolometric luminosity measurements to provide updated bolometric corrections for cool supergiants.