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The empirical upper luminosity boundary $L_{\rm max}$ of cool supergiants,often referred to as the Humphreys-Davidson limit, is thought to encodeinformation on the general mass-loss behaviour of massive stars. Further, itdelineates the boundary at which single stars will end their lives stripped oftheir hydrogen-rich envelope, which in turn is a key factor in the relativerates of Type-II to Type-Ibc supernovae from single star channels. In thispaper we have revisited the issue of $L_{\rm max}$ by studying the luminositydistributions of cool supergiants (SGs) in the Large and Small MagellanicClouds (LMC/SMC). We assemble samples of cool SGs in each galaxy which arehighly-complete above $\log L/L_{\odot}$=5.0, and determine their spectralenergy distributions from the optical to the mid-infrared using modernmulti-wavelength survey data. We show that in both cases $L_{\rm max}$ appearsto be lower than previously quoted, and is in the region of $\logL/L_{\odot}$=5.5. There is no evidence for $L_{\rm max}$ being higher in theSMC than in the LMC, as would be expected if metallicity-dependent winds werethe dominant factor in the stripping of stellar envelopes. We also show that$L_{\rm max}$ aligns with the lowest luminosity of single nitrogen-richWolf-Rayet stars, indicating of a change in evolutionary sequence for starsabove a critical mass. From population synthesis analysis we show that theGeneva evolutionary models greatly over-predict the numbers of cool SGs in theSMC. We also argue that the trend of earlier average spectral types of cool SGsin lower metallicity environments represents a genuine shift to hottertemperatures. Finally, we use our new bolometric luminosity measurements toprovide updated bolometric corrections for cool supergiants.

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