The excess of cool supergiants from contemporary stellar evolution models defies the metallicity-independent Humphreys-Davidson limit

Abstract

The Humphreys-Davidson (HD) limit empirically defines a region of high luminosities (log L > 5.5) and low effective temperatures (T < 20kK) on the Hertzsprung-Russell Diagram in which hardly any supergiant stars are observed. Attempts to explain this limit through instabilities arising in near- or super-Eddington winds have been largely unsuccessful. Using modern stellar evolution we aim to re-examine the HD limit, investigating the impact of enhanced mixing on massive stars. We construct grids of stellar evolution models appropriate for the Small and Large Magellanic Clouds (SMC, LMC), as well as for the Galaxy, spanning various initial rotation rates and convective overshooting parameters. Significantly enhanced mixing apparently steers stellar evolution tracks away from the region of the HD limit. To quantify the excess of over-luminous stars in stellar evolution simulations we generate synthetic populations of massive stars, and make detailed comparisons with catalogues of cool (T < 12.5kK) and luminous (log L > 4.7) stars in the SMC and LMC. We find that adjustments to the mixing parameters can lead to agreement between the observed and simulated red supergiant populations, but for hotter supergiants the simulations always over-predict the number of very luminous (log L > 5.4) stars compared to observations. The excess of luminous supergiants decreases for enhanced mixing, possibly hinting at an important role mixing has in explaining the HD limit. Still, the HD limit remains unexplained for hotter supergiants.

accepted for publication in MNRAS

References in corpus (42)

• Modules for Experiments in Stellar Astrophysics (MESA)
• Binary interaction dominates the evolution of massive stars
• The death of massive stars - I. Observational constraints on the progenitors of type II-P supernovae
• An eclipsing binary distance to the Large Magellanic Cloud accurate to 2 per cent
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
• Updated Electron-Conduction Opacities: The Impact on Low-Mass Stellar Models
• Slowing the Spins of Stellar Cores
• The Evolution of Massive Helium Stars Including Mass Loss
• The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
• The VLT-FLAMES survey of massive stars: Evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds
• The VLT-FLAMES survey of massive stars: atmospheric parameters and rotational velocity distributions for B-type stars in the Magellanic Clouds
• A new mass-loss rate prescription for red supergiants
• The evolution of rotating very massive stars with LMC composition
• The Initial Masses of the Red Supergiant Progenitors to Type-II Supernovae
• The Tarantula Massive Binary Monitoring: I. Observational campaign and OB-type spectroscopic binaries
• Constraining mixing in massive stars in the Small Magellanic Cloud
• Towards a better understanding of the evolution of Wolf-Rayet stars and Type Ib/Ic supernova progenitors
• Wolf-Rayet stars in the Small Magellanic Cloud: I. Analysis of the single WN stars
• Testing massive star evolution, star-formation history and feedback at low metallicity : Spectroscopic analysis of OB stars in the SMC Wing
• Why binary interaction does not necessarily dominate the formation of Wolf-Rayet stars at low metallicity
• The VLT-FLAMES Tarantula Survey III: A very massive star in apparent isolation from the massive cluster R136
• The `Red Supergiant Problem': the upper luminosity boundary of type-II supernova progenitors
• The dependence of convective core overshooting on stellar mass: semi-empirical determination using the diffusive approach with two different element mixtures
• Massive donors in interacting binaries: effect of metallicity
• The red supergiant and supernova rate problems: implications for core-collapse supernova physics
• The dependence of convective core overshooting on stellar mass: reality check, and additional evidence
• The metallicity dependence of envelope inflation in massive stars
• On the effect of rotation on populations of classical Cepheids I. Predictions at solar metallicity
• New predictions for radiation-driven, steady-state mass-loss and wind-momentum from hot, massive stars. I. Method and first results
• The Tarantula Massive Binary Monitoring: III. Atmosphere analysis of double-lined spectroscopic systems
• Spatially resolved dusty torus toward the red supergiant WOH G64 in the Large Magellanic Cloud
• A new survey of cool supergiants in the Magellanic Clouds
• A census of massive stars in NGC 346. Stellar parameters and rotational velocities
• LMC Blue Supergiant Stars and the Calibration of the Flux-weighted Gravity--Luminosity Relationship
• A new prescription for the mass-loss rates of WC and WO stars
• On the Blue Loops of Intermediate-Mass Stars
• Weighing Melnick 34: the most massive binary system known
• A dearth of young and bright massive stars in the Small Magellanic Cloud
• Stellar population of the superbubble N206 in the LMC I. Analysis of the Of-type stars
• The Tarantula Massive Binary Monitoring project: II. A first SB2 orbital and spectroscopic analysis for the Wolf-Rayet binary R145
• Testing massive star evolution, star-formation history, and feedback at low metallicity: Photometric analysis of OB stars in the SMC Wing

Cited by in corpus (8)

• A synthetic population of Wolf-Rayet stars in the LMC based on detailed single and binary star evolution models
• On the Red Supergiant Wind Kink. A Universal mass-loss concept for massive stars
• Superadiabaticity and the metallicity independence of the Humphreys-Davidson limit
• The earliest O-type eclipsing binary in the Small Magellanic Cloud, AzV 476: A comprehensive analysis reveals surprisingly low stellar masses
• Reverse Algols and hydrogen-rich Wolf-Rayet stars from very massive binaries
• Spectroscopic and evolutionary analyses of the binary system AzV 14 outline paths toward the WR stage at low metallicity
• The drastic impact of Eddington-limit induced mass ejections on massive star populations
• The symphony of pulsations and binarity among massive stars using HERMES spectroscopy and TESS photometry