Blue supergiants as tests for stellar physics

Abstract

Massive star evolution is still poorly understood, and observational tests are required to discriminate between different implementations of physical phenomenon in stellar evolution codes. By confronting stellar evolution models with observed properties of blue supergiants, such as pulsations, chemical composition and position in the Hertzsprung-Russell diagram, we aim at determining which of the criterion used for convection (Schwarzschild or Ledoux) is best able to explain the observations. We compute state-of-the-art stellar evolution models with either the Schwarzschild or the Ledoux criterion for convection. Models are for $14$ to $35M_{\odot }$ at solar or Large Magellanic Cloud metallicity. For each model, we compute the pulsation properties to know when radial modes are excited. We then compare our results with the position of blue supergiants in the Hertzsprung-Russell diagram, with their surface chemical composition, and with their variability. Our results at Large Magellanic Cloud metallicity shows only a slight preference for the Ledoux criterion over the Schwarzschild one in reproducing at the same time the observed properties of blue supergiants, even if the Schwarzschild criterion cannot be excluded at this metallicity. We check that changing the overshoot parameter at solar metallicity does not improve the situation. We also check that our models are able to reproduce the position of Galactic blue supergiant in the flux-weighted-gravity -- luminosity relation. We confirm that overall, models computed with the Ledoux criterion are slightly better in matching observations. Our results also support the idea that most Galactic $\alpha$ Cyg variables are blue supergiants of the group 2, i.e. stars that have been through a previous red supergiant phase where they have lost large amount of mass.

arXiv comment14 pages, 10 figures, accepted for publication in A&A

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