Convective core sizes in rotating massive stars

Abstract

Context. Spectroscopic studies of Galactic O and B stars show that many stars with masses above 8 M⊙ have been observed in the Hertzsprung-Russell (HR) diagram just beyond the main-sequence (MS) band, as predicted by stellar models computed with a moderate overshooting. This may be an indication that the convective core sizes in stars in the upper part of the HR diagram are larger than predicted by these models. Aims. Combining stellar evolution models and spectroscopic parameters derived for a large sample of Galactic O and B stars with the inclusion of brand-new information about their projected rotational velocities, we reexamine the question of the convective core size in MS massive stars. Methods. We computed a grid of 120 different stellar evolutionary tracks with three initial rotations at solar metallicity (Z = 0.014), spanning a mass range from 7 to 25 M⊙, and combining different values for the initial rotation rate and overshooting parameter. For the rotating models, we considered two cases, one with a moderate and one with a strong angular momentum transport, the latter imposing a solid body rotation during most of the MS phase. We confront the results with two observed features: the position of the terminal age main sequence (TAMS) in the HR diagram and the decrease of the surface rotation when the surface gravity decreases at the end of the MS phase. Results. We confirm that for stars more massive than about 8 M⊙, the convective core size at the end of the MS phase increases more rapidly with the mass than in models computed with a constant step overshoot chosen to reproduce the main sequence width in the low mass range (around 2 M⊙). This conclusion is valid for both the cases of non-rotating models and rotating models either with a moderate or a strong angular momentum transport. The increase of the convective core mass with the mass obtained from the TAMS position is, however, larger than the one deduced from the surface velocity drop for masses above about 15 M⊙. Although the observations that are available at present cannot determine the best choice between the core sizes given by the TAMS and the velocity drop, we discuss various methods of escaping this dilemma. At the moment, comparisons with eclipsing binaries seem to favor the solution given by the velocity drop. Conclusions. While we confirm the need for larger convective cores at higher masses, we find tensions among different methods for stars more massive than 15 M⊙. The use of single-aged stellar populations (non-interacting binaries or stellar clusters) would be a great asset in resolving this tension.

One size does not fit all: Evidence for a range of mixing efficiencies in stellar evolution calculations

“…Given the assumption that the members of a stellar cluster are formed at approximately the same time and with a similar initial chemical composition, the observed morphology of the colour-magnitude diagram can be used to calibrate internal mixing mechanisms in stellar models. Several studies have demonstrated that the width of the main sequence is sensitive to the mixing history of stars, and used this sensitivity to estimate the effect of overshooting and/or rotational mixing on stellar models (VandenBerg et al 2006;Castro et al 2014;Martinet et al 2021).…”

Grids of stellar models with rotation VI: Models from 0.8 to 120 $M_\odot$ at a metallicity Z = 0.006

“…A fixed value of the overshooting parameter has been adopted to extend the size of the convective cores during the H-and He-burning phases by an amount equal to 10% the pressure scale height estimated at the Schwarzschild boundary. Such an assumption of a constant overshooting parameter for the entire mass domain is of course questionable, with studies favoring an overshooting that increases with the stellar mass above about 9 M (Brott et al 2011;Castro et al 2014;Martinet et al 2021;Scott et al 2021). The present grid does not consider such a variation in the overshooting parameter for the sake of homogeneity with the physics adopted in the first paper of this series (Ekström et al 2012).…”

Stellar convective penetration: parameterized theory and dynamical simulations

“…Beyond the Sun, overshooting in massive stars with convective cores must be finely tuned as a function of stellar mass, again pointing to missing physics in our current parameterizations (Claret & Torres 2018;Jermyn et al 2018;Viani & Basu 2020;Martinet et al 2021;Pedersen et al 2021). Since core convective overshoot increases the reservoir of fuel available for nuclear fusion at each stage in stellar evolution, improved models of core convective boundary mixing could have profound impacts on the post-main sequence evolution and remnant formation of massive stars (Farmer et al 2019;Higgins & Vink 2020).…”