Massive star evolution: rotation, winds, and overshooting vectors in the mass-luminosity plane

Abstract

Context. Massive star evolution is dominated by various physical effects, including mass loss, overshooting, and rotation, but the prescriptions of their effects are poorly constrained and even affect our understanding of the main sequence. Aims. We aim to constrain massive star evolution models using the unique test-bed eclipsing binary HD 166734 with new grids of MESA stellar evolution models, adopting calibrated prescriptions of overshooting, mass loss, and rotation. Methods. We introduce a novel tool, called the mass-luminosity plane or M − L plane, as an equivalent to the traditional HR diagram, utilising it to reproduce the test-bed binary HD 166734 with newly calibrated MESA stellar evolution models for single stars. Results. We can only reproduce the Galactic binary system with an enhanced amount of core overshooting (α ov = 0.5), mass loss, and rotational mixing. We can utilise the gradient in the M − L plane to constrain the amount of mass loss to 0.5 -1.5 times the standard prescription test-bed, and we can exclude extreme reduction or multiplication factors. The extent of the vectors in the M − L plane leads us to conclude that the amount of core overshooting is larger than is normally adopted in contemporary massive star evolution models. We furthermore conclude that rotational mixing is mandatory to obtain the correct nitrogen abundance ratios between the primary and secondary components (3:1) in our test-bed binary system. Conclusions. Our calibrated grid of models, alongside our new M − L plane approach, present the possibility of a widened main sequence due to an increased demand for core overshooting. The increased amount of core overshooting is not only needed to explain the extended main sequence, but the enhanced overshooting is also needed to explain the location of the upper-luminosity limit of the red supergiants. Finally, the increased amount of core overshooting has -via the compactness parameter -implications for supernova explodability.

Theoretical investigation of the Humphreys–Davidson limit at high and low metallicity

“…The models then diverge at the TAMS with the grey model (α ov = 0.5) leaving the main sequence later. We find that increasing the core size by overshooting α ov = 0.5, which may be preferred at lower masses (∼ 20-40 M ) as found in Higgins & Vink (2019), semiconvective regions are unable to form since the envelope mass is insufficient, therefore the grey model in Fig. 3 evolves to a RSG and remains so until it explodes as a supernova, regardless of metallicity.…”

“…We later find that mixing and mass loss play a combined role in the duration of RSG/BSG phases (see Sect. 3.2), though these processes can be separated in the M-L plane as described in Higgins & Vink (2019). Lower mass stars may evolve quickly to become a RSG during He-burning, though undergo blue loops before returning to a RSG.…”

“…In Higgins & Vink (2019) we provided an analysis of a mainsequence test-bed binary HD166734 which suggested an extension of the core by overshooting of up to α ov = 0.5. For this reason, we first attempted to reproduce the HD limit with post-MS models for relatively high values of α ov = 0.5.…”

The Wolf–Rayet binaries of the nitrogen sequence in the Large Magellanic Cloud

“…Usually, CHE is explained by invoking large initial rotation, which can efficiently mix the star. However, CHE can also be thought of as a proxy for increased homogeneity of massive stars, for which evidence is currently accumulating (e.g., Ramachandran et al 2019;Higgins & Vink 2019). It should be further noted that while evolution tracks always pass through the red-supergiant phase for all progenitor masses, no red supergiants with progenitor masses 25 M have ever been observed (Humphreys-Davidson-limit: Humphreys & Davidson 1979;Davies et al 2018).…”

Wind collision and accretion simulations of the massive binary system HD 166734

“…The uniqueness of HD 166734 and the discrepancy observed in this system, lead Higgins, & Vink (2019) to model the stars with alternative parameters, following the method of Weidner & Vink (2010). Higgins, & Vink (2019) used the MESA stellar evolution code to create a calibrated grid of rotating star models, and found that the properties for the two stars in HD 166734 can be obtained with enhanced mixing by rotation and larger overshooting coefficient. Their method also demonstrated that it is very useful to plot stellar evolution tracks on the mass-luminosity plane, in addition to the HR-diagram.…”