The mass discrepancy in intermediate- and high-mass eclipsing binaries: The need for higher convective core masses

Abstract

Context. Eclipsing, spectroscopic double-lined binary star systems (SB2) are excellent laboratories for calibrating theories of stellar interior structure and evolution. Their precise and accurate masses and radii measured from binary dynamics offer model-independent constraints and challenge current theories of stellar evolution. Aims. We aim to investigate the mass discrepancy in binary stars. This is the significant difference between stellar components' masses measured from binary dynamics and those inferred from models of stellar evolution via positions of the components in the T eff -log g Kiel diagram. We study the effect of near-core mixing on the mass of the convective core of the stars and interpret the results in the context of the mass discrepancy. Methods. We fit stellar isochrones computed from a grid of mesa stellar evolution models to a homogeneous sample of eleven highmass binary systems. Two scenarios are considered, where individual stellar components of a binary system are treated independent of each other and where they are forced to have the same age and initial chemical composition. We also study the effect of the microturbulent velocity and turbulent pressure on the atmosphere model structure and stellar spectral lines, and its link with the mass discrepancy.Results. We find that the mass discrepancy is present in our sample and that it is anti-correlated with the surface gravity of the star. No correlations are found with other fundamental and atmospheric parameters, including the stellar mass. The mass discrepancy can be partially accounted for by increasing the amount of near-core mixing in stellar evolution models. We also find that ignoring the microturbulent velocity and turbulent pressure in stellar atmosphere models of hot evolved stars results in overestimation of their effective temperature by up to 8%. Together with enhanced near-core mixing, this can almost entirely account for the ∼30% mass discrepancy found for the evolved primary component of V380 Cyg. Conclusions. We find a strong link between the mass discrepancy and the convective core mass. The mass discrepancy can be solved by considering the combined effect of extra near-core boundary mixing and consistent treatment in the spectrum analysis of hot evolved stars. Our binary modelling results in convective core masses between 17 and 35% of the stellar mass, in excellent agreement with results from gravity-mode asteroseismology of single stars. This implies larger helium core masses near the end of the main sequence than anticipated so far.

Discovery of β Cep pulsations in the eclipsing binary V453 Cygni

“…However, there is often significant disagreement between the masses of O and B stars inferred from spectroscopy and stellar evolution theory, versus dynamical masses measurements in binary systems. This E-mail: astro.js@keele.ac.uk mass discrepancy was identified by Herrero et al (1992) and has been explained by invoking varying amounts of near-core mixing (Guinan et al 2000;Tkachenko et al 2014Tkachenko et al , 2016Tkachenko et al , 2020.…”

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

“…This is primarily because binary studies have the potential to provide masses and radii from the stars' relative orbital motion around a common center of mass. Moreover, accurate, absolute and model-independent masses and radii are achievable in the case of eclipsing binary systems (see e.g., Southworth et al, 2020 andTkachenko et al, 2020) owing to the ability to simultaneously model spectroscopic radial velocities together with the eclipse depths in the light curve of a binary system.…”

Massive Star Modeling and Nucleosynthesis

“…The two more used methods are either the width of the main sequence in an HR diagram (Herwig, 2000;Bressan et al, 2012;Ekström et al, 2012;Choi et al, 2016), or the velocity drop at the end of the main sequence (Brott et al, 2011a). More recently, it has been proposed to calibrate the overshoot on binary stars systems (Tkachenko et al, 2020). Some studies indicate that there could be a dependence of the overshoot with the mass of the star (see for example Castro et al, 2014;Claret and Torres, 2016), and so using a fixed value would lead either to overestimate the overshoot in the lower mass domain, or to underestimate it in the higher mass domain.…”