Asteroseismic Constraints on the Models of Hot B Subdwarfs: Convective Helium-Burning Cores

Abstract

Abstract.Asteroseismology of non-radial pulsations in Hot B Subdwarfs (sdB stars) offers a unique view into the interior of core-helium-burning stars. Ground-based and space-borne high precision light curves allow for the analysis of pressure and gravity mode pulsations to probe the structure of sdB stars deep into the convective core. As such asteroseismological analysis provides an excellent opportunity to test our understanding of stellar evolution. In light of the newest constraints from asteroseismology of sdB and red clump stars, standard approaches of convective mixing in 1D stellar evolution models are called into question. The problem lies in the current treatment of overshooting and the entrainment at the convective boundary. Unfortunately no consistent algorithm of convective mixing exists to solve the problem, introducing uncertainties to the estimates of stellar ages. Three dimensional simulations of stellar convection show the natural development of an overshooting region and a boundary layer. In search for a consistent prescription of convection in one dimensional stellar evolution models, guidance from three dimensional simulations and asteroseismological results is indispensable. Hot Subdwarf B starsSubdwarf B (sdB) stars are a class of hot (T eff = 20, 000−40, 000 K) and compact (log g = 5.0−6.2) stars with very thin hydrogen envelopes (M H < 0.01 M ) [1,2]. They form the so-called extreme horizontal branch (EHB) in the Hertzsprung-Russell diagram, where most of them quietly burn helium in their cores for ∼10 8 years. A violent process stripped them of most of their hydrogen envelope before they underwent the helium flash near the tip of the red giant branch (RGB). As a consequence they will not climb up the asymptotic giant branch (AGB) once their core helium is exhausted, but rather evolve directly to become white dwarfs.Although some sdB stars appear to be single and others occur in wide binaries, surveys have concluded that the majority are in close binaries with white dwarf or lowmass main sequence companions [3][4][5]. A small fraction of the latter exhibit eclipses, offering insight into the orbital parameters and masses of the systems. These HW Virginis stars, named after the discovery system, play a crucial role in determining the mass distribution of sdB stars. The median mass of sdB stars estimated from these binaries is 0.469 M [6].The formation scenarios for sdB stars have to explain their existence in both single and binary systems as well as introduce a process by which the hydrogen envelope of the progenitor stars can be lost. Common envelope ejection (CE) or stable Roche lobe overflow (RLOF) during binary evolution result in systems with close and wide e-mail: jtschindler@email.arizona.edu separations, respectively, [7]. Population synthesis models can well explain the occurrence of binary systems and the mass distribution of the primary and secondary components [8]. To explain the existence of single sdB stars, extreme mass loss on the RGB [9-11] as well as white dwarf mergers...

Evolutionary modelling of subdwarf B stars using MESA with the predictive mixing and convective premixing schemes

“…Other proposed solutions involve microphysics, such as changes to opacities or nuclear reaction rates. Comprehensive discussion on the subject and additional models can be found in Schindler et al (2015Schindler et al ( , 2017. Some of these methods allow the core to grow up to 0.25 M , but for younger models (with high Y c ), adequate for the mentioned seismic targets, the core masses are lower, M cc < 0.2 M .…”