Scaling of the asymptotic entropy jump in the superadiabatic layers of stellar atmospheres

TL;DR

This paper reveals a universal entropy stratification in the superadiabatic layers of stellar atmospheres, which can enhance the boundary conditions used in stellar structure models, improving their accuracy across different cool stars.

Contribution

It introduces a universal polynomial representation of the entropy stratification in stellar surface layers, aiding more accurate stellar modeling.

Findings

Universal entropy stratification when normalized by entropy jump and minimum.

Polynomial fits accurately represent the entropy profile.

Surface convection operates similarly across all cool stars.

Abstract

Stellar structure calculations are able to predict precisely the properties of stars during their evolution. However, convection is still modelled by the mixing length theory; therefore, the upper boundary conditions near the optical surface do not agree with asteroseismic observations. We want to improve how the outer boundary conditions are determined in stellar structure calculations. We study realistic 3D stellar atmosphere models to find alternative properties. We find that the asymptotic entropy run of the superadiabatic convective surface layers exhibit a distinct universal stratification when normalised by the entropy minimum and jump. The normalised entropy can be represented by a 5th order polynomial very accurately, and a 3rd order polynomial also yields accurate coefficients. This generic entropy stratification or the solar stratification, when scaled by the entropy jump and minimum, can be used to improve the modelling of superadiabatic surface layers in stellar structure calculations. Furthermore, this finding indicates that surface convection operates in the same way for all cool stars, but requires further scrutiny in order to improve our understanding of stellar atmospheres.