Spherically-symmetric model stellar atmospheres and limb darkening I: limb-darkening laws, gravity-darkening coefficients and angular diameter corrections for red giant stars

TL;DR

This paper compares plane-parallel and spherically symmetric stellar atmosphere models, revealing significant differences in limb darkening, gravity-darkening, and angular diameter corrections, especially for giant stars with low surface gravity.

Contribution

It provides new limb-darkening laws and correction factors based on spherically symmetric models, improving the accuracy of stellar observations for giant stars.

Findings

Spherical models predict different limb darkening for log g < 3.

Standard limb-darkening laws fit poorly to spherical models.

Angular diameter corrections vary significantly between geometries.

Abstract

Model stellar atmospheres are fundamental tools for understanding stellar observations from interferometry, microlensing, eclipsing binaries and planetary transits. However, the calculations also include assumptions, such as the geometry of the model. We use intensity profiles computed for both plane-parallel and spherically symmetric model atmospheres to determine fitting coefficients in the BVRIHK, CoRot and Kepler wavebands for limb darkening using several different fitting laws, for gravity-darkening and for interferometric angular diameter corrections. Comparing predicted variables for each geometry, we find that the spherically symmetric model geometry leads to different predictions for surface gravities log g < 3. In particular, the most commonly used limb-darkening laws produce poor fits to the intensity profiles of spherically symmetric model atmospheres, which indicates the need for more sophisticated laws. Angular diameter corrections for spherically symmetric models range from 0.67 to 1, compared to the much smaller range from 0.95 to 1 for plane-parallel models.