Colder and Hotter: Interferometric imaging of {\beta} Cassiopeiae and {\alpha} Leonis

TL;DR

This study uses interferometric imaging to analyze the rapid rotation and gravity darkening effects in stars {eta} Cas and {\alpha} Leo, revealing anomalies and challenging existing theoretical models with empirical data.

Contribution

It provides new interferometric observations of two stars, models their rotation and gravity darkening, and proposes revised empirical gravity darkening coefficients for rapid rotators.

Findings

Both stars are confirmed as rapidly rotating with gravity darkening anomalies.

The empirically-derived gravity darkening coefficient is 0.19, differing from von Zeipel law predictions.

The fast rotation of {eta} Cas suggests efficient core-envelope coupling over 0.5 Gyr.

Abstract

Near-infrared interferometers have recently imaged a number of rapidly rotating A-type stars, finding levels of gravity darkening inconsistent with theoretical expectations. Here, we present new imaging of both a cooler star {\beta} Cas (F2IV) and a hotter one {\alpha} Leo (B7V) using the CHARA array and the MIRC instrument at the H band. Adopting a solid-body rotation model with a simple gravity darkening prescription, we modeled the stellar geometric properties and surface temperature distributions, confirming both stars are rapidly rotating and show gravity darkening anomalies. We estimate the masses and ages of these rapid rotators on L-Rpol and HR diagrams constructed for non-rotating stars by tracking their non-rotating equivalents. The unexpected fast rotation of the evolved subgiant {\beta} Cas offers a unique test of the stellar core-envelope coupling, revealing quite efficient coupling over the past ~ 0.5 Gyr. Lastly we summarize all our interferometric determinations of the gravity darkening coefficient for rapid rotators, finding none match the expectations from the widely used von Zeipel gravity darkening laws. Since the conditions of the von Zeipel law are known to be violated for rapidly rotating stars, we recommend using the empirically-derived {\beta} = 0.19 for such stars with radiation-dominated envelopes. Furthermore, we note that no paradigm exists for self-consistently modeling heavily gravity-darkened stars that show hot radiative poles with cool convective equators.