Imaging and Modeling Rapidly Rotating Stars: Alpha Cephei and Alpha Ophiuchi

TL;DR

This study uses high-resolution interferometry and gravity darkening models to precisely determine the physical properties of rapidly rotating stars Alpha Cephei and Alpha Ophiuchi, revealing temperature gradients and proposing a new mass estimation method.

Contribution

It introduces a novel approach combining interferometric imaging and gravity darkening modeling to accurately measure stellar parameters and estimate masses of single rapid rotators.

Findings

Determined stellar inclination, radii, and temperatures with unprecedented precision.

Revealed ~2000K temperature gradient between polar and equatorial regions.

Proposed a new method for direct mass estimation using Vsin(i) and geometrical constraints.

Abstract

We present sub-milliarcseond resolution imaging and modeling of two nearby rapid rotators Alpha Cephei and Alpha Ophiuchi, obtained with the CHARA array - the largest optical/IR interferometer in the world. Incorporating a gravity darkening model, we are able to determine the inclination, the polar and equatorial radius and temperature, as well as the fractional rotation speed of the two stars with unprecedented precision. The polar and equatorial regions of the two stars have ~2000K temperature gradient, causing their apparent temperatures and luminosities to be dependent on their viewing angles. Our modeling allow us to determine the true effective temperatures and luminosities of Alpha Cep and Alpha Oph, permitting us to investigate their true locations on the H-R diagram. These properties in turn give us estimates of the masses and ages of the two stars within a few percent of error using stellar evolution models. Also, based on our gravity darkening modeling, we propose a new method to estimate the masses of single stars in a more direct way through Vsin(i) measurements and precise geometrical constraint. Lastly, we investigate the degeneracy between the inclination and the gravity darkening coefficient, which especially affects the modeling of Alpha Oph. Although incorporating Vsin(i) has lifted the degeneracy to some extent, higher resolution observations are still needed to further constrain the parameters independently.