Predicted observational effects of rapid rotation for Be stars

TL;DR

This study models how rapid rotation affects observable features of Be stars, revealing that gravity darkening and inclination significantly influence spectral and polarization measurements, aiding in understanding stellar rotation.

Contribution

It provides a comprehensive set of predictions on how rapid rotation alters Be star observables across different subtypes and disk densities, using advanced 3D radiative transfer simulations.

Findings

Gravity darkening can make stars appear more evolved.

Rotation influences Hα line profiles depending on viewing angle.

Polarization slope is highly sensitive to rotation rate.

Abstract

We conduct a systematic study on the effects of rapid rotation on predicted Be star observables. We use the three-dimensional Monte Carlo radiative transfer code, \textsc{hdust}, to model a comprehensive range of Be star subtypes at varying rotation rates. Using these models, we predict $V$ magnitude and photometric color, H$\alpha$ line profiles, and polarization at UV wavelengths as well as in the $V$-band for Be stars from B0 to B8. For each spectral subtype, we investigate the effects of disk density on the produced observables. We find that reddening and brightening effects of gravity darkening may cause rapidly-rotating stars to appear more evolved than they truly are. Rotational effects on the H$\alpha$ line profile shape may reduce line intensity for Be stars viewed at low inclinations and increase line intensity for those viewed at high inclinations. Additionally, rapid rotation can significantly impact the measured equivalent width of the line produced by a star with a moderate to high density disk, especially at high inclinations. When the star-disk system is viewed near edge-on, gravity darkening can result in stronger H$\alpha$ emission than would otherwise be expected for a disk of a given density. We also find that the competing effects of rapid rotation and H\,\textsc{i} opacity cause the slope of the polarized continuum (the polarization color) to be very sensitive to changes in the stellar rotation rate. This quantity offers a strong diagnostic for the rotation rate of Be stars.