$\Omega$-Slow Solutions and Be Star Disks

TL;DR

This paper investigates the role of rapid stellar rotation and gravity darkening in forming Be star disks, developing a novel model that explains observed emission profiles and the two-component wind structure.

Contribution

It introduces a new method for solving gravity darkened, oblated m-CAK equations and demonstrates how stellar shape and gravity darkening influence wind properties and emission profiles.

Findings

The oblate finite disk correction factor varies significantly between equator and pole.

Predicted Hα profiles match standard models for specific line-force parameters.

Fast wind component has negligible impact on Hα emission, which mainly originates from equatorial disks.

Abstract

As the disk formation mechanism(s) in Be stars is(are) as yet unknown, we investigate the role of rapidly rotating radiation-driven winds in this process. We implemented the effects of high stellar rotation on m-CAK models accounting for: the shape of the star, the oblate finite disk correction factor, and gravity darkening. For a fast rotating star, we obtain a two-component wind model, i.e., a fast, thin wind in the polar latitudes and an $\Omega$-slow, dense wind in the equatorial regions. We use the equatorial mass densities to explore H$\alpha$ emission profiles for the following scenarios: 1) a spherically symmetric star, 2) an oblate shaped star with constant temperature, and 3) an oblate star with gravity darkening. One result of this work is that we have developed a novel method for solving the gravity darkened, oblated m-CAK equation of motion. Furthermore, from our modeling we find a) the oblate finite disk correction factor, for the scenario considering the gravity darkening, can vary by at least a factor of two between the equatorial and polar directions, influencing the velocity profile and mass-loss rate accordingly, b) the H$\alpha$ profiles predicted by our model are in agreement with those predicted by a standard power-law model for following values of the line-force parameters: $1.5 \lesssim k \lesssim 3$, $ \, \alpha \sim 0.6$ and $\, \delta \gtrsim 0.1$, and c) the contribution of the fast wind component to the H$\alpha$ emission line profile is negligible; therefore, the line profiles arise mainly from the equatorial disks of Be stars.