TESS observations of Be stars: general characteristics and the impulsive magnetic rotator model

TL;DR

This study analyzes TESS light curves of Be stars, revealing their rotational characteristics and proposing the impulsive magnetic rotator model to explain their variability and surface activity.

Contribution

It introduces the impulsive magnetic rotator model and provides new insights into the rotational velocities and variability patterns of Be stars based on TESS data.

Findings

73% of Be stars are short-period variables.

Be stars rotate at about 66% of critical velocity.

Rotational amplitudes increase with effective temperature.

Abstract

In this work, we study the general characteristics of Be star light curves from TESS sectors 1-26 in the light of a model we call the "impulsive magnetic rotator". It is found that 73 percent of 441 classical Be stars are short-period variables. The light curves are characterised by non-coherent periods dominated by the fundamental and first harmonic with variable relative amplitudes. By comparing the equatorial rotational velocities obtained from the photometric fundamental periods with the projected rotational velocities,it is shown that the fundamental period is indistinguishable from the rotation period. The distributions of the projected rotational velocities and the equatorial rotational velocities for Be and non-Be stars are discussed. Using the equatorial rotational velocities of Be stars directly determined from the photometric period, it is found that, on average, Be stars rotate at about 0.66 of critical velocity. This result is independent of assumptions concerning gravity darkening. It is shown that the rotational amplitudes of Be stars increase with effective temperature and are considerably higher than those of non-Be stars. The amplitude distributions of Be and non-Be stars are also significantly different. Of particular interest is a large rise in rotational amplitudes from A0 to B5 for non-Be stars. We suspect that this may be a result of the increasing importance of a line-driven wind. Perhaps the Be stars are just those with greatest surface activity with increased mass loss aided by rapid rotation.