Most Rotational Variables Dominated by a Single Bright Feature are $\alpha^2$ CVn Stars

TL;DR

This paper identifies a rare class of rotational variable stars with unique light curves, predominantly composed of Ap/Bp stars with bright spots, suggesting specific magnetic field topologies and offering new insights into stellar surface features.

Contribution

It reveals that most UCBH light curves are from $eta^2$ CVn stars, linking their light curves to bright spots on Ap/Bp stars and proposing a connection to magnetic field structures.

Findings

Majority of UCBH stars are Ap/Bp stars with bright spots.

UCBH light curves are characteristic of $eta^2$ CVn stars.

Bright spots on Ap/Bp stars are more common than previously thought.

Abstract

We previously reported a rare class of variable star light curves isolated from a sample of 4.7 million candidate variables from the ATLAS survey. Dubbed `UCBH' light curves, they have broad minima and narrow, symmetrical maxima, with typical periods of 1-10 days and amplitudes of 0.05--0.20 mag. They maintain constant amplitude, shape, and phase coherence over multiple years, but do not match any known class of pulsating variables. A localized bright spot near the equator of a rotating star will produce a UCBH-type light curve for most viewing geometries. Most stars that exhibit rotational variability caused primarily by a single bright feature should therefore appear as UCBH stars, although a rotating bright spot is not the only thing that could produce a UCBH-type lightcurve. We have spectroscopically investigated fourteen UCBH stars and found ten of them to be Ap/Bp stars: A-type or B-type stars with greatly enhanced photospheric abundances of specific heavy elements. Rotationally variable Ap/Bp stars are referred to as $\alpha^2$ CVn variables. Most ATLAS UCBH stars are therefore $\alpha^2$ CVn stars, although only a minority of $\alpha^2$ CVn stars in the literature have UCBH light curves. The fact that $\alpha^2$ CVn stars dominate the UCBH class suggests that lone bright spots with sufficient size and contrast develop more readily on Ap/Bp stars than on any other type. The $\alpha^2$ CVn UCBH stars may be characterized by a specific magnetic field topology, making them intriguing targets for future Zeeman-Doppler imaging.