Most Rotational Variables Dominated by a Single Bright Feature Are α 2 CVn Stars

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 light curve. We have spectroscopically investigated 14 UCBH stars and found 10 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 α ² CVn variables. Most ATLAS UCBH stars are therefore α ² CVn stars, although only a minority of α ² CVn stars in the literature have UCBH light curves. The fact that α 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 α ² CVn UCBH stars may be characterized by a specific magnetic field topology, making them intriguing targets for future Zeeman-Doppler imaging.