Long period Ap stars discovered with TESS data

Abstract

The TESS space mission is producing superb data for asteroseismology, eclipsing binary stars, gyrochronology, and other fields of stellar astronomy where the data are variable light curves. We show that TESS data are excellent for astrophysical inference from peculiar stars that show variability. The Ap stars have the strongest magnetic fields of any main-sequence stars. Some have been shown to have rotation periods of months to centuries. The astrophysical cause of their slow rotation -- the braking mechanism -- is not known with certainty. These stars are rare: there are currently about 3 dozen with known periods. The magnetic Ap stars have long-lived spots that allow precise determination of their rotation periods. We show that most Ap stars with TESS data that show no low-frequency variability must have rotation periods longer than a TESS sector of 27 d. We find 60 Ap stars in the southern ecliptic hemisphere TESS data with no rotational variability, of which at most a few can be pole-on, and six likely have nearly aligned magnetic and rotation axes. Of the other 54, 31 were previously known to have long rotation periods or low projected equatorial velocities, which proves our technique; 23 are new discoveries. These are prime targets for long-term magnetic studies. We also find that 22 per cent of long-period Ap stars are roAp stars, versus only 3 per cent of the other Ap stars studied with TESS sector 1 and 2 data. This work demonstrates astrophysical inference from nonvariable stars -- "something for nothing".