Long period Ap stars discovered with TESS data

Abstract

Context. The TESS space mission has a primary goal to search for exoplanets around bright, nearby stars. Because of the high precision photometry required for the main mission, it also is producing superb data for asteroseismology, eclipsing binary stars, gyrochronology – any field of stellar astronomy where the data are variable light curves.
Aims. In this work we show that the TESS data are excellent for astrophysical inference from peculiar stars that show no variability. The Ap stars have the strongest magnetic fields of any main-sequence stars. Some Ap stars have also been shown to have rotation periods of months, years, decades and even 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.
Methods. The magnetic Ap stars have long-lived spots that allow precise determination of their rotation periods. We argue, and show, that most Ap stars with TESS data that show no low-frequency variability must have rotation periods longer than, at least, a TESS sector of 27 d. Results. From this 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 very low projected equatorial velocities, which proves our technique; 23 are new discoveries. These are now prime targets for long-term magnetic studies. We also find that 12 of the 54 (22 per cent) long-period Ap stars are roAp stars, versus only 3 per cent (29 out of 960) of the other Ap stars studied with TESS in sectors 1−13, showing that the roAp phenomenon is correlated with rotation, although this correlation is not necessarily causal. In addition to probing rotation in Ap stars, these constant stars are also excellent targets to characterise the instrumental behaviour of the TESS cameras, as well as for the CHEOPS and PLATO missions.
Conclusions. This work demonstrates astrophysical inference from nonvariable stars – we can get “something for nothing”.