Rotational modulation in A and F stars: Magnetic stellar spots or convective core rotation?

Abstract

The Kepler mission revealed a plethora of stellar variability in the light curves of many stars, some associated with magnetic activity or stellar oscillations. In this work, we analyse the periodic signal in 162 intermediate-mass stars, interpreted as Rossby modes and rotational modulation - the so-called hump & spike feature. We investigate whether the rotational modulation (spike) is due to stellar spots caused by magnetic fields or due to Overstable Convective (OsC) modes resonantly exciting g modes, with frequencies corresponding to the convective core rotation rate. Assuming that the spikes are created by magnetic spots at the stellar surface, we recover the amplitudes of the magnetic fields, which are in good agreement with theoretical predictions. Our data show a clear anti-correlation between the spike amplitudes and stellar mass and possibly a correlation with stellar age, consistent with the dynamo-generated magnetic fields theory in (sub)-surface convective layers. Investigating the harmonic behaviour, we find that for 125 stars neither of the two possible explanations can be excluded. While our results suggest that the dynamo-generated magnetic field scenario is more likely to explain the spike feature, we assess further work is needed to distinguish between the two scenarios. One method for ruling out one of the two explanations is to directly observe magnetic fields in hump & spike stars. Another would be to impose additional constraints through detailed modelling of our stars, regarding the rotation requirement in the OsC mode scenario or the presence of a convective-core (stellar age).