Rotation and spots in normal A and Am/Fm stars

Abstract

ABSTRACT Frequency analysis of long-term ultraprecise photometry can lead to precise values of rotation frequencies of rotating stars with ‘hump and spike’ features in their periodograms. Using these features, we computed the rotation frequencies and amplitudes. The corresponding equatorial rotational velocity ($V_{\rm{rot}}$) and spot size were estimated. On fitting the autocorrelation functions of the light curves with the appropriate model, we determined the star-spot decay time-scale. The $\rm \mathit{ V}_{rot}$ agrees well with the projected rotational velocity ($\rm \nu ~sin$ i) in the literature. Considering a single circular and black spot, we estimate its radius from the amplitude of the ‘spike’. No evidence for a significant difference in the average ‘spike’ amplitude and spot radius was found for Am/Fm and normal A stars. Indeed, we derived an average value of $\rm \sim\! 21\pm 2$ and $\rm \sim\! 19\pm 2$ ppm for the photometric amplitude and of $\rm 1.01\, \pm \, 0.13$ and $\rm 1.16\, \pm \, 0.12$ $R_\mathrm{ E}$ for the spot radius (where $\rm \mathit{ R}_E$ is the Earth radius), respectively. We do find a significant difference for the average spot decay time-scale, which amounts to 3.6 ± 0.2 and 1.5 ± 0.2 d for Am/Fm and normal A stars, respectively. In general, spots on normal A stars are similar in size to those on Am/Fm stars, and both are weaker than previously estimated. The existence of the ‘spikes’ in the frequency spectra may not be strongly dependent on the appearance of star-spots on the stellar surface. In comparison with G, K, and M stars, spots in normal A and Am/Fm stars are weak, which may indicate the presence of a weak magnetic field.