Rotation and spots in normal A and Am/Fm stars

Abstract

Frequency analysis of long-term ultra-precise 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 ($v_{\mathrm{rot}}$) and spot size were estimated. On fitting the autocorrelation functions of the light-curves with the appropriate model, we determined the starspot decay-time scale. The $v_{\mathrm{rot}}$ agrees well with the projected rotational velocity ($v\sin i$) 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 $\sim 21\pm 2$ and $\sim 19\pm 2\mathrm{ppm}$ for the photometric amplitude and of $1.01\pm 0.13$ and $1.16\pm 0.12R_{\mathrm{E}}$ for the spot radius (where $R_{\mathrm{E}}$ is the Earth radius), respectively. We do find a significant difference for the average spot decay-time scale, which amounts to $3.6\pm 0.2$ and $1.5\pm 0.2$ days 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 starspots 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.

arXiv comment20 pages, Published in MNRAS

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