Rotation and spots in normal A and Am/Fm stars

TL;DR

This study analyzes rotation and starspot properties in A and Am/Fm stars using precise photometry, revealing similarities in spot size but differences in decay times, and suggesting weak magnetic fields in these stars.

Contribution

It provides the first detailed comparison of starspot characteristics between normal A and Am/Fm stars, including decay times and amplitudes, using long-term photometric data.

Findings

Spot sizes are similar in A and Am/Fm stars.

Decay times are significantly longer in Am/Fm stars.

Spots are weaker compared to cooler stars, indicating weak magnetic fields.

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 ($v_{\rm rot}$) 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 $v_{\rm rot}$ agrees well with the projected rotational velocity ($v\,{\rm sin}\,i$) 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 $\rm \sim 21\pm2$ and $\sim 19\pm2\,{\rm ppm}$ for the photometric amplitude and of $\rm 1.01\,\pm\,0.13$ and $1.16\,\pm\,0.12\,R_{\rm E}$ for the spot radius (where $R_{\rm E}$ is the Earth radius), respectively. We do find a significant difference for the average spot decay-time scale, which amounts to $3.6\pm0.2$ and $1.5\pm0.2$ 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.