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تسجيل مستخدم جديدRotation and spots in normal A and Am/Fm stars
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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 $rm sim 21pm2$ and $sim 19pm2,{rm ppm}$ 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 $3.6pm0.2$ and $1.5pm0.2$ 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.
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