Rotation and spots in normal A and Am/Fm stars

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_{{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 V_{rot} agrees well with the projected rotational velocity (ν ~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 ̃ 21± 2 and ̃ 19± 2 ppm for the photometric amplitude and of 1.01 ± 0.13 and 1.16 ± 0.12 R_ E for the spot radius (where 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.

Publication : Monthly Notices of the Royal Astronomical Society, Volume 492, Issue 3, p.3143-3155
DOI : 10.1093/mnras/stz3623 
Bibcode : 2020MNRAS.492.3143T 
Keywords : stars: chemically peculiar; stars: general; stars: rotation; starspots