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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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The nominal Kepler mission provided very high-precision photometric data. Using these data, interesting phenomena such as spots, and ``hump and spike features were observed in the light curves of some normal A and metallic lined A stars (Am stars). However, the connection between such phenomena and the chemical peculiarity of the Am stars is still unclear. In order to make progress on these issues, its important to collect high-resolution spectroscopic data to determine their fundamental parameters and individual chemical abundances. In this paper, we present a spectroscopic study of a sample of ``hump and spike stars in the nominal Kepler field. We used data collected with the High Efficiency and Resolution Mercator {E}chelle Spectrograph (HERMES). We determined the spectral type of these stars and obtained the atmospheric stellar parameters such as effective temperatures, surface gravities, projected rotational, microturbulent and radial velocities. We also performed a detailed individual chemical abundance analysis for each target. We confirmed KIC 3459226 and KIC 6266219 as Am stars, KIC 9349245 as a marginal Am star, while KIC 4567097, KIC 4818496, KIC 5524045, KIC 5650229, KIC 7667560, and KIC 9272082 are non-Am stars. To estimate their evolutionary phases, all the stars were placed in the Hertzsprung-Russell (HR) diagram. Based on their spectral classification and chemical abundance pattern, we reclassified KIC 6266219 (previously treated as chemically normal) as an Am star (kA3hA7mF1) and KIC 9272082 (previously treated as Am) as non-Am.
About 22000 Kepler stars and nearly 60000 TESS stars from sectors 1-24 have been classified according to variability type. A large proportion of stars of all spectral types appear to have periods consistent with the expected rotation periods. A previous analysis of A and late B stars strongly suggests that these stars are indeed rotational variables. In this paper we have accumulated sufficient data to show that rotational modulation is present even among the early B stars. A search for flares in TESS A and B stars resulted in the detection of 110 flares in 68 stars. The flare energies exceed those of typical K and M dwarfs by at least two orders of magnitude. These results, together with severe difficulties of current models to explain stellar pulsations in A and B stars, suggest a need for revision of our current understanding of the outer layers of stars with radiative envelopes.
Weak magnetic fields have recently been detected in a number of A-type stars, including Vega and Sirius. At the same time, space photometry observations of A- and late B-type stars from Kepler and TESS have highlighted the existence of rotational modulation of surface features akin to stellar spots. Here we explore the possibility that surface magnetic spots might be caused by the presence of small envelope convective layers at or just below the stellar surface, caused by recombination of H and He. Using 1D stellar evolution calculations and assuming an equipartition dynamo, we make simple estimates of field strength at the photosphere. For most models the largest effects are caused by a convective layer driven by second helium ionization. While it is difficult to predict the geometry of the magnetic field, we conclude that the majority of intermediate-mass stars should have dynamo-generated magnetic fields of order a few gauss at the surface. These magnetic fields can appear at the surface as bright spots, and cause photometric variability via rotational modulation, which could also be wide-spread in A-stars. The amplitude of surface magnetic fields and their associated photometric variability is expected to decrease with increasing stellar mass and surface temperature, so that magnetic spots and their observational effects should be much harder to detect in late B-type stars.
A fraction of late B-type stars, the so-called HgMn stars, exhibit enhanced absorption lines of certain chemical elements, notably Hg and Mn, combined with an underabundance of He. For about a decade now the elements with anomalously high abundances in HgMn stars are known to be distributed inhomogeneously over the stellar surface. Temporal evolution of these elemental spots have been reported in a few HgMn stars, first secular evolution of the mercury spots in alpha And, and recently also a fast evolution of yttrium and strontium spots in HD 11753. The fast evolution of spots in HD 11753 is combined with a slower change in the overall abundance of the affected elements. In this paper I review what is known of elemental spots in HgMn stars and their secular and fast temporal evolution.
HD144941 is an evolved early-type metal-poor low-mass star with a hydrogen-poor surface. It is frequently associated with other intermediate helium-rich subdwarfs and extreme helium stars. Previous photometric studies have failed to detect any variability. New observations with the K2 mission show complex but periodic variations with a full amplitude of 4 parts per thousand. It is proposed that these are due to an inhomogeneous surface brightness distribution (spots) superimposed on a rotation period of 13.9+/-0.2 d. The cause of the surface inhomogeneity is not identified, although an oblique dipolar magnetic field origin is plausible.
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