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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.
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We discuss most recent spectroscopic and spectropolarimetric observations of the star HD 19400 representative of the group of PGa stars. Our high-spectral-resolution study of abundances, line profile variability, and the longitudinal magnetic field of HD 19400 discloses a remarkable similarity between this group and the group of HgMn stars.
Hot luminous stars show a variety of phenomena in their photospheres and winds which still lack clear physical explanation. Among these phenomena are photospheric turbulence, line profile variability (LPV), non-thermal emission, non-radial pulsations, discrete absorption components (DACs) and wind clumping. Cantiello et al. (2009) argued that a convection zone close to the stellar surface could be responsible for some of these phenomena. This convective zone is caused by a peak in the opacity associated with iron-group elements and is referred to as the iron convection zone (FeCZ). Assuming dynamo action producing magnetic fields at equipartition in the FeCZ, we investigate the occurrence of subsurface magnetism in OB stars. Then we study the surface emergence of these magnetic fields and discuss possible observational signatures of magnetic spots. Simple estimates are made using the subsurface properties of massive stars, as calculated in 1D stellar evolution models. We find that magnetic fields of sufficient amplitude to affect the wind could emerge at the surface via magnetic buoyancy. While at this stage it is difficult to predict the geometry of these features, we show that magnetic spots of size comparable to the local pressure scale height can manifest themselves as hot, bright spots. Localized magnetic fields could be widespread in those early type stars that have subsurface convection. This type of surface magnetism could be responsible for photometric variability and play a role in X-ray emission and wind clumping.
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.
While monitoring a sample of apparently slowly rotating superficially normal early A stars, we have discovered that HR 8844 (A0 V), is actually a new Chemically Peculiar star. We have first compared the high resolution spectrum of HR 8844 to that of four slow rotators near A0V ($ u$ Cap, $ u$ Cnc , Sirius A and HD 72660) to highlight similarities and differences. The lines of Ti II, Cr II, Sr II and Ba II are conspicuous features in the high resolution high signal-to-noise SOPHIE spectra of HR 8844 and much stronger than in the spectra of the normal star $ u$ Cap. The Hg II line at 3983.93 AA is also present in a 3.5 % blend. Selected unblended lines of 31 chemical elements from He up to Hg have been synthesized using model atmospheres computed with ATLAS9 and the spectrum synthesis code SYNSPEC48 including hyperfine structure of various isotopes when relevant. These synthetic spectra have been adjusted to the mean SOPHIE spectrum of HR 8844, and high resolution spectra of the comparison stars. Chisquares were minimized in order to derive abundances or upper limits to the abundances of these elements for HR 8844 and the comparison stars. HR 8844 is found to have underabundances of He, C, O, Mg, Ca and Sc, mild enhancements of Ti, V, Cr, Mn and distinct enhancements of the heavy elements Sr, Y, Zr, Ba, La, Pr, Sm, Eu and Hg, the overabundances increasing steadily with atomic number. This chemical pattern suggests that HR 8844 may actually be a new transition object between the coolest HgMn stars and the Am stars.
Blue horizontal-branch stars are Population II objects which are burning helium in their core and possess a hydrogen-burning shell and radiative envelope. Because of their low rotational velocities, diffusion has been predicted to work in their atmospheres. In many respects, blue horizontal-branch stars closely resemble the magnetic chemically peculiar stars of the upper main sequence, which show photometric variability caused by abundance spots on their surfaces. These spots are thought to be caused by diffusion and the presence of a stable magnetic field. However, the latter does not seem to be axiomatic. We searched for rotationally induced variability in 30 well-established bright field blue horizontal-branch stars in the solar neighbourhood and searched the literature for magnetic fields measurements of our targets. We employed archival photometric time series data from the ASAS, ASAS-SN, and SuperWASP surveys. The data were carefully reduced and processed, and a time series analysis was applied using several different techniques. We also synthesized existing photometric and spectroscopic data of magnetic chemically peculiar stars in order to study possible different surface characteristics producing lower amplitudes. In the accuracy limit of the employed data, no significant variability signals were found in our sample stars. The resulting upper limits for variability are given. We conclude that either no stellar surface spots are present in field blue horizontal-branch stars, or their characteristics (contrast, total area, and involved elements) are not sufficient to produce amplitudes larger than a few millimagnitudes in the optical wavelength region. New detailed models taking into account the elemental abundance pattern of blue horizontal-branch stars are needed to synthesize light curves for a comparison with our results.
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