Very precise satellite photometry has revealed a large number of variable stars whose variability is caused either by surface spots or by binarity. Detailed studies of such variables provide insights into the physics of these objects. We study the nature of the periodic light variability of the white dwarf EPIC 206197016 that was observed by the K2 mission. We obtain phase-resolved medium-resolution spectroscopy of EPIC 206197016 using XSHOOTER spectrograph at VLT to understand the nature of the white dwarf variability. We use NLTE model atmospheres to determine stellar parameters at individual phases. EPIC 206197016 is a hot DA white dwarf with $T_\text{eff}=78\,$kK. The analysis of the spectra reveals periodic radial velocity variations that can result from gravitational interaction with an invisible secondary whose mass corresponds to a red dwarf. The close proximity of the two stars where the semimajor axis is about $3\,R_\odot$ results in the irradiation of the companion with temperatures more than twice as high on the illuminated side compared to the nonilluminated hemisphere. This effect can explain the observed light variations. The spectra of the white dwarf show a particular feature of the Balmer lines called the Balmer line problem, where the observed cores of the lower Balmer lines are deeper than predicted. This can be attributed to either weak pollution of hydrogen in the white dwarf atmosphere by heavy elements or to the presence of a circumstellar cloud or disk.

Classical main-sequence chemically peculiar stars show light variability that originates in surface abundance spots. In the spots, the flux redistribution due to line (bound-bound) and bound-free transitions is modulated by stellar rotation and leads to light variability. White dwarfs and hot subdwarfs may also have surface abundance spots either owing to the elemental diffusion or as a result of accretion of debris. We model the light variability of typical white dwarfs and hot subdwarfs that results from putative surface abundance spots. We show that the spots with radiatively supported iron overabundance may cause observable light variability of hot white dwarfs and subdwarfs. Accretion of debris material may lead to detectable light variability in warm white dwarfs. We apply our model to the helium star HD 144941 and conclude that the spot model is able to explain most of observed light variations of this star.

Context. Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates. Aims. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation. Methods. We used phase-resolved medium-resolution X-shooter spectroscopy to understand the light variability of the stars KIC 250152017 and KIC 249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra. Results. KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.

Context. Ultraprecise space photometry enables us to reveal light variability even in stars that were previously deemed constant. A large group of such stars show variations that may be rotationally modulated. This type of light variability is of special interest because it provides precise estimates of rotational rates.Aims. We aim to understand the origin of the light variability of K2 targets that show signatures of rotational modulation.Methods. We used phase-resolved medium-resolution X-shooter spectroscopy to understand the light variability of the stars KIC 250152017 and KIC 249660366, which are possibly rotationally modulated. We determined the atmospheric parameters at individual phases and tested the presence of the rotational modulation in the spectra.Results. KIC 250152017 is a HgMn star, whose light variability is caused by the inhomogeneous surface distribution of manganese and iron. It is only the second HgMn star whose light variability is well understood. KIC 249660366 is a He-weak, high-velocity horizontal branch star with overabundances of silicon and argon. The light variability of this star is likely caused by a reflection effect in this post-common envelope binary.

Helium-peculiar magnetic chemically peculiar stars show variations of helium abundance across their surfaces. As a result of associated atmospheric scale height variations, the stellar surface becomes distorted, with helium-rich regions dented inwards. Effectively, on top of flux variations due to opacity effects, the depressed helium-rich surface regions become less bright in the optical regions and brighter in the ultraviolet. We study the observational effects of the aspherical surface on the light curves of a Cen. We simulate the light curves of this star adopting surface distributions of He, N, O, Si, and Fe derived from Doppler mapping and introducing the effect of distortion proportional to helium abundance. We show that while most of the optical and UV variations of this star result from flux redistribution due to the non-uniform surface distributions of helium and iron, the reduction of light variations due to the helium-related surface distortion leads to a better agreement between simulated optical light curves and the light curves observed with the BRITE satellites.

Context. CU Vir has been the first main sequence star that showed regular radio pulses that persist for decades, resembling the radio lighthouse of pulsars and interpreted as auroral radio emission similar to that found in planets. The star belongs to a rare group of magnetic chemically peculiar stars with variable rotational period. Aims. We study the ultraviolet (UV) spectrum of CU Vir obtained using STIS spectrograph onboard the Hubble Space Telescope (HST) to search for the source of radio emission and to test the model of the rotational period evolution. Methods. We used our own far-UV and visual photometric observations supplemented with the archival data to improve the parameters of the quasisinusoidal long-term variations of the rotational period. We predict the flux variations of CU Vir from surface abundance maps and compare these variations with UV flux distribution. We searched for wind, auroral, and interstellar lines in the spectra. Results. The UV and visual light curves display the same long-term period variations supporting their common origin. New updated abundance maps provide better agreement with the observed flux distribution. The upper limit of the wind mass-loss rate is about 10(-12) M-circle dot yr(-1). We do not find any auroral lines. We find rotationally modulated variability of interstellar lines, which is most likely of instrumental origin. Conclusions. Our analysis supports the flux redistribution from far-UV to near-UV and visual domains originating in surface abundance spots as the main cause of the flux variability in chemically peculiar stars. Therefore, UV and optical variations are related and the structures leading to these variations are rigidly confined to the stellar surface. The radio emission of CU Vir is most likely powered by a very weak presumably purely metallic wind, which leaves no imprint in spectra.

The presence of ultra-high excitation (UHE) absorption lines (e.g., O VIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near $10^6$K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star ($P=0.242035$d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultra hot wind phenomenon.

Opacity variations across stellar surfaces are the key process producing the spectral energy distribution (SED) variability in chemically peculiar (CP) stars. The opacity variations are caused by the presence of surface spots with enhanced (or depleted) abundances of chemical elements. Simulations of the SED variability of chemically peculiar stars with abundances derived from Doppler mapping provide a detailed test of the continuum (bound free) and line opacities in the model atmospheres. The effect of opacities on the SED is most pronounced in the ultraviolet region. We simulate the ultraviolet and visual SED variability of selected chemically peculiar stars using model atmospheres calculated for actual surface abundances, and compare the predicted SEDs with observational results. We show that the simulations can reliably predict the observed SED and its variability as long as complete bound free and bound bound opacities are used. Therefore, the variability of chemically peculiar stars may serve as a test of opacities included in model atmospheres.

Magnetic stars show several types of light variability which is modulated by the stellar rotation. In chemically peculiar stars, the redistribution of the flux in the surface regions with peculiar chemical composition leads to the light variability with a typical amplitude of the order of hundredths of magnitude. The most efficient processes that cause the flux redistribution are bound-bound (line) transitions of iron and bound-free (ionization) transitions of silicon. This type of light variability typically leads to a complex dependence of the amplitude on the wavelength and shows antiphase light curves in the far ultraviolet and visual regions. In hot magnetic stars, the modulation of the stellar wind by the magnetic field and the wind blanketing cause the light variability with a typical amplitude of the order of millimagnitudes. We predict the light variations in selected magnetic hot stars and compare the simulated light curves with light variations derived from observations.

The horizontally inhomogeneous chemical composition of the atmospheres of the chemically peculiar stars causes wavelength redistribution of the spectral energy in areas with increased abundance of heavier elements. Due to the rotation of the star, this usually leads to strictly periodic photometric variability in some spectral regions. We used abundance maps of the HgMn star phi Phe (HD 11753), obtained by means of the Doppler imaging, to model its photometric variability. Comparing the light curves derived from abundance maps obtained at different times, we also study how the time evolution of the surface spots affects this variability.

The initial success of the Rigidly Rotating Magnetosphere (RRM) model application to the B2Vp star sigma OriE by Townsend, Owocki & Groote (2005) triggered a renewed era of observational monitoring of this archetypal object. We utilize high-resolution spectropolarimetry and the magnetic Doppler imaging (MDI) technique to simultaneously determine the magnetic configuration, which is predominately dipolar, with a polar strength Bd = 7.3-7.8 kG and a smaller non-axisymmetric quadrupolar contribution, as well as the surface distribution of abundance of He, Fe, C, and Si. We describe a revised RRM model that now accepts an arbitrary surface magnetic field configuration, with the field topology from the MDI models used as input. The resulting synthetic Ha emission and broadband photometric observations generally agree with observations, however, several features are poorly fit. To explore the possibility of a photospheric contribution to the observed photometric variability, the MDI abundance maps were used to compute a synthetic photospheric light curve to determine the effect of the surface inhomogeneities. Including the computed photospheric brightness modulation fails to improve the agreement between the observed and computed photometry. We conclude that the discrepancies cannot be explained as an effect of inhomogeneous surface abundance. Analysis of the UV light variability shows good agreement between observed variability and computed light curves, supporting the accuracy of the photospheric light variation calculation. We thus conclude that significant additional physics is necessary for the RRM model to acceptably reproduce observations of not only sigma Ori E, but also other similar stars with significant stellar wind-magnetic field interactions.

Context. The presence of heavier chemical elements in stellar atmospheres influences the spectral energy distribution of stars. An uneven surface distribution of these elements, together with flux redistribution and stellar rotation, are commonly believed to be the primary causes of the variability of chemically peculiar (CP) stars.Aims. We aim to model the photometric variability of the CP star. Dra based on the assumption of inhomogeneous surface distribution of heavier elements and compare it to the observed variability of the star. We also intend to identify the processes that contribute most significantly to its photometric variability.Methods. We use a grid of TLUSTY model atmospheres and the SYNSPEC code to model the radiative flux emerging from the individual surface elements of. Dra with different chemical compositions. We integrate the emerging flux over the visible surface of the star at different phases throughout the entire rotational period to synthesise theoretical light curves of the star in several spectral bands.Results. The synthetic light curves in the visible and in the near-UV regions are in very good agreement with the observed variability of the star. The lack of usable far-UV measurements of the star precludes making any conclusions about the correctness of our model in this spectral region. We also obtained 194 new BVRI observations of phi Dra and improved its rotational period to P = 1.(d)716500(2).Conclusions. We show that the inhomogeneous distribution of elements, flux redistribution, and rotation of the star are fully capable of explaining the stellar variability in the visible and the near-UV regions. The flux redistribution is mainly caused by bound-free transitions of silicon and bound-bound transitions of iron.

Sigma Ori E (HD 37479) is the prototypical helium-strong star shown to harbor a strong magnetic field, as well as a magnetosphere consisting of two clouds of plasma. The observed optical (ubvy) light curve of sigma Ori E is dominated by eclipse features due to circumstellar material, however, there remain additional features unexplained by the Rigidly Rotating Magnetosphere (RRM) model of Townsend Owocki. Using the technique of magnetic Doppler imaging (MDI), spectropolarimetric observations of sigma Ori E are used to produce maps of both the magnetic field topology and various elemental abundance distributions. We also present an analysis utilizing these computed MDI maps in conjunction with NLTE TLUSTY models to study the optical brightness variability of this star arising from surface inhomogeneities. It has been suggested that this physical phenomena may be responsible for the light curve inconsistencies between the model and observations.

phi Draconis is one of the brightest known CP stars. We model its light variability using the horizontal distribution of chemical elements in the stellar atmosphere derived from abundance maps. Those elements cause redistribution of the energy from the short-wavelength part of the UV spectrum to longer wavelengths. We compute a grid of LTE model atmospheres, and synthesize a theoretical light curve. The results obtained from our computations are in a very good agreement with the observed variability of the star.

The usual photometric variability detected in Ap stars is that associated with rotation. It has long been surmised that redistribution of rotationally modulated flux in surface abundance spots is the cause of that type of variability. The redistribution is caused by bound-bound (line) and bound-free (continuum) transitions of various elements, in particular helium, silicon, iron, and (obviously) also rare-earth elements. With the availability of detailed abundance maps, complete atomic data, and detailed model atmospheres it has now become possible to simulate reliably the photometric variability due to rotation. This is demonstrated by the example of several CP stars. We generalise these result and discuss the importance of individual elements in terms of their dependence on effective temperature. We emphasise the importance of light curve prediction for testing surface abundance maps and the atomic data.

The light variability of hot magnetic chemically peculiar stars is typically caused by the flux redistribution in spots with peculiar abundance. This raises the question why some stars with surface abundance spots show significant rotational light variability, while others do not. We study the Bp star HD 64740 to investigate how its remarkable inhomogeneities in the surface distribution of helium and silicon, and the corresponding strong variability of many spectral lines, can result in one of the faintest photometric variabilities among the Bp stars. We used model atmospheres and synthetic spectra calculated for the silicon and helium abundances from surface abundance maps to predict the ultraviolet and visual light and line variability of HD 64740. The predicted fluxes and line profiles were compared with the observed ones derived with the IUE, HST, and Hipparcos satellites and with spectra acquired using the FEROS spectrograph at the 2.2m MPG/ESO telescope. We are able to reproduce the observed visual light curve of HD 64740 assuming an inhomogeneous distribution of iron correlated with silicon distribution. The light variations in the ultraviolet are hardly detectable. We detect the variability of many ultraviolet lines of carbon, silicon, and aluminium and discuss the origin of these lines and the nature of their variations. The maximum abundances of helium and silicon are not high enough to cause significant light variations. The detected variability of many ultraviolet lines is most likely of atmospheric origin and reflects the inhomogeneous elemental surface distribution. The variability of the Civ resonance lines is stronger and it probably results from the dependence of the wind mass-loss rate on the chemical composition and magnetic field orientation. We have not been able to detect a clear signature of the matter trapped in the circumstellar clouds.