Jozef Žižňovský

The light variability of the helium strong star HD 37776 as a result of its inhomogeneous elemental surface distribution

We simulate light curves of the helium strong chemically peculiar star HD 37776 assuming that the observed periodic light variations originate as a result of inhomogeneous horizontal distribution of chemical elements on the surface of a rotating star. We show that chemical peculiarity influences the monochromatic radiative flux, mainly due to bound-free processes. Using the model of the distribution of silicon and helium on HD 37776 surface, derived from spectroscopy, we calculate a photometric map of the surface and consequently the uvby light curves of this star. Basically, the predicted light curves agree in shape and amplitude with the observed ones. We conclude that the basic properties of variability of this helium strong chemically peculiar star can be understood in terms of the model of spots with peculiar chemical composition.

The nature of the light variability of the silicon star HR 7224

Context. Although photometric variations of chemically peculiar (CP) stars are frequently used to determine their rotational periods, the detailed mechanism of their light variability remains poorly understood. Aims. We simulate the light variability of the star HR 7224 using the observed surface distribution of silicon and iron. Methods. We used the TLUSTY model atmospheres calculated for the appropriate silicon and iron abundances to obtain the emergent flux and to predict the rotationally modulated light curve of the star. We also obtained additional photometric measurements and employed our own regression procedure to derive a more precise estimate of the light elements. Results. We show that the light variation of the star can be explained as a result of i) the uneven surface distribution of the elements, ii) the flux redistribution from the ultraviolet to the visible part of the spectrum, and iii) rotation of the star. We show that the silicon bound-free transitions and iron bound-bound transitions provide the main contribution to the flux redistribution, although an additional source of opacity is needed. We confirm that numerous iron lines significantly contribute to the well-known depression at 5200 A and discuss the connection between iron abundance and the value of peculiarity index a. Conclusions. The uneven surface distribution of silicon and iron is able to explain most of the rotationally modulated light variation in the star HR 7224.

Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Virginis

Context. The spectral energy distribution (SED) in chemically peculiar stars may be significantly affected by their abundance anomalies. The observed SED variations are usually assumed to be a result of inhomogeneous surface distribution of chemical elements, flux redistribution and stellar rotation. However, the direct evidence for this is still only scarce. Aims. We aim to identify the processes that determine the SED and its variability in the UV and visual spectral domains of the heliumweak star CU Vir. Methods. We used the TLUSTY model atmospheres calculated for the appropriate surface chemical composition to obtain the emergent flux and predict the rotationally modulated flux variability of the star. Results. We show that most of the light variations in the vby filters of the Stromgren photometric system are a result of the uneven surface distribution of silicon, chromium, and iron. Our models are only able to explain a part of the variability in the u filter, however. The observed UV flux distribution is very well reproduced, and the models are able to explain most of the observed features in the UV light curve, except for the region 2000−2500 A, where the amplitude of the observed light variations is higher than predicted. The variability observed in the visible is merely a faint gleam of that in the UV. While the amplitude of the light curves reaches only several hundredths of magnitude in the visual domain, it reaches about 1 mag in the UV. Conclusions. The visual and UV light variability of CU Vir is caused by the flux redistribution from the far UV to near UV and visible regions, inhomogeneous distribution of the elements and stellar rotation. Bound-free transitions of silicon and bound-bound transitions of iron and chromium contribute the most to the flux redistribution. This mechanism can explain most of the rotationally modulated light variations in the filters centred on the Paschen continuum and on the UV continuum of the star CU Vir. However, another mechanism(s) has to be invoked to fully explain the observed light variations in the u filter and in the region 2000−2500 A.

Stars with discrepant v sin i as derived from Ca II 3933 and Mg II 4481 Å lines. I. Composite-spectrum star HD 2913

Axial rotation of a star plays an important role in its evolution, physical conditions in its atmosphere and the shape of its spectrum. Methods of determining of v sin i are based on comparison of the observed profiles of spectral lines with the theoretical ones. Their accuracy depends on the type and quality of spectrograms, as well as on the algorithms used. A frequently used method is a simple comparison of one line, e.g. the Ca ii at 3933 Å or Mg ii at 4481 Å. This, however, may result in a false value of v sin i in case when low-dispersion spectra are used. We investigate the spectra of stars with a significant discrepancy of their rotational velocities introduced in various sources, and analyze the corresponding spectral region from the point of view of possible admixed features, which may mask the true line profiles. We use CCD spectra of the stars having this discrepancy, to compare with theoretical spectra. We also studied photographic spectra, obtained during the 1970s and 1980s. In this work we studied the spectra of the binary HD 2913A, and identified the spectrum of its weaker component designated as “Ab”. We estimated the effective temperature, surface gravity and projected rotational velocity of the weaker component that classify it as an early F-type Main Sequence star. The discrepancy between the values of v sin i derived from the two lines of calcium and magnesium is explained as a consequence of superposition of the pair’s spectra. The cooler component contributes by a strong Ca ii-K line, and thus significantly broadens the observed line profile.

The puzzling binary HD 143418

Context. HD 143418 was discovered recently to be a double-lined spectroscopic binary with a primary designated as a CP star. Its light displays an orbital phase coupled variability with a peak-to-peak amplitude up to 0.04 mag. Aims. The photometry available and new high dispersion spectra were investigated from a point of view of CP characteristics. Methods. A series of high resolution high S/N coude spectra was acquired from which 25 weak to strong unblended lines of Fe i and ii ,T iii ,C rii ,Z rii ,a nd Baii were selected to study spectral line variability. Two Zeeman spectra were obtained to search for a possible magnetic field of the star, and one echelle spectrum in a wide spectral region was analysed for abundance determination by means of synthetic spectra. The photometric observations were subjected to a PCA disentangling of the complex photometric behaviour. Results. We identified spectral lines of the secondary in the yellow region on the echelle as well as on two coude spectra, whose occurence belongs to an F6V star and the intensity corresponds to the luminosity ratio 0.06. Equivalent widths of the selected spectral lines of the primary component do not change within the errors of measurements. The spectra taken with a Zeeman analyser do not indicate a magnetic field. The abundance pattern does not correspond to characteristics of a variable CP2 star. The only remarkable deviation is a more than 1.1 dex deficit of scandium, one of the properties of non-variable Am stars. The photometric variability is tied to the orbital period and is due to ellipticity of the primary component and not to a putatively structured surface of the primary that is confirmed to rotate subsynchronously. The seasonal component of the light curve changes in amplitude as well as in shape. Conclusions. We conclude that the primary is a normal, mildly evolved A5V main sequence star. The seasonal variability of the orbitally modulated light curves may be related to an expected incidence of circumstellar matter originating in the tidally spinning up primary component. HD 143418 may be a prototype of a rare detached interacting close binary containing a subsynchronously rotating primary passing through its synchronisation stage.

The nature of the light variations of chemically peculiar stars CU Vir and HD 64740

The nature of the light variations of chemically peculiar stars was studied in detail only in a very few cases. To better understand the mechanisms of light variability of these stars, we study the light variations of the well-known magnetic chemically peculiar star CU Vir and one of the least amplitude variable stars HD 64740. We show that the light variability of these stars is induced by flux redistribution in spots of enhanced abundance of chemical elements (e.g., helium, silicon, iron or chromium), and by the stellar rotation. We conclude that this is a promising model for the explanation of the light variability of most chemically peculiar stars.

The mechanism of the light variability of chemically peculiar stars

Until recently, the mechanism of the light variability of chemically peculiar (CP) stars was unclear. To improve this situation, we started a theoretical and observational campaign aimed at the nature of the light variability of these stars. We use the TLUSTY model atmospheres calculated for the appropriate surface chemical composition to obtain the emergent flux and to predict the rotationally modulated light curves. We show on example of several well-studied CP stars that their light variations can be explained as a result of i) the uneven surface distribution of the elements (creating overabundant regions), ii) the flux redistribution from the ultraviolet to the visible part of the spectrum (in the overabundant regions), and iii) rotation of the star. We show that the silicon and helium bound-free transitions and iron bound-bound transitions provide the main contribution to the flux redistribution. This result is also a very precise test of modern stellar model atmospheres. We conclude that the mentioned mechanism is a very promising explanation for the light variations in CP stars of earlier spectral types.

Advanced test of the model stellar atmospheres: the nature of the light variability of magnetic chemically peculiar stars

The magnetic chemically peculiar stars exhibit both inhomogeneous horizontal distribution of chemical elements on their surfaces and the light variability. We show that the observed light variability of these stars can be successfully simulated using models of their stellar atmospheres and adopting the observed surface distribution of elements. The most important elements that influence the light variability are silicon, iron, and helium.

Atmospheric Extinction Derived from Cometary Observations

Special filters for photoelectric photometry of comets enable the study of atmospheric extinction not only in the broad band photometric systems but also in the intermediate band IHW/IAU system covering wavelengths from 365 to 514 nm. Extinction coefficients, which are characteristics of an atmospheric state, are by-products of observations of comets. Contrary to the observation of variable stars, comets have to be very frequently observed a few degrees above the horizon and so wide intervals of air masses are used in the calculation of characteristic values. Extinction measurements obtained during the observations of bright comets 1P/Halley in 1985/86, 23P/Brorsen-Metcalf in 1989 and C/Austin in 1990 are used in this study. 78 measurments in 21 observational nights cover all the parts of year with the exception of spring with cloudy weather in our mountain site and June with the shortest observational nights. Average extinction coefficients are derived in the paper and some conclusions of common validity (in central European conditions) are formulated. The results of IHW/IAU extinction study have been compared with the results of UBV extinction coefficients gained mostly as a by-product of the photoelectric photometry of variable stars (407 nights in the period from 1962 to 1995). Seasonal variations of atmospheric extinction are discussed.

Spectrum Analysis of the Binary Star 53 Aur with a CP3 Component

The CP-star 53 Aur (HD 47152, HR 2425) has been assigned in the literature different types of peculiarity. Bertaud (1959) classified it as A0p-EuCr, Cowley et al. (1969) as B9p-Eu(Cr?), Osawa (1965) as Hg and Zverko (1974) as Mn -type. MacAlister (1978) discovered its binary nature and Tokovinin (1986) first determined its orbit with a period of 13.7 y. Subsequent interferometric observations by Baize (1989) yielded new orbital elements with a period of 25.816 y, total mass of 5.8 M ⊙ and magnitude difference of two B9 components 0.3 mag. Adelman (1982) performed spectrophotometry and fitted the Balmer jump with T eff =10 500 K, while the Paschen continuum with T eff =9 500 K, log g =4 in both cases. Palmer (1965) estimated the projected rotational velocity to 325 km s −1 from the width of the Ca II-K line, while Wolff and Preston (1975) obtained 33 km s −1 from the Mg II 448.1 nm line. In this work we analyze high-dispersion spectra obtained in two distant orbital phases, namely in 1975 and in 1992 to distinguish the components of the system.