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.Comment: 11 pages, accepted for the publication in Astronomy & Astrophysic

Surprising variations in the rotation of the chemically peculiar stars CU Virginis and V901 Orionis

CU Vir and V901 Ori belong among these few magnetic chemically peculiar stars whose rotation periods vary on timescales of decades. We aim to study the stability of the periods in CU Vir and V901 Ori using all accessible observational data containing phase information. We collected all available relevant archived observations supplemented with our new measurements of these stars and analysed the period variations of the stars using a novel method that allows for the combination of data of diverse sorts. We found that the shapes of their phase curves were constant, while the periods were changing. Both stars exhibit alternating intervals of rotational braking and acceleration. The rotation period of CU Vir was gradually shortening until the year 1968, when it reached its local minimum of 0.52067198 d. The period then started increasing, reaching its local maximum of 0.5207163 d in the year 2005. Since that time the rotation has begun to accelerate again. We also found much smaller period changes in CU Vir on a timescale of several years. The rotation period of V901 Ori was increasing for the past quarter-century, reaching a maximum of 1.538771 d in the year 2003, when the rotation period began to decrease. A theoretically unexpected alternating variability of rotation periods in these stars would remove the spin-down time paradox and brings a new insight into structure and evolution of magnetic upper-main-sequence stars.Comment: 5 pages, 3 figure

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

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. We aim to identify the processes that determine the SED and its variability in the UV and visual spectral domains of the helium-weak star CU Vir. We used the model atmospheres to obtain the emergent flux and predict the rotationally modulated flux variability of the star. 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. 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. 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.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and Astrophysic

Structure of the magnetic field in the Ap star HD 187474

We reconstruct the complex magnetic field in the Ap star HD 187474 within the frame of the point field source model, where virtual magnetic charges are distributed in the stellar body. The best-fit model describes sufficiently well the observed nonsinusoidal variability of the mean magnetic field modulus and the sinusoidal behaviour of the mean longitudinal magnetic field with the phase of stellar rotation. The best fit provides discrepancy on the level of $\chi^{2}=6.10$ for all the analyzed data. We show that in HD 187474 the magnetic dipole is displaced from centre of the star by 0.055 $R_{\star}$. The dipole has a size ~0.035 $R_{\star}$. The angle between the stellar rotational axis and the magnetic dipole is \beta=37\degr

Lithium and metal abundances in long period Am binaries

High signal-to-noise CCD spectra of three bright, long period Am binaries (HD 108651, HD 116657 and HD 138213) have been obtained and the atmospheric abundances of identified chemical elements were studied. We found HD 116657 to be a new candidate as a Li deficient, long period Am binary, in addition to 16 Ori. While HD 108651 and HD 116657 have pronounced Am characteristics, HD 138213 is just a mild Am star.
These stars were further studied in a more general context related to the role of binarity in CP phenomenon. It was found that both Li abundance and the equivalent with ratio Ca $\rm{I}$ 6718/Fe $\rm{I}$ 6679 are more peculiar in systems with more pronounced eccentricities. Such behaviour is the opposite of what was previously found in Ap binaries and their anomalies. It indicates that tidal effects are of crucial importance in driving CP phenomena.

Properties and nature of Be stars

Photometric and spectroscopic monitoring of the B star resulted in the finding that this object is a new bright Be star with a clear positive correlation between the brightness and emission-line strength. The emission-line episodes are relatively short and seem to repeat frequently which makes this star an ideal target for studying the causes of the Be phenomenon. The general character of the light variations, the low v sin i = 70 km s-1 and the very pronounced line asymmetries of the \ion{He}{i} 6678 line, seen both outside and during emission-line episodes, are all attributes which make phenomenologically very similar to the well-known Be star ω CMa. Radial velocities of the deepest parts of the metallic and \ion{He}{i} 6678 absorption lines vary with a strict period of 2$^{\rm d}\!\!.$61507 over the whole time interval covered by the observations, the velocities of the broad outer wings of the same lines varying in anti-phase and with a lower amplitude. This periodicity could not be found in the radial-velocity variations of the sharp core of Hα. There is some indication of variability on a time scale of 24–29 days but our data are insufficient to prove that conclusively. A comparison of the line spectrum obtained outside emission episodes with synthetic spectra, standard dereddening of $U\!B{}V$ magnitudes and Hipparcos parallax all agree with the conclusion that is a star with the following basic properties: Teff  = 17 000 K, log $g = 3.0$ [cgs], mass of 5 $M_{\odot}$ and radius of 11 $R_{\odot}$. The strong emission-line episodes may appear regularly, in a cycle of 630 days but with different durations of individual cycles. is probably one of the first B stars for which the Be nature was predicted on the basis of the character of its light and colour changes