Jan Janík

The extremely rapid rotational braking of the magnetic helium-strong star HD 37776

Context. Light and spectrum variations of the magnetic chemically peculiar (mCP) stars are explained by the oblique rigid rotator model with a rotation period usually assumed to be stable on a long time scale. A few exceptions, such as CU Vir or 56 Ari, have been reported as displaying an increase in their rotation period. A possible increase in the period of light and spectrum variations has also been suggested from observations of the helium-strong mCP star HD 37776 (V901 Ori). Aims. In this paper we attempt to confirm the possible period change of HD 37776 and discuss a possible origin of this change as a consequence of i) duplicity; ii) precession; iii) evolutionary changes; and iv) continuous/discrete/transient angular momentum loss. Methods. We analyse all available observations of the star obtained since 1976. These consist of 1707 photometric measurements obtained in uvby(β), (U)BV, V, BTVT ,a ndHp, including 550 of our own recent observations obtained in 2006 and 2007, 53 spectrophotometric measurements of the Hei λ 4026 A line, 66 equivalent width measurements of Hei spectral lines from 23 CFHT spectrograms acquired in 1986, and 69 Hei equivalent measurements from spectral lines present in 35 SAO Zeeman spectrograms taken between 1994 and 2002. All of these 1895 individual observations obtained by various techniques were processed simultaneously by means of specially developed robust codes. Results. We confirm the previously suspected gradual increase in the 1. 5387 period of HD 37776 and find that it has lengthened by a remarkable 17.7 ± 0.7 s over the past 31 years. We also note that a decrease in the rate of the period change is not excluded by the data. The shapes of light curves in all colours were found to be invariable. Conclusions. After ruling out light-time effects in a binary star, precession of the rotational axis, and evolutionary changes as possible causes for the period change, we interpret this ongoing period increase as a braking of the star’s rotation, at least in its surface layers, due to the momentum loss through events or processes in the extended stellar magnetosphere.

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

We aim to study the stability of the periods in CU Vir and V901 Ori using all accessible observational data containing phase information. 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.

Ultraviolet and visual flux and line variations of one of the least variable Bp stars HD 64740

Context. 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. Aims. 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. Methods. 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 2.2 m MPG/ESO telescope in La Silla. Results. 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. Conclusions. The maximum abundances of helium and silicon on the surface of HD 64740 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 C iv resonance lines of carbon 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.

Period Analyses Without O-C Diagrams

We present a versatile method appropriate for the period analyses of observations containing phase information of all kinds of periodic or nearly periodic variable stars on the basis of phenomenological modelling of their phase curves and phase functions. The approach is based on rigorous application of a non-linear weighted least-squares method exploiting all available observational data and does not need an O-C diagram as an intermediate stage for period analyses. However, this approach enables us to determine precise times of extrema of light curves, to calculate ephemerides and construct plausible O-C diagrams. We substantiate the general applicability of the method on eclipsing binaries research.

λ Bootis stars in the SuperWASP survey

We have analysed around 170 000 individual photometric WASP (Wide Angle Search for Planets) measurements of 15 well-established λ Bootis stars to search for variability. The λ Bootis stars are a small group of late-B to early-F, Pop I, stars that show moderate to extreme (surface) underabundances (up to a factor 100) of most Fe-peak elements, but solar abundances of lighter elements (C, N, O and S). They are excellent laboratories for the study of fundamental astrophysical processes such as diffusion, meridional circulation, stellar winds and accretion in the presence of pulsation. From the 15 targets, 8 are variable and 7 are apparently constant with upper limits between 0.8 and 3.0 mmag. We present a detailed time-series analysis and a comparison with previously published results. From an asteroseismologic study, we conclude that the found chemical peculiarities are most probably restricted to the surface.

Reports on New Discoveries

Variability of CzeV843 Cen was detected 31 March 2016 in the FOV of V834 Cen using Dk154 telescope at La Silla.

Search for forced oscillations in binaries ? IV. The eclipsing binary V436 Per revisited

An analysis of new spectroscopic and photoelectric UBV observations, satisfactorily covering the whole orbital period of V436 Per, together with existing data allowed us to improve the knowledge of the basic physical characteristics of the binary and its components. In several aspects, our new results dier from the findings of Paper I of this series: in particular, we found that it is the star eclipsed in the secondary minimum which is slightly more massive and larger than the optical primary. We also conclude that the apsidal advance - if present at all - is much slower than that estimated in a previous study. The orbital period might be increasing by 0.28 s per year but also this finding is very uncertain and needs verification by future observations. It is encouraging to note that two completely independent sets of programs for light-curve solutions lead to identical results. A notable finding is that both binary components rotate with very similar - if not identical - rotational periods of 1 d :45 and 1 d :40, much shorter than what would correspond to a 10 d9 spin-orbit synchronization period at periastron. Rapid line-profile changes

The Kepler view of magnetic chemically peculiar stars

Magnetic chemically peculiar (mCP) stars exhibit complex atmospheres that allow the investigation of such diverse phenomena as atomic diffusion, magnetic fields, and stellar rotation. The advent of space-based photometry provides the opportunity for the first precise characterizations of the photometric variability properties of these stars. We carried out a search for new mCP stars in the Kepler field with the ultimate aim of investigating their photometric variability properties using Kepler data. As an aside, we describe criteria for selecting mCP star candidates based on light curve properties, and assess the accuracy of the spectral classifications provided by the MKCLASS code. As only very few known mCP stars are situated in the Kepler field, we had to depend largely on alternative (nonspectroscopic) means of identifying suitable candidates that rely mostly on light curve properties; in particular we relied on monoperiodic variability and light curve stability. Newly acquired and archival spectra were used to confirm most of our mCP star candidates. Linear ephemeris parameters and effective amplitudes were derived from detrended Kepler data. Our final sample consists of 41 spectroscopically confirmed mCP stars of which 39 are new discoveries, 5 candidate mCP stars, and 7 stars in which no chemical peculiarities could be established. Our targets populate the whole age range from zero-age main sequence to terminal-age main sequence and are distributed in the mass interval from 1.5 M_sun to 4 M_sun. About 25% of the mCP stars show a hitherto unobserved wealth of detail in their light curves indicative of complex surface structures. We identified light curve stability as a primary criterion for identifying mCP star candidates among early-type stars in large photometric surveys, and prove the reliability of the spectral classifications provided by the MKCLASS code.

New inclination changing eclipsing binaries in the Magellanic Clouds

Context. Multiple stellar systems are unique laboratories for astrophysics. Analysis of their orbital dynamics, if well characterized from their observations, may reveal invaluable information about the physical properties of the participating stars. Unfortunately, there are only a few known and well described multiple systems, this is even more so for systems located outside the Milky Way galaxy. A particularly interesting situation occurs when the inner binary in a compact triple system is eclipsing. This is because the stellar interaction, typically resulting in precession of orbital planes, may be observable as a variation of depth of the eclipses on a long timescale. Aims. We aim to present a novel method to determine compact triples using publicly available photometric data from large surveys. Here we apply it to eclipsing binaries (EBs) in Magellanic Clouds from OGLE III database. Our tool consists of identifying the cases where the orbital plane of EB evolves in accord with expectations from the interaction with a third star. Methods. We analyzed light curves (LCs) of 26121 LMC and 6138 SMC EBs with the goal to identify those for which the orbital inclination varies in time. Archival LCs of the selected systems, when complemented by our own observations with Danish 1.54-m telescope, were thoroughly analyzed using the PHOEBE program. This provided physical parameters of components of each system. Time dependence of the EB’s inclination was described using the theory of orbital-plane precession. By observing the parameter-dependence of the precession rate, we were able to constrain the third companion mass and its orbital period around EB. Results. We identified 58 candidates of new compact triples in Magellanic Clouds. This is the largest published sample of such systems so far. Eight of them were analyzed thoroughly and physical parameters of inner binary were determined together with an estimation of basic characteristics of the third star. Prior to our work, only one such system was well characterized outside the Milky Way galaxy. Therefore, we increased this sample in a significant way. These data may provide important clues about stellar formation mechanisms for objects with different metalicity than found in our galactic neighborhood.

Variable stars in open clusters

We present our joint efforts to study variable stars in open clusters. This includes a new catalogue, a photometric survey for new variables, and the database WEBDA. Our tools will shed more light on stellar variability in open clusters.