Viktor Votruba

Properties and nature of Be stars - 29. Orbital and long-term spectral variations of γ Cassiopeiae

A detailed analysis of more than 800 electronic high-resolution spectra of gamma Cas, which were obtained during a time interval of over 6000 days (16.84 yr) at several observatories, documents the smooth variations in the density and/or extent of its circumstellar envelope. We found a clear anticorrelation between the peak intensity and FWHM of the Hα emission, which seems to agree with recent models of such emission lines. The main result of this study is a confirmation of the binary nature of the object, determination of a reliable linear ephemeris Tmin.RV = HJD (2 452 081.9±0.6)+(203. 52±0. d 08)×E, and a rather definitive set of orbital elements. We clearly demonstrated that the orbit is circular within the limits of accuracy of our measurements and has a semi-amplitude of radialvelocity curve of 4.30 ± 0.09 km s −1 . No trace of the low-mass secondary was found. The time distribution of our spectra does not allow a reliable investigation of rapid spectral variations, which are undoubtedly present in the spectra. We postpone this investigation for a future study, based on series of dedicated whole-night spectral observations.

A hydrodynamic scheme for two-component winds from hot stars

Aims. We have developed a time-dependent two-component hydrodynamics code to simulate radiatively-driven stellar winds from hot stars. Methods. We use a time-explicit van Leer scheme to solve the hydrodynamic equations of a two-component stellar wind. Dynamical friction due to Coulomb collisions between the passive bulk plasma and the line-scattering ions is treated by a time-implicit, semianalytic method using a polynomial fit to the Chandrasekhar function. This gives stable results despite the stiffness of the problem. Results. This method was applied to model stars with winds that are both poorly and well-coupled. While for the former case we reproduce the mCAK solution, for the latter case our solution leads to wind decoupling.

Properties and nature of Be stars. XXIII. Long-term variations and physical properties of x Dra

We present an analysis of new spectroscopic observations of the bright Be star kappa Dra obtained at the Ondrejov observatory during 1992-2003 and UBV photometric observations secured at several observatories. General characteristics and a line identification of the spectrum of kappa Dra are obtained in the regions 3730-5650 A and 5850-7800 A by a comparison with the theoretical spectrum. The fundamental stellar parameters have been obtained from a comparison with a grid of NLTE model atmospheres. The best fit was found for T_eff =14\ 000 K, log g = 3.5, and v sin i = 170 km/s. These values together with a Hipparcos parallax lead to a stellar mass M= 4.8 +/- 0.8 M_S and radius R=6.4 +/- 0.5 R_S. It is encouraging to see that these values agree well with the expected evolutionary mass and radius for the effective temperature we derived. Long-term variations of kappa Dra were analysed using measurements of equivalent widths, central intensities, peak intensities of emission lines and emission peak velocity differences for H alpha, H beta, H gamma, H delta, and some helium, silicon, and iron lines. The previously reported period of 23 years in the variation of the emission strength is probably a cyclic, not a strictly periodic phenomenon. An attempt to find out a period from all available records of the H beta emission strength led to a value of 8044 +/- 167 days (22.0 years) but the phase plots show that each cycle has a different shape and length. The maximum strength of the emission lags behind the brightness maximum. This is a behaviour usually observed for long-term changes of Be stars with a positive correlation between the brightness and emission strength. Since there are obviously no published speckle observations of the star, we suggest these should be carried out. They could help to deny or confirm the possibility that the emission episodes are triggered by a periastron passage of a putative binary companion moving in an eccentric orbit with a 8044-d period, as it seems to be the case for some Be binaries. For the moment, the nature and origin of the disk around

Time-dependent spectral-feature variations of stars displaying the B[e] phenomenon - III. HD 50138

\varkappa

HD 161306: a radiatively interacting Be binary?

Dra remains unknown. From the comparison of the electronic spectra obtained at different phases of the long-term cycle and synthetic spectra it appears that there are no detectable changes in the photospheric part of the Balmer lines related to variations in the Balmer emission strength which could be attributed to an extended photosphere corresponding to inner parts of the disk, optically thick in continuum.

Weak wind effects in CNO driven winds of hot first stars

We present results of nearly six years of spectroscopic observations of the B[e] star V2028 Cyg. The presence of the cold-type absorption lines combined with a hot-type spectrum indicate the binarity of this object. Since B[e] stars are embedded in an extended envelope, the usage of common stellar atmosphere models for the analysis is quite inappropriate. Therefore, we focus on the analysis of the long-term spectral line variations in order to determine the nature of this object. We present the time dependences of the equivalent width and radial velocities of the H alpha line, [O I] 6300 A, Fe II 6427, 6433, and 6456 A lines. The bisector variations and line intensities are shown for the H alpha line. The radial velocities are also measured for the absorption lines of the K component. No periodic variation is found. The observed data show correlations between the measured quantities, which can be used in future modelling.

Spectroscopic analysis of the B/Be visual binary HR 1847

The spectrum of the Be star HD 161306 is shown to vary periodically with a period of∼100 days. The radial velocity of the He i 6678 Å emission peak varying in antiphase to the radial velocity of the Hα emission wings component suggests that the star is a binary similar to φ Per, 59 Cyg, or FY CMa, i.e. a radiatively interacting Be binary – a rare case among Be stars. This type of object is also called a φ Per-type binary or Be + sdO binaries. The range of radial-velocity variations of the strong emission peak in the helium line observed in HD 161306 is about 180 km s−1, similar to what is observed for these systems. We therefore conclude that HD 161306 may represent another case of a Be star with a hot subdwarf companion.

Is the B[e] Star V2028 Cyg a Binary

Context. During the evolution of rotating first stars, which initially consisted of only hydrogen and helium, CNO elements may emerge to their surface. These stars may therefore have winds that are driven only by CNO elements. Aims. We study weak wind effects (Gayley-Owocki heating and multicomponent effects) in stellar winds of first generation stars driven purely by CNO elements. Methods. We apply our NLTE multicomponent models and hydrodynamical simulations. Results. The multicomponent effects (frictional heating and decoupling) are important particularly for low metallicity winds, but they influence mass loss rate only if they cause decoupling for velocities lower than the escape velocity. The multicomponent effects also modify the feedback from first stars. As a result of the decoupling of radiatively accelerated metals from hydrogen and helium, the first low-energy cosmic ray particles are generated. We study the interaction of these particles with the interstellar medium concluding that these particles easily penetrate the interstellar medium of a given minihalo. We discuss the charging of the first stars by means of their winds. Conclusions. Gayley-Owocki heating, frictional heating, and the decoupling of wind components occur in the winds of evolved low-metallicity stars and the solar metallicity main-sequence stars.

Modeling of the Be Stars

We studied both components of a slightly overlooked visual binary HR 1847 spectroscopically to determine its basic physical and orbital parameters. Basic stellar parameters were determined by comparing synthetic spectra to the observed echelle spectra, which cover both the optical and near-IR regions. New observations of this system used the Ondy and Rozhen 2-m telescopes and their coude spectrographs. Radial velocities from individual spectra were measured and then analysed with the code FOTEL to determine orbital parameters. The spectroscopic orbit of HR 1847A is presented for the first time. It is a single-lined spectroscopic binary with a B-type primary, a period of 719.79 days, and a highly eccentric orbit with e = 0.7. We confirmed that HR 1847B is a Be star. Its Hα emission significantly decreased from 2003 to 2008. Both components have a spectral type B7−8 and luminosity class IV−V.

Emission features in a B[e] binary system V2028 Cyg

V2028 Cyg shows a B[e] phenomenon. Due to the presence of both cool (K7III) and hot (B4III) components in the spectra, it is supposed to be a binary. Our modelling of the time variability of H alpha line bisectors shows, however, that this hybrid spectrum can originate in one star, which is surrounded by a disc.