Miroslav Slechta

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.

A new analysis of the radial velocity variations of the eclipsing and spectroscopic binary EN Lacertae

An analysis of 1236 new electronic spectra of the eclipsing binary EN Lac from four observatories and of 994 radial velocities (RV hereafter) from photographic spectra, published by several authors, has allowed us to disentangle the RV variations due to orbital motion and due to pulsations of the star. New, accurate orbital elements as well as precise values of the three pulsation periods, already known from the previous studies, were derived. The accuracy of the orbital solution has been substantially improved after the observed RV changes were properly prewhitened for the short-term oscillations. The amplitude of the dominant RV oscillation with a period of P_1=0fd16916703 was found to vary with a 74 year cycle. The amplitudes of the two other RV oscillations, having periods of P_3=0fd 18173256 and P_2=0fd 17085554, vary on much shorter time scales of 674 d and 331 d, respectively. The value of P_2 derived here does not correspond to a one year alias of the value found by several authors from photometry but appears to be an intrinsic period. The time scales of the amplitude modulations found for P_1 and P_3 are in good agreement with previous photometric results. For the first time we present evidence of line profile variations of EN Lac. They correlate well with the short-term RV variations but they alternatively occur with periods corresponding either to the fundamental periods or to the first harmonics of P_1 to P_3. An analysis of the RV scatter along the orbital phase curve for the new spectra, obtained over a relatively short interval of time, gives some indication of a sharp increase of this scatter when the stars are approaching periastron. Since this could be a signature of forced oscillations, the effect is worth further study, though it appears rather marginal at present.

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

Absolute dimensions and apsidal motion of the eccentric binary V731 Cephei

\varkappa

Spectrum Disentangling and Orbital Solution for κ Dra

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.

A Note on the Nearest Neighbour Analysis

A detailed analysis of new and existing photometric, spectroscopic and spatial distribution data of the eccentric binary V731 Cep was performed. Spectroscopic orbital elements of the system were obtained by means of cross-correlation technique. According to the solution of radial velocities with UBVRc and Ic light curves, V731 Cep consists of two main-sequence stars with masses M1 = 2.577 (0.098) Msolar, M2 = 2.017 (0.084) Msolar, radii R1 = 1.823 (0.030) Rsolar, R2 = 1.717 (0.025) Rsolar and temperatures Teff1 = 10700 (200) K, Teff2 = 9265 (220) K separated from each other by a = 23.27 (0.29) Rsolar in an orbit with inclination of 88.70 deg (0.03). Analysis of the O-C residuals yielded a rather long apsidal motion period of U = 10000 (2500) yr compared to the observational history of the system. The relativistic contribution to the observed rates of apsidal motion for V731 Cep is significant (76 per cent). The combination of the absolute dimensions and the apsidal motion properties of the system yielded consistent observed internal structure parameter compared to the theory . Evolutionary investigation of the binary by two methods (Bayesian and evolutionary tracks) shows that the system is t = 133(26) Myr old and has a metallicity of [M/H] = -0.04(0.02) dex. The similarities in the spatial distribution and evolutionary properties of V731 Cep with the nearby open cluster NGC 7762 suggest that V731 Cep could have been evaporated from NGC 7762.

Spectroscopy of close visual binary components of the stable shell star 1 Delphini

Based on a new set of electronic spectra in a relatively wide spectral range (3500–8300 A) and using the methods of spectrum disentangling (code KOREL) and solution of RV curves (code FOTEL), we determined new orbital elements of the binary star κ Dra. The solution of the radial velocity curves for Balmer and some other strong metallic lines suggested a circular orbit and led to the following orbital elements: period P = 61.555 ± 0.029 days, epoch of periastron passage Tperiast = 49980.22 ± 0.59, RV semi-amplitude K1 = 6.81 ± 0.24km s−1, and amass function of f(m) = 0.002 Mʘ Lines of the secondary were not detected. In addition, moving absorption bumps in the violet peaks of Hα and Kβ lines were found to be phase-locked with the orbital period. Their presence suggests some kind of interaction between the binary components.

A Spectroscopic Study of HD 208905

In the nearest neighbour method of the random distribution of N objectson a sphere surface the variable (N-1) sin2(α/2) isproposed to be used instead of the angular distance α. Thenboth the expected mean and variance equal to one for large N, and both theStudents test and F-test are especially simple. An illustrative applicationshows that the superclusters – collected in the All-Sky Catalogues ofSuperclusters of Abell-ACO Clusters – need not be distributedrandomly.

Multicolour Photometry and Spectroscopy of the Slow Nova V475 Sct (Nova Scuti 2003)

Stable shell stars are ideal objects for studying basic physical principles of the formation of disks in Be stars. If these stars have a close unresolved visual companion, its contribution toward the modelling of the disk cannot be overlooked, as is sometimes done. The study aims to spectroscopically resolve close visual binary Be (shell) star 1 Del, which up to now was only resolved by speckle or micrometric measurements. The integral field spectroscopy obtained by the SINFONI spectrograph at the VLT telescope in the European Southern Observatory (ESO) in the infrared region was used; we supplemented these observations with visual spectroscopy with the Perek Telescope at the Ondrejov Observatory. Spectra of 1 Del were successfully resolved, and, for the first time, spectra of 1 Del B were obtained. We found that 1 DelA is a Be/shell star, while 1 Del B is not an emission-line object.

An Outburst Detected in the Spectrum of HD 6226

HD 208905 (SAO 19758, ADS 15466 AB, CCDM J21573+6118 AB) is a multiple system in the Cep OB2 association, (V =7.0, B1V, B − V =0.09, U −B=-0.74). The spectrum is composite showing triple absorption profiles. Based on the observed line-strengths, all the three components are believed to be early-type stars. Numerous astrometric measurements, both classical and speckle-interferometrical, can be found in the literature. These data clearly show two components, AB, separated by 300 to 400 mas. On the other hand, only very few RV data were published. Daflon in [1] presented a preliminary study of radial velocities of HD 208905 suggesting variations with a period between 13 and 27 days. Here we report new results of the RV measurements on spectra taken at McDonald, Ondřejov and Palomar Observatories from 1991 till 2005. The first orbital solution for the close pair was obtained. Twenty one spectra were successfully decomposed into three components through the use of the spectral disentangling code KOREL.