Petr Skoda

Pulsations of the Oe Star ζ Ophiuchi from MOST Satellite* Photometry and Ground-based Spectroscopy

Twenty-four days of highly precise Microvariability and Oscillations of Stars (MOST) satellite photometry obtained in mid-2004 of the rapidly rotating O9.5 V star ζ Oph have yielded at least a dozen significant oscillation frequencies between 1 and 10 cycles day-1, clearly indicating its relationship to β Cephei variables. Eight periods were found in He I λ4922 and Hβ line profile variations (LPV) of which six coincide with those from the MOST photometry. This unique photometric and spectroscopic detection of radial and nonradial pulsations leads to a plausible model in which high l-modes are excited when their frequencies in the corotating frame are similar to those of low-order radial modes. We propose that the dominant photometric 4.6 hr period (f1) corresponds to a radial first overtone excited by the κ-mechanism associated with the Fe opacity bump. No unambiguous rotational period can be identified in either the light curve or the LPV.

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.

New findings supporting the presence of a thick disc and bipolar jets in the β Lyrae system

Context. Understanding large-scale mass exchange in binaries also requires studies of complicated objects in the rapid phases of the process. β Lyr is one such object. Aims. Our goals were to analyse 52 photographic and 651 electronic spectra of β Lyr to obtain additional information about circumstellar matter and to investigate spectrophotometric information for the first time. Methods. Improved quadratic ephemeris was derived via orbital solution with the FOTEL program. The spectra were disentangled using the KOREL program. Spectrophotometric quantities of 15 stronger absorption lines of the primary were measured and corrected for the orbital continuum variations using the fluxes calculated from a fit of the light curves with the BINSYN program. Central intensities of the V and R peaks of the Hα emission line were measured and corrected for the orbital light changes using the R-band light curve numerically modelled with the program PERIOD04. Results. Disentangling of photographic and electronic spectra led to the detection of weak absorption lines originating from the pseudophotosphere of the accretion disc. This way, a rich line spectrum of the accretion disc, not limited to only two previously known Si ii 6347 and Si ii 6371 lines, was obtained. A projected rotational velocity of 180 km s −1 was estimated for the disc spectrum. Such a value agrees well with the assumption of the Keplerian rotation of the outer layers of the accretion disc. After the correction, a pronounced increase of the strength of all absorption lines around phases of the primary eclipse was found. We argue that this is due to additional absorption of the light of the primary in one of the jets and/or scattering envelope above the accretion disc of the gainer. The net intensity of the V peak of Hα shows no orbital variation, but a possible 271-d periodicity. The net intensity of the R peak shows mild orbital changes and a slow change over a cycle of about 2780 days. These results seem to support the earlier conclusion that the Hα emission originates in the jet-like structures. Conclusions. All new findings support the current picture that the circumstellar structures of β Lyr consist of a thick accretion disc, bipolar jets, and a scattering envelope above the disc.

A Multiwavelength Study of Spectral Variations in the CX Draconis Binary

An extensive collection of spectroscopic observations of the interacting binary CX Dra spanning a 23 year interval have been analyzed. This study includes a re—nement of the orbital solution of CX Dra ; equivalent width measurements that show short-, medium-, and long-term behavior of the diUerence pro- —les ; a calculation of the Balmer decrement ; velocity maps based on the velocity curves of the Ha and He I diUerence emission peaks ; trailed spectrograms of the Ha ,H b ,H e I ,and Si II lines ; and Doppler tomograms at these four wavelengths. The main conclusions are : 1. The circumstellar environment in the system changes in cycles of hundreds of days. The length of the cycles is variable. These cycles may be part of a ii super ˇˇ 4000 day cycle. 2. The equivalent widths of the diUerence Ha and He I j6678 lines are modulated with the orbital period of 6.696 days. The corresponding phase diagrams indicate a great deal of scatter, but the modula- tion is quite conspicuous when long data strings are used. The pattern of the phase diagram suggests a permanent presence of the emission in Ha ,H e I j6678, and Si II j6371 lines. 3. The radial velocities of the Ha emission peak follow an S-wave. The resulting velocity map shows that the source of the single-peaked emission lies close to the L1 point, roughly between the primary and L1 point. 4. Doppler tomograms constructed for Ha observed and diUerence pro—les show that the emission comes from a region of low velocity, a gas stream, and an accretion disk. The Hb emission arises from a region that is cospatial with the Ha source. The Doppler tomograms for He I j6678 and Si II j6371 lines suggest that emission in this gas also originates from a locus of a disk around the primary star. 5. The model based on the equivalent widths of the diUerence pro—les, the trailed spectrograms, and Doppler tomograms of the Ha ,H e I j6678, Si II j6371, and Hb lines suggests that the main source of the Ha emission is about halfway between the stars at a distance of 0.49a from the primary star, and that the He I and Si II emission sources arise from an accretion disk centered on the primary star. Subject headings : accretion, accretion disksbinaries : closecircumstellar matter ¨ stars : emission-line, Bestars : individual (CX Dra)

Notes on disentangling of spectra II. Intrinsic line-profile variability due to Cepheid pulsations ,

Context. The determination of pulsation velocities from observed spectra of Cepheids is needed for the Baade-Wesselink calibration of these primary distance markers. Aims. The applicability of the Fourier-disentangling technique for the determination of pulsation velocities of Cepheids and other pulsating stars is studied. Methods. The KOREL-code was modified to enable fitting of free parameters of a prescribed line-profile broadening function corresponding to the radial pulsations of the stellar atmosphere. It was applied to spectra of δ Cep in the H-alpha region observed with the Ondy 2-m telescope. Results. The telluric lines were removed using template-constrained disentangling, phase-locked variations of line-strengths were measured and the curves of pulsational velocities obtained for several spectral lines. It is shown that the amplitude and phase of the velocities and line-strength variations depend on the depth of line formation and the excitation potential. Conclusions. The disentangling of pulsations in the Cepheid spectra may be used for distance determination.

Properties and nature of Be stars - 27. Orbital and recent long-term variations of the Pleiades Be star Pleione = BU Tauri

Radial-velocity variations of the Hα emission measured on the steep wings of the Hα line, prewhitened for the long-time changes, vary periodically with a period of 218 d .025 ±0 d .022, confirming the suspected binary nature of the bright Be star BU Tau, a member of the Pleiades cluster. The orbit seems to have a high eccentricity over 0.7, but we also briefly discuss the possibility that the true orbit is circular and that the eccentricity is spurious owing to the phase-dependent effects of the circumstellar matter. The projected angular separation of the spectroscopic orbit is large enough to allow the detection of the binary with large optical interferometers, provided the magnitude difference primary - secondary is not too large. Since our data cover the onset of a new shell phase up to development of a metallic shell spectrum, we also briefly discuss the recent long-term changes. We confirm the formation of a new envelope, coexisting with the previous one, at the onset of the new shell phase. We find that the full width at half maximum of the Hα profile has been decreasing with time for both envelopes. In this connection, we briefly discuss Hiratas hypothesis of precessing gaseous disk and possible alternative scenarios of the observed long-term changes.

Spectra disentangling applied to the Hyades binary θ2 Tauri AB: new orbit, orbital parallax and component properties

Aims. θ 2 Tau is a detached and single-lined interferometric-spectroscopic binary as well as the most massive binary system of the Hyades cluster. The system revolves in an eccentric orbit with a periodicity of 140.7 days. Its light curve furthermore shows a complex pattern of δ Scuti-type pulsations. The secondary has a similar temperature but is less evolved and fainter than the primary. In addition, it is rotating more rapidly. Since the composite spectra are heavily blended, the direct extraction of radial velocities over the orbit of component B was hitherto unsuccessful. Our aim is to reveal the spectrum of the fainter component and its corresponding Doppler shifts in order to improve the accuracy of the physical properties of this important “calibrator” system. Methods. Using high-resolution spectroscopic data recently obtained with the Elodie (Observatoire de Haute-Provence, France) and Hermes (Roque de Los Muchachos, La Palma, Spain) spectrographs, and applying a spectra disentangling algorithm to three independent data sets including CfA spectra (Oak Ridge Observatory, USA), we derived an improved spectroscopic orbit. We next used a code based on simulated annealing and general least-squares minimization to refine the orbital solution by performing a combined astrometric-spectroscopic analysis based on the new spectroscopy and the long-baseline data from the Mark III optical interferometer. Results. As a result of the performed disentangling, and notwithstanding the high degree of blending, the velocity amplitude of the fainter component is obtained in a direct and objective way. Major progress based on this new determination includes an improved computation of the orbital parallax (still consistent with previous values). Our mass ratio is in good agreement with the older estimates of Peterson et al. (1991, 1993), but the mass of the primary is 15–25% higher than the more recent estimates by Torres et al. (1997) and Armstrong et al. (2006). Conclusions. The evolutionary status of both components is re-evaluated in the light of the revisited properties of θ 2 Tau AB. Due to the strategic position of the components in the turnoff region of the cluster, the new determinations imply stricter constraints for the age and the metallicity of the Hyades cluster. We conclude that the location of component B can be explained by current evolutionary models, but the location (and the status) of the more evolved component A is not trivially explained and requires a detailed abundance analysis of its disentangled spectrum. The improved accuracy (at the 2% level) on the stellar masses provides a useful basis for the comparison of the observed pulsation frequencies with suitable theoretical models.

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

ξTauri: a unique laboratory to study the dynamic interaction in a compact hierarchical quadruple system

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Wavelet Based Feature Extraction for Clustering of Be Stars

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.