K. Lefever

Statistical properties of a sample of periodically variable B-type supergiants : evidence for opacity-driven gravity-mode oscillations

Aims: We have studied a sample of 28 periodically variable B-type supergiants selected from the HIPPARCOS mission and 12 comparison stars covering the whole B-type spectral range. Our goal is to test if their variability is compatible with opacity-driven non-radial oscillations. Methods: We have used the NLTE atmosphere code FASTWIND to derive the atmospheric and wind parameters of the complete sample through line profile fitting. We applied the method to selected H, He, and Si line profiles, measured with the high resolution CES spectrograph attached to the ESO CAT telescope in La Silla, Chile. Results: From the location of the stars in the (log T_eff, log g) diagram, we suggest that variability of our sample supergiants is indeed due to the gravity modes resulting from the opacity mechanism. We find nine of the comparison stars to be periodically variable as well, and suggest them to be new alpha Cyg variables. We find marginal evidence of a correlation between the amplitude of the photometric variability and the wind density. We investigate the wind momentum-luminosity relation for the whole range of B spectral type supergiants, and find that the later types (>B5) perfectly follow the relation for A supergiants. Additionally, we provide a new spectral type - T_eff calibration for B supergiants. Conclusions: .Our results imply the possibility of probing internal structure models of massive stars of spectral type B through seismic tuning of gravity modes. Figures of the spectral line fits and discussion of the individual objects, Appendices A, B and Table 6 are only available in electronic form at http://www.aanda.org

Evidence for nonlinear resonant mode coupling in the β Cephei star HD 180642 (V1449 Aquilae) from CoRoT photometry

Context. We present the CoRoT light curve of the β Cep star HD 180642, assembled during the first long run of the space mission, as well as archival single-band photometry. Aims. Our goal is to analyse the detailed behaviour present in the light curve and interpret it in terms of excited-mode frequencies. Methods. After describing the noise properties in detail, we use various time series analyses and fitting techniques to model the CoRoT light curve, for various physical assumptions. We apply statistical goodness-of-fit criteria that allow us to select the most appropriate physical model fit to the data. Results. We conclude that the light-curve model based on nonlinear resonant frequency and phase locking provides the best representation of the data. Interpretation of the residuals is dependent on the chosen physical model used to prewhiten the data. Conclusions. Our observational results constitute a fruitful starting point for detailed seismic stellar modelling of this large-amplitude and evolved β Cep star.

Spectroscopic determination of the fundamental parameters of 66 B-type stars in the field-of-view of the CoRoT satellite

Aims. We determine the fundamental parameters of a sample of B stars with apparent visual magnitudes brighter than 8 in the fieldof-view of the CoRoT space mission, from high-resolution spectroscopy. Methods. We developed an automatic procedure for the spectroscopic analysis of B-type stars with winds, based on an extensive grid of FASTWIND model atmospheres. We use the equivalent widths and/or the line profile shapes of continuum normalized hydrogen, helium, and silicon line profiles to determine the fundamental properties of these stars in an automated way. Results. After thorough tests, on both synthetic datasets and very high-quality, high-resolution spectra of B stars for which we already had accurate values of physical properties from alternative analyses, we applied our method to 66 B-type stars contained in the ground-based archive of the CoRoT space mission. We discuss the statistical properties of the sample and compare them with those predicted by evolutionary models of B stars. Conclusions. Our spectroscopic results provide a valuable starting point for any future seismic modelling of the stars, should they be observed by CoRoT.

Age and Distance for the Old Open Cluster NGC 188 from the Eclipsing Binary Member V 12

We present time series radial velocity, and photometric observations of a solar-type double-lined eclipsing binary star (V 12) in the old open cluster NGC?188. We use these data to determine the spectroscopic orbit and the photometric elements for V 12. From our analysis, we determine accurate masses (Mp = 1.103 ? 0.007 M ?, Ms = 1.081 ? 0.007 M ?) and radii (Rp = 1.424 ? 0.019 R ?, Rs = 1.373 ? 0.019 R ?) for the primary (p) and secondary (s) binary components. We adopt a reddening of E B?V = 0.087 for NGC?188, and derive component effective temperatures of 5900 ? 100 K and 5875 ? 100 K, respectively, for the primary and secondary stars. From their absolute dimensions, the two components of V 12 yield identical distance moduli of V 0 ? MV = 1124 ? 009, corresponding to 1770 ? 75 pc. Both stars are near the end of their main-sequence evolutionary phase, and are located at the cluster turnoff in the color-magnitude diagram. We determine an age of 6.2 ? 0.2 Gyr for V 12 and NGC?188, from a comparison with theoretical isochrones in the mass-radius diagram. This age is independent of distance, reddening, and color-temperature transformations. We use isochrones from Victoria-Regina (VRSS) and Yonsei-Yale (Y 2) with [Fe/H] = ?0.1 and [Fe/H] = 0.0. From the solar metallicity isochrones, an age of 6.4 Gyr provides the best fit to the binary components for both sets of models. For the isochrones with [Fe/H] = ?0.1, ages of 6.0 Gyr and 5.9 Gyr provide the best fits for the (VRSS) and (Y 2) models, respectively. We use the distance and age estimates for V 12, together with best estimates for the metallicity and reddening of NGC?188, to investigate the locations of the corresponding VRSS and Y 2 isochrones relative to cluster members in the color-magnitude diagram. Plausible changes in the model metallicity and distance to better match the isochrones to the cluster sequences, result in a range of ages for NGC?188 that is more than 3 times that resulting from our analysis of V 12.

Long term photometric monitoring with the Mercator telescope - Frequencies and mode identification of variable O-B stars

Aims. We selected a large sample of O-B stars that were considered as (candidate) slowly pulsating B, beta Cep, and Maia stars after the analysis of their hipparcos data. We analysed our new seven passband geneva data collected for these stars during the first three years of scientific operations of the mercator telescope. We performed a frequency analysis for 28 targets with more than 50 high-quality measurements to improve their variability classification. For the pulsating stars, we tried both to identify the modes and to search for rotationally split modes. Methods: We searched for frequencies in all the geneva passbands and colours by using two independent frequency analysis methods and we applied a 3.6 S/N-level criterion to locate the significant peaks in the periodograms. The modes were identified by applying the method of photometric amplitudes for which we calculated a large, homogeneous grid of equilibrium models to perform a pulsational stability analysis. When both the radius and the projected rotational velocity of an object are known, we determined a lower limit for the rotation frequency to estimate the expected frequency spacings in rotationally split pulsation modes. Results: We detected 61 frequencies, among which 33 are new. We classified 21 objects as pulsating variables (7 new confirmed pulsating stars, including 2 hybrid beta Cep/SPB stars), 6 as non-pulsating variables (binaries or spotted stars), and 1 as photometrically constant. All the Maia candidates were reclassified into other variability classes. We performed mode identification for the pulsating variables for the first time. The most probable l value is 0, 1, 2, and 4 for 1, 31, 9, and 5 modes, respectively, including only 4 unambiguous identifications. For 7 stars we cannot rule out that some of the observed frequencies belong to the same rotationally split mode. For 4 targets we may begin to resolve close frequency multiplets. Based on observations collected with the p7 photometer attached to the Flemish 1.2-m mercator telescope situated at the Roque de los Muchachos observatory on La Palma (Spain). Section [see full text], including Figs. is only available in electronic form at http://www.aanda.org, and Tables 2 and 3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/463/243

Periodic Mass-loss episodes due to an oscillation mode with variable amplitude in the hot supergiant HD 50064

Aims. We aim to interpret the photometric and spectroscopic variability of the luminous blue variable supergiant HD 50064 ( V = 8.21). Methods. CoRoT space photometry and follow-up high-resolution spectroscopy with a time base of 137 d and 169 d, respectively, was gathered, analysed, and interpreted using standard time series analysis and light curve modelling methods, as well as spectral line diagnostics. Results. The space photometry reveals one period of 37 d, which undergoes a sudden amplitude change with a factor 1.6. The pulsation period is confirmed in the spectroscopy, which additionally reveals metal line radial velocity values differing by ∼30 km s ―1 depending on the spectral line and on the epoch. We estimate T eff ∼ 13 500 K, log g ∼ 1.5 from the equivalent width of Si lines. The Balmer lines reveal that the star undergoes episodes of changing mass loss on a time scale similar to the changes in the photometric and spectroscopic variability, with an average value of log M ≃ ―5 (in M ⊙ yr ―1 ). We tentatively interpret the 37 d period as the result of a strange mode oscillation.

An asteroseismic study of the β Cephei star θ Ophiuchi: spectroscopic results

The invention relates to a ballast (101) for lighting means, comprising an actively switched power factor correction stage (102), which is implemented in a Totem Pole boost topology, comprising a control circuit (103), which issues control signals for driving switches of the PFC stage (102), wherein the control circuit (103) is on a floating potential and galvanically isolated from a ground potential of the ballast (101).

Spectroscopic and photometric variability of the O9.5Vp star HD93521

Aims. The line profile variability and photometric variability of the O9.5 Vp star HD 93521 are examined in order to establish the properties of the non-radial pulsations in this star. Methods. Fourier techniques are used to characterize the modulations of the Heiλλ 5876, 6678 and Hα lines in several spectroscopic time series and to search for variations in a photometric time series. Results. Our spectroscopic data confirm the existence of two periods o f 1.75 and 2.89 hr. The line profiles, especially those a ffected by emission wings, exhibit also modulations on longer time scales, but these are epoch-dependent and change from line to line. Unlike previous claims, we find no unambiguous signature of the rota tional period in our data, nor of a third pulsation period (co rresponding to a frequency of 2.66 d −1 ). Conclusions. HD 93521 very likely exhibits non-radial pulsations with periods of 1.75 and 2.89 hr with l ≃ 8± 1 and l ≃ 4± 1 respectively. No significant signal is found in the first harm onics of these two periods. The 2.89 hr mode is seen at all epochs and in all lines investigated, while the visibility of the 1.75 hr m ode is clearly epoch dependent. Whilst light variations are detected, their connection to these periodicities is not straightforward.

Disentangling component spectra of kappa Scorpii, a spectroscopic binary with a pulsating primary. II. Interpretation of the line-profile variability

We analyse the complex short-term Si  line-profile variability of the spectroscopic binary β Cep star κ Sco after orbit subtraction, before and after spectral disentangling. We refine the known oscillation frequency of the star: f1 = 4.99922 c d −1 and detect 2 f1. Variability is also found at frequencies near f2 � 4.85 c d −1 and f3 � 5.69 c d −1 or their aliases. These frequencies are not significant if we consider the spectra alone, but they survive our selection after the consideration that they were derived previously from independent ground-based and space photometry by different teams. Moreover, we find dominant variability in the equivalent width with a frequency in the interval (0.22, 0.30) c d −1 which we interpret as the rotational frequency frot of the star. The complex window function does not allow us to determine definite values for f2, f3, frot. The variability with f1 is interpreted as a prograde non-radial oscillation mode with spherical wavenumbers (�, m) = (2, −1) or (1, −1). The additional frequencies are explained in terms of rotational modulation superposed to the main oscillation. We also point out that we cannot disprove the variability in κ Sco to originate from co-rotating structures. KOREL disentangling preserves the large-amplitude line-profile variability but its performance for complex low-amplitude variability remains to be studied in detail.

Oscillating blue stragglers, gamma Doradus stars and eclipsing binaries in the open cluster NGC 2506

Context. This is the first step in a project to combine studies of eclipsing binaries and oscillating stars to probe the interior of Blue Stragglers (BS). This may imply a way to discriminate observationally between different birth mechanisms of BS stars. Aims. We study the open cluster NGC 2506 which contains oscillating BS stars and detached eclipsing binaries for which accurate parameters can be derived. This will tightly constrain the cluster isochrone and provide an absolute mass, radius and luminosity-scale for the cluster stars along with the cluster age, metallicity and distance. The present work focuses on obtaining the light curves of the binaries and determine their orbital periods, on obtaining power spectra of the oscillating BS stars to select targets for follow-up studies, and on searching for γ Doradus type variables which are also expected to be present in the cluster. Methods. With a two-colour, dual-site photometric campaign we obtained 3120 CCD-images of NGC 2506 spread over four months. We analysed the BI time-series of the oscillating stars and used simulations to derive statistical uncertainties of the resulting frequencies, amplitudes and phases. A preliminary mode-identification was performed using frequency ratios for the oscillating BS stars, and amplitude ratios and phase differences for a population of newly detected γ Doradus stars. Results. We quadrupled the number of known variables in NGC 2506 by discovering 3 new oscillating BS stars, 15 γ Doradus stars and four new eclipsing binaries. The orbital periods of 2 known, detached eclipsing binaries were derived. We discovered a BS star with both p-mode and g-mode variability and we confronted our γ Doradus observations with state-of-the-art seismic models, but found significant discrepancy between theory and observations. Conclusions. NGC 2506 is an excellent target for asteroseismic tests of stellar models, as strong external constraints can be imposed on the models of a population of more than 20 oscillating stars of different types.