S. Jankov

Rotation and magnetic field in the Be star omega Orionis

Ori is a B2IIIe star for which rotational modulation and non-radial pulsations (NRP) have been recently investi- gated from two independent observational campaigns in 1998 and 1999. Putting the data of these 2 campaigns together, and adding data obtained in 2001, we search for multiperiodicity in the line profile variations and evidence for outbursts. From new spectropolarimetric data obtained at the Telescope Bernard Lyot (TBL, Pic du Midi, France) in 2001 we also measure the Stokes V parameter in the polarised light. We find evidence for the presence of a weak magnetic field in ! Ori sinusoidally varying with a period of 1.29 d. The equivalent widths (EW) of the wind sensitive UV resonance lines also show a variation with the same period, which we identify as the rotational period of the star. We propose an oblique rotator model and derive Bpol = 530 230 G to explain the observations. Moreover, we carry out an abundance analysis and find the star to be N- enriched, a property which is shared with other magnetic stars. We propose! Ori as the first known classical Be star hosting a magnetic field.

Rotation, pulsations and magnetic field in V 2052 Ophiuchi: A new He-strong star

V 2052 Oph is a β Cep star with v sini ∼ 60 km s −1 . The behavior of its stellar wind was studied in the ultraviolet wavelength region with the IUE satellite. It revealed periodic variations in the equivalent widths (EW) of the resonance lines of wind-sensitive ions with a period of 3.638833 d, which is identified as the rotational period. These variations are typical of magnetic stars. Therefore time-resolved circular spectropolarimetric observations were obtained with the Musicos ´ spectropolarimeter at the 2-m Telescope Bernard Lyot (TBL) to search for a magnetic field in the star. Stokes V patterns were observed, the inclination and magnetic angles were derived and a value was determined for the polar magnetic field (250 ± 190 G) using an oblique rotator dipole model. The spectroscopic information was used to search for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves. Multiperiodicity was found, corresponding to radial (f1 = 7.15 c d −1 ) and non-radial (f2 = 6.82 c d −1 , l = 3 or 4) pulsation modes. The rotation period is also detected in rotationally modulated observables because of the magnetic poles passing through the observers line of sight. We determined the stellar parameters of the star, which was found to be chemically peculiar, in particular He-enriched. This makes V 2052 Oph the first magnetic He-strong early B star with known pulsational properties.

Multi-site, multi-technique survey of gamma Doradus candidates - I. Spectroscopic results for 59 stars

We present the first results of a 2-year high-resolution spectroscopy campaign of 59 candidate γ Doradus stars which were mainly discovered from the HIPPARCOS astrometric mission. More than 60% of the stars present line profile variations which can be interpreted as due to pulsation related to γ Doradus stars. For all stars we also derived the projected rotation velocity (up to more than 200 km s −1 ). The amplitude ratios 2K/∆m for the main HIPPARCOS frequency are in the range 35−96 km s −1 mag −1 . About 50% of the candidates are possible members of binary systems, with 20 stars being confirmed γ Doradus. At least 6 stars present composite spectra, and in all but one case (for which only one spectrum could be obtained), the narrow component shows line profile variations, pointing towards an uncomfortable situation if this narrow component orig- inates from a shell surrounding the star. This paper is the first of a series concerning mode identification using both photometric and spectroscopic methods for the confirmed γ Doradus stars of the present sample.

Gravitational-darkening of Altair from interferometry

Received / Accepted Abstract. Several interferometric observations revealed that the rapid rotator Altair is a ∞attened star with a non-centrally symmetric intensity distribution. In this work we perform for the flrst time a physically consistent analysis of all interferometric data available so far, corresponding to three difierent interferometers operating in several spectral bands. These observations include new data (squared visibilities in the H and K bands from VLTI- VINCI) as well as previously published data (squared visibilities in the K band from PTI and squared visibilities, triple amplitudes, and closure phases in the visible between 520 nm and 850 nm from NPOI). To analyze these data we perform a ´ 2 minimization using an interferometry-oriented model for fast rotators, which includes Roche approximation, limb-darkening, and von Zeipel-like gravity-darkening. Thanks to the rich interferometric data set available and to this physical model, the main uniqueness problems were avoided. As a main result we show that observations can only be explained if Altair has a gravity-darkening compatible with the expected value for hot stars, i.e., the von Zeipel efiect (Tefi / g 0:25 ).

First VLTI/MIDI observations of a Be star: Alpha Arae

We present the first VLTI/MIDI observations of the Be star alpha Ara (HD 158 427), showing a nearly unresolved circumstellar disk in the N band. The interferometric measurements made use of the UT1 and UT3 telescopes. The projected baselines were 102 and 74 meters with position angles of 7 ° and 55°, respectively. These measurements put an upper limit on the envelope size in the N band under the uniform disk approximation of φmax= 4±1.5 mas, corresponding to 14 R*, assuming R*=4.8 Rȯ and the Hipparcos distance of 74 pc. On the other hand the disk density must be large enough to produce the observed strong Balmer line emission. In order to estimate the possible circumstellar and stellar parameters we have used the SIMECA code developed by Stee et al. (1995, A&A, 300, 219) and Stee & Bittar (2001, A&A, 367, 532). Optical spectra taken with the echelle instrument Heros and the ESO-50 cm telescope, as well as infrared ones from the 1.6m Brazilian telescope were used together with the MIDI spectra and visibilities. These observations place complementary constraints on the density and geometry of the alpha Ara circumstellar disk. We discuss the potential truncation of the disk by a companion and we present spectroscopic indications of a periodic perturbation of some Balmer lines.

MATISSE: perspective of imaging in the mid-infrared at the VLTI

MATISSE is foreseen as a mid-infrared spectro-interferometer combining the beams of up to four UTs/ATs of the Very Large Telescope Interferometer (VLTI) of the European Southern Observatory. The related science case study demonstrates the enormous capability of a new generation mid-infrared beam combiner. MATISSE will constitute an evolution of the two-beam interferometric instrument MIDI. MIDI is a very successful instrument which offers a perfect combination of spectral and angular resolution. New characteristics present in MATISSE will give access to the mapping and the distribution of the material (typically dust) in the circumstellar environments by using a wide mid-infrared band coverage extended to L, M and N spectral bands. The four beam combination of MATISSE provides an efficient UV-coverage : 6 visibility points are measured in one set and 4 closure phase relations which can provide aperture synthesis images in the mid-infrared spectral regime.

A spectroscopic study of southern (candidate) gamma Doradus stars. I. Time series analysis

Laboratoire Univ. d’Astroph. de Nice (LUAN), UMR 6525, Parc Valrose, 06108 Nice Cedex 02, FranceReceived 18 June 2005 / Accepted 27 October 2005Abstract. We present results of a spectroscopic study of 37 southern (candidate) γ Doradus stars based on ´echellespectra. The observed spectra were cross-correlated with the standard template spectrum of an F0-type star foran easier detection of binary and intrinsic variations. We identified 15 objects as spectroscopic binaries, including7 new ones, and another 3 objects are binary suspects. At least 12 objects show composite spectra. We coulddetermine the orbital parameters for 9 binaries, of which 4 turn out to be ellipsoidal variables. For 6 binaries, weestimated the expected time-base of the orbital variations. Clear profile variations are observed for 17 objects,pointing towards stellar pulsation. For 8 of them, we have evidence that the main spectroscopic and photometricperiods coincide. Our results, in combination with prior knowledge from the literature, lead to the classificationof 10 objects as new bona-fide γ Doradus stars, 1 object as new bona-fide δ Scuti star, and 8 objects as constantstars. Finally, we determined the projected rotational velocity with two independent methods. The resulting vsinivalues range from 3 to 135 km s

Stellar differential rotation and inclination angle from spectro-interferometry

Differential interferometry (DI) is a technique that combines high spectral resolution with high spatial resolution. It is particularly suited to probe mechanisms that induce chromatic signatures, such as large scale mass motions. We thus investigate the use of DI combined with the Fourier transform method to study the stellar surface differential rotation. We show that, compared to spectroscopy, DI has the double advantage of allowing the use of all available Fourier frequencies as well as of disentangling the stellar differential rotation rate and the inclination angle. We derive some reference analytical expressions of DI observables. It is shown that the foreseen instrumental performances of the spectro-interferometer VLTI/AMBER are high enough to study the stellar differential rotation.

First sky validation of an optical polarimetric interferometer

Aims.We present the first lab and sky validation of spectro-polarimetric equipment put at the combined focus of an optical long-baseline interferometer. We tested the polarimetric mode designed for the visible GI2T Interferometer to offer spectropolarimetric diagnosis at the milliarcsecond scale. Methods.We first checked the whole instrumental polarization in the lab with a fringe simulator, and then we observed α Cep and α Lyr as stellar calibrators of different declinations to tabulate the polarization effects throughout the GI2T declination range. Results.The difference between both linear polarizations is within the error bars and the visibilities recorded in natural light (i.e. without the polarimeter) for calibration purposes are the same order of magnitude as the polarized ones. We followed the α Cep visibility for 2 h after the transit and α Lyr for 1.5 h and detected no decrease with hour angle due to the fringe pattern smearing by instrumental polarization. Conclusions.Differential celestial rotation due to the dissymetric Coude trains of the GI2T is well-compensated by the field rotators, so the instrumental polarization is controlled over a relatively wide hour angle range (±2 h around the transit at least). Such a polarimetric mode opens new opportunities especially for studies of circumstellar environments and significantly enhances both the potential of an optical array and its ability for accurate calibration.

Non-radial pulsations in the γ Doradus star HD 195068

We present high resolution spectroscopic observations of the γ Doradus star HD 195068. About 230 spectra were collected over 2 years. Time series analysis performed on radial velocity data shows a main peak at 1.61 d −1 , a frequency not yet detected in photometry. The Hipparcos photometric 1.25 d −1 frequency is easily recovered as is 1.30 d −1 while the third photometric frequency, 0.97 d −1 , is only marginally present. The good quality of our data, which includes 196 spectra collected over seven consecutive nights, shows that both the 1.61 d −1 and intermediate 1.27 d −1 (mixture of 1.25 and 1.30 d −1 ) frequencies are present in the line profile variations. Using the Fourier-Doppler Imaging (FDI) method, the variability associated with 1.61 d −1 can be successfully modeled by a non-radial pulsation mode � = 5 ± 1, |m| = 4 ± 1. For the intermediate frequency 1.27 d −1 we deduce � = 4 ± 1, |m| = 3 ± 1. Evidence that the star is not pulsating in the radial mode (� = 0) rules out a previous classification as an RR Lyrae type star. We investigate the time variability of FDI power spectra concluding that the observed temporal variability of modes can be explained by a beating phenomenon between closely spaced frequencies of two non-radial modes. The distribution of the oscillation power within the line profile indicates that there is a significant tangential velocity component of oscillations characteristic of high radial order gravity modes which are predicted to be observed in γ Doradus type stars.