Anne-Marie Hubert

CoRoT measures solar-like oscillations and granulation in stars hotter than the Sun.

Oscillations of the Sun have been used to understand its interior structure. The extension of similar studies to more distant stars has raised many difficulties despite the strong efforts of the international community over the past decades. The CoRoT (Convection Rotation and Planetary Transits) satellite, launched in December 2006, has now measured oscillations and the stellar granulation signature in three main sequence stars that are noticeably hotter than the sun. The oscillation amplitudes are about 1.5 times as large as those in the Sun; the stellar granulation is up to three times as high. The stellar amplitudes are about 25% below the theoretic values, providing a measurement of the nonadiabaticity of the process ruling the oscillations in the outer layers of the stars.

Effects of gravitational darkening on the determination of fundamental parameters in fast-rotating B-type stars

In this paper we develop a calculation code to account for the effects carried by fast rotation on the observed spectra of early-type stars. Stars are assumed to be in rigid rotation, and the grid of plane-parallel model atmospheres used to represent the gravitational darkening are calculated by means of a non-LTE approach. Attention is paid to the relation between the apparent and parent non-rotating counterpart stellar fundamental parameters and apparent, and true Vsin i parameters as a function of the rotation rate Ω/Ω c , stellar mass, and inclination angle. It is shown that omitting of gravitational darkening in the analysis of chemical abundances of CNO elements can produce systematic overestimation or underestimation, depending on the lines used, rotational rate, and inclination angle. The proximity of Be stars to the critical rotation is revised while correcting not only the Vsin i of 130 Be stars, but also their effective temperature and gravity to account for stellar rotationally induced geometrical distortion and for the concomitant gravitational darkening effect. We concluded that the Vsin i increase is accompanied by an even higher value for the stellar equatorial critical velocity, so that the most probable average rate of the angular velocity of Be stars attains Ω/Ω c ≃ 0.88.

Fundamental parameters of Be stars located in the seismology fields of COROT

In preparation for the COROT space mission, we determined the fundamental parameters (spectral type, temperature, gravity, vsini) of the Be stars observable by COROT in its seismology fields (64 Be stars). We applied a careful and detailed modeling of the stellar spectra, taking into account the veiling caused by the envelope, as well as the gravitational darkening and stellar flattening due to rapid rotation. Evolutionary tracks for fast rotators were used to derive stellar masses and ages. The derived parameters will be used to select Be stars as secondary targets (i.e. observed for 5 consecutive months) and short-run targets of the COROT mission. Furthermore, we note that the main part of our stellar sample is falling in the second half of the main sequence life time, and that in most cases the luminosity class of Be stars is inaccurate in characterizing their evolutionary status.

The Identification of New Be Stars in GAUDI

The Ground-based Asteroseismology Uniform Database Interface (GAUDI) is the result of the preparatory work performed for the COROT satellite. In the data available in GAUDI we discovered 17 B-type stars that show emission in their Balmer lines and were not known to display such emission before, including at least 16 nonsupergiant ones. We thus reclassify those stars as Be stars. These 17 new Be stars increase the number of Be stars in the field of view of COROT by ~25%, which is important for the target selection of the mission. Moreover, ~70% of the discovered Be stars are of late subtypes. Be stars have been mostly found among early subtypes until now, but this could be due to an observational bias. Finally, one of the discovered stars is either a slowly rotating shell Be star or a Herbig Be star with a low v sin i, which makes this star especially interesting.

Review about populations of Be stars: stellar evolution of extreme stars

Among the emission-line stars, the classical Be stars known for their extreme properties are remarkable. The Be stars are B-type main sequence stars that have displayed at least once in their life emission lines in their spectrum. Beyond this phenomenological approach some progresses were made on the understanding of this class of stars. With high-technology techniques (interferometry, adaptive optics, multi-objects spectroscopy, spectropolarimetry, high-resolution photometry, etc) from different instruments and space mission such as the VLTI, CHARA, FLAMES, ESPADONS-NARVAL, COROT, MOST, SPITZER, etc, some discoveries were performed allowing to constrain the modelling of the Be stars stellar evolution but also their circumstellar decretion disks. In particular, the confrontation between theory and observations about the effects of the stellar formation and evolution on the main sequence, the metallicity, the magnetic fields, the stellar pulsations, the rotational velocity, and the binarity (including the X-rays binaries) on the Be phenomenon appearance is discussed. The disks observations and the efforts made on their modelling is mentioned. As the life of a star does not finish at the end of the main sequence, we also mention their stellar evolution post main sequence including the gamma-ray bursts. Finally, the different new results and remaining questions about the main physical properties of the Be stars are summarized and possible ways of investigations proposed. The recent and future facilities (XSHOOTER, ALMA, E-ELT, TMT, GMT, JWST, GAIA, etc) and their instruments that may help to improve the knowledge of Be stars are also briefly introduced.

Correlation Between a Light Outburst and Pulsations in a CoRoT Be Star: HD 49330

HD 49330 (B0.5IVe) has been observed with CoRoT during the LRA1 run and presented an outburst of moderate amplitude (0.03 mag). The analysis of its light curve revealed many frequencies in the Fourier spectrum typical of p and g pulsation modes with amplitude variations along the run closely correlated to the outburst. The simultaneous spectroscopic observations allowed us to identify some p and g modes. Using these results, we performed a preliminary seismic modeling of the Be star HD 49330 taking into account its rapid rotation.

Album: A Tool for the Analysis of Slitless Spectra and its Application to ESO WFI Data

ALBUM is a general-purpose tool to visualize and screen large amounts of slitless spectra∈dex slitless spectra. It was developed for a search for emission-line stars in SMC and LMC clusters. The observations were obtained with ESO’s Wide Field Imager (WFI∈dexWFI) and comprise ~8 million low-resolution spectra. The tool as well as the results of its application to the SMC part of the database are presented. The inferred frequency of Be stars is compared to the one in the higher-metallicity environment of the Milky Way.

Massive star formation in the outer Galaxy: S284

Triggered star formation by high-mass stars has been widely explored from the theoretical point of view (Elmegreen 1998), while observations do not seem to convincingly confirm it. The difficulty arises when trying to discriminate between the inherent properties of these star forming regions and the effects induced by nearby systems. In order to study this process in isolation, the candidate HII regions must present simple morphologies. This condition is most likely fulfilled in the areas slightly off the Galactic plane (Deharveng et al. 2005) and in the outer Galaxy. S284 is a giant HII region located at the Galactic anti-centre. This object is situated further out the Perseus arm (Rgal~13.5 kpc) where star formation significantly drops. S284 harbours a widely studied OB cluster in its centre: Dolidze-25. In this conference, we will present Spitzer/IRAC and H-alpha(INT) observations of this region and surroundings. The observations show a symmetical distribution of the surrounding dust and the ionised gas with respect to the central OB cluster. Moreover, the data show substructures indicative of ongoing high-mass star formation at the rim of the ionised nebula on various scales. With these data we have characterised the ionising sources, constrained the ages of some of the substructres/second generation clusters and determine the luminositiy of the youngest embedded stars and protostars. The isolated nature of S284 is a very interesting feature, also favoured by its proximity (5.5 kpc). However, what makes S284 a particularly intriguing object is the fact that it has low metallicity. This allows us to determine whether there are any fundamental differences between this low-metallicity environment and better studied high-metallicity sites of star formation. Furthermore, this site may well be a much better suited template for massive star formation in the more distant Universe.