Be stars and binaries of the Small Magellanic Cloud

Whilst the metallicity (or chemical composition of the environment) is an

important factor on the stellar evolution of Be stars and on their

rotational velocity (see the previous

news on the Be stars), other phenomenon like pulsations could help to

eject matter. Thanks to the possibilities offered by ESO’s VLT, and the GIRAFFE

multi-object spectrograph in MEDUSA mode,

and thanks to the photometric databases OGLE

et MACHO, a team of astronomers

at the Observatoire de Paris (GEPI) observed 350 hot stars (O, B, Be) in

an area of the metal-poor galaxy Small Magellanic Cloud (figure

1) close to the open cluster NGC330. The study of their spectra and lightcurves allowed to discover new binary systems, to find that

the size of a large fraction of the Be stars disks are bigger in faint metallicity environment,

and to detect for the first time pulsations in 13 Be stars of the Small

Magellanic Cloud.

Figure 1 : Champ observé du Petit Nuage de Magellan. L’amas NGC330 se trouve au milieu en bas. Le nord est en haut et l’ouest à droite.

The use of spectra allows to find indirectly that, in

the frame of Keplerian rotation mode (like planets around the Sun) the disks

of a large fraction of Be stars in the Small Magellanic Cloud are larger than in the Large

Magellanic Cloud and in the Milky Way.

The use of spectra allows to find indirectly that, in

the frame of Keplerian rotation mode (like planets around the Sun) the disks

of a large fraction of Be stars in the Small Magellanic Cloud are larger than in the Large

Magellanic Cloud and in the Milky Way. 

Moreover, it seems that the proportion of Be stars

to B stars is higher in the Small Magellanic Cloud than in the Large

Magellanic Cloud and in the Milky Way. This result could be linked to

the stellar rotational velocities higher in low metallicity area (Small

Magellanic Cloud) than at higher metallicity (Large Magellanic Cloud and

Milky Way). This result will be extended to the whole field of these galaxies.

Moreover, it seems that the proportion of Be stars

to B stars is higher in the Small Magellanic Cloud than in the Large

Magellanic Cloud and in the Milky Way. This result could be linked to

the stellar rotational velocities higher in low metallicity area (Small

Magellanic Cloud) than at higher metallicity (Large Magellanic Cloud and

Milky Way). This result will be extended to the whole field of these galaxies. 

Figure 2 : Système d’étoiles binaires SMC5_000977. A gauche, nous pouvons voir les 2 composantes spectrales correspondant aux 2 étoiles du système prises à différentes phases . A droite, nous pouvons voir les courbes de lumière montrant les éclipses mutuelles des 2 étoiles de ce système dans 2 couleurs. La période du système est de 3.128 jours. 

The combined use of spectra and lightcurves from the

photometric surveys MACHO and

OGLE allows to discover new

binary systems of stars in the Small Magellanic Cloud. In figure 2,

an example of binary system is shown. It also shows the

spectral lines of the 2 components of this binary system at 2 different

epochs. Moreover it shows the lightcurves for this system and

their mutual eclipses as time passes. The detected binaries will be

used in a forthcoming study in order to determine the effect of the

metallicity on the stellar radii and to precise the distance of the

Small Magellanic Cloud.

The combined use of spectra and lightcurves from the

photometric surveys MACHO and

OGLE allows to discover new

binary systems of stars in the Small Magellanic Cloud. In figure 2,

an example of binary system is shown. It also shows the

spectral lines of the 2 components of this binary system at 2 different

epochs. Moreover it shows the lightcurves for this system and

their mutual eclipses as time passes. The detected binaries will be

used in a forthcoming study in order to determine the effect of the

metallicity on the stellar radii and to precise the distance of the

Small Magellanic Cloud. The lightcurves have also allowed us to find short-term

variability in 13 Be stars, i.e. in 10% of the sample of Be stars that were observed in the Small Magellanic Cloud. Among these 13 stars, 9

show several short-term periods of variation, which plaids in favor of

non-radial pulsations in these stars (see figure 3). These

pulsations combined to the high stellar rotational velocities of stars could

allow the ejection of matter at the origin of the Be stars’ disks.

However, the theoretical models did not foresee the existence of

pulsations at low metallicity, typically in the Small Magellanic Cloud.

Figure 3 : Variations photométriques pour l’étoile Be multipériodique SMC5_13978 (ci-contre en haut) et pour l’étoile Be monopériodique SMC5_14212 (ci-contre en bas). Les fréquences de ces variations sont indiquées sur chaque courbe en cycles/jour. Ces variations de lumière peuvent être imputées à des pulsations de ces étoiles.

 In conclusion, while fast rotation plays a fundamental

role in the appearance of Be stars, a role even more

important in low metallicity environments, it is not always sufficient to allow matter

ejections. Other

phenomena combined with rotation, such as the presence of a magnetic field

or non-radial pulsations, could favour the creation of the circumstellar envelope of Be stars.

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Reference : Be stars and

binaries in the field of the SMC open cluster NGC330 with VLT-FLAMES

C. Martayan, M. Floquet, J.

Gutiérrez-Soto, A.-M. Hubert, M. Mekkas, C. Neiner (GEPI Observatoire de Paris, France), J.

Fabregat (Observatoire Astronomique de Valencia, Espagne) A&A,

2007, sous presse ContactChristophe Martayan(Observatoire de Paris, GEPI, & CNRS) http://www.obspm.fr

Dernière modification le 4 mars 2013