Be Stars

Among hot stars, certain B stars are known as fast rotators and for their specific characteristic of being surrounded by an anisotropic gaseous envelope. This circumstellar envelope, located in the equatorial region of the star, is fed by episodic ejections of matter: this is the Be phenomenon. However, the physical process at the origin of the ejections of gas from Be stars remains mysterious.

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: Observed field of the Small Magellanic Cloud . The open cluster NGC330 lies in the middle-bottom. North is at the top and west on the right.

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.

Figure 2: Binary system SMC5_000977. Left panel, one can see the 2 spectral components of HeI line profiles, which correspond to the 2 stars of the system, seen at 2 phases. Right panel, one can see the lightcurves in 2 colors, which show the eclipses of the 2 stars of this system. The period of this system is 3.128 days.

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: Photometric variations for the multiperiodic Be star SMC5_13978 (left, top) and for the monoperiodic Be star SMC5_14212 (left, bottom). The frequencies of these variations are indicated on each curve in cycles/day. These light variations could be due to pulsations in these stars.

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.

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

Contact
Christophe Martayan (Observatoire de Paris, GEPI, & CNRS)

Be stars and binaries of the Small Magellanic Cloud