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June 12th, 2000

The enigmatic stars of Dr. Wolf and Mr.Rayet

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The enigmatic stars of Dr. Wolf and Mr. Rayet

Credit: Image courtesy of L. Drissen1 & R. Arsenault2
1 Université Laval, Québec, Canada
2 Observatoire de Paris, Meudon, France

In the summer of 1867, French astronomers Charles Wolf and Georges Rayet observe for the first time three strange objects during a visual spectroscopic study of stars in the constellation Cygnus at the Observatoire de Paris. They wrote:

"Parmi les nombreuses étoiles dont la lumière a été étudiée à l'aide d'un prisme, on n'en connait qu'une seule, Gamma de Cassiopée, dont le spectre offre constamment des lignes brillantes. Nous avons l'honneur de signaler à l'Académie l'existence de semblables lignes dans trois étoiles de la constellation du Cygne... Leur spectre se compose d'un fond éclairé dont les couleurs sont à peine visibles. Tous trois présentent une série de lignes brillantes. L'identification des lignes lumineuses de ces étoiles avec celles des spectres des gaz incandescents nous a été impossible..."

(Comptes rendus de l'Académie des sciences, 1867, vol 65, p. 292)

These three stars were indeed very strange. First, their spectrum was full of emission lines; even today, emission line stars comprise only a tiny fraction of all known stars. Moreover, the strongest lines were caused by the presence of an element which was still unknown at the time, Helium. Helium, named after the ancient Greek god of the Sun "Helios", was later identified in spectra of the solar corona in 1868.

More than a century later and after the discovery of about 500 stars with the same spectral signatures (including 200 in our own Milky Way), Wolf-Rayet stars are still a fascinating subject of research for astronomers. This week's images show many of them discovered in M33, a nearby Galaxy. The Wolf-Rayet phase, which lasts less than 500 000 years, is in fact the "Swan song" of the most massive stars, shortly preceding their explosion as supernova. Most stars eject a very small part of their atmosphere in their surroundings: this phenomenon is called a "stellar wind". The wind of Wolf-Rayet stars is billions of times stronger than the Sun's. For comparison, at this pace the Sun would be completely devoid of gas in less than 50 000 years!

The gas expeled at speeds up to 4000 km/s from the Wolf-Rayet stars interacts with the surrounding interstellar medium, which is ionized by the strong UV radiation from the star and forms a nebula. Most Wolf-Rayet stars are thus surrounded by a ring nebula, the most famous of all being NGC 6888 in Cygnus. Many such Nebulae were discovered in Messier 33 and are featured this week.

The "color" of the gas in the nebula (see below) is a good indicator of the temperature of the central star: the bluer the gas, the hotter the central star. The high-resolution images and spectra of these nebulae reveal that these were not formed continuously, but rather by multiple episodes of mass ejection from the central star. It is important to compare the properties of Wolf-Rayet stars in the Milky Way with those of other galaxies such as M33 to better understand the evolution of massive stars, as it is strongly influenced by the environment in which they were formed. Drissen and Luc Dessart (Université Laval) will go back to the CFHT at the end of the summer to study in more detail the Wolf-Rayet stars of M33: with high quality spectrograms, they expect to determine the chemical abundances, surface temperature and mass-loss rates of these fascinating stars.

Technical description:

This week's images show several nebulae surrounding Wolf-Rayet stars in the nearby galaxy Messier 33, also known as NGC 598, located in the constellation Triangulum, the Triangle, 3 million light-years away from us. The diameter of the "bubbles" range from 30 to 100 light-years. The images were obtained with the MOS (multi-object spectrograph) attached to the CFHT in October 1992. Images in the light of Hydrogen (red) and Oxygen (blue) were combined to reveal the intricate structure of the wind-blown bubbles. Typical exposure times were about 20 minutes.

next week: The Young and Active DG Tauri

editors: François Ménard & Jean-Charles Cuillandre
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