Circumstellar Shells of Gas
Thursday, December 11, 2008
Science Update - A look at CfA discoveries from recent journals

The second brightest object in the sky outside of our solar system is the variable star CW Leo, located about 450 light-years away in the direction of the constellation of Leo (the brightest object in the sky is the southern hemisphere star called Eta Carina). CW Leo is barely visible at optical wavelengths, however, because its tremendous luminosity is emitted mostly in the infrared.

CW Leo is famous not only because it is so bright, but because it has already burned most of its hydrogen and much of its helium fuel, and in the process has produced shells of dust around the star rich in molecules, with over sixty species identified, many of them organic. In fact, the well documented, layered cocoons of dust and gas are the result of the star's ejecting about one earth-mass of material per year. CW Leo may be unusually bright, but it is nonetheless representative of the class of evolved stars in our galaxy which astronomers think are responsible for making and disbursing most of the dust and organic molecules in space. This dust is then a catalyst for more complex chemical reactions that occur, and also plays a pivotal role in preparing the interstellar medium for a next generation of stars. Astronomers are still trying to figure out, however, exactly how the star makes and blows away all this material.

SAO's Submillimeter Array (SMA) is the first facility capable of obtaining both very high spatial resolution imaging and precise velocity information at submillimeter wavelengths where many of these molecules emit their radiation. A team of nine SAO astronomers led by Nimesh Patel, along with two colleagues, have just published the first two articles in a pioneering series on the molecular envelope in CW Leo. These first papers report on the previously known sulfur-bearing molecules SiS and CS, and on CO. The first two are found to lie in a shell about sixty astronomical units from star, a distance where the ambient temperature is thought to have cooled enough for dust to solidify but where the stellar wind is still accelerating outward. The CO is discovered in an unusual, excited state indicating that it comes from much closer to the star, perhaps only a few astronomical units which, in the case of this kind of swollen evolved star, puts it very close to the star's photosphere itself. The results help to explain the detailed inner structure of the dust cocoons formed in evolved stars, and mark the beginning of a new probe of evolved stars.