Enormous stellar shell raises theoretical questions.

Enormous stellar shell raises theoretical questions

Stars tend to lose material to the space that surrounds them. Some of this loss is gradual and continuous--the so-called stellar winds. Some is abrupt -- the sudden blowing off of a surface layer that then forms a shell around the star. A group of astronomers now reports in the Nov. 15 ASTROPHYSICAL JOURNAL the discovery of an especially large, cool shell around the star R Coronae Borealis. How this shell was formed and what makes it glow are both mysteries for which current theory does not seem to have answers.

Many stars that throw off their outer layers form planetary nebulas, more or less spherical clouds of ionized gas inside which the parent star resides. This shell, composed of carbon or silicate grains, is about 100 times the size of an ordinary planetary nebula. The R Coronae Borealis shell is 8 parsecs or 26 light-years across, about 20 times as large as any shell previously found around a late-type star such as this. One of the discoverers, Frederick C. Gillett of the National Optical Astronomy Observatories' Kitt Peak National Observatory in Tucson, Ariz., points out that if the sun were in the center of this shell, the shell would encompass not only the sun's planetary system but also about 50 of the nearest star systems. Other members of the observing group are Dana E. Backman of Kitt Peak, Charles Beichman of the Jet Propulsion Laboratory in Pasadena, Calif., and Gerry Neugebauer of Caltech in Pasadena.

The astronomers believe that the shell is the result of some process of mass expulsion that took about 125,000 years and ended around 25,000 years ago, but they are not sure quite what that process was. Currently stars like R Coronae Borealis appear to be losing mass gradually. This is something of a contradiction to present theory, which expects that such stars should lose their hydrogen-rich outer layers abruptly as their central nuclear fusion mechanism starts to burn heavier elements. However, according to the nature of the shell as revealed by observations of the Infrared Astronomy Satellite, this gradual process as seen today cannot completely explain what happened in the distant past to R Coronae Borealis. "Therefore, the process that built the extended shell had to be different from the one going on today," Gillett says.

These stellar shells usually glow. They absorb radiation emitted by the stars in their centers or from the ambient "radiation field," the total contribution of other nearby stars. The radiation heats them until they glow, usually in the infrared. However, in this case, heating from the central star would be adequate only if the dust in the shell had a peculiar composition and unusually small grains. Ambient radiation doesn't seem to provide enough either, so the source of the heat remains a puzzle.

Another puzzle is that, while such a shell should be mostly hydrogen, so far the observers have not seen any. In this range of the spectrum, evidence of hydrogen is hard to pin down, but they are looking for it.