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From massive star to supernova remnant.

From massive star to supernova remnant

Among the exotic species found in the galactic zoo, Wolf-Rayet stars are both rare and distinctive. Believed to be massive stars in a late stage of their stellar evolution, these intensely luminous objects are born with about 20 to 100 times the sun's mass. Over a period of just a few million years, strong stellar winds carry away nearly half their mass. Eventually, they probably end their lives in a violent supernova explosion. Until now, however, astronomers lacked direct observational evidence for this evolutionary path.

Joy Nichols-Bohlin of the Computer Sciences Corp. in El Segundo, Calif., and Robert A. Fesen of the University of Colorado at Boulder started by looking at Wolf-Rayet stars surrounded by a ring nebula -- a shell of expanding gas very close to the star. About 5 to 10 percent of all Wolf-Rayet stars have ring nebulas.

Working with ultraviolet light measurements obtained by NASA's International Ultraviolet Explorer satellite, the researchers discovered three Wolf-Rayet stars that also appear to lie within and near the center of wispy, much more distant, expanding gaseous shells. In one case, for instance, the ring nebula surrounding the star is about 20 light-years across while the large dust shell is 160 light-years across. Each huge, distant shell may be the remnant of an earlier supernova explosion that shattered the now-invisible binary companion of each Wolf-Rayet star, the reseachers said at an American Astronomical Society meeting this week in Kansas City, Mo.

The observations fit a model for the evolution of a binary star system consisting of two massive stars orbiting each other. The more massive and faster-evolving of the two stars is the first to shift into the Wolf-Rayet phase. Within 200,000 years or so, it explodes in a supernova, sending off a gaseous cloud and collapsing into a neutron star. Its companion also soon becomes a Wolf-Rayet star and then undergoes a supernova explosion. The result is a binary system consisting of two neutron stars.

"If these huge shells that have been detected are indeed old supernova remnants, then the three Wolf-Rayet stars might be members of massive binary star systems, having entered their Wolf-Rayet phase relatively soon after the supernova explosion of one of the stars in the system," says Nichols-Bohlin. "Obviously, a great deal more work needs to be done to confirm that these are highly evolved supernova remnants rather than interstellar bubbles ejected by a single star."

Nevertheless, postulating that a binary system rather than a single star is responsible for the supernova remnant provides a tidy explanation for the presence of a ring nebula around these Wolf-Rayet stars. Such a star loses mass to its neutron-star companion, but the flow is so great that the neutron star can't accept all the material. Some forms into a gaseous cloud, which is swept out into a ring by the stellar winds. The formation of ring nebulas in massive binary systems may be an example of the same kind of phenomenon that leads to the association of planetary nebulas with binary systems consisting of less massive stars--but on a much larger scale (see box).

"Only around those Wolf-Rayet stars that show nicely formed ring nebulae do we find any evidence that there was a supernova," says Fesen. These ring nebulas appear to signal both a violent past and an explosive future.
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Title Annotation:Wolf-Rayet stars
Author:Peterson, I.
Publication:Science News
Date:Jun 11, 1988
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