Probing the ancestry of supernova 1987A.

Six years have passed since astronomers witnessed the dazzling debut of supernova 1987A, glowing with the luminosity of 100 million suns. While the brightness of this exploded star has waned, the intense interest in it remains. Astronomers are still puzzling, for example, over the character of the star that gave birth to this nearby supernova.

A new model supports speculation that the parent star did not live its life as a loner. Rather, it may have had an orbiting companion that significantly influenced its history,

The new study attempts to explain a ring of fluorescent gas surrounding supernova 1987A that formed from material ejected by the supernovas parent star more than 20,000 years before the explosion occurred. During that era, the compact parent evolved into a bloated star known as a red giant, which blows a lowspeed, high-densitywind of material into space. Later, just a few thousand years before the supernova explosion, the red giant apparently lost some of its girth, forming a blue giant, which emits a faster, lower density wind. In time, this wind caught up with the wind from the red giant. The resulting collision compressed the gas in the red giant wind, sculpting it into a shape that became visible when radiation from the supernova explosion heated it.

Had the winds blown gas equally in all directions, the collision would probably have formed a shell rather than the ring revealed by the Hubble Space Telescope in 1990. Some researchers then proposed that the gravitational tug of an unseen companion star could have concentrated the red giant wind along an equatorial ring, away from the parent star's polar regions. However, the variation in wind density along different directions seemed too small to require a companion.

In a more detailed analysis, John M. Blondin of North Carolina State University in Raleigh and Peter Lundqvist of the Stockholm Observatory in Sweden now come to a different conclusion. They find that in order for an asymmetry in the red giant wind to account for the ring as well as a pair of lobe-shaped emissions above and below the ring, the wind must have blown 20 times as much gas out along the equator of the parent star as at its poles. Such a scenario requires a companion, the astronomers report in the March 1 ASTROPHYSICAL JOURNAL.

"If you follow this model, then the only explanation I can see... is a nearby companion;' asserts Blondin. Nevertheless, he has begun work on a model in which the spin of a solitary parent star might generate a blue giant wind that preferentially blows along the equator. Roger Chevalier of the University of Virginia in Charlottesville notes that a magnetic field embedded in the blue giant wind might confine gas to a ring.

If 1987A's predecessor had a partner that survived the explosion, astronomers may spot it as the supernova continues to fade. In a Jan. 15 circular of the International Astronomical Union, Arlin RS. Crotts of Columbia University and William E. Kunkel of the Carnegie Institution of Washingtons Las Campanas Observatory in La Serena, Chile, reported that in the partner's core is still burning hydrogen, the star has less than 2.5 times the mass of the sun.