A million years ago, in the constellation Scorpius, one of a pair of binary stars erupted into a supernova. Its partner shot off into space and is called Zeta Ophiuchi, a bright, giant "runaway star." The supernova, which must have collapsed into a neutron star, has been harder to trace. Now there are two candidates. If astronomers can determine which is the missing partner, they will gain a better understanding of the dynamics of supernova explosions in binary systems and the origin of runaway stars.

One contender is a nearby neutron star, known as RX J1856.5-3754. Using the Hubble Space Telescope, Fred Walter of the State University of New York, Stony Brook, has calculated the faint star's apparent path across the sky. This path, Walter says, means that the neutron star came from the Upper Scorpius association--the group of bright, young stars in which Zeta Ophiuchi was born.

In a paper scheduled for publication in The Astrophysical Journal in January, Walter theorizes that RX J1856.5-3754 is the collapsed core of the supernova that flung Zeta Ophiuchi into space. Because the neutron star's radial velocity (its movement toward or away from Earth) is unknown, astronomers can't tell whether the path leads into the association or passes in front of it. But Walter calculates that if the neutron star came from the association, it did so 1.15 million years ago--just when Zeta Ophiuchi made its own explosive exit.

Some other astronomers, however, believe RX J1856.5-3754 may be far too old to have been Zeta Ophiuchi's companion. Marten van Kerkwijk of Utrecht University thinks a much more likely candidate for Zeta Ophiuchi's erstwhile partner is a radio pulsar--a spinning neutron star that emits a radio pulse--known as PSR J1932+1059. From the rate at which PSR J1932+1059's rotation is slowing down, Van Kerkwijk calculates that the pulsar is at most a few million years old. In another Astrophysical Journal paper, scheduled to appear in October, Ronnie Hoogerwerf, Jos de Bruijne, and Tim de Zeeuw of Leiden Observatory in the Netherlands show that PSR J1932+1059 probably also left Upper Scorpius 1 million years ago.

Walter agrees that PSR J1932+1059 could also be the former binary companion of Zeta Ophiuchi. In any case, Walter says, it's possible that the neutron star and the young pulsar blazed into being in the same part of the sky at about the same time. "There was at least one supernova in Upper Scorpius about 1 million years ago. Why not two?" agrees Adriaan Blaauw of the University of Groningen, the Netherlands, who was the first to trace Zeta Ophiuchi back to the Upper Scorpius association. "But," Blaauw says, "I don't know of any other runaway star that could have been the companion of a second supernova."

--GOVERT SCHILLING

Related sites
Walter's paper about RX J1856.5-3754
Paper about PSR J1932+1059 by Hoogerwerf and colleagues