Strange Stars? Odd features hint at novel matter. (This Week).

Exotic forms of matter never observed before in the wild may have turned up in the remnants of two collapsed stars, according to new findings publicized last week by NASA.

At an April 10 briefing at NASA headquarters in Washington, D.C., researchers argued that observations by the orbiting Chandra X-Ray Observatory indicate that the astronomical object dubbed RXJ1856.5-3754 may lack the neutrons, protons, and electrons of ordinary matter. Another object, 3C58, may contain odd forms of matter surrounded by neutrons.

Since its discovery in 1996, astronomers had thought that RXJ1856 was a neutron star, a stellar cinder composed almost solely of neutrons. Now it appears the star may be a more bizarre--and up until now hypothetical--object called a quark star, says Jeremy J. Drake of the Smithsonian Astrophysical Observatory in Cambridge, Mass.

A quark star would consist entirely of the building blocks of matter called quarks, but they wouldn't be combined into more massive particles, as they are in all matter known to date. This quark matter would include the so-called up and down quarks of which protons and neutrons are made and also "strange" quarks, which are heavier and not found in ordinary matter.

If the quark-star interpretation holds, the RXJ 1856 findings "will be an astonishing discovery of fundamental significance," says Norman K. Glendenning of Lawrence Berkeley (Calif.) National Laboratory.

Adds Michael S. Turner of the University of Chicago: "These results ... are demonstrating that the universe can be used as a laboratory to explore nature in ways that we simply cannot do on Earth."

From the new, precise measurements of RXJ1856's X-ray emissions, Drake and his colleagues were able to calculate how hot the star is. That, in turn, enabled them to use theoretical models of compact stars to infer that the star has a radius of 5 to 6 kilometers. That value is about half of what's expected if the object were a neutron star, but about right if it were a quark star. A report on the findings is scheduled to appear in the June 20 Astrophysical Journal.

The researchers admit their quark-star hypothesis isn't the only possible explanation for the data. One of RXJ1856's discoverers, Frederick M. Walter of the State University of New York at Stony Brook, notes that earlier measurements of optical and ultraviolet emissions disagree with the quark-star notion. In a report posted on the Internet (http://www. arxiv.org/abs/astro-ph/ 0204199) on the day after the NASA briefing, Walter and James M. Lattimer, also of Stony Brook, argue that RXJ1856 is a normal-size neutron star with an X-ray hot spot or, perhaps, a radiation-distorting atmosphere.

The other object discussed at the NASA briefing, 3C58, may also contain peculiar forms of matter, though not necessarily quark matter. David J. Helfand of Columbia University, a member of the team that studied the star, says that 3C58 may contain crystals of pions or kaons. These materials have previously only been seen as fleeting particles in high-energy accelerator experiments.

The big surprise of 3C58 is its relatively cool apparent temperature. The scientists say they know what the temperature should be because they know 3C58's age from records of a supernova in A.D. 1181.

If that supernova left behind a neutron star, the observed X-ray spectrum should indicate a temperature of about 2 million kelvins, Helfand says. Instead, the Chandra readings point to a temperature of less than a million kelvins. The data indicate that 3C58 is losing heat faster than standard theories of neutron star cooling permit. Previously unknown forms of matter, such as pion crystals, could account for the results, Helfand says. He and his colleagues will present those findings in a future Astrophysical Journal Letters.

Lars Bildsten of the University of California, Santa Barbara remains unconvinced by the arguments for exotic forms of matter in either 3C58 or RXJ1856. For example, 3C58 could be just an unusually weighty neutron star, he says. Before astronomers will be able to determine just how exotic these stars are, Bildsten and other skeptics say, theorists must improve the neutron star models on which the far-reaching interpretations rest.