Research News Brief
Unexpected Physics Conditions in RHIC Collisions
At the APS meeting, speakers from all four detector groups (BRAHMS, PHENIX, PHOBOS, STAR) at the Relativistic Heavy Ion Collider (RHIC) agreed that it is too early to declare a sighting of the coveted quark-gluon plasma (QGP), a primordial soup of free-ranging quarks and gluons. But presenters said they found preliminary signs of tantalizing QGP "prerequisites."
Studying the products of RHIC's collisions between near-light-speed-velocity gold-ion beams, all four detector groups measured a more equal ratio of antiprotons to protons-roughly a 3:2 ratio, according to BRAHMS measurements-than ever before seen in nuclear collisions. This is the closest reproduction yet of the matter-antimatter balance thought to prevail at the time of the Big Bang than previously achieved in the laboratory, said PHOBOS member Russell Betts (University of Illinois at Chicago). In fact, the abundance of protons and antiprotons in the collision products was surprising, raising possibilities of a new production mechanism for proton-antiproton pairs or a suppression in the production of lighter particles, said PHENIX's Sam Aronson (Brookhaven National Laboratory). Studying the highest-momentum products moving transversely to the direction of the ion beams, the groups found hints of "jet quenching," the idea that the particles lose significant energy while traveling through the collision fireball. Such a large energy loss does not occur in ordinary nuclear matter.
In addition, the STAR collaboration observed that the collision fireball expanded violently, at supersonic speeds. Voicing a minority view, STAR member John Cramer (University of Washington) speculated that such a violently exploding fireball may mean that RHIC is operating at energies higher than those required for creating a QGP, expected by some to expand more gently. However, all agree that the picture will become clearer in RHIC's next experimental run, slated to begin later this month, in which the groups expect to gather 10-100 times more data from the accelerator, which will be able to run, for the first time, at its maximum energy of 100 GeV/nucleon.
Latest Investigations of Orion Nebula Lower Odds of Planet Formation
In 1993, when the Hubble Space Telescope surveyed the Orion nebula for the first time, its images provided a substantial boost for the argument that stars with planetary systems are commonplace in the galaxy. Now, however, the most recent analyses of one of the youngest, closest and brightest nebulae cast doubt on that conclusion and suggest that planets may be far rarer than astronomers have thought.
C. Robert O'Dell, lead scientist on the first HST studies of Orion and now a research professor at Vanderbilt University, has been studying the nebula since 1964. At the APS April meeting in Washington D.C., O'Dell reported that the most recent studies of Orion indicate that the youngest and brightest stars in the cluster are so powerful that the ultraviolet radiation they produce should blast away the dust and gas surrounding newly formed stars before they can form planets. According to current estimates, it takes about 10 million years for a planet to form.
Over the last eight years, O'Dell and colleagues at the National Autonomous University of Mexico have used a combination of optical and radio telescope data to construct a detailed, three-dimensional map of the nebula. Using this map, he estimates that only 10 percent of the proplyds in the nebula are shielded from the erosive star-shine. If planetary formation times are correct, and the conditions in the Orion nebula are typical of stellar nurseries, then only one star in 10 is likely to form a planetary system, O'Dell says.
Black Holes May Take Space for a Spin
Astronomers at NASA's Goddard Space Flight Center now have direct observational evidence that at least some black holes spin like whirlpools, wrapping up the fabric of space with them. According to Tod Strohmayer, who presented the finding at the APS April meeting, there are unique patterns in one such black hole system that have previously only been observed in spinning neutron stars. With these new parameters, he was able to verify that black holes can also spin in similar fashion. "Almost every kind of object in space spins, but with black holes it's much harder to directly see that they are spinning, because they don't have a solid surface," he says. "We can, however, see light emitted from matter plunging into the black hole, which whips frantically around the black hole before it is lost forever."
Strohmayer's target was GRO J1655-40, a microquasar 10,000 light years from Earth, with a mass seven times that of our Sun. The observation also challenges existing theories about neutron star radiation, according to Strohmayer, who also observed the first paired quasiperiodic oscillations (QPOs) from a black hole-a feature common to neutron stars. QPOs are a flickering type of X-ray, thought to result from radiation emitting from the solid neutron star surface, but a black hole has no solid surface. The spin of a black hole would be caused by the angular momentum of the star that formed it, says Strohmayer, particularly if that progenitor is a spinning neutron star.
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