Number 98, October 13, 1992 by Phillip F. Schewe and Ben Stein
CIRCUMSTELLAR DISKS have been discovered around the stars V376 Cas and V633 Cas in the Cassiopoeia constellation by scientists at the Nordic Optical Telescope in the Canary Islands. The disks consist mostly of micron-sized dust grains and extend out to a radius of 500-750 astronomical units from the stars. Starlight reflected by the dust is polarized, allowing the disks to be detected at near-infrared wavelengths. Some astronomers believe that circumstellar disks should surround many young stars, and that they represent the remnants of or the raw material for planet formation. The first circumstellar disk to be detected (in the 1980s), and the only one to be imaged, surrounds the star Beta Pictoris. (Nature, 1 Oct. 1992.)
PRODUCING ANTI-HYDROGEN is the aim of several experiments. The first step, creating positrons and antiprotons, is difficult enough, but forming a stable anti-atom out of the anti-particles seems harder still. Gerald Gabrielse of Harvard uses the Low Energy Antiproton Ring (LEAR) at CERN plus his own electrostatic trap to slow (and store) antiprotons down to energies as low as 0.3 meV. Bringing them together with positrons (perhaps in a double-trap setup) is several years off. At Fermilab, Charles Munger of SLAC hopes to search for the very few anti-hydrogen atoms he suspects may be generated when a beam of antiprotons, striking a hydrogen target, creates electron-positron pairs; occasionally the positron might link up with one of the antiprotons. One direct approach involves colliding antiproton and positron beams together but, according to Gabrielse, this would most likely result in the particles bouncing off each other rather than mating. Scientists expect that anti-hydrogen, once it can be made, will be useful in the study of gravity and quantum mechanics. (Science, 25 Sept. 1992.)
HOW MANY ATOMS DOES IT TAKE TO MAKE METAL? Small clusters of atoms have neither the properties of single atoms nor those of the bulk material. Several years ago studies of mercury clusters showed that the transition to metallic behavior occurred in the size range between 20 and 70 atoms. Now two different research groups in Germany have independently studied the optical properties of mercury clusters in this transition range. Scientists at the Philipps University at Marburg (K. Rademann et al.) and at Freiburg University (Hellmut Haberland et al.) report that as the cluster size is increased, an important absorption line in the mercury spectrum---that corresponding to a transition between the 6s and 6p states in single atoms---falls off because as the cluster grows, the two states start to overlap; meanwhile another (broad) absorption peak arises, corresponding to the excitation of a collective ("plasmon") electronic state in the cluster as a whole. (26 October 1992 issue of Physical Review Letters.)
THE 500TH ANNIVERSARY OF COLUMBUS' arrival in the New World on October 12 was marked by the onset of a stepped-up effort to find even newer worlds. A 10-year Search for Extraterrestrial Intelligence (SETI) NASA program has several components, including a scan of 1000 stars by the 310-m radio dish at Arecibo and a JPL survey of the entire sky (at low sensitivity) over a wide range of radio channels. (Science, 2 October 1992.)
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