Laser guide star adaptive optics imaging polarimetry of Herbig Ae/Be stars

We have used laser guide star adaptive optics and a near-infrared dual-channel imaging polarimeter to observe light scattered in the circumstellar environment of Herbig Ae/Be stars on scales of 100 to 300 astronomical units. We revealed a strongly polarized, biconical nebula 10 arc seconds (6000 astronomical units) in diameter around the star LkHalpha 198 and also observed a polarized jet-like feature associated with the deeply embedded source LkHalpha 198-IR. The star LkHalpha 233 presents a narrow, unpolarized dark lane consistent with an optically thick circumstellar disk blocking our direct view of the star. These data show that the lower-mass T Tauri and intermediate mass Herbig Ae/Be stars share a common evolutionary sequence.     

Accreting neutron stars, black holes, and degenerate dwarf stars

During the past 8 years, extended temporal and broadband spectroscopic studies carried out by x-ray astronomical satellites have led to the identification of specific compact x-ray sources as accreting neutron stars, black holes, and degenerate dwarf stars in close binary systems. Such sources provide a unique opportunity to study matter under extreme conditions not accessible in the terrestrial laboratory. Quantitative theoretical models have been developed which demonstrate that detailed studies of these sources will lead to a greatly increased understanding of dense and superdense hadron matter, hadron superfluidity, high-temperature plasma in superstrong magnetic fields, and physical processes in strong gravitational fields. Through a combination of theory and observation such studies will make possible the determination of the mass, radius, magnetic field, and structure of neutron stars and degenerate dwarf stars and the identification of further candidate black holes, and will contribute appreciably to our understanding of the physics of accretion by compact astronomical objects.     

Space, stars, c60, and soot

Although carbon has been subjected to far more study than all other elements put together, the buckminsterfullerene hollow-cage structure, recently proposed to account for the exceptional stability of the C(60) cluster, has shed a totally new and revealing light on several important aspects of carbon's chemical and physical properties that were quite unsuspected and others that were not previously well understood. Most significant is the discovery that C(60) appears to form spontaneously, and this has particularly important implications for particle formation in combustion and in space as well as for the chemistry of polyaromatic compounds. The intriguing revelation that 12 pentagonal "defects" convert a planar hexagonal array of any size into a quasi-icosahedral cage explains why some intrinsically planar materials form quasi-crystalline particles, as appears to occur in the case of soot. Although the novel structural proposal has still to be unequivocally confirmed, this article pays particular attention to the way in which it provides convincing explanations of puzzling observations in several fields, so lending credence to the structure proposed for C(60).     

Frequencies of occultations of stars by planets, satellites, and asteroids

Calculations show that several occultations of stars by the large satellites of the outer planets, Pluto, and the large asteroids could be observed each decade with existing equipment at Earth-based telescopes. A systematic program of occultation predictions and observations is urged in order to improve our knowledge about the atmospheres, sizes, shapes, topography, and positions of these poorly understood bodies, in support of forthcoming spacecraft missions to the outer solar system.     

Near-field optics: microscopy, spectroscopy, and surface modification beyond the diffraction limit

The near-field optical interaction between a sharp probe and a sample of interest can be exploited to image, spectroscopically probe, or modify surfaces at a resolution (down to approximately 12 nm) inaccessible by traditional far-field techniques. Many of the attractive features of conventional optics are retained, including noninvasiveness, reliability, and low cost. In addition, most optical contrast mechanisms can be extended to the near-field regime, resulting in a technique of considerable versatility. This versatility is demonstrated by several examples, such as the imaging of nanometric-scale features in mammalian tissue sections and the creation of ultrasmall, magneto-optic domains having implications for highdensity data storage. Although the technique may find uses in many diverse fields, two of the most exciting possibilities are localized optical spectroscopy of semiconductors and the fluorescence imaging of living cells.     

Phonon optics and phonon propagation in semiconductors

Recent experiments involving the propagation of extremely high frequency, short-wavelength acoustic phonons in semiconductors are described. Such phonons, which play an important role in thermal energy transpor and nonradiative recombination processes, can be used as sensitive microscopic probes of electronic, defect, and interface states. Experiments on phonon transmission thrugh epitaxially grown bulk material as well as thin-film superlattice structures in the semiconductor gallium arsenide are described. Such thin-film periodic structures can be used to build frequency-selective phonon filters and reflectors, which can in turn be used to manipulate phonon diagnostic beams in technologically important materials.     

Scenes from a marriage--of optics and electronics

BALTIMORE-Exploring the common ground between optics and electronics, more than 6800 physicists, spectroscopists, and engineers gathered here from 21 to 26 May at the joint meetings of the Conference on Lasers and Electro-Optics and the Quantum Electronics and Laser Science conference. Participants unveiled new technologies that have sprung up on this common ground, such as an imaging technique that can gauge the chemical composition of materials. They also described ways to broaden that ground, such as a novel approach for integrating lasers and silicon chips-a challenge that has slowed progress toward a new generation of high-speed computers and communications.     

Two gamma-ray sources and ancient guest stars

On the basis of the fact that the youngest neutron stars such as the Crab pulsar and the Vela pulsar emit strong gamma-ray radiation, it is suggested that a few gamma-ray sources may be identified with young compact sources formed in the events of guest stars. Two such sources, 2CG 353+16 and 2CG 054+01, are identified with guest stars observed in the 14th century B.C. and A.D. 1230, respectively.     

Trapped ions, laser cooling, and better clocks

Ions that are stored in electromagnetic "traps" provid the basis for extremely high resolution spectroscopy. By using lasers, the kinetic energy of the ions can be cooled to millikelvin temperatures, thereby suppressing Doppler frequency shifts. Potential accuracies of frequency standards and clocks based on such experiments are anticipated to be better than one part in 10(15).