Few circumstellar disks have been directly observed. Here we use sensitive differential polarimetric techniques to overcome atmospheric speckle noise in order to image the circumstellar material around HD 169142. The detected envelope or disk is considerably smaller than expectations based on the measured strength of the far-IR excess from this system

We report the discovery of two faint ultracool companions to the nearby (d∼17.9 pc) young G2V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a adaptive optics instrument mounted on the Gemini North 8-meter telescope. Both objects have the same common proper motion as the primary star as seen over a 7 month baseline and have near-IR photometric colors that are consistent with an early-L classification. Near-IR spectra taken with the NIRSPEC AO instrument on the Keck II telescope reveal K I lines, FeH, and H 2 O bandheads. Based on these spectra, we determine that both objects have spectral type dL2 with an uncertainty of 2 spectral subclasses. The position of the new companions on the H-R diagram in comparison with theoretical models is consistent with the young age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity, lithium abundance and fast rotation. HD 130948B and C likely constitute a pair of young contracting brown dwarfs with an orbital period of about 10 years, and will yield dynamical masses for L dwarfs in the near future.

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AMANDA is a high-energy neutrino telescope presently under construction at the geographical South Pole. In the Antarctic summer 1995/96, an array of 80 optical modules (OMs) arranged on 4 strings (AMANDA-B4) was deployed at depths between 1.5 and 2 km. In this paper we describe the design and performance of the AMANDA-B4 prototype, based on data collected between February and November 1996. Monte Carlo simulations of the detector response to down-going atmospheric muon tracks show that the global behavior of the detector is understood. We describe the data analysis method and present first results on atmospheric muon reconstruction and separation of neutrino candidates. The AMANDA array was upgraded with 216 OMs on 6 new strings in 1996/97 (AMANDA-B10), and 122 additional OMs on 3 strings in 1997/98.

We have developed a potential-energy surface for spin-polarized K͑ 2 S͒ +K 2 ͑ 3 ⌺ u + ͒ collisions and carried out quantum dynamical calculations of vibrational quenching at low and ultralow collision energies for both bosons 39 K and 41 K and fermions 40 K. At collision energies above about 0.1 mK the quenching rates are well described by a classical Langevin model, but at lower energies a fully quantal treatment is essential. We find that for the low initial vibrational state considered here ͑v =1͒, the ultracold quenching rates are not substantially suppressed for fermionic atoms. For both bosons and fermions, vibrational quenching is much faster than elastic scattering in the ultralow-temperature regime. This contrasts with the situation found experimentally for molecules formed via Feshbach resonances in very high vibrational states.