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Long-lived emission, known as afterglow, has now been detected from about a dozen g-ray bursts. Distance determinations place the bursts at cosmological distances, with redshifts, z, ranging from ,1 to 3. The energy required to produce these bright g-ray¯ashes is enormous: up to ,10 53 erg, or 10 per cent of the rest-mass energy of a neutron star, if the emission is isotropic. Here we present optical and near-infrared observations of the afterglow of GRB990123, and we determine a redshift of z > > 1:6. This is to date the brightest g-ray burst with a well-localized position and if the g-rays were emitted isotropically, the energy release exceeds the rest-mass energy of a neutron star, so challenging current theoretical models of the sources. We argue, however, that our data may provide evidence of beamed (rather than isotropic) radiation, thereby reducing the total energy released to a level where stellar-death models are still tenable.1 Palomar Observatory 105-24,

The Palomar Testbed Interferometer (PTI) is a long-baseline infrared interferometer located at Palomar Observatory, California. It was built as a testbed for interferometric techniques applicable to the Keck Interferometer. First fringes were obtained in July 1995. PTI implements a dual-star architecture, tracking two stars simultaneously for phase referencing and narrow-angle astrometry. The three fixed 40-cm apertures can be combined pair-wise to provide baselines to 110 m. The interferometer actively tracks the white-light fringe using an array detector at 2.2 um and active delay lines with a range of +/- 38 m. Laser metrology of the delay lines allows for servo control, and laser metrology of the complete optical path enables narrow-angle astrometric measurements. The instrument is highly automated, using a multiprocessing computer system for instrument control and sequencing.Comment: ApJ in Press (Jan 99) Fig 1 available from http://huey.jpl.nasa.gov/~bode/ptiPicture.html, revised duging copy edi

We report the results of a sensitive K-band survey of Herbig Ae/ Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependence on luminosity and effective temperature. For most targets, the measured near-infrared sizes (ranging from 0.2 to 4 AU ) support a simple disk model possessing a central optically thin (dust-free) cavity, ringed by hot dust emitting at the expected sublimation temperatures (T s $ 1000-1500 K). Furthermore, we find a tight correlation of disk size with source luminosity R / L 1 = 2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0-B3 stars) are undersized compared to predictions of the ''optically thin cavity'' model, likely because of optically thick gas within the inner AU.

High-resolution images of T Tau and its infrared companion have been reconstructed from near-and mid-infrared data collected at the Hale 5 m telescope. The near-infrared (1-5 fim) results were obtained by two dimensional speckle imaging and the mid-infrared (10-20 ¡im) results were derived from shift and add procedures applied to slit scans. The spectral energy distributions of the separated components were constructed from 1 to 20 [im data collected in less than half a year (1990 September to 1991 January). The spectral energy distribution of the optical component (T Tau N) is interpreted as containing two distinct constituents, a photosphere and a surrounding disk of circumstellar material. Measurements at a number of infrared wavelengths over the period 1985 December to 1991 January show a 2 mag color-independent change in the brightness of the infrared component (T Tau S). We propose that this may have been caused by an increase in accretion onto T Tau S and model the spectral energy distribution of T Tau S as being dominated by an accretion disk.

We simulate the actions of a coronagraph matched to diffraction-limited adaptive optics (AO) systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8.lm telescopes, and identify useful parameter ranges for AO coronagraphy on these systems. We model the action of adaptive wavefront correction with a tapered, high-pass filter in spatial frequency rather than a hard low frequency cutoff, and estimate the minimum number of AO channels required to produce sufficient image quality for coronagraphic suppression within a few diffraction widths of a central bright object (as is relevant to e.g., brown dwarf searches near late-type dwarf stars). We explore the effect of varying the occulting image-plane stop size and shape, and examine the trade-off between throughput and suppression of the image halo and Airy rings. We discuss our simulations in the context of results from the 241-channel Palomar Hale AO coronagraph system, and suggest approaches for future AO coronagraphic instruments on large telescopes.

The infrared companions (IRCs) associated with several normal low-mass preÈmain-sequence (T Tauri) stars pose an interesting problem for theories of binary star formation. The IRCs have very low infrared color temperatures and large infrared excesses, which have led observers to suggest that they may be less evolved objects such as protostars. This paper presents an attempt to understand the IRCs as a class by examining a broad range of observations and applying simple arguments and models. We propose that the IRCs may represent relatively normal young low-mass stars experiencing episodes of enhanced circumstellar extinction, possibly due to rapid accretion of disk material perturbed by their gravitational inÑuence at aphelion or perihelion.

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