Coronagraphic imaging of protoplanetary-disk candidate stars using adaptive optics

A coronagraph has been equipped with adaptive optics to provide high-resolution ground-based images of the circumstellar environments of bright stars at optical wavelengths. The Adaptive Optics Coronagraph presently contains an image-motion compensation system for stabilization of the telescope field. By stopping the telescope aperture D to approximately 4 r₀, where r₀ is Fried's parameter, the maximum attainable resolution gain factor of 2.2 has been achieved. Gains measured for D/r₀ greater than 14 are below theoretical expectation and are indicative of centroid anisoplanatism, a small spatial-coherence outer scale, or both. A comparison of stabilized and unstabilized images shows that this coronagraph may detect circumstellar objects 2 magnitudes fainter than those detectable with a conventional coronagraph. Coronagraphic observations of two protoplanetary-disk candidate stars are described. R-band images of the beta Pictoris circumstellar disk obtained in 1991 provide the first reliable optical photometry of the disk 40-100 AU from the star. At 100 AU, the radial power-law dependence of the disk-midplane surface brightness changes abruptly; the brightness gradient becomes less steep within 100 AU. Further, the brightness gradients of the two disk extensions are asymmetric. The geometric thickness of the disk appears nearly constant within approximately 115 AU and increases proportionally with radius beyond approximately 115 AU. These changes in brightness gradient and disk thickness at approximately 100 AU may mark the boundary of rapid ice sublimation within which only refractory grains exist, but may also reflect a flattened grain distribution associated with planetary formation within 40 AU. Images of epsilon Sagittarii obtained in 1990 and 1992 reveal a pointlike source, presumably a star, approximately 2''.1 from epsilon Sgr. Although the star appears partially occulted by the coronagraphic mask in the V- and R-band images, it is clearly resolved in the I-band image obtained with a smaller mask. Large photometric uncertainties preclude an accurate assessment of the star's spectral type. Although the proximity of the star to epsilon Sgr supports the hypothesis that previously reported optical and UV spectra of epsilon Sgr are composite, the star appears too faint to account for the noted spectral anomalies. However, a K- or M-type companion to epsilon Sgr may explain the observed infrared excess.