Resolution of the circumstellar disk of β Pictoris at 10 and 20 µm

Abstract

The A5 dwarf β Pic is a star that may be surrounded by a proto-planetary disk. Among the stars discovered with the Infrared Astronomical Satellite (IRAS) to have far-infrared excesses from circumstellar dust^1,2, β Pic has a disk that is apparently viewed nearly edge-on, which increases its surface brightness and spatial contrast. Using coronagraphs to image the disk at optical wavelengths, it has been found to extend to at least 1,100 AU from the star^3–6 and is viewed as starlight scattered from dust. The colour of the disk light is the same as, or redder than, the direct starlight, which implies that the scattering grains are larger than ∼1 μm (refs 3, 4). Likewise, some models of radiative transfer in a dusty disk which are constrained by the IRAS scans and fluxes² have not required submicron-sized grains and indicate a possible dust-free zone near the star^5,7,. Constraints on these models have been limited partly by the use at optical wavelengths of an occulting disk to block the direct starlight and by the low spatial resolution of the 12–100 μm IRAS observations, which greatly limit information about the central region within ∼10″, or 150 AU, of the star. Here we present observations at 10 and 20 μm with ∼5″ resolution of the central ∼13″-diameter region of β Pic; these resolve the inner disk and provide important constraints for the models and a basis for removing some of their ambiguities.