An infrared image of the dust disc around beta PIC

Abstract

The star beta Pictoris is a normal main-sequence star with a remnant disc of cool dust around it. We present observations of the beta Pic dust disc in the far-infrared, obtained using the photometer (ISOPHOT) on the Infrared Space Observatory (ISO) satellite. We resolve the edge-on disc using high-resolution scans, with full widths at half-maximum of 8.7 x 2.9 arcsec^2 (+/- 1.5 arcsec) at 25 mu m (corresponding to a characteristic disc radius of 84 au) and 14.5 x 6.1 arcsec^2 (+/- 2.0 arcsec) at 60 mu m (corresponding to 140 au). The disc is just resolved in the minor axis direction at 60 mu m. Photometry, from 4.85 to 200 mu m, is combined with low-resolution spectroscopy and modelled to give an average dust temperature of 85 K, using a blackbody modified by an emissivity proportional to lambda^-1. A simple radiative transfer model is also used, with the grain radius ranging from 1 mu m to 5 mm. The dust mass is estimated to be around (3-10) x 10^-8 M_solar (0.9-2.7 M_moon), depending on the selected grain parameters.