The very compact dust disk in the Red Rectangle

Abstract

Aims. We aim to study the dust distribution in the central regions of the Keplerian disk of the Red Rectangle, the prototype of binary post-AGB stars with rotating circumbinary disks, and to compare it with the distribution of relevant molecular gas tracers. Methods. We present new high-resolution (20 milliarcseconds, mas) ALMA observations of continuum and line emissions at 0.9 mm. The maps have been analyzed by means of a simple model of dust and free–free emission that is able to reproduce the continuum data. Results. (i) We find that most of the dust emission in the Red Rectangle is concentrated in the inner disk regions, with a typical size of 250 AU in diameter and 50 AU in width. (ii) The settlement of dust grains onto inner equatorial regions is remarkable when compared with the relatively widespread gas distribution. (iii) This region is basically coincident with the warm PDR (photo-dominated region) where CI, CII, and certain molecules such as HCN are presumably formed, as well as probably PAHs (polycyclic aromatic hydrocarbons, whose emission is very strong in this source). (iv) We confirm the large size of the grains, with a typical radius ∼150 μm. The opacity of dust at 0.9 mm is deduced to be relatively large, ∼0.5. (v) We also confirm the existence of a very compact HII region in the center, for which we measure an extent of 10–15 mas (∼10 AU) and a total flux of 7–8 mJy at 0.9 mm.