The complex dust formation zone of the AGB star IRC+10216 probed with CARMA 0.25 arcsec angular resolution molecular observations

Abstract

We present low spectral resolution molecular interferometric observations at 1.2 mm obtained with the Combined Array for Research in Millimetre-wave Astronomy (CARMA) towards the C-rich AGB star IRC+10216. We have mapped the emission of several lines of SiS, H13CN, SiO and SiC2 in the ground and first excited vibrational states with a high angular resolution of 0.25 arcsec. These observations have allowed us to partially resolve the emission of the envelope at distances from the star 50 stellar radii (R), where the stellar wind is mainly accelerated. The structure of the molecular emission has been modelled with a 3D radiation transfer code. The emission of line SiS (v = 0, J = 14–13) is best reproduced with a set of maser emitting arcs arranged between 5 and 20R. The abundance of H13CN with respect to H2 decreases from 8 × 10−7 at 1–5R to 3 × 10−7 at 20R. The SiO observations are explained with an abundance 2 × 10−8 in the shell-like region between 1 and 5R. At this point, the SiO abundance sharply increases up to (2–3) × 10−7. The vibrational temperature of SiO increases by a factor of 2 due north-east between 20 and 50R. SiC2 is formed at the stellar surface with an abundance of 8 × 10−7 decreasing down to 8 × 10−8 at 20R probably due to depletion on to dust grains. Several asymmetries are found in the abundance distributions of H13CN, SiO and SiC2 which define three remarkable directions (north-east, south-southwest, and south-east) in the explored region of the envelope. There are some differences between the redshifted and blueshifted emissions of these molecules suggesting the existence of additional asymmetries in their abundance distributions along the line of sight.