CRL 618 - THE NATURE OF THE 200 KM S-1 MOLECULAR OUTFLOW

Using the IRAM interferometer, we have mapped with 2.4" x 3.4" resolution the J = 1 --> 0 HCN line emission in the proto-planetary nebula CRL 618. Our maps resolve the 200 km s-1 molecular outflow, as well as the slowly expanding circumstellar envelope, allowing a precise positioning of these components with respect to the central HII region. A model for the high velocity molecular gas is proposed: it suggests the emission from the HCN wings is produced by the strong impact of a primary outflow on a few well localised dense clumps in the inner envelope. Formation of high velocity molecules, and in particular HCN, is expected to occur in the post-shock regions: the kinematical age for the high velocity HCN molecules is estimated as < 50 years, the excitation temperature as 200 K less-than-or-equal-to T(ex) less-than-or-equal-to 1000 K. To our knowledge, this is the first object for which a post-shock formation time scale has been inferred. Our analysis shows that 70% of the HCN line emission comes from a very compact, spherically symmetric region. Furthermore, we show that the HCN J = 1 --> 0 transition in the envelope is mainly collisionally excited and strongly affected by self-absorption.