2004ApJ...602..960S

We present interferometric maps with high angular resolution, 1".1 to 1".9, of the HCO^ + ^ J=1-0 and HCN J=1-0 emission lines in the protoplanetary nebula CRL 618. These molecules are present in both the slowly (∼10-20 km/s) and rapidly (∼200 km/s) outflowing components of the molecular envelope of CRL 618. The high-velocity (HV) molecular outflow is composed of two compact (mean size ∼1") clumps aligned with the optical lobes and separated by 2".1. The distance between these clumps is now 0".5 larger than 11 yr ago, which is consistent with rapid expansion of the HV clumps. The expansion velocity in the HV outflow increases linearly with the distance to the nebular center, from which we derive a kinematical age of ∼60 yr. Our data reveal that part of the low-velocity (LV) molecular gas in CRL 618 arises in a extended structure elongated ∼11" in the direction perpendicular to the outflow, which is consistent with a dense, equatorial torus expanding at V_exp∼17.5 km/s. We have used a two-dimensional spatio-kinematical model to better constrain the structure, kinematics, and density distribution of the HV and LV components that are probed by HCN J=1-0 and HCO^ + ^ J=1-0 emission. The central H II region of CRL 618, which is traced by free-free mm-continuum emission, is unresolved in these observations. We report (1) a recent decrease of the mm-continuum flux and (2) a 3 mm-to-1 mm flux ratio consistent with optically thin free-free emission in the millimeter wavelength range. The continuum flux at 3 mm (1 mm) was 1.6±0.15 Jy (1.2±0.3 Jy) in 2000, when our observations were performed. The weakening of the millimeter continuum, as well as the decrease of the free-free turnover frequency, could result from a recent cessation of the post-AGB wind or the growth of the cavity of the central H II region (or both). From a consideration of multiwavelength datasets on CRL 618, we provide a comprehensive view of the temporal evolution of its nebular material.