1998A&A...331..361B

We present HST imaging of continuum (5500Å) and atomic line (Hα, [OI] 6300Å, [SII] 6717 and 6731Å, and [OIII] 5007Å) emissions in the protoplanetary nebula M 1-92. Ground based imaging of 2µm continuum and H₂ ro-vibrational (S(1) v=1-0 and v=2-1 lines) emission has been also performed. The 5500Å continuum is due to scattering of the stellar light by grains in a double-lobed structure comparable in extent and total density with the molecular envelope detected at mm wavelengths, which consists of two empty shells with a clear axis of symmetry. On the other hand, the optical line emission comes mainly from two chains of shocked knots placed along the symmetry axis of the nebula and inside those cavities, for which relatively high excitation is deduced (shock velocities of about 200km/s). The H₂ emission probably comes from more extended regions with representative temperature and density of 1600K and 6x10³cm^–3, intermediate in location and excitation between the atomic line knots and the very cold region detected in CO emission. We argue that the chains of knots emitting in atomic lines correspond to shocks taking place in the post-AGB bipolar flow. The models for interstellar Herbig-Haro objects seem to agree with the observations, at least qualitatively, explaining in particular that the atomic emission from the bipolar flow dominates over that from shocks propagating in the AGB shell. Models developed for protoplanetary nebula dynamics fail, however, to explain the strong concentration of the atomic emission along the symmetry axis.