The shock structure in the protoplanetary nebula M1-92: imaging of atomic and H_2 line emission
Abstract
We present HST imaging of continuum (5500 Angstroms) and atomic line (Hα , [OI] 6300 Angstroms, [SII] 6717 and 6731 Angstroms, and [OIII] 5007 Angstroms) emissions in the protoplanetary nebula M 1-92. Ground based imaging of 2$() μm continuum and H_2 ro-vibrational (S(1) v=3D1-0 and v=3D2-1 lines) emission has been also performed. The 5500 Angstroms\ continuum is due to scattering of the stellar light by grains in a double-lobed structure comparable in extent and total density with the molecular envelope detected at mm wavelengths, which consists of two empty shells with a clear axis of symmetry. On the other hand, the optical line emission comes mainly from two chains of shocked knots placed along the symmetry axis of the nebula and inside those cavities, for which relatively high excitation is deduced (shock velocities of about 200 ^km s^{-1}}). The H_2 emission probably comes from more extended regions with representative temperature and density of 1600 K and 6 10^3 cm^{-3}$, 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. jets and 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. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-2655
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- March 1998
- Bibcode:
- 1998A&A...331..361B
- Keywords:
-
- CIRCUMSTELLAR MATTER;
- STARS: AGB AND POST-AGB;
- PLANETARY NEBULAE: INDIVIDUAL: M 1-92;
- SHOCKS;
- INFRARED: ISM: LINES AND BANDS