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Astron. Astrophys. 331, 361-371 (1998)

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The shock structure in the protoplanetary nebula M 1-92:
imaging of atomic and H2 line emission *

V. Bujarrabal 1, J. Alcolea 1, R. Sahai 2, J. Zamorano 3 and A.A. Zijlstra 4

1 Observatorio Astronómico Nacional (IGN), Apartado 1143, E-28800 Alcalá de Henares, Spain (bujarrabal,j.alcolea@oan.es)
2 Jet Propulsion Laboratory, MS 183-900, 4800 Oak Grove Drive, Pasadena CA 91109, USA (sahai@lb6.jpl.nasa.gov)
3 Departamento de Astrofísica, Facultad C. Físicas, Universidad Complutense, E-28040 Madrid, Spain (jaz@astrax.fis.ucm.es)
4 European Southern Observatory, Karl Schwarzschild Strasse 2, D-85748 Garching bei München, Germany (azijlstr@eso.org)

Received 18 March 1997 / Accepted 19 September 1997


We present HST imaging of continuum (5500 Å) and atomic line (H [FORMULA], [OI] 6300 Å, [SII] 6717 and 6731 Å, and [OIII] 5007 Å) 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 5500 Å 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 [FORMULA] emission probably comes from more extended regions with representative temperature and density of 1600 K and 6 103 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. 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.

Key words: circumstellar matter – stars: AGB and post-AGB – planetary nebulae: individual: M 1 – 92 – shocks – infrared: ISM: lines and bands

* 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

© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998