STIS observations of FLIERs in NGC 7662 ^*

¹ Dip. di Astronomia, Universita' di Firenze, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: mariop@arcetri.astro.it
² IRA/CNR, Largo E. Fermi 5, 50125 Firenze, Italy
³ Astronomy Dept., FM-20, University of Washington, Seattle - WA 98195, USA
⁴ Isaac Newton Group of Telescopes, Ap. de Correos 321, 38700 Sta. Cruz de la Palma, Canary Islands, Spain

Received: 10 November 2003
Accepted: 15 March 2004

Abstract

We observed the planetary nebula NGC 7662 with STIS on board the HST to investigate the physical conditions in the microstructures within the nebula at high spatial resolution and to closely examine the physical conditions in FLIERs. We aim to reveal spectroscopic differences in small scale structures moving at about the same velocity as the surrounding nebular gases, called SLOWERs. Shocks are expected to play a role in FLIERs but not in SLOWERs. The physical conditions in the two types of microstructures have been determined better than ever. In both types of substructures, the electron density shows a moderate increase inward, while the electron temperature becomes a little lower. The ionization increases from the central parts of the FLIERs to their edges. No particular increase of the line emission typical of shock excitation, such as the [SII] 6725 Å  doublet, is seen at the interface between the ambient gas and the microcondensations. Thus, we observe no evidence for shock effects in the FLIERs. We estimate that the thickness of the post-shock layer is too small to be detected, in spite of the brightness of the nebula. For the same reason the determinations of abundances within the FLIERs done assuming the contribution of photoionization followed by ion-electron collision excitation to be dominant in producing the observed emission lines, might be considered to be acceptable. The geometrical complexity of microstructures composed of sub and subsub-structures coupled with the need to take into account charge-exchange reaction effects, prevent us from drawing a firm conclusion on the nitrogen overabundance suggested in the literature for the FLIERs relative to the surrouding ambient gas. We however argue that our HST observations do not support this enhancement.