Astron. Astrophys. Suppl. Ser. 133, 381-386

The density and shock characteristics of NGC 2818

J.P. Phillips¹ and L. Cuesta²

Send offprint request: J.P. Phillips

1 - Instituto de Astronomia y Meteorologia, Avenida Vallarta 2602, Col. Arcos Vallarta, C.P. 44130 Guadalajara, Jalisco, Mexico
e-mail: jpp@udgserv.cencar.udg.mx
2 - Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain
e-mail: lcc@ll.iac.es

Received January 26; accepted June 26, 1998

Abstract:

We report the results of narrow band imaging of the bipolar outflow source NGC 2818 in transitions of [NII], [OIII], [SII], and HI. As a consequence, we are able to assess the overall excitation properties of the shell, and determine that line ratios towards the nebular periphery are anomalous, and characteristic of planar shocks with velocity $V_{\rm s} \geq 110$ km s^-1. Pre-shock densities are likely to be modest, and of order $10^2 < n_{\rm p} < 10^3$ cm^-3.

The [SII] imaging is further used to derive a fully-sampled map of electron density, whence it is clear that the density structure is rather complex and related to various features within the excitation maps. Peak values $n_{\rm e}\sim 1.5\ 10^3$ cm^-3 occur close to the nucleus, although densities subsequently decline for major axis displacements > 7 arcsec. The minor axis distribution, on the other hand, appears to be somewhat more extended and bilobal.

It is probable that complexities in the density mapping reflect the presence of condensations and filaments within the primary shell structure, and that these are also responsible for apparent variations in density as a function of forbidden and permitted line transition.

Key words: planetary nebula: NGC 2818 -- ISM: jets and outflows