Spectral trends in planetary nebulae: The roles of radiative and shock excitation

Instituto de Astronomia y Meteorologia, Av. Vallarta 2602, Col. Arcos Vallarta, C.P. 44130 Guadalajara, Jalisco, Mexico e-mail: jpp@udgserv.cencar.udg.mx

Send offprint request to: J.P. Phillips

Received: 27 October 1997
Accepted: 19 January 1998

Abstract

We have investigated de-reddened spectral line ratios for some 538 planetary nebulae. As a result, it has proved possible to define comparative variations between differing transitions, evaluate the viability of radiative modeling for the generality of nebulae, and assess the importance of shocks in modifying low- excitation line strengths. Whilst most transitions are well represented in terms of radiative excitation, the [OI]  Å line appears to be appreciably too strong in most of the present sample; a deviation which may arise through shock interaction between the primary outflow shell and enveloping superwind material. Comparison between shock modeling and line excesses also suggests that an appreciable proportion of [SII]  Å emission may arise through shock excitation; a conclusion which, if confirmed, may have serious consequences for nebular density estimations. Some 14 nebulae are identified as likely shock candidates, whilst it is proposed that the majority of bipolar nebulae may also show spectral deviations associated with shock excitation. Line excesses for these latter sources are most consistent with shock velocities  km s^-1; values which are also comparable to observed wind velocities. Finally, sources containing FLIERs (Balick et al. 1993) are shown to be confined to highly specific spectral regimes; a result which permits us to identify three further possible FLIER sources, and propose characteristic line ratio diagnostics for the further discovery of such features.