Astron. Astrophys. 332, 721-731 (1998)

1. Introduction

The morphological appearance of planetary nebulae (PNe) has since long attracted the attention of the researchers. Various designations and/or classification schemes have then been proposed since the early work of Curtis (1918) up to the most recent ones (Balick 1987, Schwarz et al. 1993, Manchado et al. 1996). Particularly significant for the implications with the evolutionary history of PNe has been the recognition of the close correlation between the enhanced chemical abundances in He/H and/or N/O (Peimbert, 1978) and the bipolar morphology of the nebula. This as well as other properties of this class, including the properties of the central stars and their galactic location, was extensively investigated by several authors (see Corradi & Schwarz 1995, hereafter CS95, for a recent comprehensive study). Nearly all bipolar PNe belong in fact to type I PNe (Peimbert, 1978; Peimbert & Torres-Peimbert 1983) defined by having either He/H 0.125 or (N/O) -0.3. These overabundances have been related to more massive progenitors for the central stars of these PNe, as compared with those of PNe of other morphological classes. Massive AGB stars ( 2  ) are in fact expected to be substantially He and/or N enriched at their surface, following second and third dredge-up episodes as well as efficient H-burning at the base of the convective envelope (Renzini & Voli 1981). Recent calculations (Marigo et al. 1996, and 1997, private communication) fail, however, to reproduce the most extreme He overabundances measured in some bipolar PNe.

In order to better quantify these problems, we have obtained deep long-slit spectra of fourteen bipolar nebulae selected from the compilation of CS95. From these data, we have derived new, homogeneous, good quality chemical abundances. By means of these observations, we also wished to investigate the possibility of detecting chemical variations through the nebulae, which may reflect different mass loss episodes from their progenitors, or be the trace of chemical inhomogeneities in the outflows, a subject which has been little investigated so far. In paper I (Corradi et al. 1997a), the results for the bipolar PN IC 4406 were presented, illustrating in detail observations, data reduction and the analysis method. We discuss in the present paper the results for the other 13 PNe. Information about some basic properties of the nebulae can be found in Table 1 of CS95.

© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998
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