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Astron. Astrophys. 322, 975-981 (1997)

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1. Introduction

Bipolar objects form a distinctive class of planetary nebulae (PNe). Here the term "bipolar" is used to indicate nebulae which consist of an equatorial waist from which two symmetrical lobes depart in the perpendicular directions, as opposed to elliptical PNe in which the central waist is absent (Schwarz et al. 1993). These nebulae are also referred to as "butterfly" in the literature (Balick 1987 ). Besides their characteristic morphology, bipolar PNe show a number of peculiar properties (see Corradi & Schwarz 1995 - hereafter CS95 - for a recent extensive analysis and for references to previous studies): they belong to a younger Galactic disc population than elliptical objects, have the hottest central stars among PNe, chemical overabundances of helium, nitrogen, and possibly neon, outflow velocities of up to one order of magnitude larger than the typical expansion velocities of PNe, large neutral envelopes, and peculiar IRAS colours. Some of the above properties indicate that bipolar PNe are produced by more massive progenitors than PNe of the other morphological classes.

Chemically, bipolar nebulae belong to the class of type I PNe (Peimbert & Torres-Peimbert 1983) defined by having either He/H [FORMULA] or [FORMULA] (N/O) [FORMULA] -0.3. The most likely reason for this overabundance in He and/or N relative to the Sun and to most of the other PNe is that massive AGB stars ([FORMULA]   [FORMULA]  2  [FORMULA]) are expected be substantially He and/or N enriched at their surface, following second and third dredge-up episodes as well as a consequence of efficient H-burning at the base of the convective envelope (Renzini & Voli 1981). Modern calculations (Marigo et al. 1996) fail, however, to reproduce the He and N overabundances measured in several bipolar PNe, at least for the considered range of progenitor masses (0.7 [FORMULA] [FORMULA] [FORMULA][FORMULA]).

Although chemical abundances are available for many PNe, including bipolar PNe, these determinations are not all of the same accuracy (cf. Perinotto, 1991 for a critical compilation of abundances of galactic PNe). New, homogeneous, high quality chemical data are still needed to improve our knowledge on the subject. We have obtained deep long-slit spectra of about fifteen bipolar nebulae selected from the compilation of CS95. These data allow us to obtain an homogeneous set of chemical abundances for this morphological class of PNe, as well as to address the question of possible abundance variations inside the nebulae. This is an important topic, because material found at different positions in a PN could be the result of various mass loss episodes or trace the chemical inhomogeneities in the original mass outflow. In this paper, we discuss the results for the bipolar PN IC 4406, illustrating in detail observations, data reduction, and the analysis method. The results for the other nebulae will be presented in a forthcoming paper.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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