5. Discussion and conclusions
IC 4406 just meets the condition for Type I PNe of having He/H . On the other hand, its N/O abundance ratio is not strongly enhanced. According to its morphology (the equatorial waist is not very pronounced), kinematics (the polar expansion velocities are not very high), and chemistry (the He and N/O enrichment is modest), IC 4406 appears therefore to be a "moderate" bipolar PN.
Apart from the sulphur abundances, whose interpretation remains difficult, the results of the present study make IC 4406 a chemically homogeneous object. This is consistent with the idea that the nebula is the result of a single envelope ejection from the progenitor star, its elongated shape being the result of collimation processes in the chemically homogeneous, coeval ejecta.
Our abundance determinations can be compared with those from previous studies of IC 4406, at least for its central region. de Freitas Pacheco et al. (1992) found He/H=0.095, substantially lower than the values of 0.141 (Kaler 1978, 1983), 0.13-0.14 (Perinotto et al. 1994), and 0.126 (present work). Significant differences between de Freitas Pacheco et al. (1992) and our work are also found for oxygen, nitrogen, and argon, whose abundances are by a factor 1.5 higher in their study. Our abundances show better agreement with those of Kaler (1978, 1983) and Perinotto et al. (1994), although variations of up to 30 are the rule rather than the exception. These numbers give an idea of the actual uncertainty in the PNe abundances available in the literature.
Another point to be considered in the future, when a better understanding of the problem will be reached, is the question of whether the ionic abundances derived by the intense collisionally excited forbidden lines are really accurate. Recent studies (e.g. Liu et al., 1995) have in fact shown that there is a clear discrepancy between the chemical abundances for heavy ions (in particular O ) computed by collisionally excited lines and those from recombination lines, the latter ones being up to a factor of five higher. A full explanation of this discrepancy is not available yet, even taking into account the temperature fluctuations scheme of Peimbert (1967).
To our knowledge, the only other chemical studies of bipolar PNe with detailed spatial information are those by Guerrero (1995) on NGC 2440 and NGC 2818, and by Guerrero et al. (1996) on K 4-55. NGC 2818 appears to be similar to IC 4406 in showing constant abundances along its major axis. On the contrary, significant chemical variations are found in the various morphological components of NGC 2440 and K 4-55, which are interpreted as due to successive mass loss episodes of newly enriched material from their progenitors. These very different results illustrate the interest in pursuing this kind of spatially detailed chemical studies of bipolar PNe.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998