A spectroscopic study of the bipolar planetary nebula Mz 3

We have obtained a medium-resolution, deep optical long-slit spectrum of the bipolar planetary nebula Mz 3. The spectrum covers the wavelength range 3420-7400Å. Over 200 emission lines have been detected, many of them permitted and forbidden transitions from Fe⁺, Fe²⁺ and Fe³⁺ and other iron-group elements. The spectra have been used to determine nebular thermal and density structures and elemental abundances. The very rich and prominent [FeIII] emission lines observed in the optical spectrum of Mz 3 are found to originate exclusively from an unresolved emission region centred on the central star. The relative intensities of [FeIII] lines arising from the same upper level are in good agreement with the theoretical predictions. The [FeIII] lines detected in Mz 3, arising from levels of different excitation energies and critical densities, provide powerful diagnostic tools to probe the physical conditions in the central emitting region. We find that all the observed [FeIII] diagnostic line ratios consistently yield an electron temperature of T_e= 11 000 K and a density of logN_e (cm^-3) = 6.5. The latter value is close to the densities where the ratios of these [FeIII] diagnostic lines are most sensitive to density variations, suggesting that the density in the central emission core could be even higher. In contrast, all the other standard nebular density- and temperature-diagnostic line ratios, all of lower critical densities than the [FeIII] lines and therefore only useful at densities <=10⁶ cm^-3, yield consistently lower electron densities, with the resultant values correlating with their critical densities. This indicates that the central dense emission core has a highly stratified density structure such that forbidden lines of relatively low critical densities are collisionally suppressed in high-density regions. Given the highly stratified thermal and density structures of the core emission region, no reliable elemental abundances can be determined for this region, except possibly for iron, for which we find an abundance of approximately half the solar value. In contrast to the dense central core, emission from the extended bipolar lobes of Mz 3 are well represented by a mean electron temperature of 6800 K and density of 5200 cm^-3. The elemental abundances derived for this region, on a logarithmic scale where H = 12, are He = 10.93, C = 8.41, N = 8.37, O = 8.50, Ne = 7.64, S = 7.15, Cl = 5.28 and Ar = 7.14. In all cases, the abundances are close to the average values deduced for Galactic planetary nebulae. In particular, there is no evidence of He enrichment, as claimed in previous studies. The newly derived heavy-element abundances are significantly higher than those published in the literature. In those earlier studies, as a result of the contamination of emission from the dense central core, the average electron temperature in Mz 3 was significantly overestimated, resulting in grossly underestimated heavy-element abundances. However, Mz 3 does seem to have a relatively high N/O abundance ratio, in line with the result derived from the ISO observations of the far-infrared fine-structure lines.