Spatial distribution of near-infrared and optical emission properties in the bipolar nebula Menzel 3

Visiting Astronomer at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. Ground-based optical spectra combined with near-infrared spectra and images of the young bipolar planetary nebula Menzel 3 (Mz 3) reveal positional variations in extinction, excitation, density and other characteristics. Interstellar extinction is probably less than A_v = 2.65, while extinction toward the nucleus is approximately 4.6 mag. The lobes show stratified ionization, with high-excitation emission localized at high latitudes. Quantitative analysis using the CLOUDY spectral synthesis code suggests that the polar lobes 'see' an excitation source similar to a blackbody of roughly 36 000 K and 10 000 L_⊙, and chemical abundances reveal that the ejecta have He enhanced by a factor of ∼2, and N enhanced more strongly, with N/O∼1. The lobes are probably radiatively excited, but shocks may heat the 'blisters' at the polar axis. The bright, unresolved nucleus has a crowded emission spectrum distinct from the diffuse bipolar lobes, and it indicates high electron densities of 10⁶-10⁷ cm^-3. An equatorial disc-like geometry for dense gas in the nucleus is likely. The nucleus also shows a nearly power-law continuum; when corrected for reddening it may require two stars, one hot star and one cool giant, along with hot ∼900-K dust. A distance up to ∼2.5 kpc is possible. In general, the spectral characteristics of Mz 3 are similar to those of the well-studied nebula M 2-9, and the two objects may share a similar evolutionary history. However, an important difference between them is that Mz 3 appears to be mostly devoid of molecular hydrogen, while infrared H₂ lines are conspicuous in M 2-9.