Spectroscopic confirmation of the planetary nebula nature of PM1-242, PM1-318 and PM1-333 and morphological analysis of the nebulae

Abstract

We present intermediate resolution long-slit spectra and narrow-band Halpha, [NII] and [OIII] images of PM1-242, PM318 and PM1-333, three IRAS sources classified as possible planetary nebulae. The spectra show that the three objects are true planetary nebulae and allow us to study their physical properties; the images provide a detailed view of their morphology. PM1-242 is a medium-to-high-excitation (e.g., HeII4686/Hbeta ~0.4; [NII]6584/Halpha ~0.3) planetary nebula with an elliptical shape containing [NII] enhanced point-symmetric arcs. An electron temperature [Te([SIII])] of ~10250 K and an electron density [Ne([SII])] of ~2300 cm-3 are derived for PM1-242. Abundance calculations suggest a large helium abundance (He/H ~0.29) in PM1-242. PM1-318 is a high-excitation (HeII4686/Hbeta ~1) planetary nebula with a ring-like inner shell containing two enhanced opposite regions, surrounded by a fainter round attached shell brighter in the light of [OIII]. PM1-333 is an extended planetary nebula with a high-excitation (HeII4686/Hbeta up to ~0.9) patchy circular main body containing two low-excitation knotty arcs. A low Ne([SII]) of ~450 cm-3 and Te([OIII]) of ~15000 K are derived for this nebula. Abundance calculations suggest that PM1-333 is a type I planetary nebula. The lack of a sharp shell morphology, low electron density, and high-excitation strongly suggest that PM1-333 is an evolved planetary nebula. PM1-333 also shows two low-ionization polar structures whose morphology and emission properties are reminiscent of collimated outflows. We compare PM1-333 with other evolved planetary nebulae with collimated outflows and find that outflows among evolved planetary nebulae exhibit a large variety of properties, in accordance with these observed in younger planetary nebula.