SIMBAD references

We present intermediate resolution long-slit spectra and narrowband Hα, [N II], and [O III] images of PM 1-242, PM 318, and PM 1-333, 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. PM 1-242 is a medium- to high-excitation (e.g., He IIλ4686/Hβ ∼ 0.4; [N II]λ6584/Hα ∼ 0.3) planetary nebula with an elliptical shape containing [N II] enhanced point-symmetric arcs. An electron temperature [T_e([S III])] of ∼ 10250 K and an electron density [N_e([S II])] of ∼ 2300/cm3 are derived for PM 1-242. Abundance calculations suggest a large helium abundance (He/H ∼ 0.29) in PM 1-242. PM 1-318 is a high-excitation (He IIλ4686/Hβ ∼ 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 [O III]. PM 1-333 is an extended planetary nebula with a high-excitation (He IIλ4686/Hβ up to ∼ 0.9) patchy circular main body containing two low-excitation knotty arcs. A low N_e([S II]) of ∼ 450/cm3 and T_e([O III]) of ∼ 15000 K are derived for this nebula. Abundance calculations suggest that PM 1-333 is a type I planetary nebula. The lack of a sharp shell morphology, low electron density, and high excitation strongly suggest that PM 1-333 is an evolved planetary nebula. PM 1-333 also shows two low-ionization polar structures whose morphology and emission properties are reminiscent of collimated outflows. We compare PM 1-333 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.