Radio Continuum and Recombination Line Observations of the Polypolar Planetary Nebula NGC 2440

Abstract

H92α and He92α recombination lines and λ3.6 cm continuum observations have been carried out with the Very Large Array (synthesized beam 68 × 42, PA 74°) toward the polypolar planetary nebula NGC 2440. The H92α and He92α lines have been detected, for the first time in this nebula, over a region of θ14'' around its geometric center. The λ3.6 cm continuum emission is detected over a region of 65'' × 40'', similar in extent to the optical emission. Physical parameters of the nebula have been derived for the region in which the recombination lines were detected. From the line to continuum ratio, an electron temperature of 16,000 K and a single ionized helium abundance Y+0.14 are obtained. A relative extinction map of NGC 2440 has also been obtained from the ratio of the λ3.6 cm continuum and Hα images. A higher relative extinction in the central region traces the dense and dusty environment of an apparent central toroid. The velocity field obtained from the H92α line suggests that the central toroidal structure has a distorted, noncircular shape. The polypolar characteristics of this nebula are discussed and compared with those of similar objects.