VLA observations of circumnebular neutral hydrogen in IC 418

Abstract

Neutral hydrogen images of the planetary nebula IC 418 have been made with the Very Large Array. These images show H I emission and absorption in close association with the nebula. Assuming a distance of 1 kpc, the total mass of circumnebular neutral hydrogen is 0.35 +- 0.05 M_sun. Model fits to the data indicate that the neutral gas falls as a r^-2 density distribution with outflow velocity about 5 km/s less than the expansion rate of the ionized gas. The observations also indicate that there is a region devoid of H I emission between the outer edge of the H II nebula and the inner edge of the H I shell. It is suggested that this gap is comprised of molecular hydrogen and that the surrounding H I shell is produced by photodissiociation of H_2 by the interstellar radiation field. Physical parameters of the H I gas are derived.