Ionization-bounded and Density-bounded Planetary Nebulae

TL;DR

This paper introduces an observational method using radio continuum images to distinguish between ionization-bounded and density-bounded planetary nebulae, aiding in accurate estimation of ionizing photon rates.

Contribution

It presents a simple criterion based on radio images to classify planetary nebulae as ionization- or density-bounded, with application to two specific nebulae.

Findings

NGC 7027 is likely ionization-bounded.

Hb 4 is density-bounded in some directions.

The criterion helps determine nebulae ionization status.

Abstract

Planetary nebulae (and in general any photoionized region) can be classified as ionization-bounded or density-bounded. It is important to determine in which case is the planetary nebula studied to be able to estimate from nebular observations, for example, the total rate of ionizing photons produced by the central star. In this paper we present a simple observational criterion that uses radio continuum images and that allows to establish a necessary (but not sufficient) condition for the planetary nebula to be considered as ionization-bounded. We apply the criterion to two planetary nebulae: NGC 7027 is most possibly ionization-bounded, while Hb~4 is density-bounded, at least in some directions.