Shadows behind Neutral Clumps in Photoionized Regions

Abstract

Shadows behind neutral clumps in photoionized regions can have a neutral core surrounded by gas that is photoionized by the di use ux produced by the nebula. We present a simple analytic model describing the conguration of such shadows. We also present numerical gasdynamic simulations of the relaxation to the nal, steady state. These models have clear applications (e.g., to the cometary knots in the Helix Nebula) but can also be applied in other astrophysical contexts.