(5) Shadows Behind Neutral Clumps in Photoionized Regions
Canto et al. (1998) studied the regions within photoionized nebulae
in which the ionizing radiation from the central source is blocked by an
intervening neutral, opaque clump - the shadow zones. The shadow gas can be
partially ionized by diffuse ionizing radiation from the unshadowed H II region
which surrounds it and is dynamically affected by the pressure of that
surrounding gas. An analytical model for the final, steady configuration of
the shadows was derived involving a neutral core surrounded by a layer which
is ionized by the diffuse radiation of the surrounding H II region. Time-
dependent, numerical gas dynamics simulations which include radiative transfer
and the hydrogen ionization rate equation were performed to demonstrate in
detail how the shadow gas relaxes to this final state, in which the ionized
shadow gas is denser and cooler than the surrounding H II region in pressure
balance with it, with the shadow's mass substantially increased by the
accretion of gas from the surrounding H II region. These results will have
broad astrophysical application, from circumstellar to interstellar to
intergalactic H II regions. They already serve to explain, for example, the
long-tailed clumps observed in Planetary Nebulae (e.g. Helix nebula) as an
effect of the enhanced emission rate of the shadow zones relative to the
surrounding H II region gas.