The Ionization Structure of Planetary Nebulae -- IX LUMINOUS FILAMENTS
Open Access
- 1 February 1972
- journal article
- Published by Oxford University Press (OUP) in Monthly Notices of the Royal Astronomical Society
- Vol. 156 (1) , 91-101
- https://doi.org/10.1093/mnras/156.1.91
Abstract
Recent observational and theoretical evidence indicates that density condensations which are optically thick to ionizing radiation exist in planetary nebulae. Such a condensation shields the gas in a column behind it from the stellar radiation field, so that the shielded gas is ionized only by the diffuse radiation from the neighbouring gas. Since stellar radiation is characterized by an effective temperature which may exceed 1050K and diffuse radiation is emitted by a gas which does not differ significantly from 1040K, the physical conditions in the shadowed and unshadowed regions are quite different. Although the shadow free gas can be very highly ionized, the shadow zone is composed predominantly of singly ionized atoms. Viewed in the light of lines from singly ionized atoms, the shadow appears as a luminous filament. The comet-like structures seen in NGC 7293 are suggested to be optically thick condensations (‘heads’) and the associated luminous shadows (‘tails’).Keywords
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