Abstract
A probable evolutionary sequence of H II regions based on six distinct types of observed objects is suggested. Two examples which may deviate from this idealized sequence, are discussed. Even though a sizemean density relation for H II regions can be used as a rough indication of whether a nebula is very young or evolved, it is argued that such a relation is not likely to be useful for the quantitative assignment of ages to H II regions. Evolved H II regions appear to fit into one of four structural types: rings, core-halos, smooth structures, and irregular or filamentary structures. Examples of each type are given with their derived physical parameters. The energy balance in these nebulae is considered. The mass of ionized gas in evolved H II regions is in general too large to trace the nebula back to single compact H II regions. Finally, the morphological type of the Galaxy is considered from its H II region content.
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Churchwell, E. (1975). Evolved H II regions. In: Wilson, T.L., Downes, D. (eds) H II Regions and Related Topics. Lecture Notes in Physics, vol 42. Springer, Berlin, Heidelberg. https://doi.org/10.1007/3-540-07409-0_116
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DOI: https://doi.org/10.1007/3-540-07409-0_116
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