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Recent radio continuum observations of AFGL 618 have shown that the free-free emission flux density has increased by approximately a factor of two over a 2-3 year interval, whereas the spectrum has remained optically thick up to at least 12 GHz. This is interpreted as the result of expansion of a compact H II region within the molecular/dust envelope of AFGL 618. The central star of the nebula is likely to be a 1 solar-mass star now rapidly evolving into the planetary nebula stage.
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Kwok et al. (1981) studied this question.
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