Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H II Region?

Details Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H II Region? Time Variation in G24.78+0.08 A1: Evidence for an Accreting Hypercompact H II Region?

2008-01-08

arxiv.org/abs/0801.1298v1

Astronomy and Astrophysics

Astrophysics

10 pages, 1 figure, accepted in ApJ Letters

Scientific paper

10.1086/528957

Over a timescale of a few years, an observed change in the optically thick radio continuum flux can indicate whether an unresolved H II region around a newly formed massive star is changing in size. In this Letter we report on a study of archival VLA observations of the hypercompact H II region G24.78+0.08 A1 that shows a decrease of ~ 45 % in the 6-cm flux over a five year period. Such a decrease indicates a contraction of ~ 25 % in the ionized radius and could be caused by an increase in the ionized gas density if the size of the H II region is determined by a balance between photoionization and recombination. This finding is not compatible with continuous expansion of the H II region after the end of accretion onto the ionizing star, but is consistent with the hypothesis of gravitational trapping and ionized accretion flows if the mass-accretion rate is not steady.

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