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

TL;DR

This study reports a significant decrease in radio flux from a hypercompact H II region over five years, suggesting contraction likely due to variable accretion processes rather than continuous expansion.

Contribution

It provides observational evidence of flux variation in a hypercompact H II region, supporting models of ionized accretion flows and gravitational trapping.

Findings

45% decrease in 6-cm flux over five years

Approximate 25% contraction in ionized radius

Supports variable accretion and gravitational trapping models

Abstract

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.