Dynamic Massive Star Formation: Radio Flux Variability in UCHII Regions

TL;DR

This study identifies and analyzes radio flux variability in ultracompact HII regions, revealing that such variability is more common and occurs earlier in the evolution of massive star formation than previously predicted.

Contribution

It provides the largest sample of variable UCHII regions and links radio variability to early evolutionary stages of massive star formation.

Findings

44% of UCHII regions show >30% flux variability over 8 years.

Variable regions are younger and more compact, often in hypercompact stages.

High variability correlates with optically thick, early-stage HII regions.

Abstract

Context: Theoretical models of early accretion during the formation process of massive stars have predicted that HII regions exhibit radio variability on timescales of decades. However, large-scale searches for such temporal variations with sufficient sensitivity have not yet been carried out. Aims: We aim to identify HII regions with variable radio wavelength fluxes and to investigate the properties of the identified objects, especially those with the highest level of variability. Methods: We compared the peak flux densities of 86 ultracompact HII (UCHII) regions measured by the GLOSTAR and CORNISH surveys and identified variables that show flux variations higher than 30% over ~8 yr timespan between these surveys. Results: We found a sample of 38 variable UCHII regions, which is the largest sample identified to date. The overall occurrence of variability is 44$\pm$5%, suggesting that variation in UCHII regions is significantly more common than prediction. The variable UCHII regions are found to be younger than non-variable UCHII regions, all of them meeting the size criterion of hypercompact (HC) HII regions. We studied the 7 UCHII regions (the ``Top7'') that show the highest variability with variations > 100%. The Top7 variable UCHII regions are optically thick at 4--8 GHz and compact, suggesting they are in a very early evolutionary stage of HCHII or UCHII regions. There is a significant correlation between variability and the spectral index of the radio emission. No dependence is observed between the variations and the properties of the sources' natal clumps traced by submillimeter continuum emission from dust, although variable HII regions are found in clumps at an earlier evolutionary stage.