Dynamic massive star formation: Radio flux variability in UC H II regions

Context. Theoretical models of early accretion during the formation process of massive stars have predicted that H II 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. Our aim is to identify H II 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 H II (UC H II ) regions measured by the GLOSTAR and CORNISH surveys and identified variables that show flux variations higher than 30% over the ~8 yr timespan between these surveys. Results. We found a sample of 38 variable UC H II regions, which is the largest sample identified to date. The overall occurrence of variability is 44±5%, suggesting that variation in UC H II regions is significantly more common than prediction. The variable UC H II regions are found to be younger than nonvariable UC H II regions, all of them meeting the size criterion of hypercompact (HC) H II regions. We studied the seven UC H II regions that show the highest variability (the “Top7”) with variations >100%. The Top7 variable UC H II regions are optically thick at 4–8 GHz and compact, suggesting they are in a very early evolutionary stage of HC H II or UC H II 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 H II regions are found in clumps at an earlier evolutionary stage.