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

¹ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
² Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing, 100101, PR China
³ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁴ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
⁵ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Purple Mountain Observatory, and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, China
⁷ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
⁸ Department of Astronomy, Tsinghua Univerisity, 30 Shuangqing Road, Beijing 100084, China
⁹ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA

^★ Corresponding authors; yangay@nao.cas.cn; M.A.Thompson@leeds.ac.uk; J.S.Urquhart@kent.ac.uk

Received: 1 September 2024
Accepted: 22 October 2024

Abstract

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.