ATOMS: ALMA three-millimetre observations of massive star-forming regions - XIV. Properties of resolved ultra-compact H?II regions

Hydrogen radio recombination lines (RRLs) are one of the major diagnostics of the physical properties of H II regions. We use RRL H40 alpha, He40 alpha, and 3 mm continuum emission to investigate the properties of a large sample of resolved UC H II regions identified in the ATOMS survey. In total, we identify 94 UC H II regions from H40 alpha emission. The basic parameters for these UC H II regions, such as electron density, emission measure, electron temperature, ionic abundance ratio (n(He+)/n(H+)), and line width are derived. The median electron density and the median n(He+)/n(H+ )ratio of these UC H II regions derived from RRLs are similar to 9000 cm(-3) and 0.11, respectively. Within UC H II regions, the n(He+)/n(H+) ratios derived from the intensity ratio of the He40 alpha and H40 alpha lines seems to be higher in the boundary region than in the centre. The H40 alpha line width is mainly broadened by thermal motion and microturbulence. The electron temperature of these UC H II regions has a median value of similar to 6700 K, and its dependence on galactocentric distance is weak.

Automated summary

Hydrogen radio recombination lines are used to study the properties of resolved UC H II regions. A large sample of these regions were investigated and it was found that their electron density, temperature, and abundance ratios are similar to the central star regions, while the helium abundance ratio is higher in the boundary region.