Abstract
We present high-angular resolution observations, conducted with the Atacama Large Millimeter/Submillimeter Array (ALMA) in Band 6, of high-excitation molecular lines of CH3CN, CH3OH, and the H29α radio recombination line, towards the G333.6-0.2 ultracompact (UC) H ii region. Our observations reveal three hot molecular cores: A, B, and C, where emission is detected in ten components of the J=14→13 rotational ladder of CH3CN and in the CH3OH J=51,4→41,3 transition. Rotational diagram analysis of CH3CN reveals excitation temperatures ranging from 380 to 430 K. First-order moment maps of CH3CN and CH3OH reveal distinct velocity gradients in all cores, suggesting rotating structures, with core A also showing evidence of expansion motions. The H29α recombination line shows a linewidth of 30.2±0.12 km s−1, dominated by dynamical and thermal broadening, indicative of large-scale motions in ionized gas. Analysis of the ionized gas properties yields an electron density of (4.8±0.4)×105 cm−3, an emission measure of (1.23±0.06)×109 pc cm−6, and a Lyman continuum photon flux consistent with an O5–O6 V (Zero-Age Main Sequence; ZAMS) star. Our results suggest that G333.6-0.2 is in an intermediate evolutionary stage between hypercompact (HC) and ultracompact (UC) H ii regions, hosting active high-mass star formation with rotating hot cores and ionized gas dynamics.