2021A&A...650A.142M

Context. Hyper-compact (HC) or ultra-compact HII regions are the first manifestations of the radiation feedback from a newly born massive star. Therefore, their study is fundamental to understanding the process of massive (≥8M_☉) star formation.
Aims. We employed Atacama Large Millimeter/submillimeter Array (ALMA) 1.4mm Cycle 6 observations to investigate at high angular resolution (≃0.050", corresponding to 330au) the HC HII region inside molecular core A1 of the high-mass star-forming cluster G24.78+0.08.
Methods. We used the H30α emission and different molecular lines of CH₃CN and ¹³CH₃CN to study the kinematics of the ionized and molecular gas, respectively.
Results. At the center of the HC HII region, at radii ≤500au, we observe two mutually perpendicular velocity gradients, which are directed along the axes at PA=39° and PA=133°, respectively. The velocity gradient directed along the axis at PA=39° has an amplitude of 22km/s/mpc, which is much larger than the other's, 3km/s/mpc. We interpret these velocity gradients as rotation around, and expansion along, the axis at PA=39°. We propose a scenario where the H30α line traces the ionized heart of a disk-jet system that drives the formation of the massive star (≃20M_☉) responsible for the HC HII region. Such a scenario is also supported by the position-velocity plots of the CH₃CN and ¹³CH₃CN lines along the axis at PA=133°, which are consistent with Keplerian rotation around a 20M_☉ star.
Conclusions. Toward the HC HII region in G24.78+0.08, the coexistence of mass infall (at radii of ∼5000au), an outer molecular disk (from ≤4000au to ≥500au), and an inner ionized disk (≤500au) indicates that the massive ionizing star is still actively accreting from its parental molecular core. To our knowledge, this is the first example of a molecular disk around a high-mass forming star that, while becoming internally ionized after the onset of the HII region, continues to accrete mass onto the ionizing star.