The ionized heart of a molecular disk ALMA observations of the hyper-compact HII region G24.78+0.08 A1

Abstract

Context. Hyper-compact (HC) or ultra-compact H II 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 (≥8 M⊙) star formation.

Aims. We employed Atacama Large Millimeter/submillimeter Array (ALMA) 1.4 mm Cycle 6 observations to investigate at high angular resolution (≈0.′′050, corresponding to 330 au) the HC H II 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 H II region, at radii ≲500 au, 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 22 km s−1 mpc−1, which is much larger than the other’s, 3 km s−1 mpc−1. 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 (≈20 M⊙) responsible for the HC H II 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 20 M⊙ star.

Conclusions. Toward the HC H II region in G24.78+0.08, the coexistence of mass infall (at radii of ~5000 au), an outer molecular disk (from ≲4000 au to ≳500 au), and an inner ionized disk (≲500 au) 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 H II region, continues to accrete mass onto the ionizing star.