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

Abstract

The study of hyper-compact (HC) or ultra-compact (UC) HII regions is fundamental to understanding the process of massive (> 8 M_sun) star formation. 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 HII region inside molecular core A1 of the high-mass star-forming cluster G24.78+0.08. We used the H30alpha emission and different molecular lines of CH3CN and 13CH3CN to study the kinematics of the ionized and molecular gas, respectively. At the center of the HC HII region, at radii <~500 au, we observe two mutually perpendicular velocity gradients, which are directed along the axes at PA = 39 deg and PA = 133 deg, respectively. The velocity gradient directed along the axis at PA = 39 deg has an amplitude of 22 km/s mpc^(-1), which is much larger than the other's, 3 km/s mpc^(-1). We interpret these velocity gradients as rotation around, and expansion along, the axis at PA = 39 deg. We propose a scenario where the H30alpha line traces the ionized heart of a disk-jet system that drives the formation of the massive star (~20 M_sun) 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 deg, which are consistent with Keplerian rotation around a 20 M_sun star. Toward the HC HII 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 HII region, continues to accrete mass onto the ionizing star.