The ionized heart of a molecular disk. ALMA observations of the hyper-compact HII region G24.78+0.08 A1
2021
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 ~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.
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