ALMA-QUARKS: Few-thousand-year “Hatching out of the Egg:” The Supersonic Breakout of a Hypercompact H II Region from Its Parental Hot Core

Abstract The kinematic evolution of hypercompact (HC) H ii regions around young high-mass stars remains poorly understood due to complex interactions with parental environs. We present Querying Underlying Mechanisms of Massive Star Formation with Atacama Large Millimeter/submillimeter Array–Resolved Gas Kinematics and Structures (ALMA-QUARKS) 1.3 mm and ATOMS 3 mm observations (the highest resolution is ∼0.01 pc) of a deeply embedded HC H ii region (diameter ∼ 0.015 pc, electron density ∼ 2 × 10 5 cm −3 ) exhibiting a striking ≳20 km s −1 global redshift seen in optically thin H30 α and H40 α recombination lines relative to its parental hot molecular core within a hub–filament system. The 1.3 mm continuum data reveal a distinct 0.1 pc arc and a perpendicular 0.04 pc tail. We propose that this morphology arises from a dynamic champagne flow: the slow expansion of an HC H ii region into a preexisting filament forms an arc and associated low-velocity (few kilometers per second) SiO shocks. Meanwhile, in the opposite direction ionized gas escapes along a steep density gradient traced by a tail and high-velocity (20 km s −1 ) SiO emission. We reject the bow shock scenario in which ionized gas comoves with a runaway high-mass star because shocked gas in the arc aligns with the hub velocity, contradicting the bow shock prediction. Non-LTE radiative transfer modeling further rules out infall of ionized gas as the velocity shift origin. We conclude that this exceptional HC H ii region is undergoing a few-thousand-year transition phase of “hatching out of the egg:” the ionized gas of the HC H ii region has just broken out of its parental hot core and now is flowing outward supersonically. This work highlights how anisotropic density distributions induce supersonically anisotropic ionized flows that govern HC H ii region evolution.