ALMA-QUARKS: Few-Thousand-Year Hatching out of “Egg”: The Supersonic Breakout of a Hypercompact Hii Region from Its Parental Hot Core

The kinematic evolution of hypercompact Hii (HC Hii) regions around young high-mass stars remains poorly understood due to complex interactions with parental environs. We present ALMA QUARKS/ATOMS 1.3 mm/3 mm observations (the highest resolution ∼ 0.01 pc) of a deeply embedded HC Hii region (diameter ∼ 0.015 pc, electron density ∼ 2 × 105 cm−3) exhibiting a striking ≳ 20 km s−1 global redshift seen in optically thin H30α/H40α 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 HC Hii region into a pre-existing filament forms the arc and associated low-velocity (few km s−1) SiO shocks. Meanwhile, in the opposite direction ionized gas escapes along a steep density gradient traced by the tail and high-velocity (20 km s−1) SiO emission. We reject the bow shock scenario in which ionized gas co-moves 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 Hii region is undergoing a few-thousand-year transition phase of “hatching out of the egg”: the ionized gas of HC Hii region has just broken out of its parental hot core