ALMA-QUARKS: Few-Thousand-Year Hatching out of "Egg": The Supersonic Breakout of a Hypercompact H II Region from Its Parental Hot Core

TL;DR

This study probes the kinematics of a deeply embedded hypercompact H II region in a hub–filament system using ALMA-QUARKS/ATOMS data at ~0.01 pc resolution. By deblending mm recombination lines and modeling both ionized and molecular gas, the authors rule out infall and bow-shock scenarios and identify a champagne-flow breakout as the driver of the observed global redshift and arc–tail morphology. They interpret I19095 as a rare, few-thousand-year hatching-out-of-the-egg phase in which ionized gas escapes along density gradients, producing anisotropic supersonic flows and associated SiO shocks. The findings emphasize how anisotropic density distributions shape HC H II region evolution and offer a framework for recognizing rapid breakout phases in massive star formation, albeit with a brief observable window. Future high-resolution, long-baseline ALMA studies will be needed to map the inner ionized and molecular structure at au scales during this transient phase.

Abstract

The kinematic evolution of hypercompact H II (HC H II) 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 $\sim0.01$ pc) of a deeply embedded HC H II region (diameter $\sim0.015$ pc, electron density $\sim2\times10^{5}$ cm$^{-3}$) exhibiting a striking $\gtrsim20$ km s$^{-1}$ global redshift seen in optically thin H30$α$/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 HC H II 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 H II region is undergoing a few-thousand-year transition phase of "hatching out of the egg": the ionized gas of 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.

Figures (14)

• Figure 1: Velocity shifts between mm RRLs and molecular lines in high-mass star-forming molecular clumps. Vertical dashed line highlights the shift of 20 km s$^{-1}$. (a) Single-dish survey of 976 ATLASGAL clumps Kim2017. (b) Velocity shifts for 91 embedded HC and UC H ii regions, identified across 146 ATOMS molecular clumps ATOMSIII. (c) Size (radius)-velocity shift relation for 91 ATOMS HC or UC H ii regions, highlighting compactness and extreme kinematics of I19095.
• Figure 2: ATOMS view of the I19095 hub-filament system and the two embedded 3 mm cores. (a) ATOMS H$^{13}$CO$^+$$J=1-0$ 0th moment map ($3\sigma$ channel threshold), overlaid with ATOMS 3 mm contours (levels: 0.72–222 mJy beam$^{-1}$ in 5 logarithmic steps) and filament skeletons (F1–F5) color-coded by their $\rm v_{lsr}$. (b) ATOMS 3 mm continuum emission, with ellipses denoting Gaussian FWHM sizes for the cores, crosses making the core centers, boxes indicating zoom-in regions shown in Fig. \ref{['FIGURE:ATOMS-HII']}, and circles marking the fields of view of ATOMS and QUARKS.
• Figure 3: ATOMS 3 mm continuum and H40$\alpha$ line emission of the Main Core (first row) and the East Core (second row). (a1) 3 mm continuum. (a2) H40$\alpha$ 0th moment map (a3) H40$\alpha$ velocity field (1st moment). (a4) Source-averaged H40$\alpha$ (red) and H$^{13}$CO$^+$ (blue) spectra. Panels (b1)–(b4) follow the same order as (a1)–(a4). Contours (levels: 0.72–222 mJy beam$^{-1}$ in 10 logarithmic steps) trace the 3-mm continuum emission. Ellipses denote Gaussian FWHM sizes. All moment maps are generated using $3\sigma$ channel threshold.
• Figure 4: QUARKS images of Main Core and its associated structures. (a) QUARKS 1.3 mm continuum overlaid with ATOMS 3 mm contours (levels identical to Fig. \ref{['FIGURE:ATOMS-HII']}) and other extracted 1.3 mm cores (crosses; Jiao et al. in prep.). The extended dusty structures are highlighted with red, green and yellow dashed/dotted lines. (b) QUARKS 1.3 mm continuum after subtracting the CASA-fitted Main Core emission. Associated contour levels are from $6\sigma$ to 17.9 mJy beam$^{-1}$ in 10 logarithmically spaced steps. (c) Zoomed-in view of Main Core in 1.3 mm continuum with contours (levels: $6\sigma$ to 0.18 Jy beam$^{-1}$ in 10 logarithmically spaced steps). The separation between Main Core and arc vertex is indicated with white scalebar. (d) 0th moment map of C$_2$H$_5$CN $J=26-25$ overlaid with 1.3 mm contours, tracing hot molecular gas.
• Figure 5: XCLASS-deblended H30$\alpha$. (a) EM, (b) centroid velocity, and (c) line width. (d, e) source-averaged spectra: observed (black) and modeled (red) H30$\alpha$, compared with C$^{18}$O, C$_{2}$H$_{5}$CN, and SiO. The observed and modeled H30$\alpha$ spectra both include the molecular line emission. (f) SED fitted with uniform (gray) and power-law (black) density models. Archival VLA data (7 mm–6 cm; Appendix \ref{['APP:SED']}) and ATOMS 3 mm/QUARKS 1.3 mm data are shown as black and gray dots, respectively, with sizes indicating the beam size of observations.