The ALMA-ATOMS-QUARKS survey: Resolving a chemically rich massive protostellar outflow

TL;DR

This study uses ALMA ATOMS-QUARKS data to dissect the chemistry of a chemically rich massive protostellar outflow in IRAS 16272-4837 (SDC335). By mapping 35 transitions from 22 molecules across 1.3 mm and 3 mm bands, it reveals a dual kinematic structure: a high-velocity jet along the axis and a low-velocity cavity, with knots showing strong chemical activity. The authors classify molecular emission into three origins—entrainment from the core, shock-induced release from dust grains, and thermal excitation by shock heating—demonstrating that outflows act as multi-origin chemical factories during high-mass star formation. The work provides concrete observational constraints on shock chemistry, ice mantle processing, and gas-phase reactions in massive protostellar environments, informing models of chemical evolution in early stellar evolution stages.

Abstract

We present a comprehensive study on the physical and chemical structures of a chemically rich bipolar outflow in a high-mass star forming region IRAS 16272$-$4837 (SDC335), utilizing high-resolution spectral line data at 1.3 mm and 3 mm dual-bands from the ALMA ATOMS and QUARKS surveys. The high-velocity jet is enveloped by a lower-velocity outflow cavity, containing bright knots that show enhanced molecular intensities and elevated excitation temperatures. Along the outflow, we have identified 35 transitions from 22 molecular species. By analyzing the spatial distribution and kinematics of these molecular lines, we find that the molecular inventory in the outflow is regulated by three processes: (i) direct entrainment from the natal molecular core by the outflow; (ii) shock-induced release of molecules or atoms from dust grains; and (iii) thermal desorption and gas-phase reactions driven by shock heating. These results confirm that outflows are not only dynamical structures but also active chemical factories, where entrainment, shocks, and thermal processing jointly enrich the molecular content. Our findings confirmed that outflow chemistry has multi-origin nature, and provide critical insights into chemical evolution during high-mass star formation.

Figures (25)

• Figure 1: (a) CO (2--1) RGB image constructed using the ACA+TM1+TM2 combined data. Blue- and red-shifted components are integrated over velocities of $-42$ to $-12$ and $+12$ to $+42~\mathrm{km\,s^{-1}}$ relative to the systemic velocity, while the green channel traces emission within $\pm12~\mathrm{km\,s^{-1}}$. The cyan line marks the jet axis. Hot cores C1--C3 and several emission knots are visible. (b) Zoom-in of the central region outlined in (a), with 3 mm continuum in colorscale and 1.3 mm contours highlighting compact sources. Continuum contours start at 21 mJy beam$^{-1}$ and increase in steps of a factor of 1.618 (golden ratio). The beams of 3 mm (white) and 1.3 mm (grey) are shown in the lower-right corner.
• Figure 2: Integrated intensity maps of selected molecular transitions detected in SDC335. Red/Blue contours: red-shifted and blue-shifted components, with velocity ranges labeled in each panel. Background: emission integrated over the systemic velocity gap between the blue- and red-shifted ranges. Contours are drawn at 10 levels logarithmically spaced from 3$\sigma$ to the map maximum. The typical 1$\sigma$ rms in the integrated-intensity maps is 2.6 K km s$^{-1}$ for the 3 mm data and 0.7 K km s$^{-1}$ for the 1.3 mm data. Each panel lists the corresponding transition and upper energy level.
• Figure 3: PVDs extracted along the jet axis of SDC335. The background color scale in each panel shows SiO emission, tracing the full spatial and velocity extent of the outflow. Overlaid contours correspond to representative transitions of other molecules, as labeled in each panel. Negative offsets indicate the southern (blue-shifted) lobe, and positive offsets the northern (red-shifted) lobe, with 0$"$ marking the C1.
• Figure 4: (a) PCA of molecular emission along the jet axis (data within $\pm2^{\prime\prime}$ of C1 excluded). Molecules are separated into four quadrants reflecting dominant chemical–kinematic behavior: Quadrant I: COMs enhanced in shocked regions; Quadrant II: outflow tracers with low- and high-velocity components; Quadrant III: shock/high-velocity tracers; Quadrant IV: systemic-velocity or low S/N lines. (b) Schematic view of the physical and chemical structure of the bipolar outflow in SDC335. The main panel shows the spatial distribution of key molecular tracers associated with the hot core, warm envelope, and jet-driven outflows. The upper inset illustrates post-shock low-velocity outflows heating the gas and sublimating molecules from icy mantles. The lower inset depicts strong shocks destroying dust grains, releasing ice-phase material and driving Si atoms from grain cores into the gas phase to form SiO.
• Figure 5: Kendall rank correlation coefficients between integrated intensities of molecular lines, shown as a lower-triangular matrix to avoid duplication.