Molecular line emission from 1000 au scales outflows to <30 au compact structures in NGC1333 IRAS4A2

TL;DR

The study uses high-resolution ALMA observations at ≈1.3 mm to map molecular line emission around the Class 0 protostar IRAS4A2 from 1000 au-scale outflows down to compact <30 au structures. Through CASSIS LTE line identification and multi-scale imaging, the authors detect HCN, H₂CO, HCO⁺, and numerous COMs, revealing a chemically rich, spatially stratified environment with both large-scale outflows and a compact inner disk/envelope. Kinematic analyses, including Keplerian and rotating-infall models, indicate a central protostellar mass of ≈0.2 $M_igodot$ and show that different molecules trace multiple components (disk, envelope, shocks, outflow) simultaneously, complicating unambiguous origin assignments. The work also finds COMs such as CH₃OCHO, CH₃CDO, C₂H₃CN, and CH₃NCO in compact inner regions, suggesting that hot-corino chemistry is connected to hot-core-like chemistry in high-mass regions, with implications for the onset of disk formation and planet formation in the earliest stages.

Abstract

Aims. Studying protostellar objects in their earliest stages, particularly during the Class 0 phase, provides key insight into the beginnings of planet formation and dust evolution. Disentangling the various components, however, is particularly challenging. High spatial and spectral resolution observations of molecular line emission with the Atacama Large Millimeter/submillimeter Array (ALMA) are therefore crucial for probing their complex environments. Methods. In this work, we present high-resolution ($\sim$30 au) ALMA observations at 1.3 millimeters of the Class 0 protostellar system IRAS4A2. Results. We detected large, well-traced outflows in HCN (3-2), H$_{2}$CO$(2_{1~2}-1_{1~1})$, and HCO$^{+}$ (3-2), along with numerous complex organic molecules (COMs) tracing central, more compact regions. Using moment maps, we analyzed the kinematics and spatial distributions of the molecular emission, revealing a wide range of spatial scales, from compact structures within the IRAS4A2 core at $\sim$8 au in radius, to extended $\sim$5000 au outflow emission. Specifically, we find that CH$_{3}$CDO and CH$_{3}$OCHO could be both good tracers of the disk, possibly tracing its rotation. Lines of OCS (22-21), SO$_{2}~(13_{3~11}-13_{2~12})$, HCN, H$_{2}$CO, and HCO$^{+}$, show more extended structures around IRAS4A2, likely tracing the envelope, disk, accretion shocks, the base of an outflow, and the outflow itself. Conclusions. Most COMs appear to trace distinct inner regions near the central protostar, while other molecules trace more extended structures, such as the envelope or outflows. The kinematics, emission patterns, and position-velocity diagrams suggest that individual molecules trace multiple components simultaneously, making it challenging to disentangle their true origins. Altogether, these findings highlight the complex spatial distribution within the IRAS4A2 system. Abridged.

Figures (8)

• Figure 1: Position-velocity diagrams of $\rm SO_{2}$, $\rm OCS$, $\rm H_{2}CO$, and $\rm HCN$ extracted along a cut perpendicular to the jet axis. For $\rm{OCS}$ and $\rm{H_2CO}$, we fitted the emission using three kinematic models: pink lines represent the infall motion fit, blue lines correspond to the Keplerian rotation fit, and green lines indicate the rotating infalling envelope model. The labels show the mass of the central protostar derived from each fit, and the negative offset marks the north-west direction. For $\rm{SO_2}$ and $\rm{HCN}$, we overlay the model with the average of the fit parameters obtained from the models in $\rm{OCS}$ and $\rm{H_2CO}$ for comparison, as their emission is more complicated.
• Figure 2: Integrated intensities of the three representative molecular lines that were not blended with any other, showcasing the three visually identified regions they trace within the IRAS4A2 disk: $\rm C_{2}H_{3}CN$, $\rm CH_{3}CDO$ and $\rm CH_{3}OCHO$. The white contours show the 30 $\sigma$ to 60 $\sigma$ levels of the continuum emission, and the synthesized beams are indicated in the lower left corner of each image. The middle panels shows a P–V diagram of $\rm CH_{3}CDO$ and $\rm CH_{3}OCHO$, extracted along a cut perpendicular to the outflow axis. The green dashed curve represents the Keplerian rotation model from $\rm H_{2}CO$, while white contours indicate emission levels from 1.5 $\sigma$ to 3 $\sigma$, based on the Band 6 line data. The bottom panel shows the moment 1 (M1) map of $\rm CH_{3}OCHO$, indicating the velocity structure across the emitting region.
• Figure 3: Left panel: Deconvolved radii of the emission plotted as a function of excitation temperature for the confirmed detected molecules in IRAS4A2. The error bars represent the minimum and maximum radii observed for all molecular lines, while the markers indicate the median radius values. All molecules have at least one blended transition, except for methyl isocyanate, which is highlighted with a black hat on top of its marker to indicate the absence of blending. Right panel: The emission radius plotted against the upper state energy levels. Different colors and different figures represent emission from different molecules, as highlighted in the rightmost panel.
• Figure 4: Normalized molecular abundance ratios relative to methanol, for the species detected in this work. The values are compared to those observed in both high-mass star-forming regions (AFGL4176; 2019AA...628A...2B and SgrB2; 2017AA...601A..49B) and a low-mass star-forming region (IRAS16293B; 2016AA...595A.117J).
• Figure 5: CASSIS models of the spectral lines in IRAS4A2 (pink) are compared with the observed spectra (blue) at frequencies where line emission is detected. The molecules are labeled in black