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

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
³ National Research Council of Canada, Herzberg Astronomy and Astrophysics Research Centre, 5071 West Saanich Road, Victoria, BC V9E 2E7, Canada
⁴ Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801, USA
⁵ Center for Interdisciplinary Exploration and Research in Astronomy, Northwestern University, 1800 Sherman Rd., Evanston, IL 60202, USA
⁶ NSF MPS-Fellow
⁷ Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA 02138, USA
⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, PO Box 94249, 1090 GE Amsterdam, The Netherlands

^★ Corresponding author: guerra@strw.leidenuniv.nl

Received: 16 June 2025
Accepted: 15 October 2025

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, such as the envelope, outflow, and nascent disk, to characterize and understand these young systems, 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. We present high-resolution (∼30 au) ALMA observations at 1.3 mm of the Class 0 protostellar system IRAS4A2. We analyzed the gas emission surrounding this young source, tracing it from the extended outflow to the most compact inner region and identifying emission lines using the spectral analysis tool CASSIS.

Results. We detect large, well-traced outflows in HCN (3–2), H₂CO (2₁₂–1₁₁), and HCO⁺ (3–2), along with numerous complex organic molecules (COMs) such as C₂H₃CN, CH₂(OH)CHO, CH₃OCHO, and CH₃C¹⁵N that trace central, more compact regions. These molecules span upper-energy levels (E_u) ranging from 20 to 487 K and have excitation temperatures between 100 and 300 K. 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 ∼8 au in radius, to extended ∼5000 au outflow emission. Specifically, we find that CH₃CDO and CH₃OCHO could both be good tracers of the disk, possibly tracing its rotation. Lines of OCS (22–21), SO₂ (13_{3 11}–13_{2 12}), HCN, H₂CO, and HCO⁺, which have lower upper-level energies, show more extended structures around IRAS4A2, likely tracing the envelope, disk, accretion shocks, the base of an outflow, and the outflow itself. Some of these molecular lines exhibit signatures consistent with Keplerian rotation, indicating a central protostellar mass of approximately 0.2 M_⊙.

Conclusions. Most COMs appear to trace distinct inner regions near the central protostar, while other molecules like OCS, SO₂, HCN, and H₂CO 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.