[Paper Review] The impact of shocks on the chemistry of molecular clouds: high resolution images of chemical differentiation along the NGC1333-IRAS2A outflow

J. K. Jørgensen, M. R. Hogerheijde+4 more|ArXiv.org|Nov 5, 2003

Astrophysics and Star Formation Studies52 citations

TL;DR

This study investigates shock-driven chemistry in the NGC1333-IRAS2A outflow using high-resolution interferometric and single-dish millimeter/submillimeter observations. It reveals that shock heating at a 15,000 AU interface between a protostellar outflow and a dense condensation triggers significant chemical differentiation, with CH₃OH, SiO, and sulfur-bearing species enhanced by 2–4 orders of magnitude in the shocked gas, while HCO⁺ is only detected post-shock, indicating grain mantle processing and selective molecule survival under high-velocity conditions.

ABSTRACT

This paper presents a detailed study of the chemistry in the outflow associated with the low-mass protostar NGC1333-IRAS2A down to 3" (650 AU) scales. Millimeter-wavelength aperture-synthesis observations from the OVRO and BIMA interferometers and (sub)millimeter single-dish observations from the Onsala 20m telescope and CSO are presented. The interaction of the highly collimated protostellar outflow with a molecular condensation ~15000 AU from the central protostar is clearly traced by molecular species such as HCN, SiO, SO, CS, and CH3OH. Especially SiO traces a narrow high velocity component at the interface between the outflow and the molecular condensation. Multi-transition single-dish observations are used to distinguish the chemistry of the shock from that of the molecular condensation and to address the physical conditions therein. Statistical equilibrium calculations reveal temperatures of 20 and 70 K for the quiescent and shocked components, respectively, and densities near 10^6 cm^{-3}. Significant abundance enhancements of two to four orders of magnitude are found in the shocked region for molecules such as CH3OH, SiO and the sulfur-bearing molecules. HCO+ is seen only in the aftermath of the shock consistent with models where it is destroyed through release of H2O from grain mantles in the shock. N2H+ shows narrow lines, not affected by the outflow but rather probing the ambient cloud. Differences in abundances of HCN, H2CO and CS are seen between different outflow regions and are suggested to be related to differences in the atomic carbon abundance. Compared to the warm inner parts of protostellar envelopes, higher abundances of in particular CH3OH and SiO are found in the outflows, which may be related to density differences between the regions.

Motivation & Objective

• To investigate the impact of protostellar outflows on molecular cloud chemistry through high-resolution spatial and spectral analysis.
• To disentangle shock-induced chemistry from quiescent envelope chemistry in low-mass protostellar environments.
• To determine physical conditions (temperature, density) and chemical abundances in shocked and ambient gas components.
• To compare molecular abundance patterns in NGC1333-IRAS2A with other outflow regions and protostellar envelopes to assess environmental and dynamical influences.
• To explore the role of atomic carbon abundance and density in shaping observed chemical differences across outflows.

Proposed method

• Combined aperture-synthesis interferometry from Owens Valley and BIMA interferometers to resolve spatial structure at 3″ (650 AU) scales.
• Multi-transition single-dish line observations from Onsala Space Observatory and Caltech Submillimeter Observatory to probe excitation and physical conditions.
• Statistical equilibrium modeling of molecular line profiles to derive temperature and density for quiescent and shocked gas components.
• Comparison of line profiles across transitions and instruments to assess homogeneity of physical conditions within each component.
• Analysis of abundance enhancements in shock-tracing molecules (e.g., SiO, CH₃OH) relative to quiescent species (e.g., HCO⁺, N₂H⁺) to infer shock chemistry mechanisms.
• Use of N₂H⁺ as a tracer of ambient, non-shocked gas due to its narrow lines and lack of enhancement in outflowing material.

Experimental results

Research questions

• RQ1 How do shock processes alter molecular abundances in the outflowing gas compared to the ambient molecular cloud?
• RQ2 What are the physical conditions (temperature, density) in the shocked and quiescent components of the NGC1333-IRAS2A outflow?
• RQ3 Why are CH₃OH and SiO significantly enhanced in the shocked region while HCO⁺ is only detected in the aftermath of the shock?
• RQ4 How do the chemical abundances in NGC1333-IRAS2A compare to those in other outflow regions like L1157, and what might explain the differences?
• RQ5 To what extent do differences in atomic carbon abundance or local density affect the observed molecular abundance patterns in outflows?

Key findings

• The shocked gas has a temperature of ~70 K and a density of ~2×10⁶ cm⁻³, while the quiescent component is colder (~20 K) and less dense (~1×10⁶ cm⁻³).
• CH₃OH abundance is enhanced by up to ~5% of the CO abundance in the shocked region, representing a 2–4 order of magnitude increase compared to quiescent gas.
• SiO traces a narrow high-velocity component at the shock interface, indicating efficient shock-induced release of volatile species from dust grains.
• HCO⁺ is only detected in the post-shock region, consistent with models where it is destroyed by H₂O released from grain mantles during shock heating.
• N₂H⁺ shows narrow lines and is unaffected by the outflow, confirming it traces the ambient, non-shocked cloud material.
• Abundances of HCN, H₂CO, and CS in NGC1333-IRAS2A are lower than in the L1157 outflow, possibly due to differences in atomic carbon abundance in the pre-shocked gas.