The impact of shocks on the chemistry of molecular clouds. High resolution images of chemical differentiation along the NGC 1333-IRAS 2A outflow

This paper presents a detailed study of the chemistry in the outflow associated with the low-mass protostar NGC 1333-IRAS 2A down to 3\arcsec (650 AU) scales. Millimeter-wavelength aperture-synthesis observations from the Owens Valley and Berkeley-Illinois-Maryland-Association interferometers and (sub)millimeter single-dish observations from the Onsala Space Observatory 20 m telescope and Caltech Submillimeter Observatory are presented. The interaction of the highly collimated protostellar outflow with a molecular condensation ∼15 000 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 10⁶ cm^-3. The line-profiles of low- and high-excitation lines are remarkably similar, indicating that the physical properties are quite homogeneous within each component. 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. The overall molecular inventory is compared to other outflow regions and protostellar environments. 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.