Sulphur chemistry and molecular shocks: The case of NGC 1333-IRAS 2

Abstract

We present SO and SO2 observations in the region of the low mass protostar IRAS2/NGC1333. The East-West outflow originating from this source has been mapped in four transitions of both SO and SO2. In addition, CS observations published in the literature have been used. We compute the SO, SO2 and CS column densities and the physical conditions at several positions of the outflow using LTE and a non-LTE LVG approximations. The SO2/SO and CS/SO abundance ratios are compared with the theoretical predictions of a chemical model adapted to the physical conditions in the IRAS2 outflow. The SO2/SO abundance ratios are constant in the two lobes of the outflow whereas CS/SO is up to 6 times lower in the shocked gas of the East lobe than in the West one. The comparison with the chemical model allows us to constrain the age of the outflow produced by IRAS2 to >5e3yr. We find low densities and temperatures for the outflow of IRAS2 (< 1e6 cm-3 and >70 K) from SO and SO2 emission probably because the two molecules trace the cooled entrained material. The East lobe of the outflow shows denser gas compared to the West lobe. We also discuss some constraints on the depleted form of sulphur.