CHEMICAL EVOLUTION IN THE CIRCUMSTELLAR STRUCTURE OF B5 IRS1

The B5 dark cloud has been identified as a site of low-mass star forma tion. We report a survey of a selection of the molecular species model led by the B5 dynamical and chemical model of Charnley et al. at the p ositions of circumstellar HCN clumps in B5 IRS1. All of the key specie s observed yield either abundances or upper limits to abundances below both the standard and the predicted values, appearing to show evidenc e of depletion and/or destruction if the transitions observed are ther malized. Our results are not in good agreement with the model, and the y bring into question the interpretation of the structure of B5 IRS1 p roposed by Fuller et al. It was expected that HCN clump C might exhibi t a higher excitation than HCN clump A, since it appeared to be locate d within the blueshifted molecular outflow. However, there is no signi ficant difference observed between the two clumps, suggesting that the near-infrared and optical nebulosity is evidence of a reflection nebu la rather than shocked material in the outflow. Finally, it is observe d that our results are more consistent with gas-grain models than with those models excluding gas-grain interaction.