MODELING THE CHEMISTRY OF DENSE INTERSTELLAR CLOUDS .1. OBSERVATIONAL CONSTRAINTS FOR THE CHEMISTRY

A search for correlations arising from molecular line data has been made in order to place constraints on the chemical models of interstellar clouds. Observations of CO, H2CO, NH3, HC3N, HC5N, and several other species have been compiled. The column density of H2 has been obtained from determinations of Av. At 1021 H2 cm-2, N(CO) for dark clouds is a factor of 6 greater than the value for diffuse clouds. This result implies that the strength of the UV radiation field where CO shields itself from dissociation is approximately one-half the strength of the average Galactic field. Also, the dark cloud data indicate that the abundance of CO continues to increase with Av for directions with Av≤ 4 mag, although less steeply with N(H2) than is the case for diffuse clouds. For H2CO, a quadratic relationship is obtained in plots versus H2 column density. The data suggest a possible turnover at the highest values for Av (≥4-5 mag). Ammonia shows no correlation with H2, C18O, HC3N, or HC5N, in part because for dark clouds the column density of NH3 is nearly constant at 1015 cm-2. A strong correlation is found between HC5N and HC3N, indicating the chemical link between the cyanopolyynes. Possible trends are also exhibited in plots of the cyanopolyynes versus C4H, but the weak correspondences with C18O suggest that emission from the cyanopolyynes and C18O arises in separate regions.