Observational constraints for the chemistry of dark clouds

A search for correlations arising from molecular line data been made in order to place constraints on the chemical models of dense interstellar clouds. Observations of CO, H2CO, NH3, HC3N, and HC5N have been compiled. The results for CO place constraints on the importance of self-shielding and indicate that the CO abundance increases with AV for directions with AV ≤ 4 mag. For H2CO, a quadratic relationship is obtained in plots versus H2 column density. This result suggests that 6 cm measurements of H2CO sample the outer regions of dark clouds, where photodissociation is still important. 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