California and Orion A clouds emission lines

CO intensities vary weakly with column density, while the intensity of traditional dense-gas tracers such as HCN, CS, and HCO^+^ varies almost linearly with column density. N_2_H^+^ differs from all other species in that it traces only cold dense gas. The intensity of the rare HCN and CS isotopologs reveals additional temperature-dependent abundance variations.

Overall, the clouds have similar chemical compositions that, as the depth increases, are sequentially dominated by photodissociation, gas-phase reactions, molecular freeze-out, and stellar feedback in the densest parts of Orion A. Our observations also allowed us to calculate line luminosities for each cloud, and a comparison with literature values shows good agreement. We used our HCN(1-0) data to explore the behavior of the HCN conversion factor, finding that it is dominated by the emission from the outermost cloud layers.

It also depends strongly on the gas kinetic temperature. Finally, we show that the HCN/CO ratio provides a gas volume density estimate, and that its correlation with the column density resembles that found in extragalactic observations.