The Densities in Diffuse and Translucent Molecular Clouds: Estimates from Observations of C2 and from Three-dimensional Extinction Maps

Newly computed collisional rate coefficients for the excitation of C _2 in collisions with H _2 , presented recently by Najar & Kalugina, are significantly larger than the values adopted previously in models for the excitation of the C _2 molecule, a widely used probe of the interstellar gas density. With these new rate coefficients, we have modeled the C _2 rotational distributions inferred from visible and ultraviolet absorption observations of electronic transitions of C _2 toward a collection of 46 nearby background sources. The inferred gas densities in the foreground interstellar clouds responsible for the observed C _2 absorption are a factor 4–7 smaller than those inferred previously, a direct reflection of the larger collisional rate coefficients computed by Najar & Kalugina. These lower-density estimates are generally in good agreement with the peak densities inferred from 3D extinction maps for the relevant sight lines. In cases where ${{\rm{H}}}_{3}^{+}$ absorption has also been observed and used to estimate the cosmic-ray ionization rate (CRIR), our estimates of the latter will also decrease accordingly because the ${{\rm{H}}}_{3}^{+}$ abundance is a function of the ratio of the CRIR to the gas density.