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

• Published in: The Astrophysical journal Vol. 973; no. 2; pp. 143 - 157
• Main Authors: Neufeld, David A., Welty, Daniel E., Ivlev, Alexei V., Caselli, Paola, Edenhofer, Gordian, Indriolo, Nick, Obolentseva, Marta, Silsbee, Kedron, Sonnentrucker, Paule, Wolfire, Mark G.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01-10-2024; IOP Publishing
• Subjects: Absorption; Computation; Cosmic ray ionization; Cosmic rays; Diffuse molecular clouds; Electron transitions; Estimates; Excitation; Extinction; Galactic cosmic rays; Gas density; Interstellar chemistry; Interstellar clouds; Interstellar gas; Interstellar matter; Interstellar molecules; Ionization; Molecular clouds; Ultraviolet absorption
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ad7264

Newly computed collisional rate coefficients for the excitation of C2 in collisions with H2, presented recently by Najar & Kalugina, are significantly larger than the values adopted previously in models for the excitation of the C2 molecule, a widely used probe of the interstellar gas density. With these new rate coefficients, we have modeled the C2 rotational distributions inferred from visible and ultraviolet absorption observations of electronic transitions of C2 toward a collection of 46 nearby background sources. The inferred gas densities in the foreground interstellar clouds responsible for the observed C2 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 H3+ 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 H3+ abundance is a function of the ratio of the CRIR to the gas density.