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

Abstract Newly computed collisional rate coefficients for the excitation of C 2 in collisions with H 2 , presented recently by Najar &amp; 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 &amp; 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> </mml:mrow> </mml:msubsup> </mml:math> abundance is a function of the ratio of the CRIR to the gas density.