Abundances and rotational temperatures of the C2 interstellar molecule towards six reddened early-type stars

TL;DR

This study measures the abundance and rotational temperatures of the interstellar C2 molecule along six lines of sight, providing insights into the physical conditions of molecular clouds using high-resolution spectra.

Contribution

It presents new measurements of C2 column densities and excitation temperatures for six Galactic sightlines, including identification of the (4, 0) Phillips system in one case.

Findings

Determined C2 column densities for six lines of sight.

Derived excitation temperatures and physical parameters of molecular clouds.

Identified C2 (4, 0) Phillips system towards HD 147889.

Abstract

Using high-resolution (~85000) and high signal-to-noise ratio (~200) optical spectra acquired with the European Southern Observatory Ultraviolet and Visual Echelle Spectrograph, we have determined the interstellar column densities of C2 for six Galactic lines of sight with E(B- V) ranging from 0.33 to 1.03. For our purposes, we identified and measured absorption lines belonging to the (1, 0), (2, 0) and (3, 0) Phillips bands A1{\Pi}u-X1{\Sigma}+g. We report on the identification of a few lines of the C2 (4, 0) Phillips system towards HD 147889. The curve-of-growth method is applied to the equivalent widths to determine the column densities of the individual rotational levels of C2. The excitation temperature is extracted from the rotational diagrams. The physical parameters of the intervening molecular clouds (e.g. gas kinetic temperatures and densities of collision partners) were estimated by comparison with the theoretical model of excitation of C2.