The EDIBLES survey. VII. A survey of C2 and C3 in interstellar clouds

Vol. 681
6. Interstellar and circumstellar matter

The EDIBLES survey. VII. A survey of C2 and C3 in interstellar clouds

This remarkable study presents a comprehensive analysis of the two simplest carbon molecules, C2 and C3, along several dozen Galactic sight lines. The EDIBLES survey combines high spectral resolution with a high signal-to-noise ratio of typically 1000 per target. This allows the authors to detect even weak individual transitions and to cleanly separate absorption features originating from different intervening interstellar clouds.

The C2 features, which are at most only a few percent in depth, were detected with sufficient S/N so that in about one-third of the lines of sight the authors measured multiple components and could determine individual column densities and excitation temperatures. The authors show that C2 and C3 are very highly correlated, with a column density ratio of 15.5+/-1.4. They also detect for the first time the features due to 12C13C, which allows them to tease out the 12C/13C ratio from the C2 molecule, finding a ratio of about 38 for the isotopologue ratio and a value of around 79+/-8 for the 12/13 C ratio, including lines of sight with multiple components. For the thermodynamic conditions, they find a mean kinetic gas temperature of around 40 K and a mean volume density of around 200 cm^-3. Thus, the lines of sight are compatible with diffuse and translucent clouds.

The authors also study the link between C2 and the "C2-DIBs", several diffuse interstellar bands (DIBs) whose strengths were suggested to be strongly correlated with the column density of C2. The authors however show that the behaviour of these C2-DIBs (equivalent width per unit reddening) is the same in sightlines with and without C2, casting some doubt on this connection. The authors show that the apparent correlation between C2 and the C2-DIBs in the literature is due to the normalization in these studies to the 6196 DIB that is responsible for the variations and the correlations. They argue that the C2-forming region of the sampled clouds may not be well probed by the C2-DIBs that, instead, are formed in less shielded parts of the clouds.