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

Abstract

We carried out a sensitive survey of C${}_2$ and C${}_3$ using the EDIBLES data set. We also expanded our searches to C${}_4$, C${}_5$, and ${}^{13}$C${}^{12}$C isotopologue in the most molecule-rich sightlines. We fit synthetic spectra generated following a physical excitation model to the C${}_2$ (2-0) Phillips band to obtain the C${}_2$ column density ($N$) as well as the kinetic temperature ($T_{\text{kin}}$) and number density ($n$) of the host cloud. The C${}_3$ molecule was measured through its $\tilde{A}-\tilde{X}$ (000-000) electronic origin band system. We simulated the excitation of this band with a double-temperature Boltzmann distribution. We present the largest combined survey of C${}_2$ and C${}_3$ to date in which the individual transitions can be resolved. In total we detected C${}_2$ in 51 velocity components along 40 sightlines, and C${}_3$ in 31 velocity components along 27 sightlines. The two molecules are detected in the same velocity components. We find a very good correlation between $N$(C${}_2$) and $N$(C${}_3$) with Pearson $r=0.93$ and an average $N$(C${}_2$)/$N$(C${}_3$) ratio of 15.5$\pm$1.4. A comparison with the behaviour of the C${}_2$ DIBs shows that there are no clear differences among sightlines with and without detection of C${}_2$ and C${}_3$. This is in direct contrast to the better-studied non-C${}_2$ DIBs who have reduced strengths in molecule-rich environments. We also identify for the first time the $Q$(2), $Q$(3), and $Q$(4) transitions of the ${}^{13}$C${}^{12}$C (2-0) Phillips band in a stacked average spectrum, and estimate the isotopic ratio of carbon ${}^{12}$C/${}^{13}$C as 79$\pm$8. Our search for the C${}_4$ and C${}_5$ optical bands was unsuccessful.

arXiv comment31 pages, 23 figures. To appear in A&A

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