Interstellar C$_\mathsf{2}$ and CN toward the Cyg OB2 association

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Abstract
An analysis of deep optical echelle spectra towards six stars in the Cyg OB2 (VI Cygni) association is presented. Interstellar absorption lines up to in the (2,0) and (3,0) bands of the C2 A -X system are detected towards Cyg OB2 No. 12. The large number of rotational lines accurately constrains the gas-kinetic temperature T and the density n to K and cm-3. The inferred C2 column density is N(C cm-2. The detection of various lines in the (1,0) and (2,0) band of the CN A -X red system suggest a column density of N(CN- cm-2. C2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are K, cm-3, N(C cm-2 towards No. 5, and K, cm-3, N(C cm-2 towards No. 9. Marginal detections of C2 towards Cyg OB2 No. 8A indicate N(C cm-2 and K. Upper limits are N(C cm-2 toward Cyg OB2 No. 7 and No. 11. The C2 observations eliminate the possibility that the molecular material along the line of sight towards Cyg OB2 No. 12 is spread over a pathlength of several hundred parsecs of very low density cm-3. The observations provide some support to a recent chemical model which assumes a nested structure of the molecular gas. Alternatively, the C2 and CN abundances obtained towards Cyg OB2 No. 12 are in agreement with the predictions of an X-ray induced chemistry driven by an ionisation rate of - s-1. Calculated equilibrium temperatures of -50 K agree with temperatures inferred from C2. The model also reproduces the observed column densities of CO and CH, and that of H to within a factor of two. We predict a H2O+ column density of cm-2 towards Cyg OB2 No. 12 and H2O+ absorption lines which are detectable by optical absorption line techniques. We report the detection of interstellar Rubidium towards Cyg OB2 No. 12 and No. 5. Inferred column densities are N(Rb cm-2 towards No. 12 and N(Rb cm-2 towards No. 5.

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