2001A&A...375..553G

An analysis of deep optical echelle spectra towards six stars in the Cyg OB2 (VI Cygni) association is presented. Interstellar absorption lines up to J''=18 in the (2,0) and (3,0) bands of the C₂ A¹Π_u - X¹Σ_g⁺ 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 T=35K and n=(600±100)cm^–3. The inferred C₂ column density is N(C₂)=(20⁺⁴_–2)x10¹³cm^–2. The detection of various lines in the (1,0) and (2,0) band of the CN A²Π_u - X²Σ⁺ red system suggest a column density of N(CN)=(8-13)x10¹³cm^–2. C₂ absorption lines are also detected towards Cyg OB2 No.5 and No.9. Inferred parameters are T=50K, n=(600±200)cm^–3, N(C₂)=(10^+3.5_–1.5)x10¹³cm^–2 towards No.5, and T=100K, n≥800cm^–3, N(C₂)=(5.2±1)x10¹³cm^–2 towards No.9. Marginal detections of C₂ towards Cyg OB2 No.8A indicate N(C₂)≃3.3x10¹³cm^–2 and T≃100K. Upper limits are N(C₂)≤3.3x10¹³cm^–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 n≃10cm^–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 ζ=(0.6-3)x10^–15s^–1. Calculated equilibrium temperatures of T=25-50K agree with temperatures inferred from C₂. The model also reproduces the observed column densities of CO and CH, and that of H₃⁺ to within a factor of two. We predict a H₂O⁺ column density of 2x10¹²cm^–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)=(13±2)x10⁹cm^–2 towards No.12 and N(Rb)=(13±2)x10⁹cm^–2 towards No.5.