5 Conclusions

We have presented observations of interstellar absorption lines of C₂ and CN towards Cyg OB2 No. 12. The C₂ observations indicate low densities of $n = 600 \pm 100$ cm^-3 of the molecular gas. We find evidence that the chemistry of the translucent cloud towards Cyg OB2 is affected by X-rays. We reproduce the observed column densities of C₂, CN, CH, and CO, for X-ray ionisation rates of $\zeta = (\mbox{0.6$-$ 3})\times 10^{-15}$ s^-1. The observed column density of H₃⁺ is reproduced to within a factor of two. We predict large amounts of H₂O⁺ towards Cyg OB2 No. 12 which will be detectable by optical absorption line techniques.

Acknowledgements

It is a pleasure to thank Alex Dalgarno for most fruitful discussions and suggestions. The careful reading of the manuscript by the referee, Dr. S. R. Federman, is kindly acknowledged.