Interstellar C2 towards the Cyg OB2 association -: A case study of X-ray induced chemistry

Observations of interstellar C-2 are used to constrain physical conditions in the translucent material towards Cyg OB2 No. 12. The C-2 analysis suggests a gas-kinetic temperature of T-kin = 35 K and a density of n(call)/I = (n(H) + n(H2))/I = 300 cm(-3). The C-2 column density is N(C-2) = 2 x 10(14) cm(-2). The fractional abundances of C-2, CO, CH, and CN, are reproduced by a chemical model driven by ionisation rates of zeta = (0.6 - 3) x 10(-15) s(-1) from X-rays, a hydrogen number density of n 600 cm(-3) and a total hydrogen column density of 1.6 x 10(22) cm(-2). Calculated temperatures of Tkin = 25 - 50 K agree with the those inferred from C-2.