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

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 C-2 A(1)II(u) -X(1)Sigma (+)(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 = 35 K and n = (600 +/- 100) cm(-3). The inferred C-2 column density is N(C-2) = (20(-2)(+4)) 10(13) cm(-2). The detection of various lines in the (1,0) and (2,0) band of the CN A(2)II(u) -X(2)Sigma (+) red system suggest a column density of N(CN) = (8-13) x 10(13) cm(-2). C-2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T = 50K, n = (600 +/- 200) cm(-3), N(C-2) = (10(-1.5)(+3.5)) x 10(13) cm(-2) towards No. 5, and T = 100 K, n greater than or equal to 800 cm(-3), N(C-2) =(5.2 +/-1) x 10(13) cm(-2) towards No. 9. Marginal detections of C-2 towards Cyg OB2 No. 8A indicate N(C-2) approximate to 3.3 x 10(13) cm(-2) and T approximate to 100 K. Upper limits are N(C-2) less than or equal to 3.3 x 10(13) cm(-2) toward Cyg OB2 No. 7 and No. 11. The C-2 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 approximate to 10 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 zeta = (0.6-3) x 10(-15) s(-1). Calculated equilibrium temperatures of T = 25-50 K agree with temperatures inferred from C-2. The model also reproduces the observed column densities of CO and CH, and that of H-3(+) to within a factor of two. We predict a H2O+ column density of 2 x 10(12) 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) = (13 +/-2) x 10(9) cm(-2) towards No. 12 and N(Rb) = (13 +/-2) x 10(9) cm(-2) towards No. 5.