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

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

Aug 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001a%26a...375..553g&link_type=abstract

Astronomy and Astrophysics, v.375, p.553-565 (2001)

Astronomy and Astrophysics

Astrophysics

23

Ism: Abundances, Ism: Clouds, Ism: Molecules, X-Rays: General

Scientific paper

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 A1Piu - X1Sigmag+ 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 C2 column density is N(C2) = (20+4-2) 1013 cm-2. The detection of various lines in the (1,0) and (2,0) band of the CN A2Piu - X2Sigma + red system suggest a column density of N(CN) = (8-13)x 1013 cm-2. C2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T = 50 K, n = (600 +/- 200) cm-3, N(C2)=(10+3.5-1.5)x 1013 cm-2 towards No. 5, and T = 100 K, n >= 800 cm-3, N(C2)=(5.2+/- 1)x 1013 cm-2 towards No. 9. Marginal detections of C2 towards Cyg OB2 No. 8A indicate N(C2) ~ 3.3x 1013 cm-2 and T ~ 100 K. Upper limits are N(C2) <= 3.3x 1013 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 n ~ 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 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 2x 1012 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 109 cm-2 towards No. 12 and N(Rb) = (13+/- 2)x 109 cm-2 towards No. 5.

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