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Issue A&A
Volume 375, Number 2, August IV 2001
Page(s) 553 - 565
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20010769



A&A 375, 553-565 (2001)
DOI: 10.1051/0004-6361:20010769

Interstellar C $_\mathsf{2}$ and CN toward the Cyg OB2 association

A case study of X-ray induced chemistry
R. Gredel1, J. H. Black2 and M. Yan3

1  Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
    e-mail: gredel@caha.es
2  Onsala Space Observatory, Chalmers University of Technology, 43992 Onsala, Sweden
    e-mail: jblack@oso.chalmers.se
3  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

(Received 7 February 2001 / Accepted 23 May 2001 )

Abstract
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 A $^1\Pi_{\mathrm u}$ -X $^1\Sigma_{\mathrm g}^+$ 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 $n = (600 \pm 100)$ cm-3. The inferred C2 column density is N(C 2) = (20+4-2) 1013 cm-2. The detection of various lines in the (1,0) and (2,0) band of the CN A $^2\Pi_{\mathrm u}$ -X $^2\Sigma^+$ red system suggest a column density of N(CN) = (8- $13)\times 10^{13}$ cm-2. C2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T = 50 K, $n = (600 \pm 200)$ cm-3, N(C $_2)=(10^{+3.5}_{-1.5})\times 10^{13}$ cm-2 towards No. 5, and T = 100 K, $n \ge 800$ cm-3, N(C $_2)=(5.2\pm 1)\times 10^{13}$ cm-2 towards No. 9. Marginal detections of C2 towards Cyg OB2 No. 8A indicate N(C $_2) \approx 3.3\times 10^{13}$ cm-2 and $T \approx 100$ K. Upper limits are N(C $_2) \le 3.3\times 10^{13}$ 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 $n \approx 10$ 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 $\zeta = (0.6$- $3)\times 10^{-15}$ 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 $2\times 10^{12}$ 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 $) = (13\pm 2)\times 10^{9}$ cm-2 towards No. 12 and N(Rb $) = (13\pm 2)\times 10^{9}$ cm-2 towards No. 5.


Key words: ISM: abundances -- ISM: clouds -- ISM: molecules -- X-rays: general

Offprint request: R. Gredel, gredel@mpia-hd.mpg.de

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