1 Introduction

The line of sight towards the heavily reddened star Cyg OB2 No. 12 has received renewed attention recently after the detection of very large amounts of interstellar H₃⁺ (McCall et al. 1998; Geballe et al. 1999). Cyg OB2 No. 12 is classified as a B8Ia supergiant and reddened by some 10 magnitudes of visual extinction (Souza & Lutz 1980). It is generally accepted that the reddening is caused by foreground material. The star suffers from a strong stellar wind with a velocity of $V_{\rm wind} = 1400$ km s^-1 (Leitherer et al. 1982; Bieging et al. 1989). A near-infrared excess of the star found by Leitherer et al. (1982) is attributed to free-free emission from the expanding envelope. The absence of interstellar H₂O and CO₂ ices at 3 $\mu$m and 4.27 $\mu$m, respectively, suggests that the foreground molecular material is diffuse (Whittet et al. 1997).

The H₃⁺ column density towards Cyg OB2 No. 12 of N(H $_3^+) = 3.8\times 10^{14}$ cm^-2 is comparable to H₃⁺ column densities observed in dense clouds (Geballe & Oka 1996; McCall et al. 1999). The question arises what processes are responsible for the production of such large amounts of H₃⁺ in the diffuse gas, and what physical conditions prevail in the medium. In a first interpretation, McCall et al. (1998) and Geballe et al. (1999) proposed that H₃⁺forms in low density material with n = 10 cm^-3 spread over pathlengths of 400-1200 pc. The authors noted, however, that their model fails to explain the abundance of CO which the authors inferred from their CO infrared observations, and that the model is in clear disagreement with earlier C₂ observations (Gredel & Münch 1994, GM94 hereafter). A detailed model of the physical and chemical structure of the line of sight towards No. 12 was recently presented by Cecchi-Pestellini & Dalgarno (2000), who proposed a nested structure of the molecular material. In their model, H₃⁺ forms in diffuse gas of density n = 50-100 cm^-3, C₂ forms in embedded clouds at n = 7000 cm^-3 at temperatures of T = 35 K, and CO forms in dense cloudlets of n> 10⁴ cm^-3.

Observations of interstellar C₂ allow physical conditions such as the density and the temperature in the molecular material to be inferred. The theory of C₂ excitation was developed by van Dishoeck & Black (1982). It has been used by a variety of authors to measure densities and temperatures in diffuse and translucent molecular clouds (Gredel 1999 and references therein). Molecular carbon was detected towards No. 12 by Souza & Lutz (1977), in their discovery detection of C₂ in the interstellar medium. The authors observed the R(2) and Q(2) lines of the (1,0) band of the C₂ Phillips system, near 1 $\mu$m, and estimated a rotational excitation temperature of $T_{\rm {ex}} = 30$-40 K and a total C₂ column density of about N(C ₂) = 10¹⁴ cm^-2. Higher temperatures of $T_{\rm {ex}} = 65$ K were derived by Lutz & Crutcher (1983). A new search of C₂ towards various stars in the Cyg OB2 association resulted in the detection of the R(2), Q(2), and Q(4) lines towards Cyg OB2 No. 12 and No. 5 by GM94. Gredel & Münch (1994) confirmed a low C₂ rotational excitation temperature towards No. 12 but failed to detect rotational lines with J'' > 4. Accurate column densities for levels J''>4 are required if densities are to be inferred (van Dishoeck & Black 1982).

A proper modeling of the chemistry towards Cyg OB2 No. 12 requires a knowledge of the physical conditions which prevail in the line of sight. We decided to obtain a deep optical spectrum towards No. 12, with the aim of detecting rotational lines with J''>4 so that densities may be accurately determined. The observations are described in Sect. 2. Section 3 contains a summary of the C₂ analysis and the results obtained. It includes the results of the detection of interstellar CN towards No. 12 in the (1,0) and (2,0) band of the CN A $^2\Pi _{\rm u}$ - X ${^2\Sigma ^+}$ red system, and of interstellar Rubidium towards No. 12 and No. 5. A comparison of the derived C₂ and CN abundances with a chemical model driven by X-ray ionisation is given in Sect. 4.