EDP Sciences
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Volume 425, Number 1, October I 2004
Page(s) 151 - 162
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20040542

A&A 425, 151-162 (2004)
DOI: 10.1051/0004-6361:20040542

Interstellar absorption lines toward Cep OB4

R. Gredel

Max-Planck Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
    e-mail: gredel@mpia.de; gredel@caha.es

(Received 29 March 2004 / Accepted 10 June 2004)

An analysis of optical echelle spectra towards nine stars in the Cep OB4 association is presented. Interstellar absorption lines which arise in the (1, 0) and (2, 0) bands of the CN A  $^2\Pi$ - X  $^2\Sigma^+$ red system towards BD+66° 1661, BD+66° 1674, and BD+66° 1675 are used to infer accurate CN column densities  N(CN). A comparison with earlier measurements in the CN violet system allows to infer a CN Doppler  b parameter of  b(CN ) = 1.2-2.4 km s -1. Molecular carbon absorption lines which arise in the (1, 0), (2, 0) and (3, 0) bands of the C 2  A $^1\Pi_{\rm u}$ - X $^1\Sigma_{\rm g}^+$ Phillips system is used to infer gaskinetic temperatures of  $35\pm10$ K and densities of  $n = 700 \pm 200$ cm -3 towards BD+66° 1661 and BD+66° 1675, and a temperature of  $60\pm10$ K and a density of  $800 \pm 400$ cm -3 towards BD+66° 1674. The R(1) line of the (0, 0) band of the CH A $^2\Delta - {\rm X}^2\Pi$ system is detected towards 6 stars. A tight correlation exists between  N(C 2) and  N(CH).  N(CN) increases with  N(CH) and with  N(C 2). Interstellar CH + is marginally detected towards four stars. Inferred CH + column densities are significantly lower than towards other lines of sight with similar reddening. The velocity structure towards Cep OB4 and the chemical abundances suggest that CN, C 2, CH, and CH + are formed in quiescent material. A previous suggestion that the molecules form in a photon-dominated region close to the stars is not supported by the observations.

Key words: ISM: abundances -- ISM: clouds -- ISM: molecules -- ISM: lines and bands

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