Interstellar diatomic carbon and the cygnus OB2 association

Diatomic carbon in interstellar clouds through observations of the (1-0) Phillips band in the spectra of reddened stars has been investigated. C2 column densities of 1.2 x 1014 cm-2 and 8.0 x 1013 cm-2 and excitation temperatures of 45K and 60K toward Cyg OB2 No. 12 and HD147889, respectively were found. A possible anticorrelation of C2 excitation temperature with color excess may be related to ultraviolet pumping of the ground-state rotational levels. Also the interstellar medium toward and stellar properties of the Cygnus OB2 association were studied through high- and medium-dispersion spectroscopy. Trace elements are severely depleted toward Cyg OB2, and several lines of evidence indicate a high-density (near 3500 cm-3) region. Stellar winds from Cyg OB2 may be affecting some of this interstellar material. Evidence is found against the production of interstellar CH+ in the shock model of Elitzur and Watson (1978). The classification of No. 12 as B8Ia was confirmed and found to be very likely a bona fide member of Cyg OB2. Many Cyg OB2 stars are extremely luminous and massive and show evidence of rapid mass loss. The H-R diagram for Cyg OB2 is well represented by mass-losing evolutionary models, and an age of 3-4 x 106 y is indicated