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

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 00 = 18 in the (2,0) and (3,0) bands of the C2 A 1 u {X 1+ 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 =3 5 K andn = (600 100) cm 3 . The inferred C2 column density is N(C2 )=( 20 +4 2 )1 0 13 cm 2 . The detection of various lines in the (1,0) and (2,0) band of the CN A 2 u {X 2+ red system suggest a column density of N (CN) =( 8 13) 10 13 cm 2 .C 2 absorption lines are also detected towards Cyg OB2 No. 5 and No. 9. Inferred parameters are T =5 0 K,n = (600 200) cm 3 , N(C2) = (10 +3:5 1:5) 10