Observations of C3in Translucent Sight Lines

The A 1 Π u ← X 1 Σ transition of the simplest polyatomic carbon chain molecule, C 3 , at 4051.6 Å has been searched for toward reddened stars where abundant C 2 had been reported and toward other stars with high color excess. Absorption from C 3 has been detected toward 15 stars with color excess E ( B - V ) from 0.33 to 1.12. The observed C 3 column densities, ranging from 10 12 to 10 13 cm -2 , are well correlated with the corresponding C 2 column densities, with N (C 2 )/ N (C 3 ) ~ 40, indicating their close chemical relation. The carbon-rich sight line toward HD 204827 (for which no previous C 2 observation had been reported) has by far the highest C 3 and C 2 column densities. The chemistry of formation of C 3 from C 2 is discussed. A search for the next strongest 020-000 vibronic band was unsuccessful as a result of the low Franck-Condon factor and interference with a stellar line. Searches for C 4 and C 5 were negative.