Observations of C₃in Translucent Sight Lines

Abstract

The A1Πu ← X1Σ transition of the simplest polyatomic carbon chain molecule, C3, at 4051.6 Å has been searched for toward reddened stars where abundant C2 had been reported and toward other stars with high color excess. Absorption from C3 has been detected toward 15 stars with color excess E(B-V) from 0.33 to 1.12. The observed C3 column densities, ranging from 1012 to 1013 cm-2, are well correlated with the corresponding C2 column densities, with N(C2)/N(C3) ~ 40, indicating their close chemical relation. The carbon-rich sight line toward HD 204827 (for which no previous C2 observation had been reported) has by far the highest C3 and C2 column densities. The chemistry of formation of C3 from C2 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 C4 and C5 were negative.