Observations of interstellar C2 toward three heavily reddened stars

Observations have been made of the 2-0 band of the Phillips system of interstellar C2 toward the heavily obscured early-type stars VI Cygni No. 12, HD 29647, and BD +66.1675 deg. The first direct proof that the rotational excitation temperatures of interstellar C2 are nonthermal was obtained. Toward VI Cygni No. 12, the rotational distribution cannot be characterized by a single, unique excitation temperature; the distribution is consistent with radiative pumping models. A very strong linear correlation was found between N(C2) and E(B-V), which suggests that the relative abundance of C2 is insensitive to a wide variation in physical conditions.