Rotational fine-structure lines of interstellar C2 toward Zeta Persei

Nine of the rotational fine-structure lines of the 2-0 band of the Phillips system of interstellar C2 toward Zeta Persei have been detected with the McDonald Observatory 2.7 m telescope's coude echelle spectrograph and Reticon detector. The equivalent widths of the lines yield values of N(C2) equals 1.2 x 10 to the 13th per sq cm and T(rot) equals 97 K. The apparent discrepancy in the observed and predicted cloud temperature is discussed by suggesting that the level populations of C2 have been altered from those expected in pure thermal equilibrium by radiative pumping through the Mulliken and Phillips systems. Although not all of the physical constants required to calculate this effect in detail are known, arguments based on reasonable estimates of their values suggest that radiative pumping can indeed produce a rotational temperature of 97 K even though the kinetic temperature of the cloud is significantly lower than this value.