2011MNRAS.411.1857I

Interstellar absorption lines up to J"= 10 in the (2, 0) band and up to J"= 6 in the (3, 0) band of the C₂A¹Π_u-X¹Σ⁺_g system are detected toward star Cernis 52 (BD +31°640) in the Perseus molecular complex. The star lies in a reddened line of sight where various experiments have detected anomalous microwave emission spatially correlated with dust thermal emission. The inferred total C₂ column density of N(C₂) = (10.5±0.2) {x} 10¹³/cm2 is well correlated with that of CH, as expected from theoretical models, and is among the highest reported for translucent clouds with similar extinction. The observed rotational C₂ lines constrain the gas kinetic temperature T and the density n=n(H) +n(H₂) of the intervening cloud to T= 40 ±10 K and n= 250±50/cm3, respectively. This is the first determination of the gas kinetic temperature and particle density of a cloud with known anomalous microwave emission.