Interstellar C₂ in the Perseus molecular complex: excitation temperature and density of a molecular cloud with anomalous microwave emission

Interstellar absorption lines up to J"=10 in the (2,0) band and up to J"=6 in the (3,0) band of the C$_2$ $A^1\Pi_u$ - $X^1\Sigma^+_g$ system are detected toward star Cernis 52 (BD+31$^o$ 640) in the Perseus molecular complex. The star lies in a redenned line of sight where various experiments have detected anomalous microwave emission spatially correlated with dust thermal emission. The inferred total C$_2$ column density of N(C$_{2}$) = (10.5$\pm$ 0.2) x 10$^{13}$ cm$^{-2}$ is well correlated with that of CH as expected from theoretical models and is among the highest reported on translucent clouds with similar extinction. The observed rotational C$_2$ lines constrain the gas-kinetic temperature T and the density n=n(H)+n(H$_2$) of the intervening cloud to T = 40$\pm$10 K and n = 250 $\pm$ 50 cm$^{-3}$, respectively. This is the first determination of gas-kinetic temperature and particle density of a cloud with known anomalous microwave emission.