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 degrees 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 C-2 column density of N(C-2) = (10.5 +/- 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 for 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 +/- 10 K and n = 250 +/- 50 cm(-3), respectively. This is the first determination of the gas kinetic temperature and particle density of a cloud with known anomalous microwave emission.