Interstellar C_2 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.
