Interstellar C_2 in the Perseus molecular complex: excitation temperature and density of a molecular cloud with anomalous microwave emission

TL;DR

This study detects interstellar C2 absorption lines toward star Cernis 52, constrains the physical conditions of the molecular cloud with anomalous microwave emission, and provides the first temperature and density measurements for such a cloud.

Contribution

It presents the first determination of gas-kinetic temperature and density in a molecular cloud associated with anomalous microwave emission.

Findings

C2 column density correlates with CH as expected.

Gas temperature is approximately 40 K.

Particle density is about 250 cm$^{-3}$.

Abstract

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.