# The Effect of Far-infrared Radiation on the Hyperfine Anomaly of the OH 18 cm Transition

## TL;DR

This study investigates the hyperfine anomalies in the OH 18 cm transition lines in molecular clouds, revealing that the 1720 MHz absorption traces cold, dense gas influenced by far-infrared radiation.

## Contribution

It demonstrates how far-infrared radiation affects hyperfine anomalies in OH lines and identifies conditions under which the 1720 MHz line shows absorption, advancing understanding of molecular cloud environments.

## Key findings

- 1720 MHz line shows absorption against the CMB in certain regions.
- Anomalous line intensities are explained by specific physical conditions.
- Absorption features trace cold, dense gas surrounded by warm CO-dark envelopes.

## Abstract

We present observations of the four hyperfine structure components of the OH 18 cm transition (1612, 1665, 1667 and 1720 MHz) toward Heiles Cloud 2 (HCL2) in Taurus and the dense cores L183 and L169. Toward the peculiar straight structure in the northern part of HCL2, the 1720 MHz line shows absorption against the cosmic microwave background at a velocity of $\sim$ 5.5 km s$^{-1}$, whereas the 1612 MHz line shows brighter emission than expected in local thermodynamic equilibrium (LTE). Such an intensity anomaly is also observed toward TMC-1 (CP), L183, and L169, where the 1612 MHz line is stronger and the 1720 MHz line is weaker than those expected under LTE. We conduct statistical equilibrium calculations considering the effect of far-infrared (FIR) radiation from surrounding clouds and find that the absorption feature of the 1720 MHz line can be reproduced by the following conditions: gas kinetic temperature lower than about 30 K, OH column density higher than 10$^{15}$ cm$^{-2}$, H$_2$ density lower than 10$^3$ cm$^{-3}$, and an ortho-to-para ratio of H$_2$ lower than 1. Therefore, the absorption feature of the 1720 MHz line is considered to trace relatively cold and dense gas that is surrounded by a warm envelope consisting of CO-dark molecular gas.