# A Comparison between Anomalous 6-cm H$_2$CO Absorption and CO(1-0) Emission in the L1204/S140

## TL;DR

This study compares the distribution of H2CO absorption and CO emission in the L1204/S140 region, revealing differences due to their distinct excitation conditions and susceptibilities to photodissociation, enhancing understanding of molecular cloud structures.

## Contribution

It provides a detailed comparison of H2CO and CO distributions in a molecular cloud, highlighting how their differing properties affect their tracing of molecular gas.

## Key findings

- H2CO absorption is offset from CO emission peaks.
- H2CO is more easily photodissociated than CO.
- CO traces warmer, denser regions more effectively.

## Abstract

We report observations of the dust cloud L1204 with the Onsala 25-m telescope in the 6 cm (1$_{11}-1_{10}$) transition of \htco. The observed region includes the S140 H${\alpha}$ arc. This spectral line is seen here in absorption against the cosmic microwave background, indicating the presence of widespread warm molecular gas at intermediate densities. Overall, the distributions of H$_2$CO and CO (taken from the literature) are fairly similar, though significant differences exist at small scales. Most notably, while the CO peak is nearly coincident with the S140 H${\alpha}$ arc, the maximum H$_2$CO absorption is clearly separated from it by a full 10$'$ beam ($\sim$ 3 pc). We argue that these differences result from differing abundances and excitation requirements. The CO(1-0) line is more optically thick and more biased towards warm gas than the H$_2$CO 6 cm line. On the other hand, formaldehyde is more easily photodissociated and is, therefore, a poorer tracer of the molecular gas located immediately behind Photon Dominated Regions.