2007MNRAS.375..278W

Determining temperatures in molecular clouds from ratios of CO rotational lines or from ratios of continuum emission in different wavelength bands suffers from reduced temperature sensitivity in the high-temperature limit. In theory, the ratio of far-infrared (FIR), submillimetre or millimetre continuum to that of a ¹³CO (or C¹⁸O) rotational line can place reliable upper limits on the temperature of the dust and molecular gas. Consequently, FIR continuum data from the COBE/Diffuse Infrared Background Experiment (DIRBE) instrument and Nagoya 4-m ¹³COJ = 1 ⟶ 0 spectral line data were used to plot 240 µm/¹³COJ = 1 ⟶ 0 intensity ratios against 140/240 µm dust colour temperatures, allowing us to constrain the multiparsec-scale physical conditions in the Orion A and B molecular clouds.

The best-fitting models to the Orion clouds consist of two components: a component near the surface of the clouds that is heated primarily by a very large scale (i.e. ∼1kpc) interstellar radiation field and a component deeper within the clouds. The former has a fixed temperature and the latter has a range of temperatures that vary from one sightline to another. The models require a dust-gas temperature difference of 0±2K and suggest that 40-50 per cent of the Orion clouds are in the form of dust and gas with temperatures between 3 and 10K. The implications are discussed in detail in later papers and include stronger dust-gas thermal coupling and higher Galactic-scale molecular gas temperatures than are usually accepted, and an improved explanation for the N(H₂)/I(CO) conversion factor. It is emphasized that these results are preliminary and require confirmation by independent observations and methods.