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-IR , submillimeter , or millimeter continuum to that of a { } ^ { 13 } \kern - 0.8 ptCO ( or C { } ^ { 18 } \kern - 0.8 ptO ) rotational line can place reliable upper limits on the temperature of the dust and molecular gas .
Consequently , far-infrared continuum data from the COBE / DIRBE instrument and Nagoya 4-m { } ^ { 13 } \kern - 0.8 ptCO J = 1 \rightarrow 0 spectral line data were used to plot 240 \mu m / { } ^ { 13 } \kern - 0.8 ptCO J = 1 \rightarrow 0 intensity ratios against 140 \mu m /240 \mu m dust color 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 . \sim 1 kpc ) 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 varies from one sightline to another .
The models require a dust-gas temperature difference of 0 \pm 2 K and suggest that 40-50 % of the Orion clouds are in the form of dust and gas with temperatures between 3 and 10 K. These results have a number implications that are discussed in detail in later papers .
These include stronger dust-gas thermal coupling and higher Galactic-scale molecular gas temperatures than are usually accepted , an improved explanation for the N ( H _ { 2 } ) /I ( CO ) conversion factor , and ruling out one dust grain alignment mechanism .