SIMBAD references

Far-infrared (far-IR) continuum data from the COBE/DIRBE instrument were combined with Nagoya 4-m ¹³COJ = 1 ⟶ 0 spectral line data to infer the multiparsec-scale physical conditions in the OrionA and B molecular clouds, using 140 µm/240 µm dust colour temperatures and the 240 µm/¹³COJ = 1 ⟶ 0 intensity ratios. In theory, the ratio of far-IR, 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 on multiparsec scales; on such scales, both the line and continuum emission are optically thin, resulting in a continuum-to-line ratio that suffers no loss of temperature sensitivity in the high-temperature limit as occurs for ratios of CO rotational lines or ratios of continuum emission in different wavelength bands.

Two-component models fit the Orion data best, where one has a fixed temperature and the other has a spatially varying temperature. The inferred physical conditions are consistent with those determined from previously observed maps of ¹²COJ = 1 ⟶ 0 and J = 2 ⟶ 1 that cover the entire OrionA and B molecular clouds. The models require that the dust-gas temperature difference is 0±2K. If this surprising result is confirmed with independent studies and applies to much of the Galactic interstellar medium (ISM), except in unusual regions such as the Galactic Centre, then there are a number of implications. These include dust-gas thermal coupling that is commonly factors of 5-10 stronger than previously believed, Galactic-scale molecular gas temperatures closer to 20K than to 10K, an improved explanation for the N(H₂)/I(CO) conversion factor (a full discussion of this is deferred to a later paper), and ruling out at least one dust grain alignment mechanism. The simplest interpretation of the models suggests that about 40-50 per cent of the Orion clouds are in the form of cold (i.e. ∼3-10K) dust and gas, although alternative explanations are not ruled out. These alternatives include the contribution to the 240-µm continuum by dust associated with atomic hydrogen and reduced ¹³CO abundance towards the clouds' edges. Even considering these alternatives, it is still likely that cold material with temperatures of ∼7-10K still exists. If this cold gas and dust are common in the Galaxy, then mass estimates of the Galactic ISM must be revised upwards by up to 60 per cent.

The feasibility of submillimetre or millimetre continuum to ¹³CO line ratios constraining estimates of dust and molecular gas temperatures was tested. The model fits allowed the simulation of the necessary millimetre-continuum and ¹³COJ = 1 ⟶ 0 maps used in the test. In certain `hot spots' - that have continuum-to-line ratios above some threshold value - the millimetre continuum to ¹³CO ratio can estimate the dust temperature to within a factor of 2 over large ranges of physical conditions. Nevertheless, supplemental observations of the ¹³COJ = 2 ⟶ 1 line or of shorter wavelength continuum are advisable in placing lower limits on the estimated temperature. Even without such supplemental observations, this test shows that the continuum-to-line ratio places reliable upper limits on the temperature.