Comparison of 13CO Line and Far-Infrared Continuum Emission as a Diagnostic of Dust and Molecular Gas Physical Conditions: III. Systematic Effects and Scientific Implications

摘要

Far-infrared continuum data from the {\it COBE}/{\it DIRBE} instrument werecombined with Nagoya 4-m $\cOone$ spectral line data to infer themultiparsec-scale physical conditions in the Orion$ $A and B molecular clouds,using 140$\um$/240$\um$ dust color temperatures and the 240$\um$/$\cOone$intensity ratios. In theory, the ratio of far-IR, submillimeter, or millimetercontinuum to that of a $\cO$ (or $\Co$) rotational line can place reliableupper limits on the temperature of the dust and molecular gas on multi-parsecscales; on such scales, both the line and continuum emission are opticallythin, resulting in a continuum-to-line ratio that suffers no loss oftemperature sensitivity in the high-temperature limit as occurs for ratios ofCO rotational lines or ratios of continuum emission in different wavelengthbands. Two-component models fit the Orion data best, where one has afixed-temperature and the other has a spatially varying temperature. The formerrepresents gas and dust towards the surface of the clouds that are heatedprimarily by a very large-scale (i.e. $\sim 1 $kpc) interstellar radiationfield. The latter represents gas and dust at greater depths into the clouds andare shielded from this interstellar radiation field and heated by local stars.The inferred physical conditions are consistent with those determined frompreviously observed maps of $\COone$ and $\Jtwo$ that cover the entire Orion$$A and B molecular clouds. The models require that the dust-gas temperaturedifference is 0$\pm 2 $K. If this surprising result applies to much of theGalactic ISM, except in unusual regions such as the Galactic Center, then thereare a number implications.