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26 April 2024 |
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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 | | W. F. Wall
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24 Jan 2006 | | Abstract: | Far-infrared continuum data from the {it COBE}/{it DIRBE} instrument were combined with Nagoya 4-m $cOone$ spectral line data to infer the multiparsec-scale physical conditions in the Orion$ $A and B molecular clouds, using 140$um$/240$um$ dust color temperatures and the 240$um$/13CO J=1-0 intensity ratios. In theory, the ratio of far-IR, submillimeter, or millimeter continuum to that of a 13CO (or C18O) rotational line can place reliable upper limits on the temperature of the dust and molecular gas.
Two-component models fit the Orion data best, where one has a fixed-temperature and the other has a spatially varying temperature. The former represents gas and dust towards the surface of the clouds that are heated primarily by a very large-scale (i.e. $sim 1 $kpc) interstellar radiation field. The latter represents gas and dust at greater depths into the clouds and are shielded from this interstellar radiation field and heated by local stars. The models require that the dust-gas temperature difference is 0$pm 2 $K. If this surprising result applies to much of the Galactic ISM, except in unusual regions such as the Galactic Center, then there are a number implications. These include dust-gas thermal coupling that is commonly factors of 5 to 10 stronger than previously believed, Galactic-scale molecular gas temperatures closer to 20$ $K than to 10$ $K, an improved explanation for the N(H$_2$)/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% of the Orion clouds are in the form of cold (i.e. $sim 3-10 $K) dust and gas, although alternative explanations are not ruled out. | | Source: | arXiv, astro-ph/0601549 | | Services: | Forum | Review | PDF | Favorites | |
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