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Formaldehyde as a probe of physical conditions in dense molecular clouds
Mangum, Jeffrey G.; Wootten, Alwyn
AA(University of Arizona, Tucson, AZ), AB(University of Arizona, Tucson, AZ)
The Astrophysical Journal Supplement Series (ISSN 0067-0049), vol. 89, no. 1, p. 123-153 (ApJS Homepage)
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NASA/STI Keywords:
Energy Levels, H Ii Regions, Interstellar Gas, Molecular Clouds, Molecular Spectra, Radiative Transfer, Star Formation, Velocity Distribution, Molecular Spectroscopy, Space Density, Spatial Resolution, Temperature
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We present a detailed analysis describing the utility of the formaldehyde (H2CO) molecule in the derivation of the kinetic temperature and spatial density within molecular clouds. Measurements of 13 transitions from both the ortho and para species of H2CO have been made toward a sample of 11 active star formation regions. These H2CO transitions range in frequency from 211 to 365 GHz and in upper-state energy from 21 to 241 K. This range in excitation has allowed us to analyze H2CO sensitivity to both cool (TK less than or approximately equal to 50 K) and warm (TK greater than 50 K) molecular material. Using a spherical large velocity gradient (LVG) model to solve for the excitation of H2CO, we analyze the sensitivity of several ortho- and para-H2CO transition intensity ratios to the kinetic temperature and spatial density within molecular clouds. Using our measured H2CO radiation temperatures, we have constrained LVG model solutions for the kinetic temperature, spatial density, and H2CO species column density in each of the sources in our sample. In all of the regions in our sample, though, we measure kinetic temperatures greater than 50 K, significantly higher than previous estimates for many of these sources. Using our ortho- and para-H2CO measurements we have calculated the N(ortho-H2CO)/N(para-H2CO) ratio in several of the sources in our sample. Our measurements indicate that N(ortho-H2CO)/N(para-H2CO) less than 3 for most of our sources. When combined with the relatively high kinetic temperature in these objects, this N(ortho-H2CO)/N(para-H2CO) ratio suggests that dust grains might play an active role in H2CO chemistry.

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