The physics and chemistry of small molecular clouds in the Galactic plane. 2: H2CO

Details The physics and chemistry of small molecular clouds in the Galactic plane. 2: H2CO The physics and chemistry of small molecular clouds in the Galactic plane. 2: H2CO

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...437..658t&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 437, no. 2, p. 658-677

Astronomy and Astrophysics

Astrophysics

12

Abundance, Astronomical Models, Chemical Analysis, Interstellar Matter, Molecular Clouds, Radio Spectra, Ultraviolet Spectra, Chemical Elements, Flux Density, Hydrostatics, Photodissociation, Polytropic Processes, Radio Telescopes

Scientific paper

We have made extensive observations of 2 and 6 cm H2CO in all 27 of the Clemens-Barvainis small molecular clouds for which several structural models including hydrostatic equilibrium polytropes were developed in an earlier paper based on C(18)O and (13)CO observations. We have observed the 211-110 line at 2.0 mm in 11 of the CB objects and in 10 of 11 cirrus cores earlier studied in C(18)O, (13)CO, and H2CO. As with the cirrus cores, the three H2CO lines in CB objects are all well fitted by both polytropic models and ad hoc n approximately r-1 models, using the external UV fields derived in the earlier papers. The reanalysis of the cirrus cores includes the 2 mm H2CO lines as well as treating the C-12/C-13 ratio as a variable, and yields approximately 40% higher fractional abundances than the earlier analysis, as well as giving equal preference to both centrally peaked and radially flat distributions of the H2CO fractional abundance. The same central H2CO abundances are found for the CB objects, but these objects favor radially flat abundance distributions, possibly because of beam dilution of the 6 cm lines speculated as unaccounted for in the detailed estimates made using maps of every source. As before, no clear preference is shown for polytropic or r-1 structures although r-1 is favored for a subset of 11 objects with 211-110 data. The large central abundances derived for both types of object (mean value 1.4 x 10-8 for ortho H2CO) are too large by a factor 104 to be compatible with gas-phase formation of H2CO. Grain formation is indicated, as concluded earlier for cirrus cores. It is argued that photocatalysis on grains is consistent with either peaked or flat H2CO abundance distributions, but this cannot be tested conclusively within the uncertainties of determining the structures or the abundance distributions. By including consistently the effects of UV radiation fields and electron excitation, our models fit accurately all four lines of C(18)O and (13)CO, and all three lines of H2CO so far observed. The derived extinctions and H2CO abundances are definitely determined, and significant deviations from hydrostatic equilibrium are ruled out. The CB objects and cirrus cores are indistinguishable physically or chemically in terms of these characteristics, which seem to represent the conditions to be found in all small, low mass molecular clouds regardless of Galactic latitiude.

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