THE PHYSICS AND CHEMISTRY OF SMALL MOLECULAR CLOUDS IN THE GALACTIC PLANE .2. H2CO

We have made extensive observations of 2 and 6 cm H2CO in all 27 of th e Clemens-Barvainis small molecular clouds for which several structura l models including hydrostatic equilibrium polytropes were developed i n an earlier paper based on (CO)-O-18 and (CO)-C-13 observations. We h ave observed the 2(11)-1(10) line at 2.0 mm in 11 of the CB objects an d in 10 of 11 cirrus cores earlier studied in (CO)-O-18, (CO)-C-13, an d H2CO. As with the cirrus cores, the three H2CO lines in CB objects a re all well fitted by both polytropic models and ad hoc n similar to r (-1) models, using the external UV fields derived in the earlier paper s. 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 similar to 40% higher fractional abundances than the earlier analysis, as wel l as giving equal preference to both centrally peaked and radially fla t distributions of the H2CO fractional abundance. The same central H2C O abundances are found for the CB objects, but these objects favor rad ially flat abundance distributions, possibly because of beam dilution of the 6 cm lines speculated as unaccounted for in the detailed estima tes made using maps of every source. As before, no clear preference is shown for polytropic or r(-1) structures although r(-1) is favored fo r a subset of 11 objects with 2(11)-(10), data. The large central abun dances derived for both types of object (mean value 1.4 x 10(-8) for o rtho H2CO) are too large by a factor 10(4) to be compatible with gas-p hase formation of H2CO. Grain formation is indicated, as concluded ear lier for cirrus cores. It is argued that photocatalysis on grains is c onsistent with either peaked or flat H2CO abundance distributions, but this cannot be tested conclusively within the uncertainties of determ ining the structures or the abundance distributions. By including cons istently the effects of UV radiation fields and electron excitation, o ur models fit accurately all four lines of (CO)-O-18 and (CO)-C-13, an d all three lines of H2CO so far observed. The derived extinctions and H2CO abundances are definitively determined, and significant deviatio ns from hydrostatic equilibrium are ruled out. The CB objects and cirr us cores are indistinguishable physically or chemically in terms of th ese characteristics, which seem to represent the conditions to be foun d in ail small, low-mass molecular clouds regardless of Galactic latit ude.