A LARGE VELOCITY GRADIENT MODEL FOR FORMALDEHYDE SPECTRA IN DARK CLOUDS WITH APPLICATIONS TO GALACTIC ISOTOPIC ABUNDANCE RATIOS
Dark Nebula provide radio astronomers with a wealth of information. Many molecules present in these clouds have transitions which are easily seen with today's equipment. One such molecule is the ortho species of formaldehyde. The 6-cm transition is particularly interesting as it appears in absorption despite the low kinetic temperatures (10 - 50(DEGREES)K) of the clouds. This absorption makes formaldehyde a useful probe of cloud dynamics as the cooling effects are sensitive to the conditions present in the clouds.^ Initial attempts to model cloud dynamics relied on ad hoc assumptions concerning the cloud's temperature. Principally the assumption of thermal equilibrium was made. With the advent of high-speed computers, more realistic models, known as non-LTE (non Local Thermal Equilibrium), have emerged. These make the less stringent assumption of statistical equilibrium.^ The non-LTE models divide roughly into two classes. The microturbulent and Large Velocity Gradient (LVG) models. It is one of the latter class of models that will be developed here. LVG allows the simplest explanation for the linewidths and has none of the other potential problems of the other class of models. The model developed here encompassess the first 22 energy levels (in order of increasing energy) of ortho-formaldehyde and is shown to be consistent with observed spectra.^ As an application, the LVG model is used to generate theoretical corrections for radiative-trapping effects in dark clouds. These corrections are used to modify observational results to obtain the isotopic abundance ratio of ('12)C/('13)C in the galactic center and plane.^ The calculations show that the LVG model gives a straightforward method of generating theoretical profiles which fit quite well to experimental observations. In fact, the theoretical profiles match well with very general input parameters. Applying this model to the optically thin species H(,2)('13)CO and H(,2)C('18)O demonstrates that using even such thin species requires a radiative-trapping correction factor of about 8%. With this factor, the ('12)C/('13)C ratio is determined to be nearly terrestrial in the galactic plane, and approximately half terrestrial in the center. This indicates that formaldehyde in the galactic center may be undergoing some sort of chemical fractionation in the carbon isotope.^
MASSANO, WILLIAM JOSEPH, "A LARGE VELOCITY GRADIENT MODEL FOR FORMALDEHYDE SPECTRA IN DARK CLOUDS WITH APPLICATIONS TO GALACTIC ISOTOPIC ABUNDANCE RATIOS" (1982). ETD Collection for Fordham University. AAI8219252.