Formaldehyde towards compact HII regions : densities and isotope ratios.
Abstract
The first detections of the interstellar 2_{11}  2_{12} line of H_{2}^{13}CO are presented. The radial velocities of these lines, toward W33 and DR21, differ from those obtained from the 1_{10}  1_{11} line of H_{2}^{13}CO and H_{2}^{12}CO. Such differences are probably caused by gradients in the H_{2} density with radial velocity. The resulting H_{2} densities are 10^{5.1} cm^{3} (W33) and 10^{5.4} (DR21). The H_{2}CO data for Sgr B2 do not give a consistent H_{2} density. Using the derived H_{2} densities, corrections for photon trapping are obtained. The corrected (H_{2}^{12}CO/H_{2}^{13}CO) ratios are ≡100 (W33) and ≡70 (DR21). The result for Sgr B2 is obtained by correcting the measured isotope ratio in the 2_{11}  2_{12} line, 25±2. The (H_{2}^{12}CO/H_{2}^{13}CO) ratios are assumed to represent the (^{12}C/^{13}C) ratios; these are combined with double isotope measurements to estimate (^{16}O/^{18}O) and (^{14}N/^{15}N) ratios.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 November 1983
 Bibcode:
 1983A&A...127..388H
 Keywords:

 Abundance;
 Astronomical Spectroscopy;
 Formaldehyde;
 Gas Density;
 H Ii Regions;
 Molecular Clouds;
 Absorption Spectra;
 K Lines;
 Radial Velocity;
 Astrophysics