Formaldehyde towards compact HII regions : densities and isotope ratios.

The first detections of the interstellar 2₁₁ - 2₁₂ line of H₂¹³CO are presented. The radial velocities of these lines, toward W33 and DR21, differ from those obtained from the 1₁₀ - 1₁₁ line of H₂¹³CO and H₂¹²CO. Such differences are probably caused by gradients in the H₂ density with radial velocity. The resulting H₂ densities are 10^5.1 cm^-3 (W33) and 10^5.4 (DR21). The H₂CO data for Sgr B2 do not give a consistent H₂ density. Using the derived H₂ densities, corrections for photon trapping are obtained. The corrected (H₂¹²CO/H₂¹³CO) ratios are ≡100 (W33) and ≡70 (DR21). The result for Sgr B2 is obtained by correcting the measured isotope ratio in the 2₁₁ - 2₁₂ line, 25±2. The (H₂¹²CO/H₂¹³CO) ratios are assumed to represent the (¹²C/¹³C) ratios; these are combined with double isotope measurements to estimate (¹⁶O/¹⁸O) and (¹⁴N/¹⁵N) ratios.