CO Isotope Line Shapes in Dark Clouds

Extract

The ratio of ground-state densities R_o ≡ N (¹³CO) / N (C₁₈O) has been used to infer physical and chemical conditions in giant molecular clouds (Wannier et al. 1976) and dark clouds (Mahoney et al. 1976; Langer et al. 1979). In dark clouds R_o is found to vary from values near the terrestrial ratio [¹³C] [¹⁶O] / [¹²C] [¹⁸O] ∼ 5 at positions of high extinction to values ∼20 at positions of low extinction. In this paper we present high-resolution J = 1 → 0 spectra of CO, ¹³CO, and C¹⁸O at positions of high extinction in TMC-2, L134, and L134N. The C¹⁸O lines have non-Gaussian wings and are ∼ half as wide as the ¹³CO lines. We find that R_O must vary across the line, from a minimum of R₀ ∼ 4 at the peak of the C¹⁸O line to a maximum of R_O ∼ 10 in the wings, unless the ¹³CO line has peak opacity ∼5. The variation of R_O with position and with velocity is consistent with models of clouds which have a dense core with low velocity-dispersion and low fractionation, and a rarefied envelope with high velocity-dispersion and high fractionation.