CO Abundance and Isotopic Fractionation in Dark Clouds

Extract

In order to determine molecular fractional abundances, both the molecular density and hydrogen density must be known. In this study we have determined these parameters by fitting the observed intensities of J = 2 → 1 and J = 1 → 0 ¹³CO and C¹⁸O transitions using a spherical cloud LVG radiative transfer model. The kinetic temperature is determined by observations of ¹²CO and is found to be between 9 K and 13 K for our sources. The fractional abundance of CO is expected to rise rapidly between regions of low extinction and those with Av ≥ 4 mag, due primarily to the decrease in photodestruction rates (Langer 1976). The C¹⁸O fractional abundance data plotted as function of Av support a nonlinear relationship between X(C¹⁸O) and Av for Av ≤ 4 mag, with an indication of an asymptotoic value X(C¹⁸O) = 2.2 × 10 (-7) in highly obscured regions. For ¹⁶O/¹⁸O ratios of 250 (suggested by our data although possible uncorrected saturation of ¹³CO makes this a lower limit) and 700 (A. Penzias, private communication) the fractions of carbon in CO in well-shielded regions are .08 and .23, respectively.