2004ApJ...610..320L

We present measurements of the C¹⁸O/C¹⁷O abundance ratio based on observations of J=1⟶0 lines of these isotopomers toward star-forming cores in the Taurus molecular cloud. Our data set includes measurements along 648 lines of sight through these clouds, covering both low and high column density regions. We compare the integrated intensity ratio for each line of sight with a simple model of emission from a dense cloud to determine this abundance ratio. Using this model, we find that a C¹⁸O/C¹⁷O abundance ratio of 4.0±0.5 is consistent with the data. However, at low column densities, it appears that a higher abundance ratio may be more appropriate. We examine ways in which the abundance ratio might be changed in the outer parts of molecular clouds and conclude that selective photodissociation of C¹⁷O by external ultraviolet light can increase the abundance ratio. A two-phase model, incorporating a C¹⁷O-free ``sheath'' of cloud material surrounding a self-shielded inner cloud region, is fitted to the data. Using this model and an assumed sheath H₂column density of 4x10²¹/cm2, we find a lower abundance ratio of 2.8±0.4 for the material in the shielded inner cloud. This new result is consistent with recent results from ultraviolet absorption spectroscopy through translucent clouds and measurements of the ¹³C¹⁸O/¹³C¹⁷O ratio in the Ophiuchus molecular cloud.