A high-resolution study of the CO-H₂ conversion factor in the diffuse cloud MBM 40

We made CO(1-0) observations of 103 lines of sight in the core and envelope of the highlatitude cloudMBM40 to determine howthe CO-H₂ conversion factor (X_CO) varies throughout the cloud. Calibrating X_CO with CH data at similar resolution (1 arcmin for CO, 1.5 arcmin for CH) yields values of X_CO ranging from 0.6 × 10²⁰ to 3.3 × 10²⁰ cm^-2 (K km s^-1)^-1 with an average of 1.3 × 10²⁰ cm^-2 (K km ^s-1)^-1. Given that the cloud has a peak reddening of 0.24 mag, it should be classed as a diffuse rather than a translucent molecular cloud. The mass obtained from the CO data and our values of X^CO is 9.6M_⊙ for the core, 12M_⊙ for the envelope and 10M_⊙ for the periphery of the cloud. A third of the molecular mass of the cloud is found in a region with E(B - V) < 0.12 mag. With these mass estimates, we determine that the cloud is not gravitationally bound.