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 high-latitude cloud MBM 40 to determine how the CO-H2 conversion factor (XCO) varies throughout the cloud. Calibrating XCO with CH data at similar resolution (1 arcmin for CO, 1.5 arcmin for CH) yields values of XCO 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 XCO is 9.6 M⊙ for the core, 12 M⊙ for the envelope and 10 M⊙ 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.