A High-Resolution Study of the CO-H2 Conversion Factor in the Diffuse Cloud MBM 40
Abstract
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' for CO, 1.5' 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(solar) for the core, 12 M(solar) for the envelope, and 10 M(solar) 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.