A High-Resolution Study of the CO-H2 Conversion Factor in the Diffuse Cloud MBM 40

TL;DR

This study measures how the CO-H2 conversion factor varies across the diffuse cloud MBM 40, revealing significant spatial variation and providing detailed mass estimates that suggest the cloud is not gravitationally bound.

Contribution

It provides the first high-resolution mapping of X_CO variation within a diffuse cloud, improving understanding of molecular mass estimates in such environments.

Findings

X_CO varies from 0.6 to 3.3 x 10^20 cm^-2 [K km s^-1]^-1

The cloud's total molecular mass is approximately 31.6 solar masses

The cloud is not gravitationally bound based on mass estimates

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-H_2 conversion factor (X_CO) varies throughout the cloud. Calibrating X_CO with CH data at similar resolution (1' for CO, 1.5' for CH) yields values of X_CO ranging from 0.6 10^20 to 3.3 10^20 cm^-2 [K km s^-1]^-1 with an average of 1.3 10^20 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 9.6 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.