2010MNRAS.406.2713B

Two potential sites of H₂ formation have been discovered in diffuse gas at high Galactic latitude through examining the far-infrared (FIR) Hi ratio and looking for an excess over that expected from an atomic medium. We call these the Spider and Ursa Major fields. New ¹²CO and ¹³CO Five College Radio Astronomical Observatory observations are presented for both regions (53936 spectra in the Spider and 23517 spectra in Ursa Major). Although there is a correlation between FIR excess and CO emission, we find that the FIR excess peaks do not coincide with the ¹²CO emission peaks, indicating that CO might be a poor tracer of H₂ in diffuse regions. This implies (i) that the density is too small to allow CO excitation, (ii) that the CO self-shielding is insufficient or (iii) local variations of the dust properties. The ¹²CO observations are compared with HI observations from the Dominion Radio Astrophysical Observatory. We decompose the 10000 HI profiles of the Spider and the 20 302 HI profiles of Ursa Major into Gaussian components. We always find at most two narrow components and one broad component. CO always seems to appear where two HI velocity components merge or where there is a HI velocity-shear.