Multiwavelength observations of cirrus clouds in the North Celestial Loop: The transition from atomic to molecular gas

Abstract

Two potential sites of H2 formation have been discovered in diffuse gas at high Galactic latitude through examining the far-infrared (FIR) H i 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 (53 936 spectra in the Spider and 23 517 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 H i observations from the Dominion Radio Astrophysical Observatory. We decompose the 10 000 H i profiles of the Spider and the 20 302 H i 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 H i velocity components merge or where there is a H i velocity-shear. © 2010 The Authors. Journal compilation © 2010 RAS.