2011MNRAS.416.1250B

In this third paper of the series, we continue to investigate the transition between atomic and molecular gas in two potential sites of molecule formation at high Galactic latitude, which we call the Spider and Ursa Major. Using Five College Radio Astronomical Observatory ¹²CO (J= 1–0) observations and four clump identification algorithms, the clump properties are first determined in both regions, showing similar properties compared to the clumps found in translucent clouds. New higher resolution ¹²CO (J= 1–0) and ¹³CO (J= 1–0) observations from the Institut de Radioastronomie Millimétrique and new ¹²CO (J= 2–1) observations from the James Clerk Maxwell Telescope (resolution ≈ 20 arcsec) are also presented in five fields for the Spider (560 spectra) and in two fields for Ursa Major (288 spectra). Using a large velocity gradient model, physical parameters (density and column density) are determined in each field. The densities are smaller at the location of the infrared excess peaks (<200/cm3) than at the locations of the ¹²CO peaks ( ≈ 10³/cm3), confirming that a small density could explain the absence of coincidence between the infrared excess peak and the ¹²CO peak. Self-shielding is probably efficient given the computed ¹²CO column densities ( ≈ 10¹⁵/cm2).