Multiwavelength observations of cirrus clouds in the North Celestial Loop: physical parameters of molecular sites

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 (CO)-C-12 (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 (CO)-C-12 (J = 1-0) and (CO)-C-13 (J = 1-0) observations from the Institut de Radioastronomie Millimetrique and new (CO)-C-12 (J = 2-1) observations from the James Clerk Maxwell Telescope (resolution approximate to 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 cm(-3)) than at the locations of the (CO)-C-12 peaks (approximate to 10(3) cm(-3)), confirming that a small density could explain the absence of coincidence between the infrared excess peak and the (CO)-C-12 peak. Self-shielding is probably efficient given the computed (CO)-C-12 column densities (approximate to 10(15) cm(-2)).