1999A&A...344..955W

We present medium and high angular resolution I and CO observations of the galactic cirrus cloud IVC 135+54-45 and compare the line emission with intensities of the IRAS 100µm and 60µm bands. Its distance above the galactic plane is about 220pc, thus well outside the scale height of normal molecular gas (≃74pc). Thus IVC 135+54-45 offers a unique possibility to study the cold interstellar medium in the transition zone between the galactic plane and the halo. We establish a linear relation between the IRAS 100µm intensities and the proton column densities in the outer part of the cloud where the H₂ content is negligible. In the centre of the cloud we find two clumps which have more FIR-emission than can be accounted for by a linear N(I)/I₁₀₀ correlation. The clumps are connected to I filaments. A detailed analysis of one of the clumps reveals that the FIR-excess is associated mainly with the filament and to a lesser degree with the clump. This result is in contradiction to our CO observations which show strong emission in the clump and only weak emission in the filament. We have evidence that the FIR-excess in the filament is partly caused by an increased dust temperature and not due to the existence of H₂ alone. We conclude that the filament and the clump represent different epochs in the transition from atomic to molecular gas. The clump is more evolved and might be close to chemical equilibrium while formation of molecular gas has not reached equilibrium in the filament. We ascribe the increased temperature in the filament to chemical energy released by H₂ formation on dust grains.