Neutral atomic carbon in intermediate-velocity clouds

Using the Heinrich-Hertz-Telescope we have detected for the first time the [C I] P-3(1) --> P-3(0) line transition at 492 GHz of neutral atomic carbon from two intermediate-velocity clouds (IVCs): IVC 135+54-46 and the Draco Nebula. Both clouds are located at high Galactic altitudes, more than 200 p c above the Galactic plane. The width of the [C I] line is quantitatively c omparable to that of the (CO)-C-12 rather than to that of the (CO)-C-13 lin e, as commonly found for Galactic plane clouds. This indicates that neutral atomic carbon within the diffuse medium at high Galactic altitudes fills a significantly larger volume in comparison to the situation in dark or dens er cirrus clouds close to the Galactic plane. Moreover, our observations su ggest that for IVCs cooling by the [C I] line is as important as or even mo re important than that by the low level CO lines. PDR models used to approx imate the observed line intensity ratios indicate that the molecular gas in high-altitude IVCs is exposed to a weak interstellar radiation field while its density must be high.