2001ApJ...561..238H

Using the Heinrich-Hertz-Telescope we have detected for the first time the [C I] ³P₁⟶³P₀ 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 pc above the Galactic plane. The width of the [C I] line is quantitatively comparable to that of the ¹²CO rather than to that of the ¹³CO line, 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 denser cirrus clouds close to the Galactic plane. Moreover, our observations suggest that for IVCs cooling by the [C I] line is as important as or even more important than that by the low level CO lines. PDR models used to approximate 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.