Neutral Atomic Carbon in Intermediate‐Velocity Clouds

Abstract

Using the Heinrich-Hertz-Telescope we have detected for the —rst time the [C I] line tran3P 1 ] 3P 0 sition 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 diUuse medium at high Galactic altitudes —lls a signi—cantly 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 —eld while its density must be high. . . . . ..