Warm H I halos around molecular clouds.

Observations of H I have been made with the 305 m Arecibo dish toward the isolated edges of several molecular clouds. Comparison with 1.3 and 2.6 mm CO line emission indicates the existence of H I halos extending 0.5 to several pc beyond the CO boundaries. Because the halos appear in emission against the 100 K galactic H I background, the gas is evidently warmer than much of the interstellar H I gas. The observed column densities and transition zone thicknesses agree well with steady state calculations equating the formation rate of H2 on grains to its destruction by UV radiation.