PHOTOLEVITATION OF DIFFUSE CLOUDS

Radiation pressure on dust grains can raise small dusty clouds above the main gaseous disk to high Galactic latitudes. This "photolevitation" effect drives neutral gas and dust into a soft Galactic fountain, and can maintain a column density of the order of 10(20) cm-2 above the main gaseous disk. This value is defined by dust opacity and corresponds to a normal dust-to-gas ratio with cosmic abundances. The maximum height reached by the photolevitated clouds depends on the radiation field and dust-to-gas ratios. Clouds located above luminous stellar clusters or near spiral arms with intense star formation can reach several hundred parsecs in height. This model may explain some observational evidence for the presence of dust mixed with gas at high latitudes in the Milky Way and in external galaxies.