The abundance of negative hydrogen in small hot interstellar clouds

The observed intensities of the diffuse interstellar absorption band at lambda 4430 have minimum values, for given stellar distances, that are equivalent to the reddening caused by the small Ambarzumian-type clouds studied by B. Stroemgren. This indicates that there is no negative hydrogen present between these clouds, which are then identified with the hot H I phase of interstellar matter. The equilibrium density of H- inside such clouds is calculated from mean densities of neutral hydrogen atoms and free electrons, derived from radio observations for the local region outside the large Orion-arm clouds. The filling factor of the small clouds is taken to be the same as that preserved in the structure of the Gum Nebula from before its ionization. An electron temperature of 3375 K corresponding to the local degree of ionization of the small-cloud hydrogen then leads to a mean density of negative hydrogen equal to 2 x 1013 cm-2 kpc-1, in agreement with the observed diffuse-band intensities. (Auth.)