Observations of Atomic Hydrogen in Dense Interstellar Dust Clouds

Abstract

High frequency resolution 21-cm line observations of almost 100 interstellar dust clouds showed that about one-third of them contain significant amounts of cold neutral hydrogen. Simple models for the velocity distribution of hydrogen in the clouds were fitted to the observations, and spin temperatures ranging from 16 to 40 K and column densities from 7 × 10¹⁹ to 6 × 10²⁰ atom cm⁻² were found. The ratios of H i to OH and H i to dust in the clouds were found to have about 1 to 10% of their value in the general interstellar medium.