Electron and local gas densities in diffuse interstellar clouds from measurements of CA I absorption.

Electron and local gas densities in 12 diffuse interstellar clouds have been determined from absorption measurements of Ca I λ4226 and Ca II λ3934 and an assumed fractional ionization χ_ɛ = 3 × l0^-4. Individual velocity components have been analyzed separately. The values for the electron density range from 0.055 to 0.57 cm^-3, while the local gas densities are between 180 and 1900 cm^-3. The large values for the electron and local gas densities toward ι, ζ, and perhaps 23 Ori, where few hydrogen molecules occur, may arise from recently compressed gas behind a shock.