The ionization of cloud and intercloud hydrogen by O and B stars

The ionization and acceleration of neutral clouds in the neighborhood of a single O star located in a low-density intercloud medium (ICM) are analyzed. It is shown that due to ionized cloud debris, the rms electron density may increase to 1 per cu cm in the lifetime of the star. It is suggested that an O star which moves away from its primordial nebula at a velocity of 10 to 20 km/s may continuously surround itself with ionized cloud debris and may never extensively ionize a low-density ICM. The lifetime of a remote cloud exposed to relatively uniform Lyman continuum radiation is derived and evaluated as a function of position in the Galaxy. It is found that extensive O-star ionization of a low-density ICM cannot be maintained for more than a few million years if the cloud debris is not cleared away by, for example, supernovae. Various ICM models are examined in which B stars and possibly nuclei of planetary nebulae play a major role in the ionization of interstellar hydrogen. It is shown that such stars may ionize a larger volume of the Galaxy than O stars, even though the volume emissivity of their Lyman continuum radiation is only 5% to 15% of that of O stars.