Ionization of the Low-Density Interstellar Medium

The fraction of 0 and early B-type stars embedded in H ii regions has been obtained for the solar neighborhood. it is found that a substantial fraction of the OB stars are outside the faintest H ii regions detectable on the Palomar Sky Survey plates. This fraction increases from 44 percent for OS stars to 83 percent for B5 stars, and is independent of reddening for the sample considered. The hydrogen ionizing radiation due to OB stars not embedded in H ii regions is 1.3 x 10-14 photons cm 3 1 in the plane of the Galaxy. The integrated spectrum of this flux is similar to that of an O7 star. These results imply that a very important source of ionization of the "intercloud medium" is due to 0 and B stars not associated with normal H ii regions. The z distribution of the stars inside and outside H ii regions is presented. This study is based on 2883 OB stars. For M33, the ionization rate is also obtained for the interarm H ii region and ainounts to 2 x 10-13 1 and 7 x 10-14 1 at distances to the nucleus of 2' and 5', respectively. All the evidence available indicates that most of the ionization in the interarm H ii region is due to late 0 stars and early B stars, so that cloud collisions, X-rays, and cosmic rays are ruled out as substantial sources of ionization. Subject headings: early-type stars - interstellar matter - nebulae