Photoionized and photodissociated regions around main-sequence stars

Within a molecular cloud, the strong ultraviolet radiation field produced by newly formed stars dissociates and ionizes the surrounding molecular gas. The radiative flux depends on the effective temperature and metallicity of the star. Using the most recent line-blanketed atmosphere models from Kurucz, we obtain the rates of ionizing and dissociating photons from stars with effective temperatures of 7.5 x 10(3) to 5 x 10(4) K, and for metallicities between 0.01 times solar and solar. With a radiative transfer model, we then compute the basic structures and sizes of the photoionized and photodissociated regions produced by stars embedded in a molecular gas with uniform densities. Absorption of the UV flux by dust decreases the mass of H II and H I produced within the cloud, and its effects are taken into account in our model. We also discuss the constraints imposed by photodissociated regions on the number of intermediate- and high-mass stars that can form in molecular clouds.