Hydrogen emissivity in realistic nebulae - The effects of velocity fields and internal dust

Abstract

The paper presents calculations of the H-beta emissivity expected from nebulae with velocity gradients or internal dust. As has been found by Capriotti, Cox, and Mathews, Lyman line escape and destruction can prevent the 100 percent conversion of high-n Lyman lines into Ly-alpha and Balmer lines. For dusty environments such as the Orion Nebula or the general interstellar medium, the H-beta emissivity can be reduced by less than about 15 percent. Lyman line escape may cause still larger deviations in environments such as nova shells where the expansion velocities are large and velocity gradients likely. Although the partial conversion of Lyman lines only lowers the H-beta emissivity by typically less than about 10 percent under most circumstances, this introduces a systematic error in abundance measurements; the abundance of other elements relative to hydrogen will be overestimated by this amount. This effect must be considered in detail if very accurate abundance measurements are to be made. The present predictions of the deviation from case B emissivity are presented in a way in which they can be easily used by observers or incorporated into photoionization or shock codes.