A Discussion on astronomy in the ultraviolet - Interstellar gas studies in the balloon ultraviolet

Abstract

Spectral observations at a resolution of 0.1 A covering the region 2870—2740 A have been made of several stars with an objective grating spectrograph mounted on a bal­loon-borne star-stabilized platform. We report our measurements of the interstellar lines of Mg II at 2795.53 A and 2802.70 A and Mg i at 2852.13 A. There are two important astrophysical reasons for observing these interstellar lines. First, in the general (H i) interstellar medium, Mg is predominantly in the Mg”** state, so its interstellar abundance can be determined directly from the Mg ii lines if the position on the curve of growth is known. Second, observation of both Mg+ and Mg° gives information on the ionization balance and leads to a value for the interstellar electron density. Values for the interstellar Mg abundance and electron density are derived for the gas in the directions of the stars observed. The results indicate that the predominant ionizing mechanism in cool clouds is photoionization by starlight