Measuring the Ionization of O Star Winds

Abstract

We present an analysis of wind line profiles from Far Ultraviolet Spectroscopic Explorer (FUSE) spectra of two O7 supergiants in the Large and Small Magellanic Clouds (Sk -67°111 and AV 232, respectively). Model fits yield the column densities of S IV, S VI, P IV, P V, N III, and N IV, providing the first direct measurement of the ionization balance in stellar winds. The ratios of S IV/S VI and P IV/P V are consistently lower in the LMC star. IUE and Hubble Space Telescope archival spectra are also used to measure N IV and N V, but the much higher optical depth makes the N V measurements inconclusive. The velocity and optical depth distributions in the wind are qualitatively similar between the two stars, when scaled to their terminal velocities. The terminal velocities are different, with AV 232 being lower (as found previously in SMC stars and linked to lower metallicity). These are the first results from a program to investigate wind ionization and velocity structure among hot stars in local galaxies, and they demonstrate the higher accuracy in measuring column densities of less abundant ions, such as phosphorus and sulfur, observable in the FUSE range.