Evidence for Homogeneous Ionization of Helium in the Local Interstellar Medium from EUVE Spectroscopy of Hot DA Stars

We present an analysis of the Extreme Ultraviolet Explorer (EUVE) spectra of the DA stars Feige 24, MCT0455--281, HZ 43, GD 153, GD 71, and Sirius B. This sample is selected on the basis of the stars' low interstellar hydrogen column densities and because of their widely separated Galactic coordinates. With the exception of Sirius B, these stars all have a prominent He i photoionization edge and a well defined Lyman continuum from which we derive reliable interstellar column densities of neutral hydrogen and neutral helium. For all the stars in the sample the measured abundance ratios of Hei :Hi differ significantly from the cosmic abundance ratio. We interpret these measurements in terms of ionization of helium in the local interstellar medium. The most striking result is that for all the Galactic lines of sight investigated, the relative ionization of helium is nearly constant with He i/H i= 0.07 +/- 0.02. This is similar to the result found for the hot DA GD 246 (Vennes et al. 1993, ApJ, 410, L119) for which a direct measurement of the He ii column was possible. The current sample probes various Galactic latitudes and longitudes and is indicative that helium is homogeneously ionized in the local interstellar medium. In optical spectra all these stars exhibit a pure hydrogen atmosphere; in the EUV both Feige 24 and MCT0455--281 show a substantial number of metals. We do not find metals in the remainder of the objects reported here, and we provide stringent upper limits for He, C, N, and O in the photospheres of these stars. This work has been supported by NASA contract NAS5-30180.