Evidence for an ionization gradient in the local interstellar medium: EUVE observations of white dwarfs

We present an enlarged and homogeneous set of interstellar hydrogen and helium column densities obtained through continuum and ionization edges fitting of nearby hot white dwarfs observed with the Extreme Ultraviolet Explorer (EUVE). We compare the HI columns towards the targets and their locations relative to the soft X-ray cavity contours drawn by Snowden et al. (1998). We then use the inferred ionization degrees of hydrogen and helium (available towards 17 objects) as a database for a search of systematic trends in the local interstellar medium. We find that the ionization degree of helium does not show any trend, with a constant value around 40%. At variance with helium, hydrogen ionization varies within the Local Bubble: there is an ionization increase along the general direction of the Canis Major cavity. These results are consistent with hydrogen being ionized by the Canis Major hot stars Adara and Mirzam. However, the co-existence of fully neutral hydrogen and strongly ionized helium along some l-o-s remains to be explained.