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

We present an enlarged and homogeneous set of interstellar hydrogen and hel ium column densities obtained through continuum and ionization edges fittin g of nearby hot white dwarfs observed with the Extreme Ultraviolet Explorer (EWE). We compare the HI columns towards the targets and their locations r elative to the soft X-ray cavity contours drawn by Snowden et al. (1998). W e then use the inferred ionization degrees of hydrogen and helium (availabl e towards 17 objects) as a database for a search of systematic trends in th e local interstellar medium. We find that the ionization degree of helium d oes not show any trend, with a constant value around 40%. At variance with helium, hydrogen ionization varies within the Local Bubble: there is an ion ization increase along the general direction of the Canis Major cavity. The se results are consistent with hydrogen being ionized by the Canis Major ho t stars Adara and Mirzam. However, the co-existence of fully neutral hydrog en and strongly ionized helium along some 1-o-s remains to be explained.