B. Wolff et al., Evidence for an ionization gradient in the local interstellar medium: EUVEobservations of white dwarfs, ASTRON ASTR, 346(3), 1999, pp. 969-978
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.