HOT WHITE DWARFS IN THE LOCAL INTERSTELLAR MEDIUM: HYDROGEN AND HELIUM INTERSTELLAR COLUMN DENSITIES AND STELLAR EFFECTIVE TEMPERATURES FROM EXTREME-ULTRAVIOLET EXPLORER SPECTROSCOPY

摘要

We have obtained extreme-ultraviolet (EUV) spectra of the hot DA white dwarfs Feige 24, G191-B2B, MCT 0455-2812, HZ 43, GD 71, and GD 153 with the Extreme-Ultraviolet Explorer (EUVE) observatory. We use these stars as a background against which to perform a study of the local interstellar medium (LISM) absorption. We describe two important instrumental artifacts, fixed pattern noise and overlapping spectral orders, and we develop a methodology to explicitly remove these effects. Feige 24, G191-B2B, and MCT 0455 — 2812 exhibit a rich complex of heavy element transitions in their otherwise hydrogen-rich photospheres (Z ≈ 10~(-5)), while HZ 43, GD 71, and GD 153 are consistent with hydrogen atmospheres with a low metalhcity (Z ≤ 10~(-7)). The photoionization edge of interstellar He I at 504 A is clearly detected in the spectra of each of these white dwarfs. We have analyzed the EUV continuum spectra of these stars with a grid of pure-hydrogen model atmospheres and, in the case of Feige 24, G191-B2B, and MCT 0455-2812, with a grid of models incorporating new opacities of C, N, O, and Fe from the Opacity Project. We obtain new EUV continuum-based estimates of effective temperatures for our sample stars and, for the three high-metallicity white dwarfs, we establish the effects of heavy element blanketing on the EUV continuum of these objects and on the determination of effective temperatures. We find that the ratio of the neutral column densities of hydrogen and helium in the LISM is similar (n_(H I)_(He I) ≈ 14) for five out of six stars in the sample but that Feige 24 may have a substantially different value. The Feige 24 results suggest that the ionization state in the LISM is inho-mogeneous. Our measurements favor a higher degree of ionization of helium relative to hydrogen and also indicate that hydrogen in the LISM is predominantly neutral in at least two lines of sight.