The Hydrogen Ionization in the Local Interstellar Medium: Effects of New EUV Sources

Recently, several very bright Extreme-UV (EUV) sources have been revealed by the ROSAT and the Extreme Ultraviolet Explorer (EUVE) all sky surveys. Among these new EUV sources, the B2 II star Epsilon CMa, the He-rich DO white dwarf MCT 0501-289, and the H-rich DA white dwarf MCT 0455-2812 have shown strong EUV radiation fields longward of the He I ionization edge at 504 Angstroms. Since these newly discovered strong EUV sources almost certainly change our predictions of the hydrogen ionization in the Local Cloud (Bruhweiler and Cheng 1988; Cheng and Bruhweiler and Cheng 1990), we have observed these three sources with the EUVE spectrometer to obtain better estimates for their contributions to the hydrogen ionization. We also present our EUVE observations of 6 other previously-known stellar EUV ionizing sources: HZ43, G191-B2B, Feige 24, HD 149499B, Beta CMa, and Sirius B. With this almost complete observed EUV ionizing radiation field, we now have the capability to greatly improve our early interstellar ionization predictions, which were based on a theoretical ionizing flux.