The Local EUV Radiation Field

A spectrum of the local extreme ultraviolet (EUV) radiation field is presented consisting of the combined EUV spectra of 48 stellar sources measured with the spectrometers aboard the Extreme Ultraviolet Explorer (EUVE) satellite. The flux from these 48 sources represents 99% of the flux from all detected sources in the EUVE all-sky survey at the longer EUV wavelengths (>250 Angstroms)\@. In actuality, the radiation field is dominated by a few bright sources with low intervening hydrogen column density, such as the bright B2II star Adhara (epsilon CMa). Though Adhara dominates the hydrogen ionizing Lyman continuum region of the spectrum (504--912 Angstroms), the helium ionizing photons (<504 Angstroms) mostly come from a small group of nearby hot white dwarfs. The measured total flux shortward of 504 Angstroms is not sufficient to explain the level of helium ionization (>25%) now being measured in the Local Cloud. A possible source of extra helium ionizing photons would be diffuse EUV emission, either from the hot Local Bubble or the conductive interface between the Local Cloud and the Local Bubble. Limits to the diffuse EUV radiation field resulting from a total of 20 million seconds of observations in 200 different directions are also shown to be insufficient to explain the helium ionization in the Local Cloud if standard emission models are used to parameterize the diffuse flux. The ionized helium is either a ``fossil'' of a previous ionizing event that has yet to return to equilibrium, or is caused by diffuse EUV radiation with a spectral signature peculiar enough to avoid detection with the current generation of EUV instrumentation. This work is supported by NASA contract NAS5-29298.