Future EUV Observations of the Interstellar Medium

Abstract

Interstellar helium can be observed directly only in the Extreme Ultraviolet (EUV) waveband. However, high sensitivity and spectral resolution are required to disentangle the He II Lyman series lines from the many heavier element lines found in the continuum spectra of many white dwarfs. In 2001 we observed the white dwarf G191‐B2 with the world’s first high resolution EUV astrophysical spectrometer (J‐PEX), flown on a NASA sounding rocket. We were able to detect interstellar He II lines against the background of other species. Our experience underlines the promise of He II absorption line measurements in mapping structure in the Local Interstellar Medium (LISM) such as the Local Bubble and the need for many more high‐resolution EUV observations. J‐PEX will fly again in 2008 to observe the binary white dwarf Feige 24, but the limited observation time with sounding rockets makes them unattractive for future detailed studies. Here we describe the scientific case for high‐resolution EUV spectroscopy, summarize the technology that makes such measurements practical, and present a concept for a ∼3‐month orbital mission, in which the J‐PEX payload is modified for flight as a small‐satellite. Such a mission might produce sensitive high‐resolution spectra for ∼30 white dwarfs, many of which could contribute to the understanding of structure in the LISM.