Photographs of J-PEX aboard the Black Brant XI rocket before its launch.

A major outstanding question in the study of white dwarf stars concerns the relationship between the hydrogen and helium dominated groups and the interaction of these with the insterstellar medium. Of particular importance is the role He plays in the H-rich DA white dwarfs as they cool. Although He can be observed in the UV and optical bands, when present in very small quantities it is only detectable in the EUV. Furthermore, an interstellar He component can be revealed by observing its shadowing effect on a stellar EUV spectrum. However, until now, EUV instrumentation has lacked the spectral resolution capable of separating the signature of HeII from the large number of other lines (mainly Fe and Ni) present in the white dwarf spectra and distinguishing the source of HeII (photospheric or interstellar), if present.

J-PEX is a sounding rocket-borne normal incidence high resolution EUV spectrometer. Its objective is to obtain high resolution EUV spectra of white dwarf stars at EUV wavelengths. Covering the spectral range 225-245A it has a theoretical resolving power of 5000, 10 times better than EUVE. The mission is a collaboration led by the US Naval Research Laboratory, involving the University of Leicester MSSL and the US Lawrence Livermore National Laboratory.

The UK effort was to supply the high spatial resolution imaging detector together with its associated readout electronics. This MCP detector stack, constructed at Leicester, was based on newly-developed small pore (6micron channel diameter) plates and was coupled to a vernier anode readout provided by MSSL. Leicester also had responsibility for defining the scientific objectives of the first flight of the instrument, to observe the hot H-rich white dwarf G191-B2B, with the aim of determining whether or not helium is really present in the star and, if found, where it is located.

For results of the mission and further details, please see the J-PEX project pages.

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