The ionization of the local interstellar medium as revealed by Far Ultraviolet Spectroscopic Explorer observations of N, O, and Ar toward white dwarfstars

Far Ultraviolet Spectroscopic Explorer spectra of the white dwarf stars G19 1-B2B, GD 394, WD 2211-495, and WD 2331-475 cover the absorption features o ut of the ground electronic states of N I, N II, N III, O I, and Ar I in th e far-ultraviolet, providing new insights on the origin of the partial ioni zation of the local interstellar medium (LISM) and, for the case of G191-B2 B, the interstellar cloud that immediately surrounds the solar system. Towa rd these targets the interstellar abundances of Ar I, and sometimes N I, ar e significantly below their cosmic abundances relative to H I. In the diffu se interstellar medium, these elements are not likely to be depleted onto d ust grains. Generally, we expect that Ar should be more strongly ionized th an H (and also O and N, whose ionizations are coupled to that of H via char ge-exchange reactions) because the cross section for the photoionization of Ar I is very high. Our finding that Ar I/H I is low may help to explain th e surprisingly high ionization of He in the LISM found by other investigato rs. Our result favors the interpretation that the ionization of the local m edium is maintained by a strong extreme-ultraviolet flux from nearby stars and hot gases, rather than an incomplete recovery from a past, more highly ionized condition.