The Ionization of the Local Interstellar Medium as Revealed by Far Ultraviolet Spectroscopic Explorer Observations of N, O, and Ar toward White Dwarf Stars

• Published in: The Astrophysical journal 2000-07, Vol.538, p.L81-L85
• Main Authors: Jenkins, E. B, Oegerle, W. R, Gry, C, Vallerga, J, Sembach, K. R, Shelton, R. L, Ferlet, R, Vidal-Madjar, A, York, D. G, Linsky, J. L, Roth, K. C, Dupree, A. K, Edelstein, J
• Format: Article
• Language: English
• Subjects: Sciences of the Universe
• ISSN: 1538-4357; 0004-637X; 1538-4357
• DOI: 10.1086/312786

Far Ultraviolet Spectroscopic Explorer spectra of the white dwarf stars G191-B2B, GD 394, WD 2211-495, and WD 2331-475 cover the absorption features out of the ground electronic states of N I, N II, N III, O I, and Ar I in the far-ultraviolet, providing new insights on the origin of the partial ionization of the local interstellar medium (LISM) and, for the case of G191-B2B, the interstellar cloud that immediately surrounds the solar system. Toward these targets the interstellar abundances of Ar I, and sometimes N I, are significantly below their cosmic abundances relative to H I. In the diffuse interstellar medium, these elements are not likely to be depleted onto dust grains. Generally, we expect that Ar should be more strongly ionized than H (and also O and N, whose ionizations are coupled to that of H via charge-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 the surprisingly high ionization of He in the LISM found by other investigators. Our result favors the interpretation that the ionization of the local medium 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. Based on data obtained for the Guaranteed Time Team by the NASA-CNES-CSA FUSE mission operated by Johns Hopkins University. Financial support to US participants has been provided by NASA contract NAS5-32985.