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Title:
The Ionization of the Local Interstellar Medium as Revealed by Far Ultraviolet Spectroscopic Explorer Observations of N, O, and AR toward White Dwarf Stars
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.
Affiliation:
AA(Princeton University Observatory, Princeton, NJ 08544), AB(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218), AC(ISO Data Center, ESA Astrophysics Division, P.O. Box 50727, 28080 Madrid, Spain.; Laboratoire d'Astronomie Spatiale, B.P. 8, 13376 Marseille Cedex 12, France), AD(Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 94720), AE(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218), AF(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218), AG(Institut d'Astrophysique de Paris, 92 bis Boulevard Arago, Paris 7504, France), AH(Institut d'Astrophysique de Paris, 92 bis Boulevard Arago, Paris 7504, France), AI(Department of Astronomy and Astrophysics, University of Chicago, 5640 Ellis Avenue, Chicago, IL 60637), AJ(JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309), AK(Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218), AL(Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138), AM(Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA 94720)
Publication:
The Astrophysical Journal, Volume 538, Issue 1, pp. L81-L85. (ApJL Homepage)
Publication Date:
07/2000
Origin:
UCP
ApJ Keywords:
ISM: Abundances, ISM: Clouds, Ultraviolet: ISM, Stars: White Dwarfs
DOI:
10.1086/312786
Bibliographic Code:
2000ApJ...538L..81J

Abstract

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.
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