[Previous] | [Session 6] | [Next]

[6.05] A New Insight on the Ionization of the Local Interstellar Medium, as Revealed by FUSE Observations of Ar~I toward White Dwarf Stars

E.B. Jenkins (Princeton U.), W.R. Oegerle, K.R. Sembach, R.L. Shelton (JHU), A. Vidal-Madjar, R. Ferlet (IAP, France), C. Gry (ISO Data Cent.), FUSE Science Team

FUSE spectra of the white dwarf stars G191-B2B and GD394 indicate that the absorption features of interstellar Ar~I at 1048.22Å\ have equivalent widths W_\lambda\leq 10mÅ. Using the column density of neutral hydrogen N({\rm H~I})=2.3\times 10¹⁸{\rm cm}^-2 reported by Vidal-Madjar, et al. (1998: Astr. Ap., 338, 694) toward G191-B2B, we infer that Ar~I/H~I toward this star is less than 60% of the cosmic abundance ratio of Ar to H (from B stars and the Sun). In the diffuse interstellar medium, argon is not likely to be depleted onto dust grains. Instead, its apparent deficiency probably arises from the fact that the photoionization cross section of Ar~I is much larger than that of H~I, while the two atoms have nearly equal recombination rates with free electrons. In the partially ionized local interstellar medium, the argon atoms are mostly ionized and thus much less conspicuous at 1048.22Å.

For some time, the moderately large fractional ionization of He in the local medium has been difficult to understand. A flux of ionizing photons with E~>~24.6 eV and sufficient intensity to create the steady-state ionization of He has not yet been observed directly (either a diffuse component or from known, probable sources), but our argon observations favor its existence. This finding seems to rule out the proposition that the gas is recombining from a more highly ionized condition and has not yet attained equilibrium.

This research is supported by NASA contract NAS5-32985.