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E. B. Jenkins (Princeton U. Obs.)
Investigations of absorption lines with FUSE have increased our understanding of the gas-phase abundances of a selection of neutral and singly-ionized elements within the interstellar medium in our part of the Galaxy. The conditions sampled range from a low density, warm medium that is partly ionized and within about 100~pc of the Sun to more distant, dense, cool clouds that are probably very well shielded from ionizing photons with energies above 13.6 eV. Abundances of O~I relative to H~I appear to be approximately constant and near the solar ratio, and we can expect that this should not break down in partly ionized regions because the fractional ionizations of oxygen and hydrogen are coupled to each other by a very strong charge exchange reaction. The photoionization cross section of neutral argon is much greater than that of hydrogen, while its recombination rate is about the same. Thus the fact that the ratio of Ar~I to O~I in the local medium is below the cosmic reference value of Ar/O, where O~I serves as a proxy for H~I, suggests that the partial ionization of H and He in the local ISM is maintained in steady state by EUV and soft x-ray radiation, rather than an incomplete recombination from a more ionized condition within the past 106\,yr. N~I also appears to be deficient in the low-density medium, but reverts to a near solar abundance ratio for moderately large H column densities, and then shows a mild depletion at even higher column densities. A comparison of the column densities of Si~II, Fe~II and P~II in the local medium shows that the depletions of silicon and iron onto dust grains are similar in magnitude to those seen for more distant gas observed with the high-resolution spectrographs on HST.
The conclusions reported here arose from observations by the NASA-CNES-CSA FUSE mission operated by Johns Hopkins University, supported by NASA contract NAS5-32985.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.