Dust in the Ionized Medium of the Galaxy: GHRS Measurements of Aliiiand Siii

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Abstract
We present interstellar absorption-line measurements of the ions S III and Al III toward the stars β1 Sco, μ Col, ξ Per, ζ Oph, ρ Leo, and HD 18100 using archival data from the Goddard High-Resolution Spectrograph on board the Hubble Space Telescope. The ions Al III and S III trace heavily depleted and nondepleted elements, respectively, in ionized gas along the sight lines to these late-O/early-B stars. We use the photoionization equilibrium code CLOUDY to derive the ionization correction relating the ratio N(Al III) / N(S III) to the gas-phase abundance [Al/S]i [≡log (N(Al)/N(S))i-log (Al/S)☉] in the ionized gas. For spectral types considered here, the corrections range from 0.1 to 0.3 dex and are independent of the assumed ionization parameter, i.e., the ratio of ionizing photon density to mean electron density. Using the results of these photoionization models, we find [Al/S]i~-1.0 in the ionized gas toward β1 Sco, ξ Per, and ζ Oph; along the low-density path toward μ Col we find [Al/S]i ≈-0.8. Since S is not depleted onto grains, these values of [Al/S]i (≈[Al/H]i) imply that Al-bearing grains are present in the ionized nebulae around these stars. If the warm ionized medium (WIM) of the Galaxy is photoionized by OB stars, the observations of ρ Leo and HD 18100 imply [Al/S]i=-0.4 to -0.5 in the WIM and thus the presence of dust grains containing Al in this important phase of the interstellar medium. While photoionization appears to be the most likely origin of the ionization for Al III and S III, we cannot rule out confusion from the presence of hot, collisionally ionized gas along the sight lines to β1 Sco and HD 18100. We find that [Al/S]i in the ionized gas along the six sight lines is anticorrelated with the electron density and average sight-line neutral density. The degree of grain destruction in the ionized medium of the Galaxy is not much higher than in the warm neutral medium. The existence of grains in the ionized regions studied here has important implications for the thermal balance of these regions.
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