Dust in the Ionized Medium of the Galaxy: Ghrs Measurements of Al

We present interstellar absorption line measurements of the ions S III and Al III towards the stars β 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 non-depleted elements, respectively, in ionized gas along the sightlines 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 towards β Sco, ξ Per, and ζ Oph; along the low-density path towards 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 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 ISM. 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 sightlines to β Sco and HD 18100. We find that [Al/S]i in the ionized gas along the six sightlines is anti-correlated with the electron density and average sightline 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. Subject headings: ISM: abundances—dust, extinction—HII regions—Galaxy: halo—ultraviolet: ISM