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Session 40 - The Interstellar Medium.
Display session, Tuesday, June 09
We present interstellar absorption line measurements of the ions \ionS3 and \ionAl3 towards the stars \beta ^1 Sco, \mu Col, \xi Per, \zeta Oph, \rho Leo, and HD 18100 using the Goddard High Resolution Spectrograph. The ions \ionAl3 and \ionS3 trace heavily depleted and non-depleted elements, respectively, in photoionized gas along the sightlines to these late-O/early-B stars. The ionized gas towards the first four of these stars, which are nearby disk stars, is likely concentrated in their \ionH2 regions. The latter two stars are at substantial distances above the Galactic plane; the absorption along these sightlines is likely tracing the warm ionized medium (WIM) of the Galaxy.
We use the photoionization equilibrium code CLOUDY to model low-density \ionH2 about late-O/early-B stars. The ionization correction relating the ratio N(\mbox\ionAl3) / N(\mbox\ionS3) to the gas-phase abundance [Al/S](\equiv \log \N(Al)/N( S) \ - \log \ Al/S \_ødot) in the ionized gas varies from 0.15 to 0.30 dex over the range of spectral types considered here and is virtually independent of the assumed particle density.
Using the results of these photoionization models, we find [Al/S] \, \sim-1.0 in the ionized gas towards \beta ^1 Sco, \xi Per, and \zeta Oph. Along the low-density sightline towards \mu Col we find [Al/S] \, \sim-0.8. These values [Al/S] 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 \rho Leo and HD 18100 imply -0.6 \lesssim [Al/S] \lesssim -0.3 and thus the presence of dust grains containing Al in the WIM of the Galaxy. The derived gas-phase abundances of the ionized gas are generally lower in high density regions than in low density regions, similar to the observed dependence on average sightline density of neutral gas abundances. The presence of dust in \ionH2 region and diffuse ionized gas suggests grain cores are quite resilient. The existence of grains in the ionized regions studied here has important implications for the thermal balance of these regions.
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