The Galactic Warm Ionized Medium: the First Direct Measures of its Ionization and Abundances

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Hst Proposal Id #9410

Scientific paper

The warm ionized medium {WIM} is the dominant gaseous component of the Galactic halo and represents an important sink of the radiative and kinetic energy output of stars and supernovae, though the source of its ionization remains unknown. We will use stis spectroscopy of the post-AGB stars ROB 162 and ZNG 1 in the globular clusters NGC 6397 and Messier 5 to measure directly the abundances and ionization states of several key metals in the Galactic WIM. These sight lines are unique: because the two clusters also contain pulsars with published radio dispersion measurements, these are the only sight lines for which we can derive the column densities of both HI, em and ionH2, as well as the columns of multiple ionization stages of the important metals S, P, and Fe. We will use the proposed stis observations with existing use data to derive the total gas-phase abundances of S, P, and Fe for the material along these sight lines with no ionization uncertainties. We will directly measure the ionization fractions of S and P in the WIM. We will also infer the dust content of the WIM. Our study of the ionization state and dust content of the WIM will provide the best yet constraints for models of this gas. Our work will also provide the best constraint for the fundamental ``cosmic'' reference abundance {averaged over these sight lines} of the undepleted elements S and P.

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