Variable Interstellar Absorption toward HD 219188 --- Probing the Structure of an Interstellar Cloud

Physics

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Hst Proposal Id #9962 Ism And Circumstellar Matter

Scientific paper

Within the last 10 years, strong, narrow Na I absorption has appeared at v_SUN -38 km s^-1 toward the halo star HD 219188; that absorption has continued to strengthen, by a factor 2-3, over the past three years. The line of sight appears to be moving into/through a relatively cold, quiescent intermediate velocity {IV} cloud, due to the 13 mas/year proper motion of HD 219188. We propose to monitor the continuing changes in the IV absorption toward HD 219188 over the next three years with STIS echelle spectra -- making use of the rich diagnostics provided by the UV lines of various neutral and singly ionized species to determine abundances/depletions and physical conditions {temperature, density, ionization} as functions of depth within the cloud. In addition to providing a unique view of the detailed structure of an interstellar cloud, these data will yield constraints on grain scattering parameters {from the variation of ionization with depth} and on any dependence of depletion on local density.

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