0 Assigned ID: 4732 ----- Attempting Submission 1 (Wed May 24 22:22:22 GMT 2023) ----- The halo of the Milky Way is home to numerous high-velocity cloud (HVC) complexes of uncertain origin. HVCs are multi-phase and trace a considerable mass flow rate onto and away from the Galaxy. Although multiple studies have attempted to determine the origin of these clouds based on their gas-phase metallicities and kinematics, no single model is able to explain the entire population. Here we propose a systematic analysis of another key property of HVCs -- their dust content. Dust is a separate and crucial piece of information on HVC origin. The presence of dust indicates the cloud has been (at least partly) processed through star formation, allowing us to rule out certain origin models, such as condensation of coronal gas and inflow of purely extragalactic gas. Outside of the Magellanic System, dust has only been found in 2 HVC complexes. We propose an archival program to determine the UV depletion of Si, Al, and Fe in 8 HVC complexes using COS G130M/G160M & STIS E140M spectra of 18 background AGN. If dust is present, this analysis would triple the number of clouds with measured UV dust depletions (from 4 to 12), thus significantly advancing our understanding of HVC chemistry and origin. The PI Dr Frances Cashman is an expert on the ISM and IGM and on Cloudy simulations of interstellar absorbers, and the properties of interstellar gas near the Galactic Center. She has worked with a variety of UV spectra and has first-authored an AJ paper on the analysis and interpretation of UV STIS spectra and dust depletions. Dr Andrew Fox is an expert in the analysis and interpretation of COS and STIS spectra of diffuse interstellar gas. He has led seven HST GO and two AR proposals on CGM-related science and has first-authored over 20 refereed papers in relevant fields. He has experience with ionization modeling and Galactic radiation fields. He first-authored an ApJ paper on the first UV dust depletion measurement in the Magellanic Stream and first-authored an ApJL on the first UV dust depletion measurement in Complex C. Dr Rongmon Bordoloi is an expert on the Fermi Bubbles and on Galactic winds, and has first-authored several ApJ papers on these subjects. Dr David French is an expert on the ISM and CGM, as well as the analysis and interpretation of HST COS spectra. Intergalactic Medium and the Circumgalactic Medium 0 Regular false false true false false false SUPPORTED