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AR 15807 (Archival Research)

Sat Jul 20 08:57:05 GMT 2024

Principal Investigator: Adam Ritchey
PI Institution: Eureka Scientific Inc.
Investigators (xml)

Title: Improved Constraints on Grain Growth from a Survey of Phosphorus and Chlorine Depletions in the Diffuse ISM
Cycle: 27

Abstract
Numerous theoretical efforts and significant observational constraints indicate that dust formation in the outer atmospheres of evolved stars and in the cooling ejecta of supernovae is insufficient to fully account for dust production in the Milky Way. Additional grain growth in the cold interstellar medium is required. Direct constraints on the growth of dust grains in interstellar clouds comes from detailed knowledge of the depletions of different elements from the gas-phase. However, the observed depletions of phosphorus and chlorine have challenged a straightforward interpretation of the depletion data. One puzzling aspect is that the gas-phase abundances of P and Cl appear to be supersolar in low depletion sight lines. We propose a comprehensive survey of P II and Cl I absorption lines from STIS E140H and E140M spectra along over 80 sight lines probing diverse physical conditions in the local Galactic ISM. FUSE data will be examined to determine the amount of Cl II present. Our focus will be on weak, unsaturated absorption features and our use of recently-updated oscillator strengths will yield the most precise P and Cl abundances derived to date. The improved constraints on the depletion behaviors of P and Cl should offer clarity to the interpretation of gas-phase element depletions. Furthermore, our analysis of neutral chlorine fractions will allow us to disentangle the complex multi-phase gas along each line of sight and may reveal an improved correlation between Cl depletions and the molecular hydrogen fractions associated with the cold dense phase.