METAL-Z — measuring dust depletion in the low metallicity environments

A critical yet poorly understood aspect of the baryon cycle in galaxies is the depletion of metals from the gas to the dust phase via dust formation, and vice versa, the return of heavy elements from the dust to the gas phase via dust destruction. Understanding dust depletion is especially crucial in the low metallicity regime (5-10 % solar), corresponding to the typical metallicity of Damped Lyman-alpha (DLA) systems, which are used to track the chemical enrichment of the universe. Corrections for dust depletion in abundance measurements in DLAs rely on locally-derived calibrations of depletions based on abundance ratios. Yet, these calibrations do not exist at metallicities lower than that of the SMC.

In this talk, I will present results from the METAL-Z Hubble program (GO-15880), which obtained COS spectra toward UV-bright O/early B stars in local dwarf galaxies IC1613 (0.15 Z_⊙) and Sextans-A (0.08 Z_⊙), with the goal to measure dust depletions in these low metallicity environments. We combine the gas-phase abundances derived from METAL-Z with photospheric abundance measurements in young OBA stars from the literature to derive interstellar depletions of iron and sulfur, two key elements for understanding the chemical enrichment of the universe. Combined with depletion measurements available in the Milky Way, LMC (0.5 Z_⊙), and SMC (0.2 Z_⊙), the METAL-Z measurements allow us to constrain dust depletions as a function of metallicity down to 0.08 Z_⊙. These results can be used to correct metallicity measurements in DLAs.