Observing Colocated Neutral and Ionized Gas-phase Iron Depletion in the Magellanic Clouds

Depletion is the observed phenomenon where gas-phase elemental abundances are reduced through accretion onto dust grains. We measure neutral gas-phase elemental abundances (S, Fe) in the Magellanic Clouds along 33 sightlines using high-resolution UV spectroscopy (Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) and HST/Space Telescope Imaging Spectrograph (STIS)), and compare them to ionized gas-phase abundances (S, Fe) adopted from the literature for six colocated H ii regions (with the furthest separation of ≲ 3 ′ , 50 pc). Comparison of S abundances shows that S is minimally depleted in the H ii regions and surrounding diffuse interstellar medium (ISM). However, we find that the gas-phase Fe abundances in H ii regions are lower than those of the neighboring neutral ISM by >0.5 dex and >2 σ in three SMC regions. This difference is consistent with an offset in the amount of Fe depleted into dust grains. As accretion of gas-phase Fe is likely not effective at the temperatures of the H ii regions, Fe depletion into solid form would have occurred in the dense atomic or molecular clouds prior to star formation. The implied stronger depletion in the H ii regions suggests that Fe-bearing grains survive destruction in the first few million years following ionization. Our observations highlight that Fe depletion in H ii regions can be a useful tracer of Fe depletion in dense molecular clouds, which are challenging to observe directly via UV absorption.