AI 中文总结
本研究利用HST/COS、HST/STIS光谱观测麦哲伦星云33条视线，对比同位置H II区与中性ISM的S、Fe丰度，发现H II区Fe亏缺更强，可示踪难直接观测的致密分子云Fe亏缺。
AI 中文摘要
亏缺是指气相元素丰度因吸积到尘埃颗粒上而降低的观测现象。我们利用高分辨率紫外光谱（哈勃空间望远镜/宇宙起源光谱仪HST/COS和哈勃空间望远镜/空间望远镜成像光谱仪HST/STIS），沿33条视线测量了麦哲伦星云中的中性气相元素丰度（硫S、铁Fe），并将其与文献中6个同位置H II区（最大角距离小于3角分，对应50秒差距）的电离气相丰度（S、Fe）进行对比。对S丰度的对比显示，S在H II区及周围弥散星际介质中仅发生极少量亏缺。然而，我们发现H II区的气相Fe丰度可比邻近中性星际介质低0.3至2个 dex。这一差异很可能是Fe吸积到尘埃颗粒的量不同导致的。由于在H II区的温度下，气相Fe的吸积过程可能并不有效，因此Fe以固态形式的亏缺应发生在恒星形成之前的致密原子或分子云中。H II区中更强的Fe亏缺表明，含Fe尘埃颗粒在电离后的最初几百万年中能在破坏过程中存活下来。我们的观测表明，H II区中的Fe亏缺可作为致密分子云中Fe亏缺的有效示踪物，而致密分子云很难通过紫外吸收直接观测。
英文摘要
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 (HST/COS and HST/STIS), and compare them to ionized gas-phase abundances (S, Fe) adopted from the literature for six co-located H\,\textsc{ii} regions (with the furthest separation of $\lesssim3'$, 50 pc). Comparing S abundances show that S is minimally depleted in the H\,\textsc{ii} regions and surrounding diffuse ISM. However, we find that the gas-phase Fe abundances in H\,\textsc{ii} regions can be lower than those of the neighboring neutral ISM by 0.3 to 2 dex. This difference is likely 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\,\textsc{ii} regions, Fe depletion into solid form would have occurred in the dense atomic or molecular clouds prior to star formation. Stronger depletion in the H\,\textsc{ii} regions shows that Fe-bearing grains survive destruction in the first few million years following ionization. Our observations highlight that Fe depletion in H\,\textsc{ii} regions can be a useful tracer of Fe depletion in dense molecular clouds, which are challenging to observe directly via UV absorption.
