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arXiv 1302.4746astro-ph.CO
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I Zw 18中的化学富集和物理条件

Chemical enrichment and physical conditions in I Zw 18

Vianney Lebouteiller, Sara Heap, Ivan Hubeny, Daniel Kunth

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Accepted for publication in A&A. Fixed typos and references
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摘要：

节录。低金属量的恒星形成矮星系是了解星际介质化学富集的主要目标。HI区域对连续恒星形成事件释放的重元素的扩散和混合提供了重要的限制。我们的主要目标是研究HI区的富集和恒星形成历史与金属丰度演化之间的相互作用。我们用哈勃/COS观测了本地宇宙中最贫金属的恒星形成星系I Zw 18。中性气体的丰度来自远紫外吸收线（HI，CII，CII*，NI，OI，.）并与HII区的丰度进行比较。建立了计算电离结构和热过程的模型。我们调查的气体冷却HI区域通过物理诊断得出的精细结构水平的C+。我们发现，HI区的丰度是低的一个因素的~2相比，HII区。HI区和HII区之间的粉尘消耗没有差异。使用硫作为金属丰度示踪剂，我们计算出太阳的金属丰度为1/46（而HII区域为1/31）。从丰度比的研究，我们认为，C，N，O和Fe主要产生在大质量恒星。我们认为，HI信封可能包含口袋的原始气体的金属丰度基本上为零。最后，我们通过对CII* 吸收线的研究，导出了HI区的物理条件。从CII* 推导出的冷却速率与扩散中性气体中H原子的碰撞一致。假设尘埃上的光电效应是主要的气体加热机制，我们从CII* 冷却速率计算恒星形成速率。如果我们假设一个低的尘埃-气体比（比银河系的值低~2000倍），我们的测定与文献中的值吻合得很好。

英文摘要：

Abridged. Low-metallicity star-forming dwarf galaxies are prime targets to understand the chemical enrichment of the interstellar medium. The HI region provides important constraints on the dispersal and mixing of heavy elements released by successive star-formation episodes. Our primary objective is to study the enrichment of the HI region and the interplay between star-formation history and metallicity evolution. We observed the most metal-poor star-forming galaxy in the Local Universe, I Zw 18, with Hubble/COS. The abundances in the neutral gas are derived from far-UV absorption-lines (HI, CII, CII*, NI, OI, ...) and are compared to the abundances in the HII region. Models are constructed to calculate the ionization structure and the thermal processes. We investigate the gas cooling in the HI region through physical diagnostics drawn from the fine-structure level of C+. We find that HI region abundances are lower by a factor of ~2 as compared to the HII region. There is no differential depletion on dust between the HI and HII region. Using sulfur as a metallicity tracer, we calculate a metallicity of 1/46 solar (vs. 1/31 in the HII region). From the study of abundance ratios, we propose that C, N, O, and Fe are mainly produced in massive stars. We argue that the HI envelope may contain pockets of pristine gas with a metallicity essentially null. Finally, we derive the physical conditions in the HI region by investigating the CII* absorption line. The cooling rate derived from CII* is consistent with collisions with H atoms in the diffuse neutral gas. We calculate the star-formation rate from the CII* cooling rate assuming that photoelectric effect on dust is the dominant gas heating mechanism. Our determination is in good agreement with the values in the literature if we assume a low dust-to-gas ratio (~2000 times lower than the Milky Way value).

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