Dwek Eli
Observational Cosmology Lab., Code 665 NASA Goddard Space Flight Center, Greenbelt, MD 20771.
Astrophys J. 2016 Jul 10;825(2). doi: 10.3847/0004-637x/825/2/136. Epub 2016 Jul 12.
The origin and depletion of iron differ from all other abundant refractory elements that make up the composition of the interstellar dust. Iron is primarily synthesized in Type Ia supernovae (SNe Ia) and in core collapse supernovae (CCSN), and is present in the outflows from AGB stars. Only the latter two are observed to be sources of interstellar dust, since searches for dust in SN Ia have provided strong evidence for the absence of any significant mass of dust in their ejecta. Consequently, more than 65% of the iron is injected into the ISM in gaseous form. Yet, ultraviolet and X-ray observations along many lines of sight in the ISM show that iron is severely depleted in the gas phase compared to expected solar abundances. The missing iron, comprising about 90% of the total, is believed to be locked up in interstellar dust. This suggests that most of the missing iron must have precipitated from the ISM gas by cold accretion onto preexisting silicate, carbon, or composite grains. Iron is thus the only element that requires most of its growth to occur outside the traditional stellar condensation sources. This is a robust statement that does not depend on our evolving understanding of the dust destruction efficiency in the ISM. Reconciling the physical, optical, and chemical properties of such composite grains with their many observational manifestations is a major challenge for understanding the nature and origin of interstellar dust.
铁的起源和消耗与构成星际尘埃成分的所有其他丰富的难熔元素不同。铁主要在Ia型超新星（SNe Ia）和核心坍缩超新星（CCSN）中合成，并存在于渐近巨星分支（AGB）恒星的外流中。由于在Ia型超新星中搜寻尘埃的结果提供了有力证据，证明其抛射物中不存在任何大量尘埃，因此只有后两者被观测到是星际尘埃的来源。因此，超过65%的铁以气态形式注入星际介质（ISM）。然而，沿星际介质中多条视线方向的紫外线和X射线观测表明，与预期的太阳丰度相比，气相中的铁严重贫化。缺失的铁约占总量的90%，据信被锁定在星际尘埃中。这表明，大部分缺失的铁一定是通过冷吸积在预先存在的硅酸盐、碳或复合颗粒上，从星际介质气体中沉淀出来的。因此，铁是唯一一种其大部分生长需要在传统恒星凝聚源之外发生的元素。这是一个有力的说法，不依赖于我们对星际介质中尘埃破坏效率不断变化的理解。使这种复合颗粒的物理、光学和化学性质与其众多观测表现相协调，是理解星际尘埃的性质和起源的一项重大挑战。
