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arXiv 1502.06991astro-ph.EP
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小世界是由什么构成的?恒星丰度和行星的组成部分

What are little worlds made of? Stellar abundances and the building blocks of planets

Eric Gaidos

摘要:

如果太阳型恒星的光球层代表了行星形成的星周盘的组成,那么恒星元素丰度的光谱测定提供了这些行星的组成信息,包括较小的岩石行星。特别是,C/O比被认为是从盘状气体冷凝并被纳入行星的固体成分的关键决定因素。此外,行星可能会通过吸收重元素或被恒星吸积而在其宿主恒星的光球层上留下化学特征。行星的存在、不存在和组成可以通过恒星之间相对丰度的微小差异来揭示。我批判性地研究了这些方案,并表明:(i)银河系化学演化模型预测,C/O比预计将接近太阳的值,并在太阳附近的矮星之间变化不大;(ii)光谱调查的M矮星的限制与C/O $\gtrsim 1$到$<10^{-3}$的恒星的出现;(iii)星子的化学成分将由从分子云继承下来的富氧尘埃的成分控制,并在富尘环境中进行处理,而不是由具有恒星成分的气体控制。第二代更简化的微行星可以通过一些物质与尘埃贫化气体的再平衡来产生。最后,我将讨论如何微小的差异,恒星之间的相对丰度与冷凝温度可以解释的尘埃气体分离,也许在星周盘,而不是行星形成。

英文摘要:

If the photospheres of solar-type stars represent the composition of circumstellar disks from which any planets formed, spectroscopic determinations of stellar elemental abundances offer information on the composition of those planets, including smaller, rocky planets. In particular, the C/O ratio is proposed to be a key determinant of the composition of solids that condense from disk gas and are incorporated into planets. Also, planets may leave chemical signatures on the photospheres of their host stars by sequestering heavy elements, or by being accreted by the stars. The presence, absence, and composition of planets could be revealed by small differences in the relative abundances between stars. I critically examine these scenarios and show that (i) a model of Galactic chemical evolution predicts that the C/O ratio is expected to be close to the solar value and vary little between dwarf stars in the solar neighborhood; (ii) spectroscopic surveys of M dwarf stars limit the occurrence of stars with C/O $\gtrsim 1$ to $<10^{-3}$; and (iii) planetesimal chemistry will be controlled by the composition of oxygen-rich dust inherited from the molecular cloud and processed in a dust-rich environment, not a gas with the stellar composition. A second generation of more reduced planetesimals could be produced by re-equilibration of some material with dust-depleted gas. Finally, I discuss how minor differences in relative abundances between stars that correlate with condensation temperature can be explained by dust-gas segregation, perhaps in circumstellar disks, rather than planet formation.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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