arXiv 1408.18412014-08-08astro-ph.EP

行星陷阱和行星核：行星-金属丰度相关性的起源

Planet Traps and Planetary Cores: Origins of the Planet-Metallicity Correlation

Yasuhiro Hasegawa, Ralph E. Pudritz

备注：: 20 pages, 5 figures, 6 tables, accepted for publication in ApJ
DOI：: 10.1088/0004-637X/794/1/25

摘要：

大质量的系外行星优先在高金属丰度（[Fe/H]）恒星周围观察到，而低质量的系外行星则没有表现出这种效应。这种所谓的行星-金属丰度相关性通常支持这样的观点，即在$r<10$ AU观测到的大多数气体巨星是通过核心吸积过程形成的。我们调查这种现象的起源，使用半解析模型，其中标准的核心吸积发生在行星陷阱中的原恒星盘快速I型移民停止。我们重点研究了三个主要的系外行星群-热彗星，位于$r \simeq 1$ AU的系外彗星和低质量行星。我们表明，使用统计方法，行星金属丰度的相关性，以及在这些模型中再现。我们发现，有特定的过渡金属丰度值[Fe/H] $=-0.2$到$-0.4$，低于低质量人口占主导地位，并在木星人口接管。在所有观测到的金属丰度上，外行星都大大超过了热木星。通过核心吸积形成的低质量行星对金属丰度不敏感，这可能占所观察到的超级地球和热海王星的很大一部分。最后，形成大质量行星的一个控制因素是行星核心的临界质量（$M_{c,crit}$），它控制着失控气体吸积的开始。假设目前的数据在[Fe/H] $>-0.6$上是大致完整的，我们的模型预测类木行星的“平均”临界核心质量最有可能的值是$\braket{M_{c,crit}} \simeq 5 M_{\oplus}$而不是$10 M_{\oplus}$。这意味着，颗粒不透明的增生信封应发挥重要作用，降低$M_{c,crit}$。

英文摘要：

Massive exoplanets are observed preferentially around high metallicity ([Fe/H]) stars while low-mass exoplanets do not show such an effect. This so-called planet-metallicity correlation generally favors the idea that most observed gas giants at $r<10$ AU are formed via a core accretion process. We investigate the origin of this phenomenon using a semi-analystical model, wherein the standard core accretion takes place at planet traps in protostellar disks where rapid type I migrators are halted. We focus on the three major exoplanetary populations - hot-Jupiters, exo-Jupiters located at $r \simeq 1$ AU, and the low-mass planets. We show using a statistical approach that the planet-metallicity correlations are well reproduced in these models. We find that there are specific transition metallicities with values [Fe/H]$=-0.2$ to $-0.4$, below which the low-mass population dominates, and above which the Jovian populations take over. The exo-Jupiters significantly exceed the hot-Jupiter population at all observed metallicities. The low-mass planets formed via the core accretion are insensitive to metallicity, which may account for a large fraction of the observed super-Earths and hot-Neptunes. Finally, a controlling factor in building massive planets is the critical mass of planetary cores ($M_{c,crit}$) that regulates the onset of runaway gas accretion. Assuming the current data is roughly complete at [Fe/H]$>-0.6$, our models predict that the most likely value of the "mean" critical core mass of Jovian planets is $\braket{M_{c,crit}} \simeq 5 M_{\oplus}$ rather than $10 M_{\oplus}$. This implies that grain opacities in accreting envelopes should play an important role in lowering $M_{c,crit}$.

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