arXiv 2005.112092020-05-22astro-ph.EP

潮汐膨胀使低密度亚土星与核心吸积

Tidal Inflation Reconciles Low-Density Sub-Saturns with Core Accretion

Sarah Millholland, Erik Petigura, Konstantin Batygin

备注：: Accepted for publication in ApJ, 17 pages, 8 figures. Code available at this URL: https://github.com/smillholland/Sub-Saturns/
DOI：: 10.3847/1538-4357/ab959c

摘要：

虽然太阳系中没有天王星和土星大小之间的行星，但我们目前的系外行星普查包括几十颗质量和半径都很好的行星。这些亚土星表现出不同的体积密度，从~ $0.1-3\ \rm{g\ cm}^{-3}$。当简单地模拟为岩石核心上的氢/氦包层时，这种密度的多样性转化为行星包层分数的多样性，$f_\rm{env}=M_\rm{env}/M_p$的范围从~ $10\%$到~ $50\%$。具有$f_\rm{env}\sim50\%$的行星对通过核心成核吸积形成巨行星的传统模型提出了挑战，该模型预测当$M_\rm{env}\sim M_\rm{core}$时失控气体吸积的开始。在这里，我们表明，许多这些明显的$f_\rm{env}\sim50\%$行星的信封比他们似乎少丰富，占潮汐加热后。我们提出了一个新的框架，用于模拟亚土星内部，其中包括信封通货膨胀由于潮汐，这是由观察到的非零偏心率，以及潜在的偏心率。因此，当我们将我们的模型应用于已知的亚土星时，我们推断出比无潮汐估计更低的$f_\rm{env}$。我们提出了一个案例研究K2-19 b，一个中等偏心的次土星。忽略潮汐，K2-19 b似乎有$f_\rm{env}\sim50\%$，不稳定地保持在失控阈值附近;通过包括潮汐，我们发现$f_\rm{env}\sim10\%$，解决了紧张局势。通过对$4-8\ R_{\oplus}$行星的系统分析，我们发现大多数（但不是全部）类似的富氘行星具有更适度的$f_\rm{env}\sim10\%-20\%$包络。因此，许多亚土星可以被理解为经历了显著半径膨胀的亚海王星，而不是一个单独的天体类别。潮汐半径膨胀可能在其他大小的行星中起着重要作用，包括像WASP-107 b这样的超低密度行星。

英文摘要：

While the Solar System contains no planets between the sizes of Uranus and Saturn, our current exoplanet census includes several dozen such planets with well-measured masses and radii. These sub-Saturns exhibit a diversity of bulk densities, ranging from ~$0.1-3\ \rm{g\ cm}^{-3}$. When modeled simply as hydrogen/helium envelopes atop rocky cores, this diversity in densities translates to a diversity in planetary envelope fractions, $f_\rm{env}=M_\rm{env}/M_p$ ranging from ~$10\%$ to ~$50\%$. Planets with $f_\rm{env}\sim50\%$ pose a challenge to traditional models of giant planet formation by core-nucleated accretion, which predict the onset of runaway gas accretion when $M_\rm{env}\sim M_\rm{core}$. Here we show that many of these apparent $f_\rm{env}\sim50\%$ planets are less envelope rich than they seem, after accounting for tidal heating. We present a new framework for modeling sub-Saturn interiors that incorporates envelope inflation due to tides, which are driven by the observed non-zero eccentricities, as well as potential obliquities. Consequently, when we apply our models to known sub-Saturns, we infer lower $f_\rm{env}$ than tides-free estimates. We present a case study of K2-19 b, a moderately eccentric sub-Saturn. Neglecting tides, K2-19 b appears to have $f_\rm{env}\sim50\%$, poised precariously near the runaway threshold; by including tides, we find $f_\rm{env}\sim10\%$, resolving the tension. Through a systematic analysis of $4-8\ R_{\oplus}$ planets, we find that most (but not all) of the similarly envelope-rich planets have more modest envelopes of $f_\rm{env}\sim10\%-20\%$. Thus, many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects. Tidal radius inflation likely plays an important role in other size classes of planets including ultra-low-density Jupiter-size planets like WASP-107 b.

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英文摘要：

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