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arXiv 1002.4447astro-ph.EPastro-ph.SR
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GJ 436 b的内部结构模型

Interior structure models of GJ 436b

N. Nettelmann, U. Kramm, R. Redmer, R. Neuhaeuser

摘要:

GJ 436 b是第一颗在质量和半径上与海王星相似的太阳系外行星。海王星大小的行星特别有趣的性质是它们的质量Mp和半径Rp接近水行星的理论M-R关系。然而,给定Mp、Rp和平衡温度,各种内部组成是可能的。这里提出了一套广泛的内部结构模型,说明了内部成分的依赖性和可能的阶段的水发生在假定富含水的行星,如GJ 436 b的大气温度分布和平均密度的不确定性。我们展示了如何可以缩小的解决方案,如果理论约束的形成和模型大气中应用或潜在的观测约束的大气金属丰度Z1和潮汐勒夫数K2。我们通过假设三层(氢氦包层,水层,岩石核心)或两层(H/He/H2O包层,岩石核心)来模拟内部。对于水,我们使用基于FT-DFT-MD模拟的状态方程H2O-REOS。一些氢/氦的混合物似乎必须解释测量的半径。对于最热的模型,H/He的质量分数可以降低到10^-3,但仍然超过约0.7 REarth。如果出现水,它将基本上处于等离子体相或超离子相,但不是冰相。不含金属的包层模型有0.02<k2<0.2,核心质量不能从k2的测量中确定。相比之下,0.3<k2<0.82的模型要求外层的金属丰度Z1<0.89。当k2>0.3时,地核质量的不确定度减小到0.4Mp,当k2>0.5时,进一步减小到0.2Mp,地核质量和Z1成为k2的敏感函数。为了进一步缩小解决方案的范围,有必要适当处理大气层和演变。

英文摘要:

GJ 436b is the first extrasolar planet discovered that resembles Neptune in mass and radius. The particularly interesting property of Neptune-sized planets is that their mass Mp and radius Rp are close to theoretical M-R relations of water planets. Given Mp, Rp, and equilibrium temperature, however, various internal compositions are possible. A broad set of interior structure models is presented here that illustrates the dependence of internal composition and possible phases of water occurring in presumably water-rich planets, such as GJ 436b on the uncertainty in atmospheric temperature profile and mean density. We show how the set of solutions can be narrowed down if theoretical constraints from formation and model atmospheres are applied or potentially observational constraints for the atmospheric metallicity Z1 and the tidal Love number k2. We model the interior by assuming either three layers (hydrogen-helium envelope, water layer, rock core) or two layers (H/He/H2O envelope, rocky core). For water, we use the equation of state H2O-REOS based on FT-DFT-MD simulations. Some admixture of H/He appears mandatory for explaining the measured radius. For the warmest considered models, the H/He mass fraction can reduce to 10^-3, still extending over ~0.7 REarth. If water occurs, it will be essentially in the plasma phase or in the superionic phase, but not in an ice phase. Metal-free envelope models have 0.02<k2<0.2, and the core mass cannot be determined from a measurement of k2. In contrast, models with 0.3<k2<0.82 require high metallicities Z1<0.89 in the outer envelope. The uncertainty in core mass decreases to 0.4 Mp, if k2>0.3, and further to 0.2 Mp, if k2>0.5, and core mass and Z1 become sensitive functions of k2. To further narrow the set of solutions, a proper treatment of the atmosphere and the evolution is necessary.

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