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arXiv 1502.01160astro-ph.EP
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富集包层的临界核质量:H2O凝聚的作用

Critical core mass for enriched envelopes: the role of H2O condensation

J. Venturini, Y. Alibert, W. Benz, M. Ikoma

摘要:

上下文在行星形成的核心吸积情景中,迄今为止进行的大多数模拟总是假设吸积包层具有太阳成分。通过对地球上陨石雨的研究和数值模拟,我们知道微行星在穿越原行星包层时必须经历热消融和破裂。一旦原行星获得了大气层,原始包层就会富含来自星子的挥发物和硅酸盐。在形成过程中,包层成分的这种变化可能对最终的大气成分和巨行星的形成时间尺度产生重大影响。目标。研究考虑原行星的包层富集的物理意义,这是由于冰质微行星在进入核心的过程中被破坏。特别关注的是放在信封上的临界核心质量的影响,水冷凝可能发生。方法.内部结构模型的数值求解与实施更新的不透明度为所有范围的金属丰度和软件CEA计算状态方程。CEA计算任意气体混合物的化学平衡,并允许某些物质的冷凝,包括水。这意味着相变的潜热始终包含在总能量预算中。结果当在内部结构方程中考虑一个富集的包层成分时,发现临界堆芯质量显著降低。一个特别强大的减少的临界核心质量的行星,其信封金属量大于Z=0.45时,外边界条件是合适的冷凝水发生在大气层的顶层。我们表明,这种效果是定性保存时,大气是化学平衡。

英文摘要:

Context. Within the core accretion scenario of planetary formation, most simulations performed so far always assume the accreting envelope to have a solar composition. From the study of meteorite showers on Earth and numerical simulations, we know that planetesimals must undergo thermal ablation and disruption when crossing a protoplanetary envelope. Once the protoplanet has acquired an atmosphere, the primordial envelope gets enriched in volatiles and silicates from the planetesimals. This change of envelope composition during the formation can have a significant effect in the final atmospheric composition and on the formation timescale of giant planets. Aims. To investigate the physical implications of considering the envelope enrichment of protoplanets due to the disruption of icy planetesimals during their way to the core. Particular focus is placed on the effect on the critical core mass for envelopes where condensation of water can occur. Methods. Internal structure models are numerically solved with the implementation of updated opacities for all ranges of metallicities and the software CEA to compute the equation of state. CEA computes the chemical equilibrium for an arbitrary mixture of gases and allows the condensation of some species, including water. This means that the latent heat of phase transitions is consistently incorporated in the total energy budget. Results. The critical core mass is found to decrease significantly when an enriched envelope composition is considered in the internal structure equations. A particular strong reduction of the critical core mass is obtained for planets whose envelope metallicity is larger than Z=0.45 when the outer boundary conditions are suitable for condensation of water to occur in the top layers of the atmosphere. We show that this effect is qualitatively preserved when the atmosphere is out of chemical equilibrium.

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