巨型气态原行星的颗粒沉积
Grain Sedimentation in a Giant Gaseous Protoplanet
摘要：
我们计算了一颗巨大气态原行星中的颗粒沉降，例如Boss（1998）所设想的那种类型的盘不稳定性。Boss（1998）提出，这样的原行星会通过小颗粒的沉降形成核心。我们已经通过跟踪原行星收缩和演化过程中硅酸盐小颗粒的沉积来验证这一假设。我们发现，在最初的收缩过程中的原行星，这持续了一些$4\times 10^5$年，即使是非常小的（> 1微米）硅酸盐颗粒可以沉淀，创造一个核心的对流和非对流信封，虽然沉淀时间长得多，如果信封是对流，并允许颗粒被带回信封对流。由有机物质组成的颗粒在到达核心区域之前大多会蒸发，而水冰粒则会完全蒸发。这些结果表明，如果巨行星是通过引力不稳定机制形成的，那么由于颗粒的沉积，可以形成一个小的重元素核心，但它几乎完全由耐火材料组成。包括小行星捕获，我们发现核心质量在1和10 M之间的$_{\oplus}$，和一个总的高Z增强~40 M $_{\oplus}$。包层中的耐火材料将主要是水蒸气和有机残留物。
英文摘要：
We present a calculation of the sedimentation of grains in a giant gaseous protoplanet such as that resulting from a disk instability of the type envisioned by Boss (1998). Boss (1998) has suggested that such protoplanets would form cores through the settling of small grains. We have tested this suggestion by following the sedimentation of small silicate grains as the protoplanet contracts and evolves. We find that during the course of the initial contraction of the protoplanet, which lasts some $4\times 10^5$ years, even very small (> 1 micron) silicate grains can sediment to create a core both for convective and non-convective envelopes, although the sedimentation time is substantially longer if the envelope is convective, and grains are allowed to be carried back up into the envelope by convection. Grains composed of organic material will mostly be evaporated before they get to the core region, while water ice grains will be completely evaporated. These results suggest that if giant planets are formed via the gravitational instability mechanism, a small heavy element core can be formed due to sedimentation of grains, but it will be composed almost entirely of refractory material. Including planetesimal capture, we find core masses between 1 and 10 M$_{\oplus}$, and a total high-Z enhancement of ~40 M$_{\oplus}$. The refractories in the envelope will be mostly water vapor and organic residuals.
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