晉升

(中國科學院紫金山天文臺南京210008)

博士學位論文摘要選登

類地行星的形成、內(nèi)部結構與大氣逃逸

晉升?

(中國科學院紫金山天文臺南京210008)

截止到2014年4月21日,已發(fā)現(xiàn)了1 490多顆系外行星和3 705顆Kepler候選體.這從觀測角度證明了行星在銀河系中是普遍存在的.對系外行星的研究豐富并加深了人們對行星形成與演化的認識.另外,新的觀測與發(fā)現(xiàn)也不斷提出新的科學問題.本論文開展了類地行星的形成演化、內(nèi)部結構以及大氣逃逸的研究.

本文第2章針對行星形成晚期的大規(guī)模碰撞階段開展了數(shù)值模擬,發(fā)現(xiàn)類地行星(包括位于系統(tǒng)宜居區(qū)內(nèi)的行星)的形成是普遍現(xiàn)象.模擬結果表明,巨行星的較大軌道傾角不利于星子吸積，因為在高傾角巨行星的擾動下,大量星子會被散射出系統(tǒng)或直接撞向中央恒星,這使得星子盤內(nèi)部的物質(zhì)大幅度減小.加入軌道遷移的模擬表明，行星的軌道遷移是產(chǎn)生短周期類地行星的主要機制.除了遷移機制外，我們還發(fā)現(xiàn)通過碰撞合并機制形成的短周期類地行星，其動力學形成過程主要體現(xiàn)在行星胚胎間的大規(guī)模碰撞.

在第3章，我們建立了一個計算類地行星內(nèi)部結構的數(shù)值模型，并利用這個模型得到了木衛(wèi)二的3種可能結構.進一步，我們把這個內(nèi)部結構模型應用于低質(zhì)量系外行星,發(fā)現(xiàn)一些系外行星的質(zhì)量半徑關系不符合巖石類行星,它們只能用具有大氣包層或者含有大量水冰成分的結構模型來擬合.這些結果可以對系外行星的總體物質(zhì)成分做出限定.

在第4章,我們用semi-gray模型(Guillot 2010)對現(xiàn)有的計算行星大氣結構的程序做了改進，加入了恒星輻射對行星大氣上層的加熱效應.這個改進一方面使行星演化程序適用于短周期行星，另一方面可以得到行星大氣上層輻射區(qū)域的結構，這樣程序就可以模擬由恒星的X-ray與EUV輻射驅動的流體動力學大氣逃逸.本章的模擬表明,低質(zhì)量行星很容易受到大氣逃逸影響,它們有可能在演化階段被剝離全部初始大氣.相反,氣態(tài)巨行星的演化受大氣逃逸的影響較小,它們只能被剝離很少一部分大氣.本章還在行星內(nèi)核質(zhì)量、大氣質(zhì)量比例、軌道半長徑的參數(shù)空間對大氣逃逸做了研究,發(fā)現(xiàn)在行星演化早期的100 Myr內(nèi)大氣逃逸最劇烈.其后,大氣逃逸對行星演化的影響不明顯.

在第5章,將大氣逃逸模型與基于核吸積模型的行星族群綜合分析(Planetary population synthesis)結合起來(Mordasini et al.2012a,b).我們給出了在行星的半徑分布上由大氣逃逸產(chǎn)生的統(tǒng)計特征.研究發(fā)現(xiàn),在2R⊕附近行星半徑呈雙峰分布(Owen&Wu 2013),這個雙峰分布與行星族群最初的特性相關.因為在由核吸積模型產(chǎn)生的行星族群中,所有行星的大氣均為原生的H/He大氣.此外，小于4R⊕的行星的初始大氣質(zhì)量比例一般小于10%,它們的大氣層容易被全部剝離.而大質(zhì)量行星具有較大的初始大氣質(zhì)量比例與引力勢能,它們受大氣逃逸的影響很小.通過與Kepler候選體的半徑分布做比較，我們排除了不含大氣逃逸的演化模型與加熱效率為100%的大氣逃逸模型.

本文的第6章介紹了一個計算行星光譜的輻射轉移模型.這是為將來建立一套大氣反演程序所做的準備工作.

As of 2014 April 21,over 1490 con firmed exoplanets and 3705 Kepler candidates have been detected.This implies that exoplanets may be ubiquitous in the universe.In this paper,we focus on the formation,evolution,and internal structure of terrestrial planets,and the atmospheric escape of close-in planets.

In chapter 2,we investigate the dynamical evolution of planetary system after the protoplanetary disk has dissipated.We find that in the final assembly stage,the occurrence of terrestrial planets is quite common,and in 40%of our simulations finally at least one planet is formed in the habitable zone.We also find that if there is a highly-inclined giant planet in the system,a great many bodies will be either driven out of the system,or collide with the giant planet or the central star.This will lead to the difficulty in planetary accretion.Moreover,our results show that planetary migration can lead to the formation of close-in planets.Besides migration,close-in terrestrial planets can also be formed by a collisionmerger mechanism,which means that planetary embryos can kick terrestrial planets directly into orbits that are extremely close to their parent stars.

In chapter 3,we construct numerically an internal structure model for terrestrial planets,and provide three kinds of possible internal structures of Europa(Jupiter’s moon)based on this model.Then,we calculate the radii of low-mass exoplanets for various mass combinations of core and mantle,and find that some of them are inconsistent with the observed radius of rocky planets.This phenomenon can be explained only if there exists a large amount of water in the core,or they own gaseous envelopes.

In chapter 4,we improve our planetary evolution codes using the semi-gray model of Guillot(2010),which includes the incident flux from the host star as a heating source in planetary atmosphere.The updated codes can solve the structure of the top radiative zone of intensely irradiated planets,and thus can simulate the atmospheric escape of close-in planets driven by strong stellar X-ray or EUV emissions.We find that low-mass planets are sensitive to the atmospheric escape,and they could lose all their initial H/He envelopes during the evolution.On the other hand,gas giant can only lose a small fraction of their initial envelopes.We then carry out a parameter study of atmospheric escape at the planetary core mass,envelope mass fraction,and semi-major axis space.We find that the most intense phase of evaporation occurs within the early 100 Myr.Afterwards,atmospheric escape only has a small impact on the planetary evolution.

In chapter 5,we apply our new planetary evolution model to different synthetic planet populations that are directly produced by the core-accretion paradigm(Mordasini et al.2012a,b).We show that although the mass distribution of the planet populations is hardly affected by evaporation,the radius distribution clearly shows a break around 2R⊕.This break leads to a bimodal distribution in planet sizes(Owen&Wu 2013).Furthermore,the bimodal distribution is related to the initial characteristics of the planetary populations.We find that in two extreme cases,namely without any evaporation or with a 100%heating efficiency in the evaporation model,the final radius distributions show signi ficant differences compared to the radius distribution of Kepler candidates.

In chapter 6,we introduce a radiative transfer model that can calculate the radiation spectrum of close-in exoplanets.

Formation and Internal Structure of Terrestrial Planets,and Atmospheric Escape

JIN Sheng

(Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210008)

?2014-07-06獲得博士學位,導師:紫金山天文臺季江徽研究員、馬月華研究員,德國馬普天文研究所Christoph Mordasini博士、Thomas Henning教授;shengjin@pmo.ac.cn