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研究生:李見修
研究生(外文):Chien-Hsiu Lee
論文名稱:系外行星之軌道演化暨大氣逸散
論文名稱(外文):Atmospheric Blow-off and Orbital Evolution of Close-in Extra-solar Planets
指導教授:葉永烜
指導教授(外文):Wing-Huen Ip
學位類別:碩士
校院名稱:國立中央大學
系所名稱:天文研究所
學門:自然科學學門
學類:天文及太空科學學類
論文種類:學術論文
畢業學年度:94
語文別:英文
論文頁數:48
中文關鍵詞:系外行星軌道演化大氣逸散
外文關鍵詞:Atmospheric Blow-offOrbital EvolutionExtra-solar PlanetExoplanet
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我們對太陽系外行星的研究可概分為以下兩個課題:
(一) 軌道演化
當行星系統演化至晚期時,行星與宿主恆星之間藉由潮汐力的作用
,可能出現軌道衰減及主星加速自轉的現象。潮汐力作用的證據之一便是
軌道圓化效應。觀測數據顯示小軌道行星多為圓軌道,可知其經歷很強的
潮汐作用。潮汐作用與行星的質量成正比,與軌道距離成反比。因此,在
強潮汐力的影響下,重質量行星軌道迅速衰減,最後旋入主星。此一效應
可解釋目前觀測上缺乏小軌道重質量系外行星的現象。我們的模擬結果顯
示,距主星0.02 天文單位內,超過兩倍木星質量的系外行星所需的旋入
時間小於二十億年。同時,在行星旋入的過程中,行星的軌道角動量會轉
移到主星上,造成主星自轉加速。藉由小樣本的檢視,我們並未發現快速
自旋主星的存在。然而,我們發現恆星HD 209458 的自轉快於理論預測值。
依潮汐力的計算,此自轉結果相當於木星質量的系外行星自0.02 天文單
位旋入主星。然而,此初始軌道與HD 209458 現有的系外行星軌道(0.045
天文單位)過於接近,軌道無法穩定存在。因此,HD 209458 主星自轉加
速的現象無法以潮汐力的效應解釋。
(二) 大氣逸散
目前已知大部份的系外行星為類木型系外行星,主要組成成份為氫
。而恆星在來曼α波段所產生的輻射壓會對氫原子產生影響,加速系外行
星的大氣逸散。我們的研究在於估記此輻射壓的效應強度。在古典的大氣
逸散中,將氫原子逸散的機率視為一階梯函數。當氫原子的熱速度大於重
力逃脫速度時,氫原子便可逸散。但在宿主恆星輻射壓的影響下,即使氫
原子未達脫離速度,也有可能逸散,因而逸散的機率不只是一簡單的階梯
函數。在我們的模擬中,宿主恆星的重力場即可改變逸散機率函數。其效
應遠大於古典逸散。當加入主星輻射壓的效應時,透過逸散所造成的質量
損失率約為僅考慮重力影響的十倍之多。然而對小軌道的行星而言,需考
慮Roche lobe 效應。當主星的外氣層底(exobase)超過Roche lobe 時,
應以Roche lobe 當作主要逸散的場所,質量損失率也會因此減小。在我
們的估算中,HD 209458b 的外氣層底並未超過Roche lobe。由輻射壓效
應所造成的質量損失率為3×1012g/s。然而系外行星OGLE-TR-56b 的Roche
lobe 遠小於外氣層底,因此若簡單的以輻射壓效應考慮將高估其質量損失

We have studied the atmospheric blow-off and orbital evolution of close-in
extra-solar planets. Our work can be divided into two parts:
1.Orbital evolution:
We have investigated the late stage orbital evolution of close-in extra-solar
planets including the dynamic interaction between planet and star, which in-
cludes planetary orbital decay and stellar spin-up. Our result can account
for the dearth of the massive planets within 0.02AU. We also examined a
small number of statistics to determine the possibility of planet engulfment.
Although no fast rotators have been found, we have noticed a discrepancy
between the observations and the theoretical predictions for HD 209458’s ro-
tation period, which cannot be explained by the engulfment of a 1MJ planet
originally located at 0.02AU.
2.Atmospheric blow-off:
We have estimated the mass loss rate under different conditions: (i) for an
isolated planet with a classical Jeans escape; (ii) for a close-in planet with a
Jeans escape and gravitational effect from the star; (iii) for a close-in planet
subject to gravitational effects and strong radiaion pressure from the star. Our
results show that each added term (stellar gravity and radiation) could have
significant influence. In some cases, the mass loss rate was increased by a
factor of 10. In our simulation, the mass loss for HD 209458b, HD 149026b,
TrES-1b, and XO-1b could be as large as 1,000,000,000,000 g/s
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.2 Giant Planet Theories . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.1 Formation and Migration . . . . . . . . . . . . . . . . . . . . . . . 2
1.2.2 Interior Structure and Evolution . . . . . . . . . . . . . . . . . . .3
1.3 Transiting Extra-solar Planets . . . . . . . . . . . . . . . . . . . . .4
1.4 Scientific Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.5 Chapter Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2 Modification of the Rotation Periods of Stars by the Tidal
Accretion of Extra-solar Planets . . . . . . . . . . . . . . . . . . . . . 11
2.1 Tidal Interaction . . . . . . . . . . . . . . . . . . . . . . . . . . .11
2.2 Lack of Massive Close-in Extra-solar Planets . . . . . . . . . . . . . 13
2.3 Rotation Period of G-type Stars . . . . . . . . . . . . . . . . . . . .13
2.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3 Exospheric Atomic Hydrogen Escape from Close-in Hot Jupiters
under the Influence of the Stellar Radiation Pressure . . . . . . . . . . .23

3.1 Observation of the Extra-solar Planet Atmosphere . . . . . . . . . . . 23
3.2 Current Upper Atmosphere Model . . . . . . . . . . . . . . . . . . . . 24
3.3 Modified Jeans Escape under Strong Stellar Ly Emission . . . . . . . .24
3.4 Mass Loss Rate Estimation . . . . . . . . . . . . . . . . . . . . . . .26
3.5 Roche Lobe Effects on the Mass Loss Rate . . . . . . . . . . . . . . . 39
3.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4 Conclusion and Future Prospects . . . . . . . . . . . . . . . . . . . . .45
4.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
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