初生星團的生存率

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姓名

陳慧真(Hui-Chen Chen) 查詢紙本館藏

畢業系所

天文研究所

論文名稱

初生星團的生存率
(Survivability of Embedded Clusters)

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摘要(中)

本論文中，我們呈現大規模的模擬以期能了解深埋在雲氣中初生星團的生存率。一般相信，大部份的恆星都是成群誕生。紅外波段的觀測顯示深埋在雲氣中初生星團的數量遠比預期的多，這表示大部份星團在從雲氣中露出前便已解離。藉由多體運動模擬，我們研究雲氣消散是否能有效地解離初生星團。為了能有系統地研究，我們建立了一個參數空間來描述雲氣，其中包含它的質量、大小以及其擴散速率。
　　在過去的研究中，當星團和雲氣的密度分佈相同時，星團的最後狀態與恆星形成率有良好的關係。然而，我們認為恆星形成率並不足以用來描述星團和星雲的密度分佈不相同的系統。我們發現，在特定拉格朗日半徑內的初始星團－星雲質量比以及初始的動能更能被視為初生星團生存率的指標。
　　藉由比較包含與不包含質量函數的模型，我們發現其最終密度分佈並沒有顯著差別。另外，包含質量函數的模型顯示，質量沉澱的程度和初始星團－星雲質量比也有著良好關係。因此，我們建議用來描述初生星團的相關有效時間尺度的定義中，也應考慮此一初始星團－星雲質量比。

摘要(英)

In this dissertation, we present a large set of simulations in an attempt to understand the survivability of embedded star clusters. It is commonly believed that most stars form in groups. Infrared observations show that there are more embedded star clusters than expected and most clusters dissolve before they have a chance to expose. By means of N-body simulations, we study the effectiveness of gas removal on dissolving an embedded cluster. We systematically survey on the parameter space describing the natal cloud including its mass, its size and the gas removal rate.
In previous studies, the final stage of the cluster is well correlated with the star formation efficiency (SFE) for systems with the same initial density profile for cluster and cloud. However, we deem that the SFE alone is not enough to address systems with clouds and clusters are of different initial density profiles. Instead, we find that the initial cluster-cloud mass ratio at a certain Lagrangian radius and the initial kinetic energy are better indicators for the survivability of embedded clusters.
We compare the simulations of models with and without mass functions and find that there is no significant difference between the final density profiles of the two models. Moreover, simulations on models with mass function show that the level of mass segregation is well correlated with the cluster-cloud mass ratio again. Therefore, we propose that the relevant effective timescale for an embedded cluster should be defined in terms of the cluster-cloud mass ratio.

關鍵字(中)

★ 多體運動模擬
★ 星團
★ 分子雲
★ 恆星形成率

關鍵字(英)

★ star clusters
★ molecular clouds
★ SFE
★ Nbody simulations

論文目次

Contents
Abstract (Chinese) i
Abstract ii
1 Introduction 1
1.1 Molecular Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Stellar Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2.1 Embedded Clusters . . . . . . . . . . . . . . . . . .. . . . . . 5
1.2.2 Mass Function . . . . . . . . . . . . . . . . . . . . . . .. . . . 6
1.2.3 Mass Segregation . . . . . . . . . . . . . . . . . . . . . . . . . 7
1.3 Relevant Timescales . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.4 Cluster Destruction . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.5 Star Ratio in Stellar Clusters . . . . . . . . . . . . . . . . . . . . . 11
2 Clusters in a Molecular Cloud 16
2.1 Model for Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2 Model for Cloud. . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3 Parameters and Other Conditions . . . . . . . . . . . . . . . . . . . . 18
2.3.1 Cloud Mass . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.3.2 Cloud Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.3.3 Dispersing Timescale. . . . . . . . . . . . . . . . . . . . . . . . 20
3 Cloud Dispersion 22
3.1 General Result. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2 Cluster and Cloud with the Same Density Profile . . . . . . . . . . . . . 26
3.2.1 Different Dispersing Timescale. . . . . . . . . . . . . . . . . . . . 27
3.2.2 Expansion Ratio of rhm and Bound Mass Fraction . . . . . . . . . . 28
3.3 Cluster and Cloud with Different Density Profiles . . . . . . . . . . . . . 30
3.3.1 Cluster-Cloud Mass Ratio . . . . . . . . . . . . . . . . . . . . . . 31
3.3.2 Initial Kinetic Energy or Virial Energy . . . . . . . . . . . . . . . . 33
4 Dynamics 37
4.1 Mass Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
4.2 Mass Segregation . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
4.3 Initial Equilibrium State. . . . . . . . . . . . . . . . . . . . . . . . . 45
5 Concluding Remarks 51
A Symbols 57
B Two-body Relaxation Time 58

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指導教授

高仲明(Chung-Ming Ko)

審核日期

2009-7-18

推文