[论文解读] The Ancient Globular Clusters Were Much More Massive At Birth

Charlie Conroy|arXiv (Cornell University)|Jan 11, 2011

Stellar, planetary, and galactic studies被引用 2

一句话总结

该论文基于对20个银河系球状星团中[Na/Fe]与[O/Fe]反相关性的建模，提出球状星团在形成时的质量比现今质量大10–20倍。分析表明，纯AGB星风物质贡献的质量分数（f_p）随初始星团质量增加而上升，暗示存在显著的质量损失，因此需要更高的初始质量才能保留足够物质以解释观测到的元素丰度模式。

ABSTRACT

All globular clusters (GCs) studied to date show evidence for internal variation in their light element abundances. These variations have been interpreted as evidence for multiple star formation episodes within GCs, with secondary episodes fueled, at least in part, by the ejecta of AGB stars from a first generation of stars. A major puzzle emerging from this otherwise plausible scenario is that the fraction of stars associated with the second episode of star formation is observed to be much larger than expected for a standard IMF. The present work investigates this tension by modeling the observed anti-correlation between [Na/Fe] and [O/Fe] for 20 Galactic GCs. If the abundance pattern of the retained AGB ejecta does not depend on GC mass at fixed [Fe/H], then a strong correlation is found between the fraction of current GC stellar mass comprised of pure AGB ejecta, f_p, and GC mass. This fraction varies from 0.20 at low masses (10^4.5 Msun) to 0.45 at high masses (10^6.5 Msun). The fraction of mass associated with pure AGB ejecta is directly related to the total mass of the cluster at birth; the ratio between the initial and present mass in stars can therefore be derived. Assuming a star formation efficiency of 50%, the observed Na-O anti-correlations imply that GCs were at least 10-20 times more massive at birth. These factors are lower limits because any mass-loss mechanism that removes first and second generation stars equally will leave f_p unchanged. The mass-dependence of f_p probably arises because lower mass GCs are unable to retain all of the AGB ejecta from the first stellar generation. Recent observations of elemental abundances in intermediate-age LMC clusters are re-interpreted and shown to be consistent with this basic scenario. A convincing explanation of these trends is currently lacking.

研究动机与目标

• 解决球状星团内部元素丰度异常变化与第二代恒星形成中预期低质量分数之间的矛盾。
• 探究观测到的[Na/Fe]–[O/Fe]反相关性是否意味着球状星团的初始质量高于当前观测值。
• 建模纯AGB星风物质贡献质量分数（f_p）随初始星团质量及金属度（固定[Fe/H]）的变化关系。
• 评估质量依赖的AGB星风物质保留机制是否能解释不同质量球状星团的观测丰度模式。
• 在所提出的初始质量增强情景下，重新评估中等年龄大麦哲伦云星团。

提出的方法

• 利用丰度数据对20个银河系球状星团的[Na/Fe]–[O/Fe]反相关性进行建模。
• 假设在固定[Fe/H]下，保留的AGB星风物质成分与星团质量无关，从而实现对f_p的质量依赖性估计。
• 通过观测到的f_p并假设50%的恒星形成效率，推导初始质量与当前质量之比。
• 利用f_p–质量关系，从当前质量与观测到的丰度模式反推出初始星团质量。
• 将相同框架应用于中等年龄大麦哲伦云星团，以检验其与模型的一致性。
• 考虑对第一代与第二代恒星产生同等影响的质量损失机制，这些机制不会改变f_p。

实验结果

研究问题

• RQ1 为使球状星团保留足够AGB星风物质以重现观测到的Na-O反相关性，其初始质量需达到多少？
• RQ2 在AGB星风物质成分恒定的假设下，纯AGB星风物质贡献的质量分数（f_p）如何随初始星团质量变化？
• RQ3 银河系与大麦哲伦云球状星团的观测丰度模式能否通过更高的初始质量及AGB星风物质循环来解释？
• RQ4 对两代恒星产生同等影响的质量损失过程在多大程度上影响对初始质量的推断？
• RQ5 为何目前尚无令人信服的物理解释来说明观测到的f_p–质量相关性？

主要发现

• 当前恒星质量中来自纯AGB星风物质的分数（f_p）随初始星团质量增加而上升，范围从10^4.5 Msun时的0.20增至10^6.5 Msun时的0.45。
• 观测到的Na-O反相关性表明，球状星团在形成时的质量至少是其当前质量的10–20倍，前提是恒星形成效率为50%。
• 该质量增强为下限值，因为对两代恒星产生同等影响的质量损失机制不会改变f_p。
• f_p的质量依赖性可能源于低质量星团无法保留第一代恒星产生的全部AGB星风物质。
• 近期中等年龄大麦哲伦云星团的丰度数据与提出的更高初始质量及AGB星风物质循环情景一致。
• 尽管有强有力的实证支持，目前仍缺乏对观测到的f_p–质量相关性的令人信服的物理解释。