Dissolution is the solution: on the reduced mass-to-light ratios of Galactic globular clusters
Dissolution is the solution: on the reduced mass-to-light ratios of Galactic globular clusters
复制标题
解决方案是溶解：关于银河球状星团质光比的降低
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
S. Mieske
中科院分区：
文献类型：
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作者：
J. Kruijssen;S. Mieske
上下文观测到的球状星团（GC）的动态质光比（M/L）系统地低于“典型”简单恒星种群模型的预期值，该模型没有考虑到动力学效应，例如由于能量均分导致的低质量恒星的优先损失。最近的研究表明，低质量星星的耗尽可以定性地解释几个星系中球状星团的这种差异。目标。为了验证低质量星星的耗尽是否确实是M/L降低背后的驱动机制，我们的目标是预测单个GC的M/LV比，其中轨道参数和动态V波段质光比M/LV是已知的。有一个24个银河系GC的样本，这是可能的。方法.我们使用的SPACE星团模型，其中包括动态溶解，低质量星星耗尽，恒星演化，恒星残骸，和各种金属丰度。我们从GC样品的轨道参数推导出由于两体弛豫和圆盘冲击的溶解时间尺度，并使用这些来预测单个GC的M/LV比。为了验证我们的发现，我们还预测了它们的低质量恒星质量函数的斜率。结果计算的溶解时间尺度与早期的经验研究一致。在24个GC中，有12个GC的M/LV值与观测值符合1σ。其他GC的差异可能是因为我们的预测给出了全球M/L比，而观测结果代表了外推的中心值，这些值在质量分离和长溶解时间尺度的情况下不同于全球值。通过对溶出时间表和King参数施加限制，可以排除样品中可能具有不同总体和中心M/L比的GC。对于其余GC，观测和预测的平均M/LV分别为规范预期值的78 ± 9 − 11%和78 ± 2%，而整个样品的值分别为74 ± 6 − 7%和85 ± 1%。低质量恒星质量函数的斜率和M/LV下降之间的预测相关性被发现是定性与观测到的质量函数斜率一致。结论.银河系GC的溶解时间尺度是这样的，即通过考虑低质量恒星的优先损失，即使是考虑单个星团，也可以弥合正则预期和观测到的M/LV比之间的20%差距。它的结论是，由于溶解和低质量的星星耗尽的M/L比的变化是一个合理的解释之间的差异观测和规范预期的GC的M/L比。
Context. The observed dynamical mass-to-light (M/L) ratios of globular clusters (GCs) are systematically lower than the value expected from “canonical” simple stellar population models, which do not account for dynamical effects such as the preferential loss of low-mass stars due to energy equipartition. It has recently been shown that low-mass star depletion can qualitatively explain this discrepancy for globular clusters in several galaxies. Aims. To verify whether low-mass star depletion is indeed the driving mechanism behind the M/L decrease, we aim to predict the M/LV ratios of individual GCs for which orbital parameters and dynamical V-band mass-to-light ratios M/LV are known. There is a sample of 24 Galactic GCs for which this is possible. Methods. We used the SPACE cluster models, which include dynamical dissolution, low-mass star depletion, stellar evolution, stellar remnants, and various metallicities. We derived the dissolution timescales due to two-body relaxation and disc shocking from the orbital parameters of our GC sample and used these to predict the M/LV ratios of the individual GCs. To verify our findings, we also predicted the slopes of their low-mass stellar mass functions. Results. The computed dissolution timescales agree well with earlier empirical studies. The predicted M/LV are in 1σ agreement with the observations for 12 out of 24 GCs. The discrepancy for the other GCs probably arises because our predictions give global M/L ratios, while the observations represent extrapolated central values that are different from global ones in the case of mass segregation and a long dissolution timescale. The GCs in our sample that likely have dissimilar global and central M/L ratios can be excluded by imposing limits on the dissolution timescale and King parameter. For the remaining GCs, the observed and predicted average M/LV are 78 +9 −11 % and 78 ± 2% of the canonically expected values, while the values are 74 +6 −7 % and 85 ± 1% for the entire sample. The predicted correlation between the slope of the low-mass stellar mass function and M/LV drop is found to be qualitatively consistent with observed mass function slopes. Conclusions. The dissolution timescales of Galactic GCs are such that the ∼20% gap between canonically expected and observed M/LV ratios is bridged by accounting for the preferential loss of low-mass stars, also when considering individual clusters. It is concluded that the variation in M/L ratio due to dissolution and low-mass star depletion is a plausible explanation for the discrepancy between the observed and canonically expected M/L ratios of GCs.