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arXiv 0910.0730astro-ph.GA
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为什么简单的恒星星族模型不能再现银河系疏散星团的颜色

Why Simple Stellar Population models do not reproduce the colours of Galactic open clusters

A. E. Piskunov, N. V. Kharchenko, E. Schilbach, S. Röser, R. -D. Scholz, H. Zinnecker

备注：
4 pages, 3 figures, accepted for publication in Astronomy and Astrophysics Letters, revised version after language editing and with an additional reference to Cervino and Luridiana (2004)
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摘要：

(...)我们寻找一个解释的分歧之间的观察到的综合颜色的650个本地银河系星团和理论的颜色，目前的SSP模型。我们检查的假设，即观测到的和理论的颜色，这是$(B$ $-$ $V)\approx 0.3$和$(J$ $-$ $K_s)\approx 0.8$之间的系统偏移，是由于忽略了潜在的质量函数的离散性。使用Monte Carlo模拟，我们构建人工集群的同时代的恒星采取的质量分布定义的Salpeter初始质量函数（IMF），并比较它们与相应的“连续IMF”SSP模型。如果考虑到IMF的离散性，该模型完全符合观察结果，并能够自然地解释在经验颜色-年龄关系中观察到的一些红色“离群值”。我们发现，\textit{systematic}之间的连续和离散的IMF颜色的偏移达到最大值约0.5在$(B$ $-$ $V)$为一个集群质量$M_c=10^2 m_\odot$在年龄$\log t\approx 7$，并大大减少，但不完全减少到约百分之一的幅度在$\log t >7.9$在集群质量$M_c> 10^5 m_\odot$。在年轻的时候，它仍然存在，甚至在大规模的集群，并为$M_c \leqslant 10^4 m_\odot$，它是大于0.1等的$(B$ $-$ $V)$。只有年龄为$\log t< 7.5$的超大质量星系团（$M_c>10^6 m_\odot$）的偏移量才很小（大约为0.04等），并且小于河外星系团颜色的典型观测误差。

英文摘要：

(...) We search for an explanation of the disagreement between the observed integrated colours of 650 local Galactic clusters and the theoretical colours of present-day SSP models. We check the hypothesis that the systematic offsets between observed and theoretical colours, which are $(B$$-$$V)\approx 0.3$ and $(J$$-$$K_s)\approx 0.8$, are caused by neglecting the discrete nature of the underlying mass function. Using Monte Carlo simulations, we construct artificial clusters of coeval stars taken from a mass distribution defined by an Salpeter initial mass function (IMF) and compare them with corresponding "continuous-IMF" SSP models. If the discreteness of the IMF is taken into account, the model fits the observations perfectly and is able to explain naturally a number of red "outliers" observed in the empirical colour-age relation. We find that the \textit{systematic} offset between the continuous- and discrete-IMF colours reaches its maximum of about 0.5 in $(B$$-$$V)$ for a cluster mass $M_c=10^2 m_\odot$ at ages $\log t\approx 7$, and diminishes substantially but not completely to about one hundredth of a magnitude at $\log t >7.9$ at cluster masses $M_c> 10^5 m_\odot$. At younger ages, it is still present even in massive clusters, and for $M_c \leqslant 10^4 m_\odot$ it is larger than 0.1 mag in $(B$$-$$V)$. Only for very massive clusters ($M_c>10^6 m_\odot$) with ages $\log t< 7.5$ is the offset small (of the order of 0.04 mag) and smaller than the typical observational error of colours of extragalactic clusters.

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