利用M67的CMD形态来测试太阳丰度
On using the CMD morphology of M67 to test solar abundances
摘要：
疏散星团M67具有太阳金属丰度，年龄约为4 Gyr。关闭质量接近太阳金属丰度恒星在主序星演化过程中由于氢燃烧循环的发展而形成对流核的最小质量。M67的色星等图（CMD）在转折点附近的形态显示出明显的钩状特征，这是靠近转折点的恒星具有对流核的直接迹象。VandenBerg等人研究了使用M67关闭的形态来限制太阳金属丰度的可能性，特别是CNO元素，在过去几年中，太阳丰度已经向下修正了30%以上。在这里，我们扩展了他们的工作，填补了他们分析中的空白。为此，我们计算等时线适用于M67使用新的（低金属丰度）和旧的（高金属丰度）的太阳丰度和研究是否特征关闭CMD的M67可以复制或不。我们还研究了其他本构物理的重要性，确定存在这样一个钩子，特别是元素扩散，过冲和核反应率。我们发现，使用新的太阳丰度测定，与低CNO丰度，使得它更难以再现的特征CMD的M67。这一结果与VandenBerg等人的结果一致。然而，模型的本构物理变化，特别是过冲，可能会影响和改变这一结果，以至于使用低CNO太阳丰度模型构建的等时线也可以重现M67的关闭形态。我们的结论是，只有当所有影响关断形态的因素都完全在控制之下（而事实并非如此），M67才能用来限制太阳丰度。
英文摘要：
The open cluster M67 has solar metallicity and an age of about 4Gyr. The turn-off mass is close to the minimum mass for which solar metallicity stars develop a convective core during main sequence evolution as a result of the development of hydrogen burning through the CNO-cycle. The morphology of the color-magnitude-diagram (CMD) of M67 around the turn-off shows a clear hook-like feature, direct sign that stars close to the turn-off have convective cores. VandenBerg et al. investigated the possibility of using the morphology of the M67 turn-off to put constraints on the solar metallicity, particularly CNO elements, for which solar abundances have been revised downwards by more than 30% over the last few years. Here, we extend their work filling in the gaps in their analysis. To this aim, we compute isochrones appropriate for M67 using new (low metallicity) and old (high metallicity) solar abundances and study whether the characteristic turn-off in the CMD of M67 can be reproduced or not. We also study the importance of other constitutive physics on determining the presence of such a hook, particularly element diffusion, overshooting and nuclear reaction rates. We find that using the new solar abundance determinations, with low CNO abundances, makes it more difficult to reproduce the characteristic CMD of M67. This result is in agreement with results by VandenBerg et al. However, changes in the constitutive physics of the models, particularly overshooting, can influence and alter this result to the extent that isochrones constructed with models using low CNO solar abundances can also reproduce the turn-off morphology in M67. We conclude that only if all factors affecting the turn-off morphology are completely under control (and this is not the case), M67 could be used to put constraints on solar abundances.
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