The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3
The Chemical Composition Contrast between M3 and M13 Revisited: New Abundances for 28 Giant Stars in M3
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重新审视M3和M13的化学成分对比：M3中28颗巨星的新丰度
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发表时间：
2003
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通讯作者：
J. Fulbright
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作者：
C. Sneden;R. P. Kraft;P. Guhathakurta;R. Peterson;J. Fulbright
我们报告了球状星团M3中23颗明亮红巨星成员的新化学丰度，这些数据是基于凯克一号望远镜获得的高分辨率（R = 45，000）光谱。的意见，其中涉及使用多狭缝的HIRES凯克我摄谱仪，详细描述。结合这些数据与以前报道的小样本的M3巨星获得的利克3米望远镜，我们比较了金属丰度和[X/Fe]比的28 M3巨星与35-星星样本中的类似金属丰度集群M13，和银河系晕场星的[Fe/H] <-1。对于原子序数为A ≥ A（Si）的元素，我们在M3、M13或晕场样品中得到的[X/Fe]比值差别不大。这三个星团都表现出C的耗尽，并从红巨星分支“凸点”的水平开始推进演化状态，但在星团中看到的约0.7-0.9 dex的整体耗尽大于与场星相关的耗尽。O、Na、Mg和Al的行为在三个恒星样品中有明显的不同。场晕巨星和次矮星的Na与Mg呈正相关，这与II型超新星爆发或流体静力碳燃烧的预测一致。M3和M13都显示出ON、NeNa和MgAl循环的高温质子捕获合成的证据，而在晕场星中没有这种合成的证据。但是，M3中这种极端的质子捕获合成的程度比M13中的要小：M3巨星只表现出适度的O缺乏和相应的Na增加，Al的极端过度丰度没有那么高，具有低Mg和相应的高Na的恒星较少，并且没有迹象表明O耗尽是进化状态的函数，就像M13所声称的那样。NGC 6752和M13中的巨星系满足相同的O丰度与（25 Mg +26 Mg）/24 Mg比值的关系，该比值衡量了Mg稀有同位素对丰度的相对贡献。这表明，这些丰度比出现在3-6 M星团恒星的喷射物质中，这些物质随后被用来形成目前正在演化的低质量星团恒星的大气。它还表明，在最进化的M13巨星中看到的低氧丰度是在这些相同的中等质量星团恒星中的热底部O到N过程中产生的。因此，某些丰度比对光度的依赖性要求混合，但在比巨星或主序星更热的环境中更早的核合成过程是以前在主序星附近看到的变化所要求的。早期过程的性质和位置受到所观察到的Mg同位素比的限制，但没有被精确定位。
We report new chemical abundances of 23 bright red giant members of the globular cluster M3, based on high-resolution (R ∼ 45,000) spectra obtained with the Keck I telescope. The observations, which involve the use of multislits in the HIRES Keck I spectrograph, are described in detail. Combining these data with a previously reported small sample of M3 giants obtained with the Lick 3 m telescope, we compare metallicities and [X/Fe] ratios for 28 M3 giants with a 35-star sample in the similar-metallicity cluster M13, and with Galactic halo field stars having [Fe/H] < -1. For elements having atomic number A ≥ A(Si), we derive little difference in [X/Fe] ratios in the M3, M13, or halo field samples. All three groups exhibit C depletion with advancing evolutionary state beginning at the level of the red giant branch "bump," but the overall depletion of about 0.7–0.9 dex seen in the clusters is larger than that associated with the field stars. The behaviors of O, Na, Mg, and Al are distinctively different among the three stellar samples. Field halo giants and subdwarfs have a positive correlation of Na with Mg, as predicted from explosive or hydrostatic carbon burning in Type II supernova sites. Both M3 and M13 show evidence of high-temperature proton-capture synthesis from the ON, NeNa, and MgAl cycles, while there is no evidence for such synthesis among halo field stars. But the degree of such extreme proton-capture synthesis in M3 is smaller than it is in M13: the M3 giants exhibit only modest deficiencies of O and corresponding enhancements of Na, less extreme overabundances of Al, fewer stars with low Mg and correspondingly high Na, and no indication that O depletions are a function of advancing evolutionary state, as has been claimed for M13. We have also considered NGC 6752, for which Mg isotopic abundances have been reported by Yong et al. Giants in NGC 6752 and M13 satisfy the same anticorrelation of O abundances with the ratio (25Mg + 26Mg)/24Mg, which measures the relative contribution of rare to abundant isotopes of Mg. This points to a scenario in which these abundance ratios arose in the ejected material of 3–6 M⊙ cluster stars, material that was then used to form the atmospheres of the presently evolving low-mass cluster stars. It also suggests that the low oxygen abundance seen among the most evolved M13 giants arose in hot bottom O-to-N processing in these same intermediate-mass cluster stars. Thus, mixing is required by the dependence of some abundance ratios on luminosity, but an earlier nucleosynthesis process in a hotter environment than giants or main-sequence stars is required by the variations previously seen in stars near the main sequence. The nature and the site of the earlier process is constrained but not pinpointed by the observed Mg isotopic ratio.