CARBON AND OXYGEN ISOTOPIC RATIOS FOR NEARBY MIRAS
CARBON AND OXYGEN ISOTOPIC RATIOS FOR NEARBY MIRAS
复制标题
附近 MIRAS 的碳和氧同位素比
DOI：
10.3847/0004-637x/825/1/38
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发表时间：
2016
期刊：
影响因子：
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通讯作者：
O. Straniero
中科院分区：
文献类型：
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作者：
K. Hinkle;T. Lebzelter;O. Straniero
本文报道了46颗MIRA和SRA型可变渐近巨星(AGB)的碳、氧同位素比值。测量了1 5~2 5μm谱线的振动-转动第一和第二泛音CO谱线，得到了12C/13C、16O/17O和16O/18O的同位素比值。文中还讨论了与现有文献中已有的单星测量和各种演化恒星样本的比较。不同初始质量的AGB恒星的太阳组成模型被用来解释我们的结果。我们发现，大多数M星都有主序列质量≤2 M⊙，并且没有经历过大规模的第三次挖泥事件。我们样本中的四颗S类型恒星的前身质量略大一些。在样本中的六颗C星中，有三颗有明确的证据表明它们的来源与TDU的发生有关。与太阳系形成前的AGB恒星中富氧的太阳系前颗粒进行比较，揭示了星际介质化学成分的变化。以我们的长周期变星(LPV)样本为代表的当代低质量⊙星，显示出较大的16O/17O比率扩散，类似于第一组太阳系前颗粒的扩散，并与第一次挖掘后质量为1.2-2M的AGB星的理论预期一致。相比之下，现代LPVs的16O/18O比值肯定小于第一组谷物的16O/18O。这很可能是由于银河系的化学演化导致星际介质中16O/18O比值随时间减少的结果。在我们的样本中，有一颗恒星的氧成分类似于起源于经历额外混合的AGB恒星的第二组太阳系前颗粒。这可能表明，在高金属含量时，超混合过程受到阻碍，或者在低金属含量时，同样有利于超混合过程。与富氧颗粒类似，我们的样本中没有恒星显示出热底燃烧的证据，这是大质量AGB恒星的预期。
Carbon and oxygen isotopic ratios are reported for a sample of 46 Mira and SRa-type variable asymptotic giant branch (AGB) stars. Vibration–rotation first and second-overtone CO lines in 1.5–2.5 μm spectra were measured to derive isotopic ratios for 12C/13C, 16O/17O, and 16O/18O. Comparisons with previous measurements for individual stars and with various samples of evolved stars, as available in the extant literature, are discussed. Models for solar composition AGB stars of different initial masses are used to interpret our results. We find that the majority of M-stars have main sequence masses ≤2 M⊙ and have not experienced sizable third dredge-up (TDU) episodes. The progenitors of the four S-type stars in our sample are slightly more massive. Of the six C-stars in the sample three have clear evidence relating their origin to the occurrence of TDU. Comparisons with O-rich presolar grains from AGB stars that lived before the formation of the solar system reveal variations in the interstellar medium chemical composition. The present generation of low-mass AGB stars, as represented by our sample of long period variables (LPVs), shows a large spread of 16O/17O ratios, similar to that of group 1 presolar grains and in agreement with theoretical expectations for the composition of mass 1.2–2 M⊙ stars after the first dredge-up. In contrast, the 16O/18O ratios of present-day LPVs are definitely smaller than those of group 1 grains. This is most probably a consequence of the the decrease with time of the 16O/18O ratio in the interstellar medium due to the chemical evolution of the Milky Way. One star in our sample has an O composition similar to that of group 2 presolar grains originating in an AGB star undergoing extra-mixing. This may indicate that the extra-mixing process is hampered at high metallicity, or, equivalently, favored at low metallicity. Similarly to O-rich grains, no star in our sample shows evidence of hot bottom burning, which is expected for massive AGB stars.