An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613
An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613
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缺氢碳星 HD 137613 中 18O 极度过量
DOI：
10.1086/430110
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
K. D. Gordon
中科院分区：
文献类型：
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作者：
G. C. Clayton;Falk Herwig;T. Geballe;Martin Asplund;E. D. Tenenbaum;C. Engelbracht;K. D. Gordon
我们根据 2.3-2.4 μm 的 CO 第一泛音带光谱，报告了在缺氢碳 (HdC) 恒星 HD 137613 中发现了独特的大量过量 18O，其中清楚地存在 12C18O 的三个强吸收带。 12C16O 的条带也存在，但没有看到 13C16O 或 12C17O 的条带。我们估计同位素比为 16O/18O ≲ 1。该比值的太阳值约为 500。氦核燃烧和氦壳闪燃都不能产生 HD 137613 中观察到的氧和碳同位素比率。然而，观测到的丰度与在氦核燃烧结束后不久在早期渐近巨分支 (AGB) 恒星的宽氦壳层外层发现的丰度之间存在显着的相似性。目前尚不清楚外层直至氦壳层是如何丢失的，但一定涉及某种增强质量损失的机制。 HD 137613可能是一颗后早期AGB恒星，其光球层是前He燃烧壳层的外层。 HdC 恒星异常的元素丰度与 R Coronae Borealis (RCB) 恒星类似，但 HdC 恒星不会产生导致亮度下降的尘埃云。观察到的其他 RCB 或 HdC 恒星均未显示出明显的 18O。
We report the discovery of a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrum of the first-overtone bands of CO at 2.3-2.4 μm in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present, but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O ≲ 1. The solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He shell of early-asymptotic giant branch (AGB) stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He shell could be lost, but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed show significant 18O.