球状星团中B型后渐近巨星分支星的铁丰度：M 13中的Barnard 29和omega Cen中的ROA 5701
Iron abundances of B-type post-Asymptotic Giant Branch stars in globular clusters: Barnard 29 in M 13 and ROA 5701 in omega Cen
摘要：
用模式大气技术分析了球状星团中的两颗B型后渐近巨星支（post-AGB）星：M 13中的Barnard 29和ω Cen中的ROA 5701的高分辨光学和紫外光谱。光谱是由欧洲南方天文台2.2米望远镜上的FEROS和2.7米麦克唐纳望远镜上的2d-Coudé摄谱仪获得的，而紫外观测则来自HST上的GHRS。轻元素（C，N，O，Mg，Al和S）和Fe的丰度已被确定的光谱，而紫外线数据提供额外的Fe丰度估计从Fe III吸收线在1875-1900 nm波长区域。巴纳德29和ROA 5701的金属丰度相对于年轻的B型恒星都不足。这些结果与相应星系团的金属丰度以及先前对这些天体的研究结果一致。衍生的丰度模式表明，恒星并没有经历气尘分离，与以前的建议相反，虽然它们可能是在第三次挖掘开始之前从AGB演变而来的。然而，来自HST光谱的Fe丰度低于从相应的集群的金属丰度预期，由0.5德克斯巴纳德29和0.8德克斯ROA 5701。在已知金属丰度的环境中，如麦哲伦云，也发现了类似的系统性丰度不足的其他B型恒星。这些结果表明，Fe III紫外线可能会产生丰度值，通常为0.6 dex，因此这种估计应谨慎对待。
英文摘要：
High resolution optical and ultraviolet spectra of two B-type post-Asymptotic Giant Branch (post-AGB) stars in globular clusters, Barnard 29 in M 13 and ROA 5701 in omega Cen, have been analysed using model atmosphere techniques. The optical spectra have been obtained with FEROS on the ESO 2.2-m telescope and the 2d-Coudé spectrograph on the 2.7-m McDonald telescope, while the ultraviolet observations are from the GHRS on the HST. Abundances of light elements (C, N, O, Mg, Al and S) plus Fe have been determined from the optical spectra, while the ultraviolet data provide additional Fe abundance estimates from Fe III absorption lines in the 1875-1900 Å wavelength region. A general metal underabundance relative to young B-type stars is found for both Barnard 29 and ROA 5701. These results are consistent with the metallicities of the respective clusters, as well as with previous studies of the objects. The derived abundance patterns suggest that the stars have not undergone a gas-dust separation, contrary to previous suggestions, although they may have evolved from the AGB before the onset of the third dredge-up. However, the Fe abundances derived from the HST spectra are lower than those expected from the metallicities of the respective clusters, by 0.5 dex for Barnard 29 and 0.8 dex for ROA 5701. A similar systematic underabundance is also found for other B-type stars in environments of known metallicity, such as the Magellanic Clouds. These results indicate that the Fe III ultraviolet lines may yield abundance values which are systematically too low by typically 0.6 dex and hence such estimates should be treated with caution.
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