An Analysis of Ultraviolet Spectra of Extreme Helium Stars and New Clues to Their Origins
An Analysis of Ultraviolet Spectra of Extreme Helium Stars and New Clues to Their Origins
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极端氦星的紫外光谱分析及其起源的新线索
DOI：
10.1086/498674
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
Armagh Observatory
中科院分区：
文献类型：
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作者：
G. Pandey;D. Lambert;C. Jeffery;N. K. R. I. Bangalore;UT Austin;Armagh Observatory
通过哈勃太空望远镜太空望远镜成像光谱仪七颗极氦星（EHes）的紫外光谱确定了包括重元素Y和Zr在内的约18种元素的丰度：LSE 78、BD +10 2179、V1920 Cyg、HD 124448、PV Tel、LS IV-1 2和FQ Aqr。分析了BD +10 2179、V1920 Cyg和HD 124448的新光谱，并重新分析了LSE 78、HD 124448和PV Tel的已发表谱线列表。丰度分析是使用LTE线形成和LTE模型大气，特别是为这些EHes。由紫外光谱导出的恒星参数与由光学光谱导出的恒星参数符合得很好。七种EHE的采用丰度来自UV和光学分析的组合。公布的另外10个EHes的结果提供了以几乎均匀的方式获得的总共17个EHes的丰度，这是有记录以来最大的样本。EHe的初始金属丰度由Al到Ni的元素丰度表示，Fe被用来代表初始金属丰度。铁的丰度范围从大约太阳能到大约1/100太阳能。EHe演化的线索包含在H、He、C、N、O、Y和Zr丰度中。两个新的结果是：（1）一些恒星的O丰度接近于预测的初始丰度，但N丰度表明初始C，N和O几乎完全转化为N的CNO循环;（2）7颗具有紫外光谱的恒星中有3颗显示出Y和Zr的强烈增强，归因于s过程。所观察到的成分进行了讨论的预期从吸积的一个He白色矮星的C-O白色矮星。定性协议似乎是可能的，除了一个问题可能会提出的那些恒星中的O丰度接近初始O丰度。
Abundances of about 18 elements including the heavy elements Y and Zr are determined from Hubble Space Telescope Space Telescope Imaging Spectrograph ultraviolet spectra of seven extreme helium stars (EHes): LSE 78, BD +10 2179, V1920 Cyg, HD 124448, PV Tel, LS IV-1 2, and FQ Aqr. New optical spectra of BD +10 2179, V1920 Cyg, and HD 124448 were analyzed, and published line lists of LSE 78, HD 124448, and PV Tel were analyzed afresh. The abundance analyses are done using LTE line formation and LTE model atmospheres especially constructed for these EHes. The stellar parameters derived from an EHe's UV spectrum are in satisfactory agreement with those derived from its optical spectrum. Adopted abundances for the seven EHes are from a combination of the UV and optical analyses. Published results for an additional 10 EHes provide abundances obtained in a nearly uniform manner for a total of 17 EHes, the largest sample on record. The initial metallicity of an EHe is indicated by the abundance of elements from Al to Ni; Fe is adopted to be the representative of initial metallicity. Iron abundances range from approximately solar to about 1/100 solar. Clues to EHe evolution are contained within the H, He, C, N, O, Y, and Zr abundances. Two novel results are (1) the O abundance for some stars is close to the predicted initial abundance yet the N abundance indicates almost complete conversion of initial C, N, and O to N by the CNO cycles; and (2) three of the seven stars with UV spectra show a strong enhancement of Y and Zr attributable to an s-process. The observed compositions are discussed in light of expectations from accretion of an He white dwarf by a C-O white dwarf. Qualitative agreement seems likely except that a problem may be presented by those stars in which the O abundance is close to the initial O abundance.