[论文解读] Abundances in bulge stars from high-resolution, near-IR spectra I. The CNO elements observed during the science verification of CRIRES at VLT

N. Ryde, B. Edvardsson+5 more|ArXiv.org|Feb 12, 2009

Stellar, planetary, and galactic studies参考文献 39被引用 25

一句话总结

本研究利用甚大望远镜上的CRIRES，首次对银河系银晕中的三颗巨星进行了高分辨率近红外C、N、O元素丰度分析，不确定度低于0.20 dex。研究发现C+N/Fe的[Fe]为太阳值，且O、Ti和Fe的丰度与光学数据一致，表明银晕星与厚盘星之间无显著差异，同时高[S/Fe]值表明银晕早期恒星形成速率较高。

ABSTRACT

The formation and evolution of the Milky Way bulge is not yet well understood and its classification is ambiguous. Constraints can, however, be obtained by studying the abundances of key elements in bulge stars. The aim of this study is to determine the chemical evolution of CNO, and a few other elements in stars in the Galactic bulge, and to discuss the sensitivities of the derived abundances from molecular lines. High-resolution, near-IR spectra in the H band were recorded using VLT/CRIRES. Due to the high and variable visual extinction in the line-of-sight towards the bulge, an analysis in the near-IR is preferred. The CNO abundances can all be determined simultaneously from the numerous molecular lines in the wavelength range observed. The three giant stars in Baade's window presented here are the first bulge stars observed with CRIRES. We have especially determined the CNO abundances, with uncertainties of less than 0.20 dex, from CO, CN, and OH lines. Since the systematic uncertainties in the derived CNO abundances due to uncertainties in the stellar fundamental parameters, notably Teff, are significant, a detailed discussion of the sensitivities of the derived abundances is included. We find good agreement between near-IR and optically determined O, Ti, Fe, and Si abundances. Two of our stars show a solar [C+N/Fe], suggesting that these giants have experienced the first dredge-up and that the oxygen abundance should reflect the original abundance of the giants. The two giants fit into the picture, in which there is no significant difference between the O abundance in bulge and thick-disk stars. Our determination of the S abundances is the first for bulge stars. The high [S/Fe] values for all the stars indicate a high star-formation rate in an early phase of the bulge evolution.

研究动机与目标

• 利用高分辨率近红外光谱，确定银河系银晕恒星中C、N和O的化学演化过程。
• 评估所测C、N和O丰度对恒星参数（尤其是有效温度）的敏感性。
• 将近红外丰度结果与光学测定的O、Ti、Fe和Si丰度进行比较。
• 通过硫丰度测量，探究银晕的恒星形成历史。
• 通过分析巴德窗口中恒星的元素丰度，澄清银河系银晕的形成情景。

提出的方法

• 利用甚大望远镜上的CRIRES光谱仪，在H波段获取了高分辨率近红外光谱。
• 基于观测波长范围内分子线（CO、CN、OH）的丰度，推导出C、N、O及其他元素的丰度。
• 仔细确定了恒星大气参数（Teff、log g、[Fe/H]、微湍流），以最小化系统性不确定度。
• 定量评估了丰度测定对恒星参数不确定度（尤其是Teff）的敏感性。
• 将丰度结果与光学光谱数据进行比较，以验证近红外测量的可靠性。
• 首次利用近红外谱线测定了银晕恒星中的硫丰度。

实验结果

研究问题

• RQ1 基于高分辨率近红外光谱，这三颗银河系银晕巨星的C、N和O丰度分别是多少？
• RQ2 所测C、N和O丰度对恒星参数不确定度（尤其是有效温度）的敏感性如何？
• RQ3 近红外测得的O、Ti、Fe和Si丰度是否与以往的光学测定结果一致？
• RQ4 银晕恒星中硫丰度揭示了银晕早期恒星形成速率的何种信息？
• RQ5 基于CNO和α-元素丰度，银晕与厚盘恒星群体之间是否存在显著的化学差异？

主要发现

• 利用CO、CN和OH分子线，三颗银晕巨星的C、N和O丰度测定不确定度均低于0.20 dex。
• 其中两颗表现出太阳值的[C+N/Fe]比值，表明这些巨星已经历首次混合过程，其氧丰度反映了原始成分。
• 由近红外光谱测得的O、Ti、Fe和Si丰度与光学测定结果高度一致，验证了近红外方法在高消光视线中的可靠性。
• 三颗恒星均显示高[S/Fe]值，表明银晕早期恒星形成速率较高，与一次短暂而强烈的恒星形成爆发一致。
• 未在银晕与厚盘恒星之间发现氧丰度的显著差异，挑战了银晕具有独立化学起源的观点。
• 本研究首次测定了银晕恒星的硫丰度，为银晕化学演化提供了新的约束。