[论文解读] Analysis of chemical abundances in planetary nebulae with [WC] central stars. II. Chemical abundances and the abundance discrepancy factor

J. García–Rojas, M. Peña+4 more|arXiv (Cornell University)|Sep 6, 2013

Stellar, planetary, and galactic studies参考文献 101被引用 25

一句话总结

本研究利用高分辨率光谱测光法分析了12个具有[WC]型中心恒星的行星状星云中的化学丰度，比较了由碰撞激发线（CELs）和光学复合线（ORLs）得到的离子丰度。研究发现，丰度差异因子（ADF(O++) = 1.2–4）在不同星云性质下保持稳定，且由ORL得到的氧丰度超过太阳值，C/O比值表明约一半样本的前身星初始质量≥4 M⊙。

ABSTRACT

(Abridged) We present the abundance analysis of 12 PNe ionized by [WC]-type stars and wels obtained from high-resolution spectrophotometric data. Our main aims are to determine the chemical composition of the PNe and to study the behaviour of the abundance discrepancy problem (ADF) in this type of planetary nebulae. The detection of a large number of optical recombination lines (ORLs) and collisionally excited lines (CELs) from different ions were presented previously. Most of the ORLs were reported for the first time in these PNe. Ionic abundances were derived from the available CELs and ORLs, using previously determined physical conditions. Based on these two sets of ionic abundances, we derived the total chemical abundances in the nebulae using suitable ICFs (when available). In spite of the [WC] nature of the central stars, moderate ADF(O^++), in the range from 1.2 to 4, were found for all the objects. We found that when the quality of the spectra is high enough the ORLs O^++/H^+ abundance ratios obtained from different multiplets excited mainly by recombination are very similar. Possible dependence of ADFs with some nebular characteristics were analysed, finding no correlation. Abundances derived from CELs were corrected by determining the t^2 parameter. O abundances for PNe, derived from ORLs, are in general larger than the solar abundance. We derived the C/O ratio from ORLs and N/O and alpha-element/O ratios from CELs and found that these PNe are, in average, N-and C-richer than the average of large PN samples. About half of our sample is C-rich (C/O>1). The alpha-elements grow in lockstep with O abundance. Comparing the N/O and C /O ratios with those derived from stellar evolution models, we estimate that about half of our PNe have progenitors with initial masses > 4 M_sun. No correlation was found between the stellar [WC]-type and the nebular abundances.

研究动机与目标

• 确定由[WC]型及弱发射线中心恒星电离的12个行星状星云的化学成分。
• 研究具有[WC]型中心恒星的星云中丰度差异因子（ADF）的行为特征。
• 比较由碰撞激发线（CELs）和光学复合线（ORLs）得到的离子丰度，并利用电离校正因子（ICFs）计算总丰度。
• 评估恒星类型对星云丰度的影响，并通过与恒星演化模型比较推断前身星质量。
• 利用行星状星云数据研究银河系中氧丰度梯度，并与电离氢区梯度及银河系化学演化模型进行比较。

提出的方法

• 在6.5米Magellan望远镜（LCO）上利用MIKE光谱仪获取高分辨率光谱测光数据。
• 利用论文I中的物理条件（Te, ne）从CELs和ORLs推导离子丰度。
• 应用电离校正因子（ICFs）计算总元素丰度，特别针对He、C、N、O、Ne、S、Cl、Ar、Mg、Fe和Ni。
• 采用两种方法计算温度涨落参数t²：通过最小化He⁺/H⁺比值的卡方值，以及通过ORL与CEL离子丰度的一致性。
• 将ADF(O++)计算为ORL推导的O++丰度与CEL推导的O++丰度之比，以量化差异程度。
• 将观测到的N/O和C/O丰度比与恒星演化模型的预测进行比较，以估算前身星质量。

实验结果

研究问题

• RQ1 具有[WC]型中心恒星的行星状星云中，丰度差异因子（ADF）的大小和行为特征如何？
• RQ2 在这些星云中，由光学复合线（ORLs）得到的离子丰度与由碰撞激发线（CELs）得到的离子丰度相比如何？
• RQ3 ADF是否与星云性质（如表面亮度、直径、电离度或密度）存在相关性？
• RQ4 ORL推导的氧丰度高于太阳值，这对星云和恒星演化模型有何影响？
• RQ5 行星状星云中的氧丰度梯度与电离氢区及银河系化学演化模型中的梯度相比如何？

主要发现

• O++的丰度差异因子（ADF(O++)）在1.2至4之间，表明ORL与CEL丰度之间存在中等程度的差异，与典型行星状星云一致，但不极端。
• 不同多重态（如1、2、10、20、3d-4f）的ORL推导的O++/H+丰度比值一致，证实多重态1可代表以复合激发为主导的激发过程。
• 由ORL推导的氧丰度超过太阳值，且这一差异无法完全由太阳分馏或现有化学演化模型解释。
• 由ORL推导的C/O比值显示，约一半样本为碳富集（C/O > 1），表明其前身星初始质量≥4 M⊙。
• 未发现星云化学丰度与[WC]恒星子型之间存在相关性，表明在本样本中[WC]现象与前身星质量无关。
• 行星状星云中的氧丰度梯度比电离氢区更平缓且更分散，而银河系化学演化模型无法再现行星状星云的梯度，提示存在未被考虑的过程，如迁移或核合成后的富集。