[论文解读] Galactic planetary nebulae with Wolf-Rayet nuclei. II. A consistent observational data set

M. Peña, G. Stasińska+1 more|ArXiv.org|Jan 30, 2001

Stellar, planetary, and galactic studies参考文献 31被引用 41

一句话总结

本研究针对34个银河系行星状星云（含[WCG]中央星）提供了高分辨率光谱测光数据，系统推导了星云的物理条件、元素丰度及膨胀速度。研究发现，星云激发度与电子温度梯度强烈相关，这一现象与标准光致电离模型的预测相悖；同时未发现沃尔夫-拉叶现象存在质量偏好，尽管两个天体（M 1-32 和 M 1-25）表现出异常低的Ne/O比值。

ABSTRACT

We present high resolution spectrophotometric data for 34 galactic planetary nebulae with [WC] nuclei (WRPNe).The sample includes PNe with early and late [WC] stars and some WELS. Physical conditions and chemical abundances have been derived and expansion velocities were estimated for most objects of the sample. A statistical study was developed to find fundamental relationships between the nebular and central star parameters.We found evidence for a strong unexpected electron temperature gradient in WRPNe which is related to nebular excitation. Abundance ratios indicate that there seems to be no preferential stellar mass for the WR phenomenon to occur in the nucleus of a planetary nebula. The PNe M 1-25 and M 1-32 were found to have very small Ne/O ratios.Our data confirm the trend of the electron density decreasing with decreasing spectral type, which was interpreted as evidence that [WC] stars evolve from late to early [WC] types. On the other hand, the expansion velocities do not show the increase with decreasing spectral type, that one might expect in such a scenario. Two objects with very late [WC] central stars, K 2-16 and PM 1-188, do not follow the general density sequence, being of very low density for their spectral types. We suggest that the stars either underwent a late helium flash (the ``born again'' scenario) or that they have had a particularly slow evolution from the AGB. The 6 WELS of our sample follow the same density vs. [WC]-type relation as the bona fide WRPNe, but they tend to have smaller expansion velocities. The comparison between the WRPNe in the Magellanic Clouds and in the Galaxy indicates that metallicity affects the [WR] phenomenon in central stars of planetary nebulae.

研究动机与目标

• 整理34个银河系行星状星云（含[WCG]中央星）的统一、高分辨率光谱测光数据集。
• 以一致、系统的方式推导星云的物理条件（电子密度、温度）、元素丰度及膨胀速度。
• 通过分析星云性质与光谱类型的关系，研究[WCG]恒星的演化序列。
• 探讨弱发射线星（WELS）的性质及其与真实WCG型行星状星云的关系。
• 将银河系WCG型行星状星云与大、小麦哲伦云中的同类天体进行比较，评估金属丰度在[WCG]现象中的作用。

提出的方法

• 从墨西哥圣佩德罗-马科斯天文台（Observatorio Astronómico Nacional, SPM, B.C., México）获取高分辨率光谱测光数据。
• 利用诊断线比值（如[S ii]、[O ii]、[O iii]）测定星云中的电子密度与温度。
• 采用标准经验方法，从发射线强度推导元素丰度。
• 对大多数天体，通过线轮廓半高全宽（HWHM）一致估算膨胀速度。
• 对推导出的参数进行统计分析，识别样本中的相关性与趋势。
• 结合星云与恒星数据，重新审视‘重生’演化情景及演化序列，尤其针对晚期型[WCG]恒星。

实验结果

研究问题

• RQ1 WCG型行星状星云中，星云电子温度与光谱类型之间存在何种关系？是否与标准光致电离模型的预测相矛盾？
• RQ2 WELS是否遵循与真实WCG型行星状星云相同的密度-光谱类型序列？这对它们的演化状态有何启示？
• RQ3 为何两个天体（M 1-32 和 M 1-25）表现出异常低的Ne/O丰度比？
• RQ4 是否存在[WR]现象在行星状星云核中具有质量依赖阈值的证据？
• RQ5 金属丰度如何影响不同星系中[WCG]光谱类型及N/O比值的分布？

主要发现

• 在WCG型行星状星云中发现强烈的电子温度梯度，随星云激发度升高而上升，与简单光致电离模型的预测相悖。
• 未发现沃尔夫-拉叶现象在WCG型行星状星云中存在恒星质量偏好，表明该过程并非受质量限制。
• 两个天体M 1-32 和 M 1-25表现出极低的Ne/O比值（≤0.13），这一特征难以用现有演化模型解释。
• 电子密度随[WCG]光谱类型从晚期向早期降低的一般趋势得到证实，支持从晚期到早期[WCG]类型的演化序列。
• 膨胀速度并未随光谱类型降低而增加，与简单演化序列的预期相矛盾。
• 两个极晚期[WCG]恒星K 2-16 和 PM 1-188表现出与其类型不符的异常低密度，暗示可能存在‘重生’演化或极慢的AGB演化。