[论文解读] A hydrodynamical study of multiple-shell planetary nebulae. III. Expansion properties and internal kinematics: Theory versus observation

D. Schoênberner, R. Jacob+6 more|arXiv (Cornell University)|Apr 1, 2014

Astrophysics and Star Formation Studies参考文献 60被引用 26

一句话总结

本研究利用高分辨率阶梯光谱仪光谱法测量了多壳层行星状星云中壳层与激波后壳层的独立膨胀速度，发现由于辐射场和密度梯度的演化，壳层激波前缘随时间加速，而星云边缘则先减速后加速——这一现象挑战了传统认为边缘速度代表星云真实膨胀速度的假设。

ABSTRACT

We present the result of a study on the expansion properties and internal kinematics of round/elliptical planetary nebulae of the Milky Way disk, the halo, and of the globular cluster M15. The purpose of this study is to considerably enlarge the small sample of nebulae with precisely determined expansion properties. To this aim, we selected a representative sample of objects with different evolutionary stages and metallicities and conducted high-resolution echelle spectroscopy. In most cases, we succeeded in detecting the weak signals from the outer nebular shell which are attached to the main line emission from the bright nebular rim. Next to the measurement of the motion of the rim gas by decomposition of the main line components into Gaussians, we were able to measure separately, for most objects for the first time, the gas velocity immediately behind the leading shock of the shell, i.e. the post-shock velocity. We more than doubled the number of objects for which the velocities of both rim and shell are known and confirm that the overall expansion of planetary nebulae is accelerating with time. There are, however, differences between the expansion behaviour of the shell and the rim. This observed distinct velocity evolution of both rim and shell is explained by radiation-hydrodynamics simulations, at least qualitatively. Because of the time-dependent boundary conditions, a planetary nebula will never evolve into a simple self-similar expansion. Also the metal-poor objects behave as theory predicts: The post-shock velocities are higher and the rim flow velocities are equal or even lower compared to disk objects at similar evolutionary stage. We detected, for the first time, in some objects an asymmetric expansion behaviour: The relative expansions between rim and shell appear to be different for the receding and approaching parts of the nebular envelope.

研究动机与目标

• 扩展具有精确测量膨胀特性的行星状星云样本，特别是多壳层系统。
• 区分明亮边缘与微弱外层壳层的动力学行为，尤其是激波前沿之后的后激波速度。
• 将辐射流体动力学模拟的理论预测与不同金属丰度和演化阶段星云中的观测速度演化进行对比。
• 评估观测到的膨胀行为是否与行星状星云中自相似或弹道膨胀的假设相矛盾。
• 研究再电离与金属丰度在动力学演化中的作用，特别是在低金属丰度和古老星云中的影响。

提出的方法

• 对银河系盘面、晕区及M15球状星团中具有代表性的圆形/椭圆形行星状星云进行了高分辨率阶梯光谱观测。
• 通过高斯分解强发射线，分离出明亮边缘与微弱外层壳层的速度成分。
• 通过分析谱线轮廓的蓝移成分，测量后激波速度——即激波前沿后方气体的瞬时速度。
• 将观测到的速度轮廓与辐射流体动力学模拟进行比较，以解释恒星辐射、星风-星风相互作用及上游密度梯度的时变效应。
• 分析具有高温、微弱中心星的天体的动力学行为，以探测再电离的迹象并评估其对激波速度的影响。
• 通过比较高、低金属丰度星云，评估金属丰度的影响，重点关注后激波速度与边缘速度的差异。

实验结果

研究问题

• RQ1 在多壳层行星状星云中，边缘与外层壳层的膨胀速度如何随时间演化？
• RQ2 后激波速度在多大程度上超过边缘速度？其差异由何驱动？
• RQ3 低金属丰度行星状星云的动力学特性与相同演化阶段的太阳金属丰度天体相比有何不同？
• RQ4 再电离在具有快速演化中心星的星云中，对观测到的激波速度起何作用？
• RQ5 观测到的动力学行为是否与行星状星云中自相似或弹道膨胀的假设相矛盾？

主要发现

• 外层壳层的后激波速度从年轻星云中的约20 km s⁻¹增加到年老、高温系统的约40 km s⁻¹，其驱动力来自电离辐射增强和上游密度梯度的变化。
• 边缘速度最初低于AGB星风速度（10–15 km s⁻¹），随后因中心星收缩导致星风功率增加而加速至最高30 km s⁻¹。
• 年轻星云中边缘与后激波速度之差从约25 km s⁻¹减少至演化系统中的约15 km s⁻¹，表明动力学主导地位随时间演变。
• 具有高温、微弱中心星的星云中观测到的高后激波速度（40–50 km s⁻¹）与先前复合气体的再电离一致，而不仅限于持续的星风相互作用。
• 低金属丰度星云在相同演化阶段表现出比太阳金属丰度天体更高的后激波速度和更低的边缘速度，与理论预测一致。
• 行星状星云的真实膨胀速度应由后激波速度而非边缘速度代表，因为在年轻系统中，边缘速度可能低估真实膨胀速度达7倍之多。