Aller L H, Hung S, Feibelman W A
Physics and Astronomy Department, University of California, Los Angeles, CA 90095, USA.
Proc Natl Acad Sci U S A. 1999 May 11;96(10):5366-71. doi: 10.1073/pnas.96.10.5366.
NGC 6537 is an unusually high excitation point symmetric planetary nebula with a rich spectrum. Its kinematical structures are of special interest. We are here primarily concerned with the high resolution spectrum as revealed by the Hamilton echelle Spectrograph at Lick Observatory (resolution approximately 0.2 A) and supplemented by UV and near-UV data. These extensive data permit a determination of interstellar extinction, plasma diagnostics, and ionic concentrations. The photoionization models that have been used successfully for many planetary nebulae are not entirely satisfactory here. The plasma electron temperature of a photoionization model cannot much exceed 20,000 K, but plasma diagnostics show that regions emitting radiation of highly ionized atoms such as [NeIV] and [NeV] are much hotter, showing that shock excitation must be important, as suggested by the remarkable kinematics of this object. Hence, instead of employing a strict photoionization model, we are guided by the nebular diagnostics, which reveal how electron temperature varies with ionization potential and accommodates density effects. The predictions of the photoionization model may be useful in estimating ionization correction factor. In effect, we have estimated the chemical composition by using both photoionization and shock considerations.
NGC 6537是一个激发度异常高的点对称行星状星云，光谱丰富。其运动学结构特别引人关注。我们在此主要关注由利克天文台的汉密尔顿阶梯光栅光谱仪所揭示的高分辨率光谱（分辨率约为0.2埃），并辅以紫外和近紫外数据。这些广泛的数据使得我们能够确定星际消光、等离子体诊断以及离子浓度。已成功应用于许多行星状星云的光电离模型在此并不完全令人满意。光电离模型中的等离子体电子温度不能大幅超过20,000 K，但等离子体诊断表明，发射诸如[NeIV]和[NeV]等高电离原子辐射的区域要热得多，这表明激波激发必定很重要，正如该天体显著的运动学所暗示的那样。因此，我们并非采用严格的光电离模型，而是以星云诊断为指导，星云诊断揭示了电子温度如何随电离势变化并考虑了密度效应。光电离模型的预测在估计电离校正因子方面可能有用。实际上，我们通过同时考虑光电离和激波因素来估计化学成分。
