Keenan Francis P, Aller Lawrence H, Espey Brian R, Exter Katrina M, Hyung Siek, Keenan Michael T C, Pollacco Don L, Ryans Robert S I
Department of Pure and Applied Physics, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland.
Proc Natl Acad Sci U S A. 2002 Apr 2;99(7):4152-5. doi: 10.1073/pnas.062032299. Epub 2002 Mar 19.
Recent R-matrix calculations of electron impact excitation rates in K v are used to derive the nebular emission line ratio R = I(4122.6 A)/I(4163.3 A) as a function of electron density (N(e)). This ratio is found to be very sensitive to changes in N(e) over the density range 10(3) to 10(6) cm(-3), but does not vary significantly with electron temperature, and hence in principle should provide an excellent optical N(e) diagnostic for the high-excitation zones of nebulae. The observed value of R for the planetary nebula NGC 7027, measured from a spectrum obtained with the Hamilton Echelle spectrograph on the 3-m Shane Telescope, implies a density in excellent agreement with that derived from [Ne iv], formed in the same region of the nebula as [K v]. This observation provides observational support for the accuracy of the theoretical [K v] line ratios, and hence the atomic data on which they are based. However, the analysis of a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope, reveals that the [K v] 4122.6 A line in this object is badly blended with Fe ii 4122.6 A. Hence, the [K v] diagnostic may not be used for astrophysical sources that show a strong Fe ii emission line spectrum.
最近利用R矩阵对钾离子（K v）中电子碰撞激发率的计算结果，推导出星云发射线比率R = I(4122.6埃)/I(4163.3埃)作为电子密度（N(e)）的函数。发现该比率在10³至10⁶厘米⁻³的密度范围内对N(e)的变化非常敏感，但随电子温度变化不显著，因此原则上应为星云的高激发区提供出色的光学N(e)诊断方法。通过使用3米 Shane 望远镜上的汉密尔顿阶梯光栅光谱仪获得的光谱测量的行星状星云 NGC 7027的R观测值，意味着其密度与在星云同一区域形成的[Ne iv]推导出的密度高度一致。这一观测结果为理论[K v]线比率的准确性以及其所基于的原子数据提供了观测支持。然而，对共生星RR Telescopii的高分辨率光谱进行分析（该光谱是使用位于3.9米英澳望远镜上的伦敦大学学院阶梯光栅光谱仪获得的），结果显示该天体中的[K v] 4122.6埃线与Fe ii 4122.6埃线严重混合。因此，[K v]诊断方法可能不适用于显示强Fe ii发射线光谱的天体物理源。
