NGC 6153: a super-metal-rich planetary nebula?
NGC 6153: a super-metal-rich planetary nebula?
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DOI：
10.1046/j.1365-8711.2000.03167.x
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发表时间：
2000-03
影响因子：
4.8
通讯作者：
Xiaowei Liu;P. Storey;M. Barlow;I. J. Danziger;M. Cohen;M. Bryce
中科院分区：
文献类型：
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作者：
Xiaowei Liu;P. Storey;M. Barlow;I. J. Danziger;M. Cohen;M. Bryce
我们已经获得了行星状星云NGC 6153的深光谱，既沿着它的小轴，也通过均匀扫描整个星云的长缝。扫描光谱与星云总H β通量相结合，得到了光谱中所有谱线的积分通量（类似于400），其中富含C、N、O和Ne离子的强复合谱线。从NGC 6153的中心恒星探测到微弱的O vi λ 3811发射线，这表明NGC 6153的核心表面缺乏氢。利用光学数据，结合ISO LWS 43-197 μ m光谱和存档IUE和IRAS LRS光谱，研究了热学和密度结构，并从不同激发机制产生的谱线中推导出重元素丰度。在所有情况下，从多个光学复合谱线（ORLs）得到的C2+/H+、N2+/H+、O2+/H+和Ne2+/H+丰度始终比从光学、紫外或红外（IR）碰撞激发谱线（CELs）得到的相应值高约10倍，而不管后者的激发能或临界密度如何。温度敏感的光学禁线和温度不敏感的红外精细结构线之间的一致性排除了温度波动是造成ORL丰度和CEL丰度之间巨大差异的原因。我们提出了一种新的[O II]复合系数的计算结果，如果这些线在Balmer跳变温度下仅由复合激发，则观测到的和预测的[O II] λ λ 7320, 7330禁线强度很好地吻合。重组激发在激发[N ii] lambda 5754线时也很重要，如果不加以解释，将导致从观测到的(lambda 6548+lambda 6584)/lambda 5754比中高估[N ii]温度。对重组阶梯中高达7g的C ii谱线的分析表明，它们的变红校正相对强度与重组理论预测的相对强度非常吻合。沿着星云小轴拍摄的长缝光谱的空间分析得到了变化的[O iii]温度，而6000 K的氢巴尔默跳变温度在整个星云中几乎是恒定的，比[O iii]温度低2000-3000 K。观察到的高n巴尔默线衰减表明，氢谱线是由电子密度为2000(-1000)(+2000)cm（-3）的材料产生的，这与光学和红外禁线密度诊断一致，该诊断得出沿小轴的平均视距电子密度在2000和4000 cm（-3）之间变化。虽然He I和He ii重组线绘制的He/H比值在整个星云中是恒定的，在距离中心15弧秒的半径范围内，C2+/H+和O2+/H+重组线的丰度减少了2-3倍，这表明丰度梯度的存在。我们考虑了各种假设来解释各种热、密度和丰度诊断的观察行为。经验星云模型包含两个密度和温度不同的组成部分，能够解释许多观测到的模式，但前提是其中一个组成部分明显缺乏氢。其中一个模型很好地拟合了观测到的线强度和模式，该模型将500-K的h耗尽物质（假定是从密集的中性夹杂物中蒸发出来的）嵌入在9500-K的“正常”丰度物质中。另一种模型在物理上似乎更合理，它具有高密度(2 × 10(6) cm(-3))，完全电离，缺氢节嵌入“正常”成分，尽管该模型未能充分解释观察到的低（6000 K）氢巴尔默跳变温度。然而，观察到的orl和cel在不确定性范围内产生相同的重元素丰度比的事实，在缺氢结模型的背景下没有明显的解释。
We have obtained deep optical spectra of the planetary nebula NGC 6153, both along its minor axis and by uniformly scanning a long slit across the whole nebula. The scanned spectra, when combined with the nebular total H beta flux, yield integrated fluxes for all the lines (similar to 400) in our spectra, which are rich in strong recombination lines from C, N, O and Ne ions. A weak O vi lambda 3811 emission line from the central star has been detected, suggesting that the nucleus of NGC 6153 has a hydrogen-deficient surface. The optical data, together with the ISO LWS 43-197 mu m spectrum and the archival IUE and IRAS LRS spectra, are used to study the thermal and density structure and to derive the heavy-element abundances from lines produced by different excitation mechanisms. In all cases, the C2+/H+, N2+/H+, O2+/H+ and Ne2+/H+ abundances derived from multiple optical recombination lines (ORLs) are consistently higher, by about a factor of 10, than the corresponding values deduced from optical, UV or infrared (IR) collisionally excited lines (CELs), regardless of the excitation energies or critical densities of the latter. The agreement between the temperature-sensitive optical forbidden lines and the temperature-insensitive IR fine-structure lines rules out temperature fluctuations as the cause of the large difference between the ORL and CEL abundances.We present the results of a new calculation of recombination coefficients for [O II] which lead to good agreement between the observed and predicted [O II] lambda lambda 7320, 7330 forbidden line intensities if these lines are solely excited by recombination at the Balmer jump temperature. Recombination excitation is also found to be important in exciting the [N ii] lambda 5754 line, which, if unaccounted for, would lead to an overestimated [N ii] temperature from the observed (lambda 6548+lambda 6584)/lambda 5754 ratio. Analysis of a number of C ii lines arising from levels as high as 7g in the recombination ladder reveals excellent agreement between their reddening-corrected relative intensities and those predicted by recombination theory. Spatial analysis of the long-slit spectra taken along the nebular minor axis yields a varying [O iii] temperature, whereas the hydrogen Balmer jump temperature of 6000 K is approximately constant across the nebula, and is 2000-3000 K lower than the [O iii] temperature. The observed high-n Balmer line decrement indicates that the hydrogen lines arise from material having an electron density of 2000(-1000)(+2000) cm(-3), consistent with the optical and IR forbidden-line density diagnostics, which yield average line-of-sight electron densities along the minor axis varying between 2000 and 4000 cm(-3).While the He/H ratio mapped by He I and He ii recombination lines is constant within 5 per cent across the nebula, the C2+/H+ and O2+/H+ recombination-line abundances decrease by a factor of 2-3 over a radius of 15 arcsec from the centre, pointing to the presence of abundance gradients. We consider a variety of hypotheses to account for the observed behaviour of the various thermal, density and abundance diagnostics. Empirical nebular models containing two components with differing densities and temperatures are able to account for many of the observed patterns, but only if one of the components is significantly hydrogen-deficient. One such model, which gives a good fit to the observed line intensities and patterns, has 500-K H-depleted material, presumed to be evaporating from dense neutral inclusions, embedded in 9500-K material with 'normal' abundances. An alternative model, which appears more physically plausible on a number of grounds, has high-density (2x10(6) cm(-3)), fully ionized, H-deficient knots embedded in the 'normal' component, although this model fails to account adequately for the observed low (6000 K) hydrogen Balmer jump temperature. However, the observed fact that the ORLs and CELs yield heavy-element abundance ratios that are identical within the uncertainties finds no obvious explanation in the context of H-deficient knot models.