Properties of Dust Grains in Planetary Nebulae. I. The Ionized Region of NGC 6445
Properties of Dust Grains in Planetary Nebulae. I. The Ionized Region of NGC 6445
复制标题
行星状星云中尘埃颗粒的特性。
DOI：
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发表时间：
1999
期刊：
影响因子：
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通讯作者：
G. Ferland
中科院分区：
文献类型：
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作者：
P. V. van Hoof;G. C. Van de Steene;D. Beintema;P. Martin;S. Pottasch;G. Ferland
影响行星状星云光谱演化的因素之一是发出红外连续谱的尘埃颗粒的命运。已经提出了几种方法，要么破坏颗粒，要么将它们从电离区移除。为了测试这些过程是否有效，我们研究了演化星云NGC 6445的新红外光谱。这些数据表明，来自晶粒的热发射非常冷，并且与Hβ相比具有低通量。电离区的模型构建，使用光电离代码云90.05。基于这个模型，我们从气相元素的耗尽中表明，在NGC 6445的电离区域中可能发生了很小的颗粒破坏。我们还认为，星云中的尘埃气体分离是不合理的。最有可能的结论是，颗粒存在于NGC 6445的电离区域内，并且颗粒发射的低温和通量是由中心星星的低光度和颗粒的低光学深度造成的。这意味着该星云中的大部分含硅颗粒能够在硬紫外光子照射下存活至少数千年，这与先前发表的结果相矛盾。光学和红外诊断线比率之间的比较给出了星云中存在t2效应的边缘指示。然而，证据并不令人信服，差异也可以解释的光谱，孔径校正，已应用的绝对通量校准，或碰撞截面的不确定性。光致电离模型允许基于模型大气精确确定中心星星温度。得到的值为184 kK，与基于黑体近似的所有已公布的赞斯特拉温度的平均值非常一致。用LWS拍摄的离源光谱清楚地显示了温度为24 K的温暖卷云成分以及温度为7 K的非常冷的成分的存在。由于我们的观测只涵盖了天空的一小部分，因此尚不清楚7 K分量的延伸程度以及它是否对COBE拍摄的远红外绝对分光光度计（FIRAS）光谱有显著贡献。因为我们的视线是在银河平面上，所以非常冷的部分可能是一个没有恒星的核心。
One of the factors influencing the spectral evolution of a planetary nebula is the fate of the dust grains that are emitting the infrared continuum. Several processes have been proposed that either destroy the grains or remove them from the ionized region. To test whether these processes are effective, we study new infrared spectra of the evolved nebula NGC 6445. These data show that the thermal emission from the grains is very cool and has a low flux compared to Hβ. A model of the ionized region is constructed, using the photoionization code CLOUDY 90.05. Based on this model, we show from depletions in the gas-phase elements that little grain destruction can have occurred in the ionized region of NGC 6445. We also argue that dust-gas separation in the nebula is not plausible. The most likely conclusion is that grains are residing inside the ionized region of NGC 6445 and that the low temperature and flux of the grain emission are caused by the low luminosity of the central star and the low optical depth of the grains. This implies that the bulk of the silicon-bearing grains in this nebula were able to survive exposure to hard-UV photons for at least several thousands of years, contradicting previously published results. A comparison between optical and infrared diagnostic line ratios gives a marginal indication for the presence of a t2 effect in the nebula. However, the evidence is not convincing and the differences could also be explained by uncertainties in the absolute flux calibration of the spectra, the aperture corrections that have been applied, or the collisional cross sections. The photoionization model allows an accurate determination of the central star temperature based on model atmospheres. The resulting value of 184 kK is in good agreement with the average of all published Zanstra temperatures based on blackbody approximations. The off-source spectrum taken with LWS clearly shows the presence of a warm cirrus component with a temperature of 24 K as well as a very cold component with a temperature of 7 K. Since our observation encompasses only a small region of the sky, it is not clear how extended the 7 K component is and whether it contributed significantly to the Far-Infrared Absolute Spectrophotometer (FIRAS) spectrum taken by COBE. Because our line of sight is in the Galactic plane, the very cold component could be a starless core.