High rydberg state carbon recombination lines toward Cassiopeia A: physical conditions and a new class of models
High rydberg state carbon recombination lines toward Cassiopeia A: physical conditions and a new class of models
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朝向仙后座 A 的高里德伯态碳重组线：物理条件和一类新模型
DOI：
10.1086/174441
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发表时间：
1994
期刊：
影响因子：
--
通讯作者：
W. Erickson
中科院分区：
文献类型：
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作者：
H. Payne;K. Anantharamaiah;W. Erickson
我们给出了英仙座ARM云中沿CAS方向观测到的物理条件的模型，并将碳射电复合线强度的理论预测与14至775 MHz频率范围内的观测结果进行了比较。然后，将最佳拟合的模式参数与波长为21 cm的HI吸收线观测结果和低频氢复合线观测结果相结合，以评估这些云中的热平衡和气压平衡。对现有复合谱线数据的关键重新检查表明，最低频率的碳谱线观测将谱线中的积分光学深度低估了高达3倍。这是因为在基线减去过程中，从加压加宽的谱线轮廓中去掉了大的洛伦兹翼。部分或全部复合线光学厚度起源于分子云的模型不能给出令人满意的结果。基于复合线强度标准计算的模型(Salem&Brocklehurst，1979；Walmsley&Watson，1982)压力很高，并且失去了热平衡。我们修正了Salem&Brocklehurst(1979)和Walmsley&Watson(1982)所描述的谱线强度的理论计算，在大主量子数n处施加了不同的边界条件，而不是像Gulyaev&Nefedov()所建议的那样，假定根据热力学平衡填充的能级数目无限多，而是对临界主量子数ncr以上的能级布居施加一个截止。我们遵循Hummer&Mihalas(1988)提出的占据概率公式来计算ncr。利用这些新的谱线强度计算，我们发现，除复合谱线宽度外，所有的CaS A复合谱线和2 1cmHI吸收谱线数据都可以归因于一个物理条件是典型的星际介质冷中性介质的区域。我们给出了一个模型，在这个模型中，这个区域在36K附近处于热平衡状态，并且有一个合理的星际压力。在该模型中，多环芳香族氢分子的光致电离是加热该区域的主要机制。
We present models of physical conditions in Perseus arm clouds observed in the direction of Cas A. Theoretical predictions of carbon radio recombination line intensities are compared with observations spanning the frequency range 14 to 775 MHz. Best-fitting model parameters are then combined with the results of lambda 21 cm H I absorption line observations, and low-frequency hydrogen recombination line observations, to evaluate the thermal and pressure balance in these clouds. A critical reexamination of the available recombination line data shows that the lowest frequency carbon line observations have underestimated the integrated optical depths in the line by up to a factor of 3. This is due to the removal of large Lorentzian wings from the pressure broadened line profiles during baseline subtraction. Models where some or all of the recombination line optical depth originates in molecular clouds do not give satisfactory results. Models based on standard calculations of recombination line intensites (Salem & Brocklehurst 1979; Walmsley & Watson 1982) have high pressures and are out of thermal balance. We modified the theoretical calculation of line intensities, described by Salem & Brocklehurst (1979) and Walmsley & Watson (1982), by imposing a different boundary condition at large principal quantum number n. Instead of assuming an infinite number of levels populated according to thermodynamic equilibrium, we impose a cutoff in level populations above a critical principal quantum number n cr , as suggestd by Gulyaev & Nefedov (1989). We followed the occupation probability formalism presented by Hummer & Mihalas (1988) to calculate n cr . Using these new line intensity calculations, we find that, with the exception of the recombination line width, all of the Cas A recombination line and lambda 21 cm H I absorption line data can be attributed to a region where the physical conditions are typical of the cold neutral medium of the interstellar medium. We show a model in which this region is in thermal balance at a temperature near 36 K and has a reasonable interstellar pressure. In this model, the photoionization of polycyclic aromatic hydrogen molecules is the dominant mechanism for heating the region.