Non-LTE models for synthetic spectra of type Ia supernovae - III. An accelerated lambda-iteration procedure for the mutual interaction of strong spectral lines in SN Ia models with and without energy deposition
Non-LTE models for synthetic spectra of type Ia supernovae - III. An accelerated lambda-iteration procedure for the mutual interaction of strong spectral lines in SN Ia models with and without energy deposition
复制标题
DOI：
10.1051/0004-6361/201322253
复制
发表时间：
2013-07
影响因子：
6.5
通讯作者：
A. Pauldrach;T. Hoffmann;P. Hultzsch
中科院分区：
文献类型：
--
作者：
A. Pauldrach;T. Hoffmann;P. Hultzsch
上下文在Ia型超新星（SN Ia）的包层中，大量不同元素的谱线在其热多普勒宽度内重叠，并且由于这些谱线中多达20%的谱线光学深度大于1，这个问题变得更加严重。在这种光学厚的情况下，λ迭代的停滞是非LTE问题的迭代解中固有的基本物理问题之一，并且λ迭代收敛的失败是至关重要的一点，其物理意义必须完全理解。目标。我们讨论了一个一般性的问题，涉及到通常的非LTE迭代方案收敛时，属于不同的物理过渡多个强不透明度走到一起，即使只有一个单一的过程达到高光学深度的收敛受损的情况下，通常的非LTE迭代方案的失败超新星喷出物的物理条件下的辐射传输。的收敛问题是独立的选定的频率和深度的网格间距，独立的辐射传输是否解决在comoving或观察者的框架，并独立于是否使用一个共同的完全线性化方案或传统的加速λ迭代（ALI）。当在综合非LTE模型的框架中处理Ia型超新星包络的现实描述所需的所有数百万条线转换时，问题出现了。这个问题的唯一解决方案是一个完整的线性化的方法，同时考虑所有元素的所有离子，或建立ALI技术的充分推广，考虑到不同元素的强谱线的相互作用，从而解冻的“卡住”状态的迭代。方法.超新星Ia大气层的物理性质受到喷出物高速膨胀的强烈影响，而喷出物的高速膨胀主导了所有波长范围光谱的形成。因此，考虑到强烈偏离局部热力学平衡（LTE）的流体动力学爆炸模型和现实的模型大气是必要的合成和分析的光谱。在这方面，我们在对Ia超新星中的辐射传输进行建模时发现的最大挑战之一是，紫外线中的辐射能量必须仅通过光谱线转移到光学区域才能离开喷出物。然而，当使用标准程序时，模型向可能的状态的收敛受到损害。我们报告的改进，在我们的方法计算合成光谱的超新星Ia方面（i）一个改进的和复杂的处理成千上万的强线，相互作用错综复杂的“伪连续”完全形成的多普勒频移谱线;（ii）一个改进的和扩大的原子数据库;及（iii）列入能量沉积内喷出物所产生的放射性衰变主要是56镍和56钴。我们表明，我们已经开发了一个ALI程序的相互作用的强光谱线出现在大气中的超新星Ia解决了长期存在的问题，转移到光学制度的辐射能量从紫外线。因此，我们的新方法构成了更精细模型的基础，例如包括能量沉积的模型。在这方面，我们还显示了合成光谱所采用的各种方法释放的能量，并将它们与观测结果进行比较。我们详细讨论的诊断技术的应用程序的例子，一个标准的Ia型超新星，计算和观察到的光谱的比较显示，在早期阶段的能量沉积的考虑内的光谱形成区域的喷出物不定性改变的形状的紧急光谱。结论.我们的调查结果导致一个更好的理解光谱的形状如何从根本上改变的函数在喷出物的深度，并显示不同的紧急光谱是如何形成的结果的特殊物理性质的超新星Ia喷出物和由此产生的特殊性的辐射传输。这一知识提供了一个重要的洞察过程中提取信息从观察到的超新星Ia光谱，因为这些光谱是一个复杂的产品，许多不可观测的超新星Ia光谱特征，因此分析并行的可观测的超新星Ia光谱特征。
Context. In type Ia supernova (SN Ia) envelopes a huge number of lines of different elements overlap within their thermal Doppler widths, and this problem is exacerbated by the circumstance that up to 20% of these lines can have a line optical depth higher than 1. The stagnation of the lambda iteration in such optically thick cases is one of the fundamental physical problems inherent in the iterative solution of the non-LTE problem, and the failure of a lambda iteration to converge is a point of crucial importance whose physical significance must be understood completely. Aims. We discuss a general problem related to radiative transfer under the physical conditions of supernova ejecta that involves a failure of the usual non-LTE iteration scheme to converge when multiple strong opacities belonging to different physical transitions come together, similar to the well-known situation where convergence is impaired even when only a single process attains high optical depths. The convergence problem is independent of the chosen frequency and depth grid spacing, independent of whether the radiative transfer is solved in the comoving or observer’s frame, and independent of whether a common complete-linearization scheme or a conventional accelerated lambda iteration (ALI) is used. The problem appears when all millions of line transitions required for a realistic description of SN Ia envelopes are treated in the frame of a comprehensive non-LTE model. The only solution to this problem is a complete-linearization approach that considers all ions of all elements simultaneously, or an adequate generalization of the established ALI technique that accounts for the mutual interaction of the strong spectral lines of different elements and which thereby unfreezes the “stuck” state of the iteration. Methods. The physics of the atmospheres of SN Ia are strongly affected by the high-velocity expansion of the ejecta, which dominates the formation of the spectra at all wavelength ranges. Thus, hydrodynamic explosion models and realistic model atmospheres that take into account the strong deviation from local thermodynamic equilibrium (LTE) are necessary for the synthesis and analysis of the spectra. In this regard one of the biggest challenges we have found in modeling the radiative transfer in SN Ia is the fact that the radiative energy in the UV has to be transferred only via spectral lines into the optical regime to be able to leave the ejecta. However, convergence of the model toward a state where this is possible is impaired when using the standard procedures. We report on improvements in our approach of computing synthetic spectra for SN Ia with respect to (i) an improved and sophisticated treatment of many thousands of strong lines that interact intricately with the “pseudo-continuum” formed entirely by Doppler-shifted spectral lines; (ii) an improved and expanded atomic database; and (iii) the inclusion of energy deposition within the ejecta arising from the radioactive decay of mostly 56 Ni and 56 Co. Results. We show that an ALI procedure we have developed for the mutual interaction of strong spectral lines appearing in the atmospheres of SNe Ia solves the long-standing problem of transferring the radiative energy from the UV into the optical regime. Our new method thus constitutes a foundation for more refined models, such as those including energy deposition. In this regard we furthermore show synthetic spectra obtained with various methods adopted for the released energy and compare them with observations. We discuss in detail applications of the diagnostic technique by example of a standard type Ia supernova, where the comparison of calculated and observed spectra revealed that in the early phases the consideration of the energy deposition within the spectrum-forming regions of the ejecta does not qualitatively alter the shape of the emergent spectra. Conclusions. The results of our investigation lead to an improved understanding of how the shape of the spectrum changes radically as function of depth in the ejecta, and show how different emergent spectra are formed as a result of the particular physical properties of SNe Ia ejecta and the resulting peculiarities in the radiative transfer. This knowledge provides an important insight into the process of extracting information from observed SN Ia spectra, since these spectra are a complex product of numerous unobservable SN Ia spectral features, which are thus analyzed in parallel to the observable SN Ia spectral features.