CONSTRUCTING POLYNOMIAL SPECTRAL MODELS FOR STARS
CONSTRUCTING POLYNOMIAL SPECTRAL MODELS FOR STARS
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DOI：
10.3847/2041-8205/826/2/l25
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发表时间：
2016-03
期刊：
影响因子：
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通讯作者：
H. Rix;Yuan-Sen Ting (丁源森);C. Conroy;D. Hogg
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文献类型：
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作者：
H. Rix;Yuan-Sen Ting (丁源森);C. Conroy;D. Hogg
恒星光谱取决于恒星参数和几十种光球层元素丰度。将这些∼10-40模型标签同时拟合到观测光谱被认为是不可行的，因为从头算光谱模型网格计算的数量随呈指数级增长。相反，我们建议为每个波长的模型通量构造一个阶的多项式谱模型。建立这种近似只需要最少的+计算：例如，可以从最少的231个从头算光谱模型计算中构造出同时适合=20个标签的二次光谱模型(∼=2)；在实践中，稍微大一些的随机选择的模型(PSM 300-1000)会导致更好的性能。这样的PSM只能在标签空间的一部分上是一个很好的近似值，这将因情况而异。然而，以远地点调查为例，单一的二次型PSM对这次调查大部分时间内的精确从头算光谱模型提供了一个非常好的近似：对于该调查中的随机标签，PSM将通量近似到10−3以内，并将丰度恢复到精确模型的∼0.02Dex RMS以内。这种巨大的加速使光谱的多标记拟合能够与恒星光谱的计算昂贵的从头计算模型同时进行，例如非LTE模型。PSM还可以同时拟合观测参数，例如频谱的连续谱或线扩展函数。
Stellar spectra depend on the stellar parameters and on dozens of photospheric elemental abundances. Simultaneous fitting of these ∼ 10–40 model labels to observed spectra has been deemed unfeasible because the number of ab initio spectral model grid calculations scales exponentially with . We suggest instead the construction of a polynomial spectral model (PSM) of order for the model flux at each wavelength. Building this approximation requires a minimum of only + calculations: e.g., a quadratic spectral model ( = 2 ) to fit = 20 labels simultaneously can be constructed from as few as 231 ab initio spectral model calculations; in practice, a somewhat larger number (∼300–1000) of randomly chosen models lead to a better performing PSM. Such a PSM can be a good approximation only over a portion of label space, which will vary case-by-case. Yet, taking the APOGEE survey as an example, a single quadratic PSM provides a remarkably good approximation to the exact ab initio spectral models across much of this survey: for random labels within that survey the PSM approximates the flux to within 10−3 and recovers the abundances to within ∼0.02 dex rms of the exact models. This enormous speed-up enables the simultaneous many-label fitting of spectra with computationally expensive ab initio models for stellar spectra, such as non-LTE models. A PSM also enables the simultaneous fitting of observational parameters, such as the spectrum’s continuum or line-spread function.