Ia型超新星的双总体贝叶斯分层模型
Two-population Bayesian hierarchical model of type Ia supernovae
AI总结：
针对Ia型超新星标准化产生的哈勃残差起源不明问题，提出双总体贝叶斯分层模型，发现两类超新星总体的光变参数与消光特征差异，无需内禀弥散即可解释参数分布。
摘要：
当前采用的Ia型超新星标准化方法会产生哈勃残差，其物理起源尚未得到解释。本文提出一种全新的Ia型超新星贝叶斯分层模型，基于该模型给出哈勃残差的完整物理解释：模型中描述超新星内禀性质与外源性（尘埃相关）性质的潜变量源自两个总体。将模型拟合哈勃流中超新星的SALT2光变曲线参数后，我们发现强（4σ）证据表明存在两个重叠但截然不同的总体，二者主要通过平均SALT2形状参数（拉伸量）x₁区分。其中以慢衰减型超新星为主的总体（平均x₁更高）内禀颜色更蓝（平均SALT2颜色c=-0.11），尘埃红化程度是另一总体的两倍（平均红化E(B-V)=0.10）；另一总体以快衰减型超新星为主（平均x₁更低），c=-0.04，E(B-V)=0.05。两个超新星总体推断出的消光系数R_B均服从宽分布（弥散度为0.9），均值为4.1，与银河系中测得的平均消光定律高度吻合。我们还发现，超新星数据更支持选择性消光E(B-V)呈峰状（双尾）分布，而非普遍采用的指数模型（最大值固定为0的单尾分布）。本模型从消光性质及上述两个总体的差异出发，完整解释了超新星光变曲线参数的分布，无需引入任何内禀弥散。
英文摘要：
The currently used standardisation of type Ia supernovae results in Hubble residuals whose physical origin is unaccounted for. Here, we present a complete physical interpretation of the Hubble residuals based on a novel Bayesian hierarchical model of type Ia supernovae in which latent variables describing intrinsic and extrinsic (dust related) supernova properties originate from two populations. Fitting the model to SALT2 light curve parameters of supernovae in the Hubble flow we find strong ($4σ$) evidence for the presence of two overlapping, but distinct, populations differentiated primarily by their mean SALT2 shape parameter (stretch) x_1. Supernovae from the population with predominantly slow decliners (higher average x_1) are found to be intrinsically bluer (mean SALT2 colour c=-0.11) and twice as reddened by dust (mean reddening E(B-V)=0.10) than those from the opposite population dominated by fast decliners (lower average x_1) with c=-0.04$ and E(B-V)=0.05. The inferred extinction coefficient R_B in both supernova populations follows a broad (scatter 0.9) distribution with a mean of 4.1, which coincides closely with the mean extinction law measured in the Milky Way. We also find that the supernova data favour a peaked (two-tailed) distribution of selective extinction E(B-V) over the commonly adopted exponential model (one-tailed with maximum fixed at 0). Our model provides a complete explanation of the distribution of supernova light curve parameters in terms of extinction properties and the above-mentioned differences between the two populations, without the need for introducing any intrinsic scatter.
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