Title: Do spectra improve distance measurements of Type Ia supernovae? Show Chinese title

Title: 超新星Ia的距离测量会因光谱而改善吗？

Abstract: [Abridged] We investigate the use of a wide variety of spectroscopic measurements to determine distances to low-redshift Type Ia supernovae (SN Ia). We consider linear models for predicting distances to SN Ia using light-curve width and color parameters (determined using the SALT2 light-curve fitter) and a spectroscopic indicator, and evaluate the resulting Hubble diagram scatter using a cross-validation procedure. We confirm the ability of spectral flux ratios alone at maximum light to reduce the scatter of Hubble residuals by ~10% with respect to the standard combination of light-curve width and color. When used in combination with the SALT2 color parameter, the color-corrected flux ratio R^c(6420/5290) at maximum light leads to an even lower scatter, although the improvement has low statistical significance (<2 sigma) given the size of our sample (26 SN Ia). We highlight the importance of an accurate relative flux calibration and the failure of this method for highly-reddened objects. Comparison with synthetic spectra from 2D delayed-detonation explosion models shows that the correlation of R(6630/4400) with SN Ia absolute magnitudes can be largely attributed to intrinsic color variations and not to reddening by dust in the host galaxy. We consider flux ratios at other ages, as well as the use of pairs of flux ratios, revealing the presence of small-scale intrinsic spectroscopic variations in the iron-group dominated absorption features around ~4300 A and ~4800 A. The best flux ratio overall is the color-corrected R^c(4610/4260) at t=-2.5d from maximum light, which leads to ~30% lower scatter with respect to the standard combination of light-curve width and color. We examine other spectroscopic indicators related to line-profile morphology, but none appear to lead to a significant improvement over the standard light-curve width and color parameters. Show Chinese abstract

Abstract: 【摘要】我们研究了多种光谱测量方法用于确定低红移Ia型超新星（SN Ia）的距离。我们考虑了使用SALT2光变曲线拟合器确定的光变曲线宽度和颜色参数以及光谱指示器来预测SN Ia距离的线性模型，并通过交叉验证程序评估由此产生的哈勃图散度。我们证实了单一最大亮度下光谱通量比值能够使哈勃残差的散度相对于标准的光变曲线宽度和颜色组合降低约10%。当与SALT2颜色参数结合使用时，最大亮度下的色校正通量比值R^c(6420/5290)会导致更低的散度，尽管由于样本大小（26个SN Ia）的影响，这种改进的统计显著性较低（<2σ）。我们强调了准确的相对通量校准的重要性以及此方法对于高度消光物体的失效。与来自二维延迟引爆爆炸模型的合成光谱进行比较表明，R(6630/4400)与SN Ia绝对亮度的相关性主要归因于固有颜色变化而非宿主星系尘埃消光。我们还考虑了其他时间点的通量比值以及成对通量比值的使用，揭示了在大约4300 Å和4800 Å附近铁族主导吸收特征中的小尺度固有光谱变化。总体而言，最好的通量比值是从最大亮度前-2.5天开始的色校正R^c(4610/4260)，与标准的光变曲线宽度和颜色组合相比，其散度降低了约30%。我们还检查了与线轮廓形态相关的其他光谱指示器，但没有发现它们能显著改善标准的光变曲线宽度和颜色参数。