Ia型超新星的消光改正

Extinction Correction for Type Ia Supernovae

Ia型超新星因其在宇宙学上起着标准烛光的作用而备受关注,正确的消光改正才能还原其本征亮度和颜色,然后才能在宇宙学中恰当地应用。综述了不同环境中的尘埃对Ia型超新星消光的影响,并讨论Ia型超新星绝对星等的校正和Rv值偏小的原因。Ia型超新星绝对星等的弥散主要受两个方面的影响,一是由前身星状态所导致的固有弥散,二是寄主星系内尘埃的消光。利用与距离无关的参数对绝对星等校正后,可以将弥散降低到0.1 mag以下。Ia型超新星的内禀色指数呈现出随时间演化的一致性,使得其红化比较容易计算。大量的观测和分析表明,Ia型超新星在寄主星系中的Rv值在低红化时,其情形比较接近银河系;而在红化严重时偏小,多在2.0以下,低于银河系的平均值3.1。一种可能的解释是Ia型超新星的星周尘埃对星光的多重散射,另一种可能是Ia型超新星寄主星系的尘埃与典型的银河系内星际尘埃的差异。

Type Ia supernovae （SNe Ia） are widely used as a standard candle in cosmology. They have very high （as bright as an entire galaxy） and almost constant luminosity. Their absolute magnitude in visual bands presents very small dispersion of around 0.1 mag after modified according to their declining rate and correction for the extinction. Their intrinsic color index shows very homogeneous law of variation in a period after the maximum light. Understanding the extinction of SNe Ia is a key to its application in cosmology. In principle, there are four sources of extinction to SNe Ia in extra-galaxies： （1） the Galactic foreground interstellar extinction, （2） the intergalactic extinction, （3） the circumstellar extinction, and （4） the interstellar extinction of the host galaxy, with the later two contributors being sig- nificant and unclear. The extinction is calculated by comparing the observed colors and the intrinsic colors derived from their light-variation properties or from the dispersion in the Hubble diagram. The selective extinction ratio Rv is found generally smaller than the typical Galactic value 3.1, although different methods and samples yield a range of values with some of them being consistent with the Galactic case. The extinction curve from ultra-violet to near-infrared is obtained for several SNe Ia with abundant observations, which can either be fit by the Galactic extinction law but with low Rv value or by a power law. This fact may imply that the dust in the SNe Ia host galaxies tends to have smaller average size than the Galactic one. Meanwhile, a multiple-scattering model in the circumstellar envelope can also explain the difference in Rv.