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Ia型超新星多样性的观测研究
其他题名Observational Study of the Diversity of Type Ia Supernovae
张居甲
学位类型博士
导师白金明
2015-07-01
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业天体物理
关键词Ia型超新星:Sn 2012fr Sn 2011hr Sn 2013dy Iip型超新星:Sn 2013am 多样性
摘要本文是一篇介绍Ia型超新星多样性的观测及研究的博士论文。Ia型超新星是恒星世界最为壮丽的高能爆发现象之一,不仅与恒星形成和演化、宇宙金属增丰以及星系化学演化等息息相关,更因其在揭示宇宙膨胀命运的独特作用而倍受关注。Ia型超新星前身星及其爆发环境是研究观测多样性、提高测距精度的关键环节,然而目前无法获得其前身星系统的直接观测资料。庆幸的是,Ia型超新星在爆发的极早期保留了区分前身星系统性质的部分信息, 因而在早期进行观测,成为研究这类超新星前身星的重要途径。近年来,我们基于丽江2.4米望远镜,从国内‘清华大学-国家天文台超新星巡天’以及国际上其他巡天组发现的近百颗超新星候选体中证认出84颗超新星,包括一批处于爆发早期的Ia型超新星样本。结合国内外多台望远镜的后续监测数据,我们发现其中一些Ia型超新星在现有的分类体系中难以将其归入已知的光谱和测光亚类,表明它们可能具有不同的前身星性质或者不同的爆发机制。 本文将重点介绍三颗有代表性的特殊Ia型超新星,包括: (1)窄线型的SN 2012fr:总的来说,这是一颗光变曲线轮廓基本正常、下降速度较慢、亮度较高的Ia型超新星。早期光谱中存在非常强的、分离的SiIIλ6355以及Ca II等吸收线的高速成分。对比研究表明,存在两种不同轮廓的Si II λ6355吸收:一种像SN 1991T那样,吸收深度较小、半高全宽较大;另一种像SN 2012fr这样,吸收深度较大、半高全宽较小。这两种轮廓的谱线的等值宽度都较小,属于‘浅硅’的范畴,但是分别代表着不同的抛射物结构。此外,这两种超新星还有着其他明显不同的观测性质,比如早期紫外–光学颜色演化以及二次电离铁线的强度。因此,我们建议将那些有着类似SN 2012fr性质的‘浅硅’型Ia归入到一类新型Ia,因其光谱中的窄线吸收而命名为窄线型Ia。这些窄线型Ia有着相似的光变曲线下降率、光谱轮廓以及诞生环境。因为窄线型Ia和类91T 型Ia 普遍较亮,其样本数量将随着红移的增加而增加,所以窄线型Ia的发现,对于减小高红移处浅硅型Ia的内部光度弥散,提高使用这类Ia开展测距时的精度有着重要意义。 (2)贫钙且明亮的SN 2011hr:这是一颗爆发于星爆星系NGC 2691中的明亮Ia型超新星。其峰值光度以及爆炸时合成的镍的质量都很大。在球对称以及同组分的假设下,该超新星爆炸时的总的抛射物质量高达~1.8M⊙,表明它可能来自超钱德拉塞卡质量极限的爆炸。另一方面,其光谱里中等质量元素一直很弱。特别是Ca II的吸收线不仅很弱,而且强度快速下降,表明该Ia的抛射物中,钙等中等质量元素的含量特别少,绝大部分物质都燃烧生产了镍。种种迹象表明这可能是一颗超钱氏质量极限的Ia,为了验证这一假设,还需要深入开展光谱模拟工作,计算爆炸时各元素丰度及其分布,来对前身星性质以及爆炸机制进行限制。 (3)富碳的SN 2013dy:SN 2013dy极早期的光谱中出现了非常明显的、强度可与Si IIλ6355比拟的C IIλ6580吸收。如此强的残留碳的吸收还是第一次出现在正常的Ia型超新星光谱。多样性研究表明,该Ia在Branch et al. (2009)分类框架下位于‘核心正常’和‘浅硅’型Ia之间;在Wang et al. (2009)分类体系中则处于‘正常速度’与类91T/99aa型Ia的边界。这些观测特征表明这颗早期碳含量丰富的Ia 型超新星具有正常样本的主要特征,并位于Ia型超新星光谱分类的过渡区域。 另一方面,由于IIP型超新星平台中期的抛射速度与此时V波段的绝对星等有着较好的线性关系,可以用作距离的指示器,因此本文也介绍了我们对这类超新星的观测研究结果。通过对SN 2013am的观测,发现这颗IIP型超新星的光谱特征以及抛射速度都和低速且低光度的IIP型超新星SN 2005cs类似,但是却拥有着比后者更高的光度以及更多的56Ni。这一研究观测表明,低速IIP型超新星内部可能有着更为丰富的光度多样性。通过系统研究,找出这种多样性与光谱特征的联系,有助于减小这些低速样本开展测距时的系统误差。 这些研究表明,无论是Ia型超新星还是IIP型超新星,都有着非常丰富的多样性,预示着它们的前身星性质以及爆炸机制存在很多差异,值得我们继续开展观测和理论研究。而这些研究对于提高超新星测量河外距离的精度来说是必不可少的。
其他摘要This paper is a thesis about the observational research of diversity of type Ia supernova. As one of the most energetic events of stellar evolution the explosion of Type Ia supernovae (SNe Ia) has attracted increasing attention and studies of this phenomenon expanded enormously during the last three decades. SNe Ia plays an important role in astrophysics and are crucial to the studies of stellar evolution, galaxy chemical evolution and cosmology. However, the basic mechanism of SNe Ia is still uncertain even it had been used to determine the cosmical parameters. In brief, the arguments are mainly focusing on the progenitor system of SNe Ia, which is generally thought to be thermonuclear explosions of accreting carbon–oxygen white dwarfs in close binaries; however, the nature of progenitor is still unclear. Although it is hard to observe the progenitor directly, fortunately spectra taken shortly after explosion can give us much information about the circumstellar environment and progenitor of SNe Ia. For the fast variety of SNe Ia’s early spectra just a sparse well-sampled objects on hand. In the PhD.phase, we have identified 79 candidates of SNe from several transient surveys basing on the spectra observation at Lijiang 2.4m telescope with YFOSC (Yunnan Faint Object Spectragraph and Camera). And the following spectroscopy and photometry observations focus on the SNe Ia at the extremely early phase after identification. Based on these observations, we found some peculiar events of SNe Ia which can not be well classified in the current spectral classification systems. The well-observed samples of SNe Ia included in this thesis are: (1)Narrow-lined SN Ia–SN 2012fr: Extensive optical and ultraviolet (UV) observations of the type Ia supernova (SN Ia) 2012fr are presented in Zhang et al.(2014a). It has a relatively high luminosity, with an absolute B-band peak magnitude of about ?19.5 mag and a smaller post-maximum decline rate than normal SNe Ia [e.g., ?m15(B) = 0.85 ± 0.05 mag]. Based on the UV and optical light curves, we derived that a56Ni mass of about 0.88 M⊙ was synthesized in the explosion. The earlier spectra are characterized by noticeable high-velocity features (HVFs) of Si II λ6355 and Ca II with velocities in the range of ~ 22,000–31,000 km/s. At around the maximum light, these spectral features are dominated by the photospheric components which are noticeably narrower than normal SNe Ia. The post-maximum velocity of the photosphere remains almost constant at ~12,000 km/s for about one month, reminiscent of the behavior of some luminous SNe Ia like SN 1991T. SN 2012fr resides on the border between the “shallow silicon”and “core-normal”subclasses in the Branch et al. (2009) classification scheme, and on the border between normal and “high-velocity”SNe Ia in the Wang et al. (2009a) system. We proposed that SN 2012fr may represent a subset of the SN 1991T-like SNe Ia viewed ina direction with a clumpy or shell-like structure of ejecta, in terms of a significant level of polarization reported in Maund et al. (2013). However, SN 2012fr also shows key dissimilarities with the 91T-like event, such as the HVFs, the UV-optical colors, the narrow profile of Si II lines. Therefore, we suggest that SN 2012fr might stands for a new subclass named as narrow-lined SNe Ia. The further observation and study for this new subclass is essential to reveal its nature. (2)Calcium-poor SN Ia–SN 2011hr: Photometric and spectroscopic observations of type Ia supernova (SN Ia) 2011hr in the starburst galaxy NGC 2691 are presented in Zhang et al.(2015a). Based on the distance (D = 60 ± 10 Mpc) derived from serval methods with larger uncertainty, we estimate the peak brightness (i.e., MB = ?20.0 ± 0.4 mag; Lmax = 2.27 ± 0.70 × 10^43 erg/s) and the mass of synthesised56Ni (i.e., M(56Ni)=1.16±0.35 M⊙). Its luminosity excesses to the anticipation of B-band decline rate(i.e., ?m15(B)=0.92±0.05 mag). The early spectra are featureless and dominated by the lines of irongroup elements which resembles that of SN 1991T. The absorption feature of intermediate-mass elements (IMEs, e.g., Si and Ca) and also oxygen are rather weak in SN 2011hr throughout our spectroscopic observations. Furthermore, the lines of Ca II infrared-triplet disappeared at just about two months after B-band maximum while most of SNe Ia showing strong absorption of such feature even in that of 91T-like events and also super-luminous SNe Ia. It might suggest that SN 2011hr is very poor of IMEs (esp., Ca) in the ejecta. On the other hand, the similar ejecta velocities of Fe and Si might revel well-mixed abundance of these elements that is distinguish to the stratification structure of SN 1991T. Thus, SN 2011hr is a bright SN Ia sharing the major similarities with SN 1991T and remains difference to the latter. The further modelling work about this SN Ia is essential to revel its nature. (3)Cabron-rich SN Ia–SN 2013dy:Extensive observations of SN 2013dy are presented in Zhang et al.(2015b), including a larger set of optical observations since the peak brightness to a half of year, and the ultraviolet photometry around the B-band maximum. The earlier observations published in Zheng et al.(2013) are also included in the analysis. Their earliest spectrum reveals the strongest C II absorption line which is ever found in a normal SN Ia. Based on the distance (i.e., D=19.5±3.0 Mpc) derived from the Hubble-follow of host galaxy and the width-luminosity relation of SN Ia (i.e., ?m15(B)=0.87±0.03 mag; Phillips 1993), we estimate the peak brightness (i.e., MB= ?19.39 ± 0.35 mag; Lmax= 1.62 ± 0.45 × 10^43erg s?1) and the mass of synthesised56Ni (i.e., M(56Ni)=0.76±0.21M⊙). SN 2013dy resides on the border of “core-normal” and “shallow silicon” (i.e., R(Si)=0.05±0.02) subclasses in the Branch et al. (2009) classification scheme, or on the border of the “normal-velocity” SNe Ia (i.e., vmax Si II≈10,300 km/s) and 91T/99aa-like events in the Wang et al. (2009) system. The observations suggest that SN 2013dy is a bright SN Ia sharing the major similarities of normal and locates on the transitional region of nominal spectroscopic subclasses of SNe Ia. Furthermore, the observations of peculiar–SN 2013en (Liu et al. 2015)and 99aa-like event – iPTF 14bdn (Smitka et al. 2015) are also presented at here. SN 2013en is similar as the typical Iax type supernova SN 2005hk in both photometry and spectroscopy which could help us to understand the nature of this subclass of SNe Ia. Based on the combined UV-optical spectra of iPTF 14bdn, we found a strong evidence of the transition of Fe III to Fe II (and also other iron group elements). That is the first time to show such signatures in the UV spectra. On the other hand, we also presented dense optical and ultraviolet observations for the nearby type II-plateau supernova (SN IIP) 2013am in the nearby spiral galaxy M65. The early spectra are characterized by relatively narrow PCygni features, with ejecta velocities much lower than observed in normal SNe IIP (i.e., ~2000 km/s vs. ~5000 km/s in the middle of the plateau phase). Moreover, prominent Ca II absorptions are also detected in SN 2013am at relatively early phases. These spectral features are reminiscent of those seen in the low-velocity and low-luminosity SN IIP 2005cs. However, SN 2013am exhibits different photometric properties, having shorter plateau phases and brighter light-curve tails if compared to SN 2005cs. Adopting RV=3.1 and a mean value of total reddening derived from the photometric and spectroscopic methods(i.e., E(B?V ) = 0.55±0.19 mag), we find that SN 2013am may have reached an absolute V-band peak magnitude of ?15.83±0.71 mag, and produced a 56Ni mass of 0.016+0.010 ?0.006M⊙in the explosion. These parameters are close to those derived for SN 2008in and SN 2009N which have been regarded as “gap-filler” objects linking the faint SNe IIP to the normal ones. This indicates that some low-velocity SNe IIP may not necessarily result from the low-energetic explosions. The low expansion velocities could be due to a lower metallicity of the progenitor stars, a larger envelope mass ejected in the explosion, or the effect of viewing angle where these SNe were observed at an angle away from the polar direction. The diversities of SNe Ia and SNe IIP would revel that there are different progenitor system and exploding mechanism of each group. To understand these diversities is essential to the measurement of distance.
学科领域天文学
语种中文
文献类型学位论文
条目标识符http://ir.ynao.ac.cn/handle/114a53/9467
专题丽江天文观测站(南方基地)
作者单位中国科学院云南天文台
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张居甲. Ia型超新星多样性的观测研究[D]. 北京. 中国科学院研究生院,2015.
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