| 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. |
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