Spectral sequences of Type Ia supernovae. I. Connecting normal and sub-luminous SN Ia and the presence of unburned carbon
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Type Ia supernovae are generally agreed to arise from thermonuclear explosions of carbon-oxygen white dwarfs. The actual path to explosion, however, remains elusive, with numerous plausible parent systems and explosion mechanisms suggested. Observationally, type Ia supernovae have multiple subclasses, distinguished by their lightcurves and spectra. This raises the question whether these reflect that multiple mechanisms occur nature, or instead that explosions have a large but continuous range of physical properties. We revisit the idea that normal and 91bg-like supernovae can be understood as part of a spectral sequence, which changes temperature dominate. Specifically, we find that a single ejecta structure is sufficient to provide reasonable fits of both the normal type Ia supernova SN~2011fe and the 91bg-like SN~2005bl, provided that the luminosity and thus temperature of the ejecta are adjusted appropriately. This suggests that the outer layers of the ejecta are similar, thus providing some support of a common explosion mechanism. Our spectral sequence also helps to shed light on the conditions under which carbon can be detected pre-maximum SN~Ia spectra -- we find that emission from iron can fill in the carbon trough cool SN~Ia. This may indicate that the outer layers of the ejecta of events which carbon is detected are relatively metal poor compared to events where carbon is not detected.Keywords:
Pair-instability supernova
Sequence (biology)
Carbon fibers
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Progenitors of Type Ia supernovae (SNe) have been predicted to modify their ambient circumstellar (CSM) and interstellar environments through the action of their powerful winds. While there is X-ray and optical evidence for circumstellar interaction in several remnants of Type Ia SNe, widespread evidence for such interaction in Type Ia SNe themselves has been lacking. We consider prospects for detection of CSM shells that have been predicted to be common around Type Ia SNe. Such shells are most easily detected in Na I absorption lines. Variable (declining) absorption is expected to occur soon after the explosion, primarily during the SN rise time, for shells located within 1 - 10 pc of a SN. The distance of the shell from the SN can be determined by measuring the time scale for line variability.
Line (geometry)
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Type Ia supernovae (SNe Ia) come in a large range of luminosities, as determined mostly by the amount of 56Ni produced in the explosion. Nevertheless, they can be normalized and used as standard candles, which suggests that they share a similar origin. The thermonuclear explosion of a Chandrasekhar-mass (MCh) white dwarf accreting mass from a main sequence or red giant companion (the single degenerate scenario) is a favourite configuration, but the presence of SNe Ia that result from the merging of two white dwarfs of total mass exceeding MCh is supported by rate studies. SNe of the spectroscopically peculiar 1991bg class are the least luminous SNe Ia. They produce ∼0.1 M⊙ of 56Ni, which is difficult to reconcile with hydrodynamic explosion models. Here, the properties of the inner ejecta of SN 1991bg are investigated by means of synthetic nebular spectroscopy. In order to reproduce the transformation of the spectra from broader, [Fe ii]+[Fe iii] lines at day ∼120 to narrow, [Fe iii] lines at day ≳210, the innermost region must deviate significantly in density from the prediction of MCh models. In particular, a substantially lower density is required in the innermost ≈3000 km s−1 in order to provide the needed increase of ionization with time. This leads to a mass deficit of ∼0.15 M⊙ in the region inside ≈3000 km s−1 with respect to MCh models, and points to a different type of explosion. Early-time studies require a low explosion kinetic energy and lack of burning products in the outer layers. When combined with the results from this paper, the merger scenario may be a viable candidate for 1991bg-like SNe.
Cosmic distance ladder
Chandrasekhar limit
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Observations have yielded at least four unusual supernovae, which look spectroscopically very much like Type Ia SNe but have prominent narrow Hα emission lines, and have thus been dubbed, Type IIa. The following four SNe have been studied extensively: SN 1997cy, SN 1999E, SN 2002ic, SN 2005gj. We seek to shed some light on the source of the explosion by closely examining the environments of these SNe. We measure the metallicity and star formation rates for three of the host galaxies (the galaxy spectrum for the SN 2005gj host galaxy was still contaminated with emission from the SN) and compared them to those of more well-studied SNe. The measurements show low metallicity and star formation rates, the former does not agree well with classification as a Type Ia. While these measurements do not strongly indicate a particular progenitor system, they may be able to form a profile of possible galactic characteristics to look for future objects of this type.
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A parameterized supernova synthetic-spectrum code is used to study line identifications in the photospheric-phase spectra of the peculiar Type Ia SN 1991T, and to extract some constraints on the composition structure of the ejected matter. The inferred composition structure is not like that of any hydrodynamical model for Type Ia supernovae. Evidence that SN 1991T was overluminous for an SN Ia is presented, and it is suggested that this peculiar event probably was a substantially super-Chandrasekhar explosion that resulted from the merger of two white dwarfs.
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Supernovae Type Iax (SNe Iax) are less energetic and less luminous than typical thermonuclear explosions. A suggested explanation for the observed characteristics of this subclass is a binary progenitor system consisting of a CO white dwarf primary accreting from a helium star companion. A single-degenerate explosion channel might be expected to result in a dense circumstellar medium (CSM), although no evidence for such a CSM has yet been observed for this subclass. Here we present recent Spitzer observations of the SN Iax 2014dt obtained by the SPIRITS program nearly one year post-explosion that reveal a strong mid-IR excess over the expected fluxes of more normal SNe Ia. This excess is consistent with 1E-5 M_solar of newly formed dust, which would be the first time that newly formed dust has been observed to form in a normal Type Ia. The excess, however, is also consistent with a dusty CSM that was likely formed in pre-explosion mass-loss, thereby suggesting a single degenerate progenitor system. Compared to other SNe Ia that show significant shock interaction (SNe Ia-CSM) and interacting core-collapse events (SNe IIn), this dust shell in SN 2014dt is less massive. We consider the implications that such a pre-existing dust shell has for the progenitor system, including a binary system with a mass donor that is a red giant, a red supergiant, and an asymptotic giant branch star.
Red giant
Circumstellar dust
Pair-instability supernova
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The most favored progenitor scenarios for Type Ia supernovae (SNe Ia) involve the single-degenerate (SD) scenario and the double-degenerate scenario. The absence of stripped hydrogen (H) in the nebular spectra of SNe Ia challenges the SD progenitor models. Recently, it was shown that pure deflagration explosion models of Chandrasekhar-mass white dwarfs, ignited off-center, reproduce the characteristic observational features of 2002cx-like SNe Ia very well. In this work we predict, for the first time, the amount of stripped H for the off-center, pure deflagration explosions. We find that their low kinetic energies lead to inefficient H mass stripping (≲ 0.01 M☉), indicating that the stripped H may be hidden in (observed) late-time spectra of SN 2002cx-like SNe Ia.
Chandrasekhar limit
Deflagration
Stripping (fiber)
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The occurrence and properties of Type Ia supernovae (SN Ia's) in single-degenerate binary systems (white dwarf [WD] + nondegenerate companion) is examined for galaxies of different types and as a function of redshift. The rates and characteristics (peak luminosities, expansion velocities of the ejecta) expected from the explosion of mass-accreting WDs in symbiotic systems and "helium star cataclysmics" are found to be different from those arising in another class of candidate systems: cataclysmic-like (contact) systems (CLSs), where a CO WD accretes hydrogen on a thermal timescale from a Roche lobe-filling main-sequence or subgiant companion. We derive the evolution of the SN Ia rate and properties resulting from the thermonuclear explosion of sub-Chandrasekhar mass WDs in such systems when they detonate a helium layer accumulated from steady burning of hydrogen at the surface. A fraction of CLSs are believed to form a subset of the observed luminous supersoft X-ray sources (SSSs). Sub-Chandrasekhar explosions from CLSs are disfavored in all types of galaxies at redshifts z ≳ 1. On the other hand, CLSs in which the WD succeeds to grow to the Chandrasekhar mass are more likely found in spiral galaxies and absent from early-type galaxies. SN Ia statistics could (if the uncertainties still involved are reduced) help to discriminate among proposed SN Ia scenarios. The range of variation of the characteristics of SN Ia's in the CLS scenario should be narrower than in symbiotics. The predicted correlation between peak luminosity and velocity of the ejecta in SN Ia's coming from these systems is weak. For CLSs, the distinction between the characteristics of SN Ia's respectively arising from sub-Chandrasekhar and from Chandrasekhar-mass explosions should be sharp, since all sub-Chandrasekhar explosions would be produced by low-mass WDs.
Chandrasekhar limit
Subgiant
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Some stars end their lives in spectacular explosive deaths called supernovae. We understand this process happens in predominantly two ways, the collapse of the stellar core in massive stars or the thermonuclear explosion of white dwarf (WD) stars at or near the Chandrasekhar mass (MCh). This thesis focuses on the thermonuclear supernovae, called Type Ia (SNe Ia). Despite the decades of theoretical work and observational constraints of SNe Ia coming from WDs, the nature of the progenitor systems and explosion scenario is unknown. The fundamental questions still unanswered are: \Do SNe Ia come from WDs with non-degenerate companions or WD companions? and \How/why do WDs explode? To investigate the nature of SNe Ia, I performed time series radiative transfer calculations of various hydrodynamic ejecta models. Since we cannot observe the explosion itself, spectra are tools to
probe the progenitor properties. My aims were to determine ejecta mass diagnostics of SNe Ia by comparing radiative transfer calculations of four hydrodynamic ejecta models in the mass range of 1.0-1.7 MCh. Because nebular spectra are dominated by emission lines from the innermost part of the ejecta, I also investigate the physics of nebular SN Ia spectra to determine the properties of the progenitor and explosion physics. The luminosity of SNe
Ia is powered by the radioactive decay of 56Ni and subsequent decay of 56Co. Gamma-rays produced during these decays scatter in the ejecta before either escaping or being absorbed. This thesis contains work modeling the gamma-ray flux and energy deposition function. To date, SN2014J is the only SN Ia with gamma-ray observations. Detailed gamma-ray modeling will be compared to all future observations to add additional constraints on the production of 56Ni and ejecta density structure.
Thermonuclear Fusion
Chandrasekhar limit
Radioactive decay
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While the progenitors of Type Ia supernovae (SNe Ia) have long been thought to be thermonuclear explosions of white dwarf stars, what triggers the explosion are still a topic of debate. This thesis considers constraints on single-degenerate progenitors of SNe Ia based on the presence of a Roche-lobe filling companion. The ejecta strips material from the companion, that maybe detectable via Hα emission during the nebular phase. Using the full structure from simulations produces line widths are larger than those produced in simple models. The structure formed by the ejecta-companion interaction produce a broken reverse shock that may be visible in X-rays via the Fe Kα line at the age of Tycho's supernova remnant (SNR). If the similar structures in Tycho’s SNR are produced this way then the companion star must have been massive, with M ~ 2 Mo. Detections of circumstellar material within the supernova provides another way to indirectly probe the companion star. Mass loss through winds or novae are expected to shape the circumsteller medium for single-degenerate progenitors and the velocities, v ~ 100 km s-1 appear to be consistent with recurrent nova shells, a model that is tested by analysing simulations of RS Ophiuchi. Models of RS Ophiuchi can explain the absorption lines seen around the 2006 outburst if the mass loss is 10−6 Mo yr-1,. The circumsteller medium is shown to produce in the velocity and relative strengths of the features seen in SN 2006X. However, whether density in the shells is high enough to produce the required recombination timescale and to overcome ionization by γ-rays for shells at 5 × 1016 cm remains uncertain.
Line (geometry)
Thermonuclear Fusion
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Temporal variability of narrow absorption lines in high-resolution spectra of Type Ia supernovae (SNe Ia) is studied to search for circumstellar matter. Time series which resolve the profiles of absorption lines such as Na I D or Ca II H&K are expected to reveal variations due to photoionisation and subsequent recombination of the gases. The presence, composition, and geometry of circumstellar matter may hint at the elusive progenitor system of SNe Ia and could also affect the observed reddening law. To date, there are few known cases of time-varying Na I D absorption in SNe Ia, all of which occurred during relatively late phases of the supernova evolution. Photoionisation, however, is predicted to occur during the early phases of SNe Ia, when the supernova peaks in the ultraviolet. We therefore attempt to observe early-time absorption-line variations by obtaining high-resolution spectra of SNe before maximum light. We have obtained photometry and high-resolution spectroscopy of SNe Ia 2013gh and iPTF 13dge, to search for absorption- line variations. Furthermore, we study interstellar absorption features in relation to the observed photometric colours of the SNe. Results. Both SNe display deep Na I D and Ca II H&K absorption features. Furthermore, small but significant variations are detected in a feature of the Na I D profile of SN 2013gh. The variations are consistent with either geometric effects of rapidly moving or patchy gas clouds or photoionisation of Na I gas at R \approx 1019 cm from the explosion. Our analysis indicates that it is necessary to focus on early phases to detect photoionisation effects of gases in the circumstellar medium of SNe Ia. Different absorbers such as Na I and Ca II can be used to probe for matter at different distances from the SNe.
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