The influence of chemical composition on models of Type Ia supernovae

Type Ia supernovae are bright stellar explosions distinguished by standardizable light curves that allow for their use as distance indicators for cosmological studies. Despite the highly successful use of these events in this capacity, many fundamental questions remain. Contemporary research investigates how properties of the progenitor system that follow from the host galaxy such as composition and age influence the brightness of an event with the goal of better understanding and assessing the intrinsic scatter in the brightness. We provide an overview of these supernovae and proposed progenitor systems, all of which involve one or more compact stars known as white dwarfs. We describe contemporary research investigating how the composition and structure of the progenitor white dwarf systematically influences the explosion outcome assuming the progenitor is a single white dwarf that has gained mass from a companion. We present results illustrating some of these systematic effects from our research.     

Properties of SN Ia progenitors from light curves and spectra

With recent advances in theory and observations, direct connections emerge between the progenitors of Type Ia Supernovae (SNe Ia) and the observed light curves and spectra. A direct link is important for our understanding of the supernovae physics, the diversity of SNe Ia and the use of SNe Ia for high-precision cosmology because the details of the explosion depends sensitively on the initial conditions and the explosion scenario(s) realized in nature. Do SNe Ia originate from SD- or DD systems, and do they lead to M_Ch mass explosions or dynamical mergers? Does the statistical distribtion of SNe Ia depend on their environment which can be expected to change with redshift? In this contribution, we will exam from the theoretical point of view the tell-tails for this connection, their consistency with the observations, and future directions. In a first section, we present the physics of the explosion, light curves and spectral formation in a nutshell to help understanding the connection. For details of the progenitor evolution and explosion physics, we refer to reviews and the other contributions in this issue. Each of the topical sections starts with a brief general review followed by a more detailed discussion of specific results. Because the youth of the field, some bias is unavoidable towards results obtained within our collaborations (and FSU). The imprint of the metallicity, progenitor stars and properties such as the central density of the exploding WD are presented. IR spectroscopy, polarimetry and imaging of SNR remnants are discussed as a tool to test for the WD properties, magnetic fields and asymmetries. We discuss different classes of Type Ia supernovae, and their environment. Possible correlations between the spectroscopic and light curve properties of SN Ia are discussed. Finally, the overall emerging picture and future developments are discussed.     

Neutron excess number and nucleosynthesis of heavy elements in a type Ia supernova explosion

Type Ia supernovae produce very powerful burst of light, which can be observed to high redshift. This fact is very attractive for cosmological applications. For supernova light curve modeling, it is very important to know the amount of Fe and Ni, formed during the explosion. In this paper, we explore both the chemical composition of the ejected supernova shells and the possibility of weak r-process under increased neutron excess number based on a set of trajectories of tracer particles, calculated in a hydrodynamic model of SNIa explosion. It is shown that no r-process elements are synthesized in the considered supernova model, even for an increased neutron excess number (Y_e ～ 0.4) because of the slow evolution of temperature and density along chosen trajectories. The results of explosive nucleosynthesis are discussed.     

Towards an understanding of Type Ia supernovae from a synthesis of theory and observations

Motivated by the fact that calibrated light curves of Type Ia supernovae (SNe Ia) have become a major tool to determine the expansion history of the Universe, considerable attention has been given to, both, observations and models of these events over the past 15 years. Here, we summarize new observational constraints, address recent progress in modeling Type Ia supernovae by means of three-dimensional hydrodynamic simulations, and discuss several of the still open questions. It will be be shown that the new models have considerable predictive power which allows us to study observable properties such as light curves and spectra without adjustable non-physical parameters. This is a necessary requisite to improve our understanding of the explosion mechanism and to settle the question of the applicability of SNe Ia as distance indicators for cosmology. We explore the capabilities of the models by comparing them with observations and we show how such models can be applied to study the origin of the diversity of SNe Ia.     

Optical Readout Time Projection Chamber (O-TPC) for a study of oxygen formation in stellar helium burning

We are developing an Optical Readout Time Projection Chamber (O-TPC) detector for the study of the ¹²C(α, γ) ¹⁶O reaction that determines the ratio of carbon to oxygen in helium burning. This ratio is crucial for understanding the final fate of a progenitor star and the nucleosynthesis of elements prior to a Type II supernova; an oxygen rich star is predicted to collapse to a black hole, and a carbon rich star to a neutron star. Type Ia supernovae (SNeIa) are used as standard candles for measuring cosmological distances with the use of an empirical light curve-luminosity stretching factor. It is essential to understand helium burning that yields the carbon/oxygen white dwarf and thus the initial stage of SNeIa. The O-TPC is intended for use with high intensity photon beams extracted from the HIγS/TUNL facility at Duke University to study the ¹⁶O(γ, α) ¹²C reaction, and thus the direct reaction at energies as low as 0.7 MeV. We are conducting a systematical study of the best oxygen containing gas with light emitting admixture(s) for use in such an O-TPC. Preliminary results with CO₂+TEA mixture were obtained.     

Supernovae and cosmology

The extreme luminosity and their fairly unique temporal behaviour have made supernovae a superb tool to measure distances in the universe. As complex astrophysical events they provide interesting insights into explosion physics, explosive nucleosynthesis, hydrodynamics of the explosion and radiation transport. They are an end product of stellar evolution and provide clues to the stellar composition. Since they can be observed at large distances they have become critical probes to further explore astrophysical effects, like dust properties in external galaxies and the star formation history of galaxies. Some of the astrophysics interferes with the cosmological applications of supernovae. The local velocity field, distorted by the gravitational attraction of the local large scale structure, and the reddening law appear at the moment the major limitations in the accuracy with which cosmological parameters can be determined. These absorption effects can introduce a secondary bias into the observations of the distant supernovae, which needs to be carefully evaluated. Supernovae have been used for the measurement of the Hubble constant, i.e. the current expansion rate of the universe, and the accelerated cosmic expansion directly inferred from the apparent faintness of the distant supernovae.     

Birthrates and delay times of Type Ia supernovae

Type Ia supernovae (SNe Ia) play an important role in diverse areas of astrophysics, from the chemical evolution of galaxies to observational cosmology. However, the nature of the progenitors of SNe Ia is still unclear. In this paper, according to a detailed binary population synthesis study, we obtained SN Ia birthrates and delay times from different progenitor models, and compared them with observations. We find that the Galactic SN Ia birthrate from the double-degenerate (DD) model is close to those infe...     

超新星研究进展

本文先介绍超新星巡天和分类,简要地论述历史超新星SN 1006一千年,接着讨论核心塌缩超新星物理和具体事例（SN1987A 20年、 SN2006gy、 2008D）及超新星与γ射线暴的联系,文章重点讨论SNIa 和宇宙学,评述了SNIa在宇宙学中的应用和哈勃常数的确定,最后指出超新星研究目前存在的问题。     

Re-evaluation of the ^23Mg（p,γ） ^24Al reaction rate

Based on a new screening Coulomb model, this paper discusses the effect of electron screening on proton capture reaction of 23Mg. The derived result shows that, in some considerable range of stellar temperatures, the effect of electron screening on resonant reaction is prominent; on the non-resonant reaction the effect is obvious only in the low stellar temperatures. The reaction rates of ^23Mg（p,γ） ^24Al would increase 15%-25% due to the fact that the electron screening are considered in typical temperature range of massive mass white dwarfs, and the results undoubtedly affect the nucleosynthesis of some heavier nuclei in massive mass white dwarfs.     

A review of direct numerical simulations of astrophysical detonations and their implications

Multi-dimensional direct numerical simulations (DNS) of astrophysical detonations in degenerate matter have revealed that the nuclear burning is typically characterized by cellular structure caused by transverse instabilities in the detonation front. Type Ia supernova modelers often use onedimensional DNS of detonations as inputs or constraints for their whole star simulations.While these one-dimensional studies are useful tools, the true nature of the detonation is multi-dimensional. The multi-dimensional structure of the burning influences the speed, stability, and the composition of the detonation and its burning products, and therefore, could have an impact on the spectra of Type Ia supernovae. Considerable effort has been expended modeling Type Ia supernovae at densities above 1×10⁷ g·cm^-3 where the complexities of turbulent burning dominate the flame propagation. However, most full star models turn the nuclear burning schemes off when the density falls below 1×10⁷ g·cm^-3 and distributed burning begins. The deflagration to detonation transition (DDT) is believed to occur at just these densities and consequently they are the densities important for studying the properties of the subsequent detonation. This work will review the status of DNS studies of detonations and their possible implications for Type Ia supernova models. It will cover the development of Detonation theory from the first simple Chapman–Jouguet (CJ) detonation models to the current models based on the time-dependent, compressible, reactive flow Euler equations of fluid dynamics.     

Deflagrations and detonations in thermonuclear supernovae

We study a type Ia supernova explosion using three-dimensional numerical simulations based on reactive fluid dynamics. We consider a delayed-detonation model that assumes a deflagration-to-detonation transition. In contrast with the pure deflagration model, the delayed-detonation model releases enough energy to account for a healthy explosion, and does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between simulations and observations, and makes a delayed detonation the mostly likely mechanism for type Ia supernovae.     

Neutrinos,supernovae, and the origin of the heavy elements

Stars of～8-100 M_⊙end their lives as core-collapse supernovae(SNe). In the process they emit a powerful burst of neutrinos,produce a variety of elements, and leave behind either a neutron star or a black hole. The wide mass range for SN progenitors results in diverse neutrino signals, explosion energies, and nucleosynthesis products. A major mechanism to produce nuclei heavier than iron is rapid neutron capture, or the r process. This process may be connected to SNe in several ways. A brief review is presented on current understanding of neutrino emission, explosion, and nucleosynthesis of SNe.     

Nucleosynthesis in thermonuclear supernovae

We review our understanding of the nucleosynthesis that occurs in thermonuclear supernovae and their contribution to Galactic Chemical evolution. We discuss the prospects to improve the modeling of the nucleosynthesis within simulations of these events.     

Type Ia supernovae at high z

Type Ia supernovae are very powerful probes for cosmology and clear tracers of the past history of the universe. Two independent high-redshift supernova collaborations (the High-Z Team and the Supernova Cosmology Project) have presented this year evidence that we live in a low-matter density universe whose expansion is being accelerated by the presence of a dominant vacuum energy density. The Supernova Cosmology Project by using the searches performed at various z for cosmological purposes has measured high-z supernova rates as well. Those measurements provide unvaluable information on the cosmic star formation history, on the efficiency in producing supernovae out of stars, and on the involved timescale to explosion. The cosmic background due to supernova emission can also be calculated in agreement with the measured rates.     

Supernovae Ia in 2017: a long time delay from merger/accretion to explosion

I use recent observational and theoretical studies of type Ia supernovae (SNe Ia) to further constrain the viable SN Ia scenarios and to argue that there must be a substantial time delay between the end of the merger of the white dwarf (WD) with a companion or the end of mass accretion on to the WD and its terminal explosion. This merger/accretion to explosion delay (MED) is required to allow the binary system to lead to a more or less spherical explosion and to prevent a pre-explosion ionizing radiation. Considering these recent results and the required MED, I conclude that the core degenerate scenario is somewhat more favorable over the other scenarios, followed by the double degenerate scenario. Although the single degenerate scenario is viable as well, it is less likely to account for common (normal) SN Ia. As all scenarios require substantial MED, the MED has turned from a disadvantage of the core degenerate scenario to a challenge that theory should overcome. I hope that the requirement for a MED will stimulate the discussion of the different SN Ia scenarios and the comparison of the scenarios to each other.     

Double Degenerate Stars

Regardless of the formation mechanism, an exotic object, the double degenerate star (DDS), is introduced and investigated, which is composed of baryonic matter and some unknown fermion dark matter. Different from the simple white dwarfs (WDs), there is additional gravitational force provided by the unknown fermion component inside DDSs, which may strongly affect the structure and the stability of such kind of objects. Many possible and strange observational phenomena connecting with them are concisely discussed. Similar to the normal WD, this object can also experience thermonuclear explosion as type Ia supernova explosion when DDS's mass exceeds the maximum mass that can be supported by electron degeneracy pressure. However, since the total mass of baryonic matter can be much lower than that of WD at Chandrasekhar mass limit, the peak luminosity should be much dimmer than what we expect before, which may throw a slight shadow on the standard candle of SN Ia in the research of cosmology.     

Spallation model for the titanium-rich supernova remnant cassiopeia A

Titanium-rich subluminous supernovae are rare and challenge current SN nucleosynthesis models. We present a model in which ejecta from a standard supernova is impacted by a second explosion of the neutron star (a quark nova), resulting in spallation reactions that lead to (56)Ni destruction and (44)Ti creation under the right conditions. Basic calculations of the spallation products shows that a delay between the two explosions of ～5 days reproduces the observed abundance of (44)Ti in Cas A and explains its low luminosity as a result of the destruction of (56)Ni. Our results could have important implications for light curves of subluminous as well as superluminous supernovae.     

Supernova constraints on neutrino mass and mixing

In this article I review the constraints on neutrino mass and mixing coming from type-II supernovae. The bounds obtained on these parameters from shock reheating, r-process nucleosynthesis and from SN1987A are discussed. Given the current constraints on neutrino mass and mixing the effect of oscillations of neutrinos from a nearby supernova explosion in future detectors will also be discussed.     

强激光天体物理学研究

用激光等离子体相互作用对天体物理过程进行模拟研究已成为当前世界物理和天文学家深感兴趣的重要前沿领域.文章比较了强激光作用下产生的等离子体与天体物理条件下的等离子体之间在内部物理过程的相似性,论述了由前者模拟后者的物理依据,即相似性原则和定标规律.在此基础上,回顾和评述了当前已经在高离化态光谱学、类天体等离子体状态方程和辐射不透明度以及流体动力学不稳定性等方面开展的强激光天体物理学的研究,这些研究对于理解超新星、白矮星、中子星以及巨行星、褐矮星等领域的天体物理过程起到了极大的作用,并正在成为联系天体物理理论模拟和观测的中间桥梁.     

Living with lambda

This talk presents a brief overview of recent results pertaining to the cosmological constant ‘A’. I summarize the observational situation focussing on observations of high redshift Type Ia supernovae which suggest A > 0. Observations of small angular anisotropies in the cosmic microwave background complement Type Ia supernovae observations and both CMB and Sn can be combined to place strong constraints on the value of A. The presence of a small A-term increases the age of the universe and slows down the formation of large scale structure. I also review recent theoretical attempts to generate a small A-term at the current epoch and a model independent approach for determining the cosmic equation of state.