Cody Raskin

On Type Ia Supernovae From The Collisions of Two White Dwarfs

ABSTRACT We explore collisions between two white dwarfs as a pathway for making Type IaSupernovae (SNIa). White dwarf number densities in globular clusters allow 10 100redshift . 1 collisions per year, and observations by (Chomiuk et al.2008) of globularclusters in the nearby S0 galaxy NGC 7457 have detected what is likely to be a SNIaremnant. We carry out simulations of the collision between two 0.6M white dwarfsat various impact parameters and mass resolutions. For impact parameters less thanhalf the radius of the white dwarf, we nd such collisions produce ˇ 0.4 M of 56 Ni,making such events potential candidates for underluminous SNIa or a new class oftransients between Novae and SNIa.Key words: hydrodynamics { nuclear reactions, nucleosynthesis, abundances {(stars:) white dwarfs { (stars:) supernovae: general. 1 INTRODUCTIONType Ia supernovae (henceforth SNIa) play a key role inastrophysics as premier distance indicators for cosmology(Phillips 1993; Riess et al.1998; Perlmutter et al.1999), asdirect probes of low-mass star formation rates at cosmologi-cal distances (Scannapieco et al.2005; Mannucci et al.2006;Maoz 2008) and as signi cant contributors to iron-groupelements in the cosmos (Wheeler et al.1989; Timmes etal.1995; Feltzing et al.2001; Strigari 2006). Our current un-derstanding is that there are two major progenitor systemsfor these events. The rst possibility, the single-degeneratescenario, consists of a carbon-oxygen white dwarf in a bi-nary system evolving to the stage of central ignition by massoverow from a low-mass stellar companion (Whelan & Iben1973; Nomoto 1982; Hillebrandt & Niemeyer 2000). The sec-ond possibility, the double-degenerate scenario, consists ofthe merger of two white dwarfs in a binary system (Iben& Tutukov 1984; Webbink 1984; Yoon et al.2007). It is un-known at what relative frequency both of these channelsoperate (Livio 2000; Maoz 2008).Collisions between two white dwarfs, are likely to hap-pen less frequently than binary mergers. However, as dis-cussed in Timmes (2009) and Rosswog et al.(2009), theywill occur in globular clusters where the stellar densities areextremely high. For a typical globular cluster velocity dis-persion of ˇ5-10 km s

Remnants of Binary White Dwarf Mergers

We carry out a comprehensive smooth particle hydrodynamics simulation survey of double-degenerate white dwarf binary mergers of varying mass combinations in order to establish correspondence between initial conditions and remnant configurations. We find that all but one of our simulation remnants share general properties such as a cold, degenerate core surrounded by a hot disk, while our least massive pair of stars forms only a hot disk. We characterize our remnant configurations by the core mass, the rotational velocity of the core, and the half-mass radius of the disk. We also find that some of our simulations with very massive constituent stars exhibit helium detonations on the surface of the primary star before complete disruption of the secondary. However, these helium detonations are insufficiently energetic to ignite carbon, and so do not lead to prompt carbon detonations.

Spectra of Type Ia Supernovae from Double Degenerate Mergers

The merger of two white dwarfs (aka double-degenerate merger) has often been cited as a potential progenitor of Type Ia supernovae. Here we combine population synthesis, merger, and explosion models with radiation-hydrodynamics light-curve models to study the implications of such a progenitor scenario on the observed Type Ia supernova population. Our standard model, assuming double-degenerate mergers do produce thermonuclear explosions, produces supernova light curves that are broader than the observed type Ia sample. In addition, we discuss how the shock breakout and spectral features of these double-degenerate progenitors will differ from the canonical bare Chandrasekhar-massed explosion models. We conclude with a discussion of how one might reconcile these differences with current observations.

56Ni Production in Double-degenerate White Dwarf Collisions

We present a comprehensive study of white dwarf collisions as an avenue for creating type Ia supernovae. Using a smooth particle hydrodynamics code with a 13-isotope, α-chain nuclear network, we examine the resulting 56Ni yield as a function of total mass, mass ratio, and impact parameter. We show that several combinations of white dwarf masses and impact parameters are able to produce sufficient quantities of 56Ni to be observable at cosmological distances. We find that the 56Ni production in double-degenerate white dwarf collisions ranges from sub-luminous to the super-luminous, depending on the parameters of the collision. For all mass pairs, collisions with small impact parameters have the highest likelihood of detonating, but 56Ni production is insensitive to this parameter in high-mass combinations, which significantly increases their likelihood of detection. We also find that the 56Ni dependence on total mass and mass ratio is not linear, with larger-mass primaries producing disproportionately more 56Ni than their lower-mass secondary counterparts, and symmetric pairs of masses producing more 56Ni than asymmetric pairs.

Prompt Ia Supernovae Are Significantly Delayed

The time delay between the formation of a population of stars and the onset of type Ia supernovae (SNe Ia) sets important limits on the masses and nature of SN Ia progenitors. Here, we use a new observational technique to measure this time delay by comparing the spatial distributions of SNe Ia to their local environments. Previous work attempted such analyses encompassing the entire host of each SN Ia, yielding inconclusive results. Our approach confines the analysis only to the relevant portions of the hosts, allowing us to show that even so-called prompt SNe Ia that trace star formation on cosmic timescales exhibit a significant delay time of 200-500 million years. This implies that either the majority of Ia companion stars have main-sequence masses less than 3 M ☉, or that most SNe Ia arise from double white dwarf binaries. Our results are also consistent with a SNe Ia rate that traces the white dwarf formation rate, scaled by a fixed efficiency factor.

Using Spatial Distributions to Constrain Progenitors of Supernovae and Gamma-Ray Bursts

We carry out a comprehensive theoretical examination of the relationship between the spatial distribution of optical transients and the properties of their progenitor stars. By constructing analytic models of star-forming galaxies and the evolution of stellar populations within them, we are able to place constraints on candidate progenitors for core-collapsesupernovae(SNe),long-durationgamma-raybursts,andSNeIa.Inparticular,wefirstconstructmodels of spiral galaxies that reproduce observations of core-collapse SNe, and we use these models to constrain the minimum mass for SNe Ic progenitors to � 25 M� . Second, we lay out the parameters of a dwarf irregular galaxy model, which we use to show that the progenitors of long-duration gamma-ray bursts are likely to have masses above � 43M� .Finally,weintroduce anewmethodforconstraining thetimescale associatedwithSNeIaand applyittoour spiral galaxy modelsto show how observations can better beanalyzed to discriminate betweenthe leading progenitor models for these objects. Subject headingg methods: analytical — gamma rays: bursts — stars: evolution — supernovae: general Online material: color figures

RAPID OPTIMAL SPH PARTICLE DISTRIBUTIONS IN SPHERICAL GEOMETRIES FOR CREATING ASTROPHYSICAL INITIAL CONDITIONS

Creating spherical initial conditions in smoothed particle hydrodynamics simulations that are spherically conformal is a difficult task. Here, we describe two algorithmic methods for evenly distributing points on surfaces that when paired can be used to build three-dimensional spherical objects with optimal equipartition of volume between particles, commensurate with an arbitrary radial density function. We demonstrate the efficacy of our method against stretched lattice arrangements on the metrics of hydrodynamic stability, spherical conformity, and the harmonic power distribution of gravitational settling oscillations. We further demonstrate how our method is highly optimized for simulating multi-material spheres, such as planets with core–mantle boundaries.

EXAMINING THE ACCURACY OF ASTROPHYSICAL DISK SIMULATIONS WITH A GENERALIZED HYDRODYNAMICAL TEST PROBLEM

We discuss a generalization of the classic Keplerian disk test problem allowing for both pressure and rotational support, as a method of testing astrophysical codes incorporating both gravitation and hydrodynamics. We argue for the inclusion of pressure in rotating disk simulations on the grounds that realistic, astrophysical disks exhibit non-negligible pressure support. We then apply this test problem to examine the performance of various smoothed particle hydrodynamics (SPH) methods incorporating a number of improvements proposed over the years to help SPH better address problems noted in modeling the classical gravitation only Keplerian disk. We also apply this test to a newly developed extension of SPH based on reproducing kernels called CRKSPH. Counterintuitively, we find that pressure support worsens the performance of traditional SPH on this problem, causing unphysical collapse away from the steady-state disk solution even more rapidly than the purely gravitational problem, whereas CRKSPH greatly reduces this error.

Shadows of our Former Companions: How the Single-Degenerate Binary Type Ia Supernova Scenario Affects Remnants

Here we present three-dimensional high resolution simulations of Type Ia supernova in the presence of a non-degenerate companion. We find that the presence of a nearby companion leaves a long-lived hole in the supernova ejecta. In particular, we aim to study the long term evolution of this hole as the supernova ejecta interacts with the surrounding interstellar medium. Using estimates for the x-ray emission, we find that the hole generated by the companion remains for many centuries after the interaction between the ejecta and the interstellar medium. We also show that the hole is discernible over a wide range of viewing angles and companion masses.