URCA RESEARCH PROPOSAL

INVESTIGATING THE FORMATION OFCLUMPY EJECTA IN YOUNG TYPE IA

SUPERNOVA REMNANTS

ASHTON DYER

Abstract. High-resolution radio and X-ray images of several young Type Ia supernovaremnants (SNRs) such as SN 1006 and Tycho show unexpected protrusions or knotsbeyond the mean shock radius. The mechanism for formation of these knots has beendebated in recent years, yielding two major theories; hydrodynamic instabilities, andclumps of higher density ejecta formed in the early stages of the SN explosion. Clumpsformed very early would carry clues to the process by which Type Ia supernovae explode,but might also account for a discrepancy between theoretical and observed distancesbetween the forward shock (FS) and contact discontinuity (CD) in young SNRs, whichhas been used to argue for efficient shock acceleration. Recent studies have producedvarying results, largely due to the variety of techniques used and parameters taken intoconsideration during simulation. We intend to perform hydrodynamic simulations ofthe evolution of a remnant from an age of a few years to hundreds of years, using theVH-1 hydrocode in one, two, and three dimensions. By running lower resolution, lowerdimensionality simulations over a large range of initial conditions, we will identify the mostsignificant parameters to study with higher resolution 3D simulation. By determining themechanism that causes the development of knots on young SNRs, insight will be gainedon the early states of supernovae and observed distances between the FS and CD may bejustified.

1. Introduction

The expanding gaseous remains of supernovae, supernova remnants (SNRs), act as emis-saries of past events and probes of otherwise unlit regions of space. Within their complex,ever evolving structure, there lie imprints of their state at young age, in turn yielding

Date: June 17, 2013.1

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insight into the details of stars prior to death, and the mechanisms involved in supernovaexplosions. Numerical simulation of the evolution of supernova remnants has been usedfor decades in hopes of extracting the information they contain. With the increase incomputational power made available over this time period, SNR models have evolved fromone-dimensional hydrodynamic simulations with no more than 100 cells (Gull 1973), tothree-dimensional relativistic hydrodynamics and MHD simulations where high resolutionsimulations run mesh sizes in the hundredths of parsecs. This growth in technology hasallowed for more complicated models of SNRs and through those models, greater under-standing of SNR formation and expansion. Yet, with more than forty years of developmentsin the field, there is no uniformly accepted model for type Ia SNRs.

There are currently several details observed in type Ia SNRs that are not understood,many of which are observed in recent high-resolution radio and X-ray images of young typeIa SNRs. Observational data has shown a smaller distance between the forward shock (FS)and contact discontinuity (CD) than simulations predicted. Moreover a new morphologicalfeature has been revealed; protrusions of dense ejecta exist along the CD, extending beyondthe FS as is visible in Figure 1 (Warren et al. 2005). Currently two major theories existin hopes of replicating these observations.

Figure 1: Recent images of Tycho (left) and SN 1006 (right) from the Chandra X-ray observa-tory. The heterogeneous ’fleecy’ ejecta protrusions can be seen on the surfaces of both. Credit:NASA/CXC/Rutgers/K.Eriksen et al. (Tycho), NASA/CXC/Middlebury College/F.Winkler (SN1006)

The first is the theory that small inhomogeneities exist in the SNR and in time hydrody-namic instabilities around the inhomogeneities evolve into the fleecy texture observed. Theearly stages of this evolution are agreed to evolve from Rayleigh-Taylor (RT) instabilities,from which RT fingers evolve. Simulations that yield high compression at the FS haveshown these RT fingers are capable of protruding beyond the FS, similar to the observed

URCA RESEARCH PROPOSAL 3

SNRs (Warren & Blondin 2013). Moreover, it is predicted that higher resolution simu-lations may yield the formation of fleecy ejecta at a younger age in the SNR simulation,further validating this theory.

The second theory is that in the early stages of the SNR, within seconds of the explosion,clumps of high density ejecta exist throughout the SNR, possibly created by the 56Ni bubbleeffect. If this is the case, it would reveal information as to the mechanisms behind typeIa supernova explosions. Preliminary two-dimensional studies of this theory found thatthe initial density perturbations would have to be approximately 100 times as dense asthe surrounding ejecta for them to stay together long enough to pierce the CD (Wang &Chevalier 2001). More recent simulations have been run in three-dimensions and yielded amore feasible relative density range of 2.5-5 (Orlando et al. 2012).

2. Objectives

The scope of this study is to determine what is required for the second theory to repro-duce the observed state of young SNRs. Initially SNR evolution will be simulated usingvarious heterogeneous density distributions in agreement with the second theory. Datacollected from the simulation will then be compared to observation using methods simi-lar to those used by Warren & Blondin (2013) in their recent paper on the evolution ofthe first theory in which they concluded that hydrodynamic instabilities were capable ofreproducing observed results. These techniques will compare two major aspects of theSNRs. The first will be a general likeness of the forms of the SNR’s ejecta, includingcomparison of how far any protrusions extend beyond the contact discontinuity, and howlong these protrusions survive. Since observations of Tycho and SN 1006 reveal these pro-trusion extending beyond the mean forward shock radius, this element will be importantto investigate. Secondly, a more quantitative analysis of the structure of the SNR will becarried out. This will include comparison of the significant radial features of the SNR aswell as measurements of how far beyond the CD any protrusions extend. By using thesame techniques as Warren & Blondin (2013) to test the initial clumpy ejecta theory, adefinite comparison can be made between the two models. This comparison may providequantitative reasoning to believe one model is more accurate than the other.

3. Proposed Work

Three-dimension simulations will be run with VH-1 hydrocode to evolve a young SNRfrom a few days old until an age of 1000 yr. VH-1 solves Euler’s fluid dynamics equationsusing the finite difference method in spherical-polar coordinates. Early simulations, withvarying density perturbations, will be run in a conical wedge to reduce computation time.An additional homogeneous density simulation will be run as a control, to allow for com-parison to other simulations. From these trials, the density distribution that appears tobe the best candidate for replicating observations will be run in a full 4π steradians for amore in depth analysis.

The initial state of the SN, for both clumped and non-clumped simulations, will be mod-eled using a spherically symmetric exponential density profile as proposed by Dwarkadas

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& Chevalier (1998). Initial conditions such as the mass and total energy of the SNR willbe selected to create SNR similar to Tycho and SN 1006, and the interstellar medium wellbe assumed to be uniform in density. Effective adiabatic indices of 5/3, 4/3, and 6/5 willbe used in each setup to analyze the evolution of that setup for varying assumptions aboutthe compressibility and energy loss of the SNR. Density perturbations chosen will includethat used by Orlando et al. (2012) and Wang & Chevalier (2001) to allow for comparisonof our numerical model to former findings.

To quantify the likeness between the simulations and observed SNRs two aspects ofmorphology will be analyzed. The first will be the fleecy appearance of the thermal emis-sions. Comparisons will be executed in a manner similar to that used by Warren & Blondin(2013), through discussion of isosurfaces and comparison of observational images to pro-jections made by the simulation data. The second morphological element considered in theanalysis will be the radii of the reverse shock, forward shock, and contact discontinuity.Most SNR models yield ratios of RCD/RFS . 0.88, when the observed ratio is believedto be ∼ 0.93 (Warren et al. 2005). By comparing the ratios of these radial structures,a quantitative comparison can be made between our simulation, observational data, andprevious models.

4. Summary

Numerical simulations of the evolution of SNRs have greatly improved, allowing formore accurate simulations of these astrophysical bodies. Coupled with high resolutionobservations of emission spectra, the need has arisen for further refinements in our modelsof SNR evolution. By simulating the evolution of young type Ia SNRs from hundreds ofseconds to ages comparable to Tycho and SN 1006, we hope to verify the likely hood thatinitial density perturbations are required to reproduce observations. In doing so we willlearn more about the state of type Ia SNRs at very early ages, yielding insight into theprocesses that occur during the explosion of these SN.

References

[1] Dwarkadas V.V., & Chevalier R.A. 1998, ApJ, 497, 807-823[2] Gull S.F. 1973, MNRAS, 161, 47-69[3] Orlando, S., Bocchino, F., Miceli, M., et al. 2012, Apj, 749:156[4] Wang, C., & Chevalier, R.A. 2001, ApJ, 549, 1119-34[5] Warren, D. C., & Blondin, J. M. 2013, MNRAS, 429, 3099-3113[6] Warren, J.S., Hughes, J.P., Badenes, C., et al. 2005, ApJ, 634, 376-389