# Computational Astrophysics

Our primary scientific and computational focus is on tera- to exa-scale simulation of supernovae of both classes in the Universe.

Core collapse supernovae are the death throes of massive stars, more than 8-10 times the mass of our sun. They are a dominant source of elements in the Universe, without which life would not be possible. Our group is focused on ascertaining the core collapse supernova mechanism - i.e., how the explosions of these stars are initiated. Core collapse supernovae are three-dimensional, multi-physics events. Three-dimensional general relativistic radiation magnetohydrodynamics simulations must be performed to ascertain definitively the supernova mechanism and to predict all of the associated supernova observables. Core collapse supernovae are driven by neutrinos (radiation) and perhaps magnetic fields. Thus, our group has developed discretizations, solution algorithms, and codes for the solution of the multi-dimensional neutrino (radiation) transport equations and the three-dimensional magnetohydrodynamics and Poisson equations, the latter for the star's self gravity. This is ultimately a petascale to exascale computational problem.

Members of our group are also engaged in research on Type Ia supernovae - in particular, the mechanism whereby white dwarf stars (the endpoint of stellar evolution for stars less than 8 times the mass of the Sun) end their lives in stellar explosions as well. Understanding these stellar explosions is particularly important in light of the fact they provide the means to probe the evolution of the Universe as a whole. Indeed, observations of Type Ia supernovae and conclusions based on them have led to the startling fact our universe is expanding at an accelerated rate, which has significant implications for its future and ultimate fate. Type Ia supernovae are driven by thermonuclear runaway. The challenge here is the modeling of a turbulent flame, centimeters thick, in a white dwarf star the size of the Earth. Thus, three-dimensional simulations of chemically reactive flows are required, with realistic sub-grid models.

**Group Leader: **Anthony Mezzacappa

**Email:** mezzacappaa@ornl.gov

**Phone:** 865-574-6113

**Fax: ** 865-576-8746

**Secretary: **Patty Boyd

**Email:** p7v@ornl.gov

**Phone:** 865-576-1747

**Fax: ** 865-574-0405

**Staff**

- Reuben Budiardja (NICS) (reubendb@utk.edu )
- Christian Cardall (cardallcy@ornl.gov)
- Eirik Endeve (endevee@ornl.gov)
- Michael Guidry (guidry@utk.edu)
- Raphael Hix (raph@ornl.gov)
- Eric Lingerfelt (lingerfeltej@ornl.gov)
- Bronson Messer (bronson@ornl.gov )
- Anthony Mezzacappa (mezzacappaa@ornl.gov)

**Postdocs**

- Eric Lentz (lentzej@ornl.gov)
- Suzanne Parete-Koon (paretekoonst@ornl.gov)

**Graduate Students**

- Austin Chertkow (chertkowma@ornl.gov)
- Austin Harris (harrisja@ornl.gov)
- Ryan Landfield (landfieldre@ornl.gov)
- Helena Pais (hpais@utk.edu)
- Thomas Papatheodore (papatheodotl@ornl.gov)

## Computation:

Our group has been engaged in the development of (1) physics-based preconditioners for the solution of the large, sparse linear systems of equations underpinning the numerical solution of the neutrino Boltzmann kinetic equations, in collaboration with the CSMD Applied Mathematics Group (Ed D'Azevedo), (2) strategies for successful tera- to peta-scale data management (including parallel I/O), (3) visualization of multi-dimensional scalar, vector, and tensor supernova data, in close collaboration with the NCCS Visualization Task (Ross Toedte, Sean Ahern), and (4) strategies for adaptive mesh refinement for memory-intensive applications involving radiation transport.

## Our Codes

The following codes have been developed or are under development by our group:

Agile-BOLTZTRAN: A code for fully general relativistic neutrino (radiation) hydrodynamics for spherically symmetric stellar collapse and core collapse supernova simulation. Agile-BOLTZTRAN couples the general relativistic neutrino Boltzmann kinetic equations to the general relativistic hydrodynamics equations and exact Einstein equations for gravity. It also implements a detailed nuclear equation of state for the nucleons, nuclei, electrons, positrons, and photons in the stellar core (Lattimer-Swesty), as well as a realistic set of neutrino interactions with these stellar core constituents.

CHIMERA: A code that couples multigroup flux-limited diffusion neutrino transport (a sophisticated approximation of Boltzmann transport) along radial rays (the ray-by-ray-plus approximation) to three-dimensional hydrodynamics, a nuclear burning network, Newtonian self gravity with a spherical general relativistic correction, an industry standard nuclear equation of state (Lattimer-Swesty, Shen, Wilson), and with state of the art neutrino interactions. Two-dimensional multi-physics simulations of core collapse supernovae have been performed with CHIMERA (see News and Highlights) and three-dimensional simulations are underway.

GenASiS: A code that couples six-dimensional (three space, three momentum space) special relativistic Boltzmann neutrino transport to three-dimensional special relativistic magnetohydrodynamics, Newtonian self gravity, a nuclear equation of state, and state of the art neutrino opacities. Two-dimensional, multi-physics simulations of core collapse supernovae will be performed later this year. Three-dimensional simulations with GenASiS will require petascale to exascale computing resources.

Bellerophon: Info to follow.

Members of our group engaged in simulations of Type Ia supernovae use the University of Chicago ASC Center's Flash Code.

## Our Collaborators

The ORNL computational supernova effort is the center of a national collaboration involving eight senior investigators at five institutions across the U.S. This collaboration is known as the Petascale Supernova Initiative (PSI). PSI collaborators outside of ORNL and UTK are:

- John Blondin (NCSU)
- Stephen Bruenn (Florida Atlantic University)
- George Fuller (UCSD)
- Pedro Marronetti (Florida Atlantic University)

## News and Highlights of Ongoing Research

Ongoing 2D Core Collapse Supernova Simulations:

A revised picture of core collapse supernova explosions is emerging from our ongoing 2D core collapse supernova simulations. We have discovered that the core collapse supernova shock wave is likely reenergized to initiate an explosion at much later times than previously anticipated. The shock wave must exit the iron core and enter the oxygen layer before shock revival can occur. In the oxygen layer, the density of the star drops off dramatically, which gives the shock less to plow through. In addition, in the oxygen layer, nuclear burning can occur, aiding the shock energetically. The delay to explosion is naturally set by the time it takes for the shock to reach the oxygen layer. The previously discovered stationary accretion shock instability (SASI) causes large-scale distortions of the shock, causing it to reach the oxygen layer sooner in certain directions, thereby precipitating the onset of explosion. In this new picture, we have obtained explosions over a range of stellar progenitors, between 10 and 20 Solar masses. This had not been accomplished before.

Ongoing 3D Simulations:

Another major step forward this year was the initiation of a 3D multi-physics core collapse supernova simulation (with the CHIMERA code). We are currently running a low-resolution run for testing purposes on the LCF. A higher-resolution run is planned for the LCF 250 TF T2O period. This will be the first 3D multi-physics core collapse supernova simulation with multi-frequency neutrino transport.

## Select Images

The stream lines in this image show the two counter rotating flows that may be established below the supernova shock wave (the surface in the image) by the instability of the shock in a core collapse supernova explosion. The innermost flow accretes onto the central object, known as the proto-neutron star, spinning it up. This may be the mechanism whereby pulsars (spinning neutron stars) are born. [Blondin and Mezzacappa, Nature 445, 58 (2007)] |
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This is an image of the supernova remnant known as Cassiopeia A. It is the remnant of a core collapse supernova explosion that occurred approximately 300 years ago in our galaxy. The image was taken with the Chandra X-Ray Observatory. The different colors in the image correspond to different elements. The outer green edge marks the supernova shock wave and is about 10 light years across. |
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This is an image of supernova 1987A (SN1987A) in the Large Magellanic Cloud, a dwarf galaxy near our Milky Way galaxy. The image was taken by the Hubble Space Telescope. The close up shows the famous rings associated with this supernova. This is the only core collapse supernova explosion for which the intense flux of neutrinos (radiation) emitted by the proto-neutron star were detected. These neutrinos help power the explosion, and their detection in this case confirmed the basic paradigm modelers work with today. |
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In these images, the development of the supernova shock wave instability, a key component of the core collapse supernova explosion mechanism, is shown. Nonspherical distortions of the shock (the surface in the image) grow, leading to grossly aspherical explosions. The entropy of the stellar core fluid below the shock wave is shown here, which also illustrates the turbulent nature of the flow in an exploding stellar core. The Shock wave instability was discovered under the auspices of the SciDAC Terascale Supernova Initiative. [Blondin, Mezzacappa, and DeMarino, Ap.J. 584, 971 (2003)] |