# Presentation: 803360

## Adaptive Mesh Simulations of Astrophysical Detonations Using the ASCI Flash Code

**Bruce Fryxell**
- University of Chicago
**ACAT 2000 **
**Fermilab - Oct. 16, 2000**

## Astrophysical Detonations

**Collaborators****:**
- 	**Alan Calder****	****	****Paul Ricker**
**	****Jonathan Dursi****	****Bob Rosner**
**	****Don Lamb****	****	****Frank Timmes**
**	****Peter MacNeice****	****Henry Tufo**
**	****Kevin Olson****	****	****Mike Zingale**

## Overview

**The Flash problem**
**Description of the Flash code**
**Helium detonations on neutron stars                  (X-ray bursts)**
**Carbon detonations in white dwarfs               (Type Ia supernovae)**

## What Is the FLASH Problem?

-     To simulate the accumulation matter onto the surface of compact stars, nuclear ignition of the accreted (and possibly underlying stellar) material, and the subsequent evolution of the star’s interior, surface, and exterior
- X-ray bursts (on neutron star surfaces)
- Novae (on white dwarf surfaces)
- Type Ia supernovae (in white dwarf interiors)

- Credit: NASA/HST artist

## The Astrophysical Problem

**Type Ia Supernova**
** ****(White Dwarf)**

**X-ray Burst (Neutron Star****)**
**Nova (White Dwarf)**

- White dwarf:  R ~ REarth
- Neutron star: R ~ RChicago

## Description of FLASH Code

**Fully compressible hydrodynamics**
**Piecewise-Parabolic Method (Colella & Woodward)**
**Adaptive mesh refinement **
**Block-structured AMR (PARAMESH)**
**One, two or three spatial dimensions**
**Cartesian, cylindrical, or spherical**
**Equation of state for degenerate matter**
**Reaction networks for nuclear burning**
**Thermal conduction**
**Poisson Solver for self gravity**

## Description of FLASH Code

**Portable**
**ASCI Red, ASCI Blue Pacific, Nirvana**
**Cray T3E**
**Beowulf clusters**
**Workstations**
**Fully parallel**** **
**MPI, HDF 5**
**Scales to 1024+ processors**
**0.25 TFLOPS on 6000 processors of ASCI Red**
**2000 Gordon Bell finalist**
**Object-oriented framework**

## X-ray Burst Simulation

**Accretion of hydrogen-rich material onto the surface of a neutron star**
**When the hydrogen reaches the surface, it burns “quietly” to helium forming a narrow layer**
**After the helium reaches a sufficient depth (about 100 m), it becomes unstable**
**A thermonuclear runaway initiates at a point at the base of the helium layer**
**A burning front (detonation or deflagration) propagates across the surface of the neutron star, burning the entire layer to iron**

## Initial Model Parameters

**Initial neutron star initial model generated from one-dimensional stellar evolution calculation**
**R = 14 km,  M = 1.4 M******
**g = 9.48 x 10****13 ****cm s****-2**
**Accretion rate**** = 2 x 10****-11**** ****M******** ****/ yr**
**Conditions in helium layer at time of runaway**
**h = 110 m**
**M = 1.22 x 10****25**** g **
******base**** = 4 x 10****8**** g cm****-3**
**T****base**** = 1 x 10****8**** ****o****K**
**Computational grid**
**Two-dimensional cylindrical coordinates (axi-symmetric)**
**2 km wide x 1.5 km high**
**Minimum zone size = 1 m x 1 m**
**Effective resolution 2000 x 1500**

## X-ray Burst Simulation

## X-ray Burst Simulation

## X-ray Burst Simulation

**X-ray Burst Simulation**

## Density Evolution

## Adaptive Mesh Behavior

## Carbon Detonations in White Dwarfs

**Want to study the small-scale structure which develops behind the detonation front in a Type Ia supernova explosion**
**An initial planar detonation is subject to a multi-dimensional instability which produces a cellular structure behind the front, resulting in pockets of unburned material**
**Initial model chosen to represent the conditions in the interior of a white dwarf:****                                                ****	****C****12  ****at a density of 4 x 10****8**** g cm****-3**
**Three-dimensional Cartesian grid**
**Domain size – 12.8 x 12.8 x 256 cm**
**Grid size (if fully refined) – 256 x 256 x 5120**

## Carbon Detonations in White Dwarfs 

**Pressure**
**Evolution**

## Carbon Detonations in White Dwarfs 

**Pressure**
**Evolution**

## Carbon Detonations in White Dwarfs 

**Pressure**

**Max Pressure**

**Si 28 Ashes**

## Carbon Detonations in White Dwarfs 

**Carbon Detonations in White Dwarfs **

**Cross-section of Pressure**

## Carbon Detonations in White Dwarfs 

**Volume Rendered Image of Pressure**