STELLAR BOMB - The Department of Astronomy & Astrophysics ...

from its partner at a probable rate of about⅓0 of an Earth-mass per year. If it’s muchslower than this, the dwarf ’s stellar windprevents the gas from reaching the surface;if it’s any faster, the gas will flash-fuse ratherthan accumulate.As a white dwarf tips the scale toward1.44 Suns, its carbon ignites somewhereinside. Before 2004, no one could figure outhow to make a carbon-oxygen star detonate,so theorists first invoked turbulentthermonuclear fusion. These models failedto match the energy and element mix oftype Ia blasts. Models that followed aperiod of turbulent burning with a detonationbetter matched reality, but theoristssimply decided where and when the explosionwould occur and inserted it into thesimulation. “I sometimes refer to this as the‘Here, a miracle occurs’ mechanism,” saysthe University of Chicago’s Don Lamb.For this reason, Wolfgang Hillebrandtand his group at the Max Planck Institutefor Astrophysics in Munich, Germany, trieda different tack. They found that modelsusing turbulent burning alone can bettermatch observations, but, to do so, thedwarf ’s thermonuclear fires must ignite inabout 100 different points at once. That’svery unlikely. Says Lamb: “We worry onemiracle has been replaced by another.”In 2004, a team led by Alan Calder atthe University of Chicago including Lamb,stumbled onto a way to blow up a whitedwarf. Thanks to the U.S. Department ofEnergy’s computational resources, the teamhad the hardware to simulate an entirewhite-dwarf star. After ignition, a narrowfront of nuclear flame expanded throughthe star, leaving behind a 10-billion-degreeash bubble. When this bubble brokethrough the dwarf ’s crust, less than 10 percentof the star’s mass had been fused —too little to disrupt the dwarf or produce astrong explosion. “It looked like it might bea dud,” Lamb recalls.Then, team member Tomasz Plewa performedadditional 2-D simulations to seewhat happens after the bubble breeches thestar’s surface. The nuclear ash erupts, movingat around 6.7 million mph (10.8 millionkm/h), just shy of orbital speed. The hotcloud hugs the dwarf ’s billion-degree surfaceand rapidly spreads. As it does so, itplows up cooler, unfused surface matter.The superheated ash-cloud wraps aroundthe white dwarf and meets itself at the pointopposite its breakout. The collision compressesall of the unfused surface material,which explodes and rips the star apart.The model, called “gravitationally confineddetonation,” is the most completedescription of a type Ia supernova to date —and the only one in which a full-scaledetonation naturally occurs. “It’s a verypromising model for most type Ia supernovae,”Lamb says. “It was a serendipitous discovery.And it is a perfect example of howlarge-scale numerical simulations can leadto discoveries of complex, non-linearphenomena that are very difficult toimagine ahead of time,” he adds.Seventy-four years after astronomersconnected supernovae with stellardeaths, the universe’s most powerfulexplosions still tax astrophysicists. Yet,even the most complete simulationsdon’t yet capture the complex environmentof an exploding star. Modelersare beginning to probe how neutrinoemission, magnetic fields,and rotation affect the picture.Observers watch and catalognew events, using them both ascosmic yardsticks and to findholes in current understanding.And new facilities designed tocapture neutrinos and gravitationalwaves — signals that directlyescape an exploding star’s core —one day soon may give us a glimpse ofa supernova’s chaotic heart.ONLINEEXTRASee movies of supernova simulationsat www.astronomy.com/toc.www.astronomy.com 43ASY-SE0307.indd 431/30/07 11:06:06 AM