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

STELLAR BOMBCompanion starAccretingwhite dwarfA WHITE DWARF gobbles gasfrom its binary companion andgains mass. ASTRONOMY: ROEN KELLYWHEN THE DWARF nears amass threshold, the star’scarbon ignites. A 10-billion-degree bubbleof nuclear ash, seenhere 1 secondafter ignition,starts rising tothe surface.There’s enough acoustic power to blow thestar apart half a second after core bounce inBurrows’ simulation.How important this process is remainsan open question. It’s the accreting materialthat keeps a lid on the explosion, preventingneutrinos from moving the shock out. “Ifthe neutrino mechanism worked, we wouldhave seen it in our model,” Burrows says.The sound waves push streams ofaccreting matter to one side of the corewhile energizing the shock on the oppositeside. So, by creating a path of least resistance,sound may help neutrinos revitalizea stalled shock. “It’s unproven,” he says, “butvery interesting.” Moreover, the oscillatingcore could be a prominent source of gravitationalradiation.Shattered dwarfsLarge-scale computer simulations are alsoproviding new insights into how whitedwarfs, the end state of low-mass stars,destroy themselves as type Ia supernovae.Brighter and more uniform than corecollapseexplosions, type Ia events areimportant probes of the distant universe.The discoveries of dark energy and cosmicacceleration add urgency to decipheringhow they work.A Sun-like star ends its days as a whitedwarf, with the star’s carbon-oxygen-richcore crushed to Earth’s size. Most shine forbillions of years, gradually cooling untilthey fade into dark stellar cinders. Electronpressure prevents further collapse, but itworks only if the dwarf weighs less than1.44 Suns — the so-called Chandrasekharlimit. Exceed that, and collapse resumesuntil the dwarf becomes a neutron star.In 1960, University of Cambridgeastronomer Fred Hoyle and Caltech’s WilliamFowler realized a white dwarf nearthis limit could be a giant thermonuclearbomb. Place a white dwarf in closeproximity to a normal star, andthe dwarf can gain mass untilit nears the 1.44-Sun thresholdand explode. The dwarfgobbles up hydrogen gasTHE BUBBLE BREECHES thedwarf’s surface 1.4 seconds afterignition in this simulation. Thehot cloud of fusion products isn’tmoving fast enough to go intoorbit. Instead, the dwarf’s gravityconfines the bubble to thestar’s surface (blue).TOMASZPLEWA, ALANCALDER, DEANTOWNSLEY, SHI-MON ASIDA, TRIDI-VESH JENA, FANG PENG,IVO SEITENZAHL, JIM TRU-RAN, GEORGE JORDAN, CARLOGRAZIANI, JU ZHANG, ALEXEIKHOKHLOV, ANDREY ZHIGLO, ALEXEIPOLUDNENKO, BRONSON MESSER, ED BROWN,AND DONALD LAMB (U.S. DEPARTMENT OF ENERGYADVANCED SIMULATION AND COMPUTING PROGRAM,ALLIANCE FLASH CENTER, AND THE ARGONNE NATIONALLABORATORY FUTURES LAB)THE EXPANDING BUBBLE hugs the dwarf’ssurface and plows some of the star’s unfusedmaterial ahead of it. We view the cloud 1.56seconds after ignition — the last moment ofthe 3-D simulation by Calder’s team. Followingup with 2-D models, the astronomersshowed this cloud wraps around the star inless than half a second. The cloud meetsitself on the dwarf’s opposite side. When itdoes so, the unfused surface matter thebubble plowed up crashes together andexplodes, destroying the star.ASY-SE0307.indd 421/30/07 11:05:19 AM

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