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Thousands of “time bomb” stars might dot our galaxy

Sept. 6, 2011
Courtesy of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics
and World Science staff

In the Hol­ly­wood block­bust­er “Speed,” a bom­b on a bus is rigged to go off if the bus slows down be­low 50 miles per hour. The premise—slow down and you ex­plode—makes for a great ac­tion mov­ie plot.

It al­so hap­pens to have a cos­mic equiv­a­lent, some as­tro­no­mers say.

In this artist's con­cep­tion, a su­per­no­va ex­plo­sion is about to ob­lit­er­ate an or­bit­ing Saturn-like plan­et. (Cred­it: Da­vid A. Aguilar (CfA))


New re­search in­di­cates some old stars might be held to­geth­er only by their rap­id spins—so when they slow down, they ex­plode. Thou­sands of these “time bom­bs” could be scat­tered through­out our gal­axy, as­tro­no­mers claim.

“We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this re­search sug­gests that we’ve been look­ing for the wrong signs. Our work points to a new way of search­ing” for them, said as­t­ro­phys­i­cist Ros­anne Di Ste­fano of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass.

Di Ste­fano and col­leagues stud­ied a type of stel­lar ex­plo­sion called a Type 1a super­nova. It oc­curs when an old, com­pact star known as a white dwarf be­comes un­sta­ble. A white dwarf is a stel­lar rem­nant that has stopped nu­clear fu­sion, the pro­cess that pow­ers ac­tive stars. A white dwarf can weigh up to 1.4 times as much as our Sun; any heav­i­er, and it would start to shrink rap­idly un­der its own gra­vity, trig­ger­ing run­away nu­clear fu­sion that next blows it up.

There are the­o­ret­ic­ally two ways a white dwarf can ex­ceed the weight lim­it and ex­plode, or go “super­nova.” It can ac­cu­mu­late gas from a do­nor star, or two white dwarfs can col­lide. Most as­tro­no­mers fa­vor the first sce­nar­i­o as the more typ­i­cal ex­plana­t­ion. But we’d ex­pect to see cer­tain signs if that is cor­rect, and we don’t for most cases, Di Ste­fano and col­leagues say.

For ex­am­ple, we should de­tect small amounts of hy­dro­gen and he­li­um gas near the ex­plo­sion, but we don’t. That gas would come from mat­ter that was­n’t ac­cu­mu­lated by the white dwarf, or from the dis­rup­tion of the com­pan­ion star in the ex­plo­sion. As­tro­no­mers al­so have looked for the do­nor star af­ter the super­nova fad­ed from sight, with­out suc­cess.

Di Ste­fano and col­leagues sug­gest white dwarf spin might solve this puz­zle. A pro­cess of spin­ning up, then spin­ning down, could cre­ate a long de­lay be­tween the gas ac­cu­mula­t­ion and the ex­plo­sion. As a white dwarf gets heav­i­er, it al­so spins faster. If it ro­tates fast enough, its spin can help sup­port its struc­ture, al­low­ing it to cross the 1.4-solar-mass bar­ri­er. Then it would be called a “super-Chan­dra­sek­har-mass star” (the 1.4 lim­it is called the Chan­dra­sek­har mass af­ter the as­tronomer who first cal­cu­lat­ed it.)

Once gas ac­cu­mula­t­ion stops, the white dwarf should grad­u­ally slow down. Even­tu­al­ly, the spin is­n’t enough to coun­ter­act gra­vity, lead­ing to the super­nova. “Our work is new be­cause we show that spin-up and spin-down of the white dwarf have im­por­tant con­se­quenc­es,” ex­plained Di Ste­fano.

The whole process could pro­duce a de­lay of up to a bil­lion years be­tween the end of gas ac­cu­mula­t­ion, or ac­cre­tion, and the ex­plo­sion, Di Ste­fano said. This would al­low the com­pan­ion star to age and evolve in­to a sec­ond white dwarf, and sur­round­ing gas to dis­si­pate.

In our gal­axy, sci­en­tists es­ti­mate there are three Type 1a supe­rnovae every thou­sand years. If a typ­i­cal supe­r-Chan­drasekhar-mass white dwarf takes mil­lions of years to spin down and ex­plode, then cal­cula­t­ions sug­gest that there should be doz­ens of pre-ex­plo­sion sys­tems with­in a few thou­sand light-years of Earth. Those “super­nova pre­cur­sors” will be hard to find, but up­com­ing sur­veys should be able to spot them, Di Ste­fano said. “We’re look­ing for­ward to hunt­ing them out,” added study co-author Ras­mus Voss of Rad­boud Uni­vers­ity Nij­me­gen, The Neth­er­lands.

This re­search ap­pears in a pape­r in the Sept. 1 is­sue of the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters.


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In the Hollywood blockbuster “Speed,” a bomb on a bus is rigged to go off if the bus slows down below 50 miles per hour. The premise—slow down and you explode—makes for a great action movie plot. It also happens to have a cosmic equivalent, some astronomers say. New research indicates some old stars might be held together only by their rapid spins—so when they slow down, they explode. Thousands of these “time bombs” could be scattered throughout our galaxy, astronomers claim. “We haven’t found one of these ‘time bomb’ stars yet in the Milky Way, but this research suggests that we’ve been looking for the wrong signs. Our work points to a new way of searching for” them, said astrophysicist Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Di Stefano and colleagues studied a type of stellar explosion called a Type Ia supernova. It occurs when an old, compact star known as a white dwarf becomes unstable. A white dwarf is a stellar remnant that has stopped nuclear fusion, the process that powers active stars. A white dwarf can weigh up to 1.4 times as much as our Sun; any heavier, and it would start to shrink rapidly under its own gravity, triggering runaway nuclear fusion that next blows it up. There are theoretically two ways a white dwarf can exceed the weight limit and explode, or go “supernova.” It can accumulate gas from a donor star, or two white dwarfs can collide. Most astronomers favor the first scenario as the more typical explanation. But we’d expect to see certain signs if that is correct, and we don’t for most cases, Di Stefano and colleagues say. For example, we should detect small amounts of hydrogen and helium gas near the explosion, but we don’t. That gas would come from matter that wasn’t accumulated by the white dwarf, or from the disruption of the companion star in the explosion. Astronomers also have looked for the donor star after the supernova faded from sight, without success. Di Stefano and colleagues suggest white dwarf spin might solve this puzzle. A process of spinning up, then spinning down, could create a long delay between the gas accumulation and the explosion. As a white dwarf gets heavier, it also spins faster. If it rotates fast enough, its spin can help support its structure, allowing it to cross the 1.4-solar-mass barrier. Then it would be called a “super-Chandrasekhar-mass star” (the 1.4 limit is called the Chandrasekhar mass after the astronomer who first calculated it.) Once gas accumulation stops, the white dwarf should gradually slow down. Eventually, the spin isn’t enough to counteract gravity, leading to a Type Ia supernova. “Our work is new because we show that spin-up and spin-down of the white dwarf have important consequences. Astronomers therefore must take angular momentum of accreting white dwarfs seriously, even though it’s very difficult science,” explained Di Stefano. Angular momentum is a form of momentum associated with the spin of an object. The spin-down process could produce a delay of up to a billion years between the end of gas accumulation, or accretion, and the explosion, Di Stefano said. This would allow the companion star to age and evolve into a second white dwarf, and any surrounding material to dissipate. In our Galaxy, scientists estimate there are three Type Ia supernovae every thousand years. If a typical super-Chandrasekhar-mass white dwarf takes millions of years to spin down and explode, then calculations suggest that there should be dozens of pre-explosion systems within a few thousand light-years of Earth. Those “supernova precursors” will be hard to find, but upcoming surveys should be able to spot them, Di Stefano said. “We’re looking forward to hunting them out,” added study co-author Rasmus Voss of Radboud University Nijmegen, The Netherlands. This research appears in a paper in the Sept. 1 issue of the journal Astrophysical Journal Letters.