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New type of exploding star may be a dud

Nov. 21, 2012
Courtesy of the University of Chicago
and World Science staff

A type of oddly dim, ex­plod­ing star is probably a sort of dud—one the could none­the­less throw light on the na­ture of the mys­te­ri­ous “dark en­er­gy” pe­r­vad­ing space, as­tro­no­mers say.

The seem­ingly failed out­bursts are said to come from vari­ants of ex­plod­ing stars called type Ia su­pe­r­novae, which are found by the thou­sands. 

This im­age, based on su­per­com­puter sim­u­la­tions, shows the asym­met­ric sur­face char­ac­ter­is­tic of a white dwarf that fails to b­low up. (Cour­te­sy of Brad Gal­lagher, George Jor­dan/Flash)


Most type 1a su­pe­r­novae look si­m­i­lar to one an­oth­er. For this rea­son they’re used as cos­mic dis­tance in­di­ca­tors. If you saw many lamps at many dif­fer­ent dis­tances, but knew they were all of same type of lamp, you could use their dif­fer­ing bright­nesses to fig­ure out the dis­tance to all of them just by know­ing the dis­tance to one. To as­tro­no­mers, type Ia su­pe­r­novae serve as such lamps, or “s­tan­dard can­dles.” 

But these blasts re­veal more than dis­tances: da­ta on their bright­ness has al­so pe­r­suaded as­tro­no­mers that the ex­pan­sion of the uni­verse is ac­cel­er­at­ing. Sci­en­tists have dubbed the un­known cause be­hind this ac­celera­t­ion “dark en­er­gy.”

About 20 type Ia su­pe­r­novae look pe­cu­liar, though.

They're “a lit­tle bit odd,” said George Jor­dan, a com­puta­t­ional sci­ent­ist at the Uni­vers­ity of Chi­ca­go. Com­par­ing odd type Ia su­pe­r­novae to nor­mal ones may let as­t­ro­phys­i­cists more pre­cisely de­fine the na­ture of dark en­er­gy, he added. Jor­dan and col­leagues have con­clud­ed that the pe­cu­liar type Ia su­pe­r­novae are probably “white dwarf” stars that failed to det­o­nate. 

“They ig­nite an or­di­nary flame and they burn, but that is­n't fol­lowed by a trig­ger­ing of a detona­t­ion wave that goes through the star,” Jor­dan said. These find­ings were based on sim­ula­t­ions on In­trep­id, the Blue Gene/P supe­rcom­puter at Ar­gonne Na­t­ional Lab­o­r­a­to­ry. De­tails are to ap­pear in the jour­nal As­t­ro­phys­i­cal Jour­nal Let­ters.

Type Ia su­pe­r­novae are blasts that hap­pen to white dwarfs—s­tars that have shrunk to Earth size af­ter hav­ing burned most or all of their nu­clear fu­el. Most or all white dwarfs oc­cur in bi­na­ry sys­tems, those that con­sist of two stars or­bit­ing one an­oth­er.

The pe­cu­liar type Ia su­pe­r­novae are an­y­where from 10 to 100 times faint­er than nor­mal ones, which are brighter and there­fore more easily de­tected. As­t­ro­phys­i­cists have es­ti­mat­ed that they may ac­count for some 15 pe­r­cent of all type Ia su­pe­r­novae.

The first of the dim su­pe­r­novae was dis­cov­ered in 2002, not­ed Rob­ert Fish­er, a phys­i­cist at the Uni­vers­ity of Mas­sa­chu­setts Dart­mouth and co-author of the pa­pe­r. Called SN 2002cx, it's con­sid­ered the most pe­cu­liar type Ia su­pe­r­no­va known. The dim­mest of the lot, how­ev­er, was dis­cov­ered in 2008. “If the bright­ness of a stand­ard su­pe­r­no­va could be thought of as a sin­gle 60-watt light bulb, the bright­ness of this 2008 su­pe­r­no­va would be equiv­a­lent to a small frac­tion of a sin­gle can­dle or a few doz­en fire­flies,” Fish­er not­ed.

Sci­en­tists have been sim­ulating type Ia su­pe­r­no­va ex­plo­sions for years based on a sce­nar­i­o called “gravita­t­ionally con­fined detona­t­ion.“ A white dwarf starts to burn near its cen­ter. The ig­ni­tion point burns out­ward, float­ing to­ward the sur­face like a bub­ble. Af­ter it breaks the sur­face, a cas­cade of hot ash flows around the star and col­lides with it­self on the op­po­site end, trig­ger­ing a detona­t­ion.

“We took the nor­mal GCD sce­nar­i­o and asked what would hap­pen if we pushed this to the lim­its and see what hap­pens when it breaks,” Jor­dan said. In the failed detona­t­ion sce­nar­i­o, the white dwarf ex­pe­ri­ences more ig­ni­tion points that are clos­er to the co­re, which fu­els more burn­ing than in the detona­t­ion sce­nar­i­o.

“The ex­tra burn­ing causes the star to ex­pand more, pre­vent­ing it from achiev­ing tem­pe­r­a­tures and pres­sures high enough to trig­ger detona­t­ion,“ ex­plained co-author Dan­iel van Rossum of The Uni­vers­ity of Chi­ca­go's Flash Cen­ter.

So in­stead of blow­ing apart, the white dwarf re­mains mostly in­tact, though some pieces burn away. This failed detona­t­ion sce­nar­i­o looks quite si­m­i­lar to the pe­cu­liar type Ia ex­plo­sions, the re­search­ers said. The sim­ula­t­ions, they added, re­sulted in phe­nom­e­na that as­tro­no­mers now can look for or have al­ready found in their tel­e­scop­ic ob­serva­t­ions. These phe­nom­e­na could in­clude white dwarfs that dis­play un­usu­al com­po­si­tions, asym­met­ric sur­face char­ac­ter­is­tics and a kick that sends the stars fly­ing off at speeds of hun­dreds of miles per sec­ond. 

“No one had ev­er sug­gested that white dwarfs could be kicked at such ve­lo­ci­ties,” said Ha­gai Pe­rets of the Tech­nion-Is­rael In­sti­tute of Tech­nology, a col­lab­orator in the work.

Nor­mal type Ia su­pe­r­novae look fairly un­iform, but the asym­met­ric char­ac­ter­is­tics of their pe­cu­liar cousins means the lat­ter would of­ten look much dif­fer­ent from one an­oth­er, de­pend­ing on their view­ing an­gle from Earth. The asym­met­ric ex­plo­sion al­so pro­duces the kick, the re­search­ers said, which may be pow­er­ful enough to free the white dwarf from a stel­lar com­pan­ion's gra­vity or even from its home gal­axy.

“I had nev­er heard of such strange white dwarfs,” Perets said. But when he con­ducted a lit­er­a­ture search, he added, he found re­ports of white dwarfs with prope­rties that this ir­reg­u­lar com­po­si­tion could ex­plain.

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A type of oddly dim, exploding star is probably a sort of dud—one the could nonetheless throw light on the nature of the mysterious “dark energy“ pervading space, astronomers say. The seemingly halfhearted outbursts are said to come from variants of exploding stars called type Ia supernovae, which are found by the thousands. Most type 1a supernovae look similar to one another. For this reason astrophysicists use them as cosmic distance indicators. If you saw many lamps at many different distances, but knew they were all of same type of lamp, you could use their differing brightnesses to figure out the distance to all of them just by knowing the distance to one. To astronomers, type Ia supernovae serve as such lamps, or “standard candles.“ But these blasts reveal more than distances: data on their brightness has also persuaded astronomers that the expansion of the universe is accelerating. Scientists have dubbed the unknown cause behind this acceleration “dark energy.“ About 20 type Ia supernovae look peculiar, though. They're “a little bit odd,“ said George Jordan, a computational scientist at the University of Chicago. Comparing odd type Ia supernovae to normal ones may permit astrophysicists to more precisely define the nature of dark energy, he added. Jordan and colleagues have concluded that the peculiar type Ia supernovae are probably “white dwarf“ stars that failed to detonate. “They ignite an ordinary flame and they burn, but that isn't followed by a triggering of a detonation wave that goes through the star,“ Jordan said. These findings were based on simulations on Intrepid, the Blue Gene/P supercomputer at Argonne National Laboratory. Details are to appear in the journal Astrophysical Journal Letters. Type Ia supernovae are blasts that happen to white dwarfs—stars that have shrunk to Earth size after having burned most or all of their nuclear fuel. Most or all white dwarfs occur in binary systems, those that consist of two stars orbiting one another. The peculiar type Ia supernovae are anywhere from 10 to 100 times fainter than normal ones, which are brighter and therefore more easily detected. Astrophysicists have estimated that they may account for some 15 percent of all type Ia supernovae. The first of the dim supernovae was discovered in 2002, noted Robert Fisher, a physicist at the University of Massachusetts Dartmouth and co-author of the paper. Called SN 2002cx, it's considered the most peculiar type Ia supernova known. The dimmest of the lot, however, was discovered in 2008. “If the brightness of a standard supernova could be thought of as a single 60-watt light bulb, the brightness of this 2008 supernova would be equivalent to a small fraction of a single candle or a few dozen fireflies,“ Fisher noted. Scientists have been simulating type Ia supernova explosions for years based on a scenario called “gravitationally confined detonation.“ A white dwarf starts to burn near its center. The ignition point burns outward, floating toward the surface like a bubble. After it breaks the surface, a cascade of hot ash flows around the star and collides with itself on the opposite end, triggering a detonation. “We took the normal GCD scenario and asked what would happen if we pushed this to the limits and see what happens when it breaks,“ Jordan said. In the failed detonation scenario, the white dwarf experiences more ignition points that are closer to the core, which fuels more burning than in the detonation scenario. “The extra burning causes the star to expand more, preventing it from achieving temperatures and pressures high enough to trigger detonation,“ explained co-author Daniel van Rossum of The University of Chicago's Flash Center. So instead of blowing apart, the white dwarf remains mostly intact, though some pieces break off. This failed detonation scenario looks quite similar to the peculiar type Ia explosions, the researchers said. The simulations, they added, resulted in phenomena that astronomers now can look for or have already found in their telescopic observations. These phenomena include white dwarfs that display unusual compositions, asymmetric surface characteristics and a kick that sends the stars flying off at speeds of hundreds of miles per second. “This was a completely new discovery,“ Perets said. “No one had ever suggested that white dwarfs could be kicked at such velocities.“ Normal type Ia supernovae look fairly uniform, but the asymmetric characteristics of their peculiar cousins means the latter would often look much different from one another, depending on their viewing angle from Earth. The asymmetric explosion also produces the kick, which may be powerful enough to free the white dwarf from a stellar companion's gravity or even from its home galaxy. “I had never heard of such strange white dwarfs,“ Perets said. But when he conducted a literature search, he added, he found reports of white dwarfs with properties that this irregular composition could explain.