NEWS FEATURE

News Feature: How to light a cosmic candleAstronomers are still struggling to identify the companions that help whitedwarf stars self-destruct in violent supernovae explosions.

Nadia DrakeScience Writer

Billions of years ago, an ill-fated star not sodifferent from our Sun began to age andballoon outward. As it grew, the star’s glowdarkened to a deep, foreboding red, and itsouter layers sloughed off into space. Eventu-ally, the star’s nuclear furnace blinked out. Allthat remained was a dense, lifeless core aboutthe size of Earth: a white dwarf star.

That was the first time the star died. How-ever, a spectacular resurrection was at hand.Material stolen from a nearby stellar

companion ignited the dwarf’s carbon andoxygen layers and triggered a ferociousthermonuclear explosion that flung smol-dering material outward at about 10% ofthe speed of light. Decaying radioactive

elements transformed the billowing mate-rial into a blinding beacon of light.In its second death, the star blazed with the

brilliance of three billion suns.Photons expelled by this terminal stellar

spasm zoomed across space. For 21 millionyears, they traveled from their home in thePinwheel Galaxy through dust and cloudsuntil—quite improbably—they collidedwith a telescope perched atop a mountainin southern California.It was August 24, 2011, and the Palomar

Transient Factory had just seen the nearest,freshest type 1a supernova that had yet beendetected (1, 2). In the days and weeks thatfollowed, that twice-dead star—now calledsupernova 2011fe—was the most studied ob-ject in the sky. Ordinary in every way, it wasjust what astronomers needed to learn moreabout how white dwarf stars explode.Type 1a supernovae have been observed

for millennia, but in recent decades, theirpredictable brightness has made then invalu-able as cosmic distance markers. In the late1990s, Nobel Prize-winning observationsbased on these stellar explosions revealedthat the Universe is expanding at anaccelerating rate, propelled by a poorlyunderstood phenomenon dubbed dark en-ergy (3, 4). However, the processes thatproduce type 1a supernovae are still funda-mentally mysterious. “That’s a scary thing,”says astrophysicist Brad Tucker of Austra-lian National University and the Universityof California, Berkeley. “These are verypowerful tools in cosmology, but we reallydon’t know what’s going on with them.”he says.Before 2011fe exploded, astronomers did

not have much observational evidence thatexploding white dwarfs were responsible forthe supernovae. Just as the discovery an-swered one question, it also added tinder toa debate that burns brightly today: Whatkind of starry companion is donating mate-rial to the doomed dwarf?“People are now really embracing the idea

there may be more than one way to makea type 1a supernova,” says Peter Nugent, an

In 1572, the Danish astronomer Tycho Brahe observed and studied the explosion ofa star that became known as Tycho’s supernova. More than four centuries later, Chandra’simage of the supernova remnant shows an expanding bubble of multimillion-degreedebris (green and red) inside a more rapidly moving shell of extremely high energy electrons(filamentary blue). Image courtesy of NASA/CXC/Rutgers/J. Warren and J. Hughes, et al.

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astrophysicist at Lawrence Berkeley NationalLaboratory in California.

The Supernova Next DoorEvery night, the Palomar Transient Factorytakes hundreds of images of the sky. Softwaresubtracts those images from one another, andflags anything that changes between shots—what astronomers call an astrophysical tran-sient. These objects include variable stars,gamma ray bursts, and supernovae. Two anda half years ago, however, Nugent was siftingthrough the images himself because the sys-tem’s software had crashed the night before.That’s when he discovered 2011fe.The explosion was remarkably close and,

although it was just 11 hours old, the bal-looning debris cloud was already big enoughto fill the orbit of Jupiter. It was noon inBerkeley, so he asked a colleague to aima telescope in the Canary Islands at thegrowing spot of light. That observationrevealed the classic spectrum of a normal,young 1a supernova, containing silicon, cal-cium, a little bit of iron—and no hydrogen.Many of the major space observatories and

telescope arrays on the ground soon joinedin, eager to gather as much data as possibleabout the earliest stages of the star’s finalperformance. However, it was a tiny tele-scope in Mallorca, known by the acronymPIRATE, that garnered a crucial observation.

Quite by chance, PIRATE had looked at thePinwheel Galaxy just 4 hours after 2011fewent off. The image showed no trace of asupernova. The only possibility was that theexploding star was extremely dense and verysmall—less than 2% of the Sun’s diameter(5). It was the strongest evidence yet that type1a supernovae erupt from white dwarfs.

The Unusual SuspectCarbon-oxygen white dwarfs are dense,containing roughly a Sun’s mass of materialsqueezed into an Earth-size object. In thesestars, the inward crush of gravity is coun-teracted by electron degeneracy pressure, aquantum mechanical property that con-strains how tightly packed electrons can be(this is the reason white dwarfs are calleddegenerate stars). Degeneracy pressure usu-ally prevents a catastrophic collapse, leavingthe stable white dwarf to slowly fade awayover billions of years.However, if the star crosses a crucial

threshold around 1.4 solar masses, it becomesmassive enough to both overwhelm de-generacy pressure and begin fusing carbonnuclei, causing a runaway reaction thatends in a type 1a supernova. “Once thatprocess starts, for the most part, we think itjust continues. It’ll burn through the entirestar,” says astronomer Ryan Foley of theUniversity of Illinois, Urbana–Champaign.

A solitary white dwarf is in no danger ofexceeding this mass threshold, but a dwarf ina binary star system is. When two gravita-tionally bound stars circle one another, thedwarf can steal material from its companion,growing and growing until it explodes. In anormal galaxy, this kind of kleptomaniacalrelationship ends catastrophically every 200years or so.A decade ago, many astronomers favored

a model in which the dwarf’s companion waslarge and gassy, such as a red giant star.“Everybody was settled with that. That’s whatwe taught in our introductory astrophysicsclasses,” Foley says. However, some type 1asupernovae seem to have taken about 10billion years to grow up and die—much toolong to be explained by a shorter-lived redgiant binary system. The discrepancy sug-gests other partners must be involved.“There’s quite strong evidence that many,

if not most, type 1a supernovae come froma different system, with two white dwarfs,”says astronomer Alexei Filippenko of theUniversity of California, Berkeley. If they’reclose enough, two dwarfs will slowly spiral intoward one another as they emit gravitationalwaves—ripples in the fabric of space-timethat draw energy from the stars. As theycome together, either the more massive dwarfsteals material from its companion until itexplodes and obliterates both of them or thetwo dwarfs collide and are annihilated.The double-degenerate scenario provoked

skepticism within the field for years. Earlymodels of their death spiral could not explainhow the dwarfs transferred mass or cametogether quickly enough to explode. Plus, theexplosion physics weren’t quite right: scien-tists could not make the dwarfs in theirmodels shine brightly enough or explain themany layers of chemical elements emergingover the course of the explosion.However, new observations and better

models are now changing astronomers’minds. “Almost everything has been flip-ped on its head,” says Nugent.

Missing CompanionsOne of the most telling flaws in the case forlarge companion stars is that astronomershave not seen much evidence of them aroundtype 1a supernovae.When a dwarf detonates, the explosion

should tear some of the hydrogen gas froma nondegenerate companion star and fling itoutward. “And yet, we don’t see any evidencefor that gas,” Filippenko says. Indeed, one ofthe hallmarks of a 1a explosion is a lack ofhydrogen gas flying outward at high speed.If an explosion is caught early enough, like

2011fe, models suggest that astronomers

Supernova 2011fe was discovered just hours after it exploded in the Big Dipper. Studiesby the Nearby Supernova Factory of its spectrum as it evolved over time have produceda benchmark atlas of data by which to measure all future type Ia supernovas. Imagecourtesy of B. J. Fulton (Las Cumbres Observatory Global Telescope Network, Goleta, CA).

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should also see what’s called a shock break-out. This is when a large companion star actsas a roadblock for some of the supernovaejecta; instead of flying off into space, thematerial piles up behind the companion.Eventually, the material will heat up andproduce a rapid brightening that appears asan abnormal bump in the supernova’s earlylight curves. However, no one has foundmuch evidence for shock breakouts.Astronomers also expected to find beaten-

up former companions within the debris ofancient 1a supernovae. “If we find an actualcompanion in the remnant, it’s the best proofwe’re ever going to get to finding out whatthe progenitor system is,” says WolfgangKerzendorf, an astronomer at the Universityof Toronto, Canada.Rocketing through space with peculiar

speeds and spins and possibly carryingchemical scars from the explosion, these starsshould be odd enough to identify. However,with one possible exception (6), scientistshave not seen them. Kerzendorf has scruti-nized the remnants of Tycho’s supernova,which exploded in 1572, Kepler’s supernovaof 1604, and the remains of a supernova thatexploded in the year 1006. Of those, Tycho’sis the only remnant with a possible com-panion star, but the claim is disputed.Conversely, a white dwarf companion

would not survive the explosion, so therewould be no leftover star to find. That’s whatAshley Pagnotta and Bradley Schaefer, thenboth at Louisiana State University in BatonRouge, concluded in 2012 after studying asupernova remnant called SNR 0509-67.5 inthe Large Magellanic Cloud (7).However, absence of evidence is not

evidence of absence, and there has been nodirect confirmation of a white dwarf com-panion. Even archival Hubble images of2011fe could only rule out companionsdimmer than a Sun-like star (8). Observa-tions of 2014J, a recent supernova some 11.5

million light-years away in the Cigar Galaxy,are more constraining—but they do not ruleout dimmer nondegenerate stars like redm-dwarfs, which could easily evade detection.“M-dwarfs are the most common kind of starin the galaxy,” says J. Craig Wheeler, an as-trophysicist at the University of Texas atAustin. “How often would white dwarfs andm-dwarfs pair up? The answer is a lot. Thereare billions of each of them,” he says.There is at least one strong, recent piece

of evidence for the traditional scenario: asupernova called PTF 11kx, surrounded bycomplex shells of gas and ejected materialthat suggest it blew up with the help of a redgiant companion (9). Foley has also studieddistant supernova remnants and found thatabout 20% have tell-tale outflows of sodiumgas that hint at a relatively large and gassycompanion star (10). “The simplest expla-nation for that is they come from single de-generate systems,” he says.With a growing number of observations

supporting each scenario, many scientistsnow strongly suspect that doomed whitedwarfs could be dancing with a varietyof companions. “There’s reasonable cir-cumstantial evidence for both channels,”say Saurab Jha, an astrophysicist at Rut-gers University. That’s a real surprise, headds, because it would mean that differentcombinations of ingredients and variedcooking times could produce remarkablysimilar type 1a supernovae.

However, the race to accept multipleprogenitor systems worries Kerzendorf.“We have to try to find one model thatfits all. I’m not saying it’s impossible thatthere are two progenitor systems,” hesays. Kerzendorf adds, “But if you openthat door, then there could be three. Andthen every single supernova could haveits own progenitor system. That mightnot be the truth.”

Illuminating Dark EnergyAlthough scientists do not fully understandhow to explode a white dwarf, the dead stars’role as cosmic milestones is on solid ground.However, a better understanding of howthese candles light up will make distancemeasurements more accurate and helpscientists figure out whether dark energyhas changed over time. To do that, scien-tists need to find some really old, distantsupernovae and be confident that theyunderstand the mechanics of the explo-sions. “The concern is that maybe the starsystems that are exploding in type 1asupernovae are different,” says Jha, “butwe don’t know how they’re going tobe different.”Still, scientists are confident that they will

crack the mystery of these twice-dead can-dles. “It’s just a hard slog, the way sciencesometimes is,” Wheeler admits, “but thereare so many people getting so much data,I suspect we’ll figure it out.”

1 Nugent PE, et al. (2011) Supernova SN 2011fe from an explodingcarbon-oxygen white dwarf star. Nature 480(7377):344–347.2 Nugent PE, et al. (2011) Young type 1a supernova PTF 11kly inM101. The Astronomer’s Telegram. Available at http://www.astronomerstelegram.org/?read=3581. Accessed July 25, 2014.3 Perlmutter S, et al. (1998) Measurements of Ω and Λ from 42high-redshift supernovae. Astrophysical Journal 517(2):565–586.4 Riess AG, et al. (1998) Observational evidence from supernovae foran accelerating universe and a cosmological constant. Astron J116(3):1009–1038.5 Bloom JS, et al. (2012) A compact degenerate primary-starprogenitor of SN 2011fe. Astrophysical Journal Letters 744(2):L17.

6 Ruiz-Lapuente P, et al. (2004) The binary progenitor of TychoBrahe’s 1572 supernova. Nature 431(7012):1069–1072.7 Schaefer BE, Pagnotta A (2012) An absence of ex-companion starsin the type Ia supernova remnant SNR 0509-67.5. Nature 481(7380):164–166.8 Li W, et al. (2011) Exclusion of a luminous red giant as acompanion star to the progenitor of supernova SN 2011fe. Nature480(7377):348–350.9 Dilday B, et al. (2012) PTF 11kx: A type Ia supernova with asymbiotic nova progenitor. Science 337(6097):942–945.10 Foley R, et al. (2012) Linking Type 1a supernova progenitors andtheir resulting explosions. ApJ 752:101.

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