The star that re­fuses to burn out

Long-lived su­per­nova de­fies un­der­stand­ing of how ce­les­tial bod­ies evolve.

Los Angeles Times - - CALIFORNIA - AMINA KHAN amina.khan@la­ Twit­ter: @am­i­nawrite

Long-lived su­per­nova de­fies sci­en­tists’ un­der­stand­ing of how ce­les­tial bod­ies evolve.

Talk about go­ing out with a bang — and then an­other bang. Astronomers at Las Cum­bres Ob­ser­va­tory have dis­cov­ered a su­per­nova that has been shin­ing for years in­stead of mere months, sur­viv­ing far beyond its ex­pected life­span.

The strange and still-go­ing stel­lar ex­plo­sion, de­scribed in the jour­nal Na­ture, de­fies sci­en­tists’ un­der­stand­ing of dy­ing stars and may force them to re­think their ideas of how stars evolve.

“The su­per­nova of­fers astronomers their great­est thrill: some­thing they do not un­der­stand,” Stan Woosley of UC Santa Cruz, who was not in­volved in the study, wrote in a com­men­tary.

The su­per­nova known as iPTF14hls didn’t seem like any­thing out of the or­di­nary when it was picked up in Septem­ber 2014 by the In­ter­me­di­ate Palo­mar Tran­sient Fac­tory tele­scope near San Diego. The su­per­nova sits some 500 mil­lion light-years away in the con­stel­la­tion Ursa Ma­jor. Astronomers checked in on it ev­ery so of­ten, even­tu­ally clas­si­fy­ing the bright ob­ject as a type II-P su­per­nova that was al­ready be­gin­ning to dim.

Su­per­novas are pow­er­ful ex­plo­sions, putting out the bright­ness of around 100 mil­lion suns, but that bea­con in the night sky lasts only 100 days or so be­fore fad­ing away. There are a few that might last more than 130 days, but those are very rare. So af­ter a while, astronomers largely ig­nored iPTF14hls, ex­pect­ing it would ul­ti­mately dis­ap­pear into the night.

That changed af­ter UC Santa Bar­bara un­der­grad­u­ate stu­dent Zheng Chuen Wong, who was in­tern­ing at the ob­ser­va­tory, was asked to go through old data and see whether any­thing un­usual stuck out. He no­ticed iPTF14hls still shin­ing some 135 to 140 days since be­ing dis­cov­ered — and it wasn’t fad­ing. It was get­ting brighter.

“When he showed it to me, my first re­ac­tion was, ‘Well that can’t be a su­per­nova — it must be some­thing else,’ ” lead au­thor Iair Ar­cavi, an as­tro­physi­cist with UC Santa Bar­bara and Las Cum­bres, a global ob­ser­va­tory net­work based in Go­leta, Calif.

Ar­cavi fig­ured they’d get a spec­trum of the star, split­ting its light into a rain­bow to read its chem­i­cal fin­ger­print, and they’d fig­ure out what it re­ally was.

“And I was re­ally shocked when it did look like a su­per­nova in the spec­trum,” he said. “In fact, it looked like the most com­mon type of su­per­nova. It was the last thing I was ex­pect­ing to see.”

They kept watch­ing. The su­per­nova ul­ti­mately stayed bright for more than 600 days, blow­ing away the com­pe­ti­tion. And yet, in spite of its ad­vanced age, it looked like a su­per­nova that was just 2 months or so old.

It was as jar­ring as meet­ing a 600-year-old hu­man — one who looked no older than 60, Ar­cavi said.

On top of that, iPTF14hls did not fol­low the nor­mal path for a su­per­nova, Woosley pointed out. It ap­peared to vary in bright­ness by as much as 50%, im­ply­ing that it might be ex­plod­ing and fad­ing over and over again. It was far more lu­mi­nous than a typ­i­cal type II-P su­per­nova, and it had clearly let off far more en­ergy than its peers sim­ply by out­last­ing them.

“It means there’s def­i­nitely an­other way for stars to ex­plode that doesn’t de­stroy them com­pletely so they can do it mul­ti­ple times,” Ar­cavi said.

What was feed­ing this mys­te­ri­ous se­ries of ex­plo­sions? Ar­cavi and his col­leagues nar­rowed down the pos­si­bil­i­ties. Per­haps a fast-spin­ning neu­tron star, a mag­ne­tar, was be­hind the re­peated ex­plo­sions — but that would mean the su­per­nova should have started out far brighter than this one did. Per­haps the col­lid­ing shells of high-speed mat­ter seen in an­other kind of su­per­nova, known as type IIn, were caus­ing this su­per­nova to shine — but iPTF14hls wasn’t pro­duc­ing the X-rays and ra­dio waves that would be ex­pected if that were hap­pen­ing.

In­stead, sci­en­tists think this su­per­nova may be a pul­sa­tional-pair in­sta­bil­ity su­per­nova. In this sce­nario, the in­side of a mon­ster star with the mass of 100 suns be­comes so hot that some of its en­ergy turns into mat­ter and an­ti­mat­ter. If that hap­pens, the core be­comes un­sta­ble, trig­ger­ing an ex­plo­sion that blows off the star’s outer lay­ers, Ar­cavi said. If true, iPTF14hls would be the first ex­am­ple of this kind of su­per­nova ever seen.

The re­searchers looked back in their data from 1954 and found a lu­mi­nous ob­ject right where iPTF14hls sits in the sky. In their data from 1993, that ob­ject was gone. It seems iPTF14hls has been slowly flick­er­ing for more than half a cen­tury.

Even with this the­ory, not ev­ery­thing lines up, Ar­cavi said. For ex­am­ple, the star should have blown off all of its hy­dro­gen in its mid-cen­tury episode, but the star clearly has plenty of hy­dro­gen left.

“I’ve never seen any­thing like this,” Ar­cavi said. “Usu­ally when you see a weird su­per­nova or some­thing for the first time there’s some ex­pla­na­tion and then you find more and you work it out. But this one is just de­fy­ing all of the ex­pla­na­tions we have. There’s no model or the­ory that can fully ex­plain this.”

The orig­i­nal star it­self is some­thing of a sur­prise, he pointed out. Jumbo-sized stars were more com­mon in the uni­verse’s past, when they were made of mostly hy­dro­gen; the more traces of heavy el­e­ments in a star’s body, the smaller it tends to be. And those mas­sive stars did not live long — on the or­der of 100 mil­lion years or so, un­like our smaller sun, which has al­ready lived 4.6 bil­lion years.

“That also tells us some­thing about how stars are formed,” Ar­cavi said. “Also, if these things are com­mon, then what­ever kind of ele­men­tal abun­dances they’re eject­ing into their gal­axy could have a sig­nif­i­cant ef­fect on the over­all abun­dances of el­e­ments that we should be see­ing.”

Among the most re­mark­able things about iPTF14hls is how un­re­mark­able it looks, Ar­cavi added.

“This is one of the things I love most about this su­per­nova, that its spec­trum is so vanilla,” he said.

That means that any ex­pla­na­tion for this su­per­nova also has to ac­count for more com­mon su­per­novas — and the stars that birth.

“A bet­ter un­der­stand­ing of iPTF14hls could pro­vide in­sight into the evo­lu­tion of the most mas­sive stars, the pro­duc­tion of the bright­est su­per­novae and pos­si­bly the birth of black holes that have masses near 40 so­lar masses — such as those as­so­ci­ated with the first di­rect de­tec­tion of grav­i­ta­tional waves,” Woosley wrote.

It also hints that there may be many other longlived su­per­novas that have es­caped no­tice be­cause they’ve been mas­querad­ing as more typ­i­cal stel­lar ex­plo­sions. There may be more of them al­ready recorded in the ar­chives, wait­ing to be found.

The sci­en­tist said they’re con­tin­u­ing to ob­serve this su­per­nova to see whether, as time goes by, they’ll be able to peer past the ex­pand­ing outer lay­ers as they spread and be­come more trans­par­ent, to see the en­ergy source within be­fore it fades com­pletely.


AN ARTIST’S rendering of a su­per­nova. Most fade af­ter 100 days or so, but a su­per­nova known as iPTF14hls has been shin­ing far beyond its ex­pected life­span.

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