Nadia Drake, Type 1a Supernovae - Why Our Standard Candle Isnt Really Standard

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Type 1a supernova 1994D exploded near the outskirts of galaxy NGC 4526. (NASA/ESA/Hubble/High-Z Supernova Search Team)

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Type 1a Supernovae: Why Our Standard Candle Isnt Really Standard by Nadia Drake

When I joined Phenomena, Carl Zimmer asked: What obsesses you? Among my obsessions, I answered, are type 1a supernovae. Here we go.

How can an astronomical object of such crucial cosmological importance remain so fundamentally mysterious?

When a runaway thermonuclear explosion rips through a white dwarf star and blows the star to bits, its called a type 1a supernova. These explosions are incredibly violent

and incredibly bright, sometimes outshining entire galaxies. Thought to occur about once every two centuries in a galaxy like the Milky Way, these stellar cataclysms are

relatively frequent events.

The star doing the exploding is a white dwarf with a fairly standard mass, so the supernovas brightness is predictable. And because luminosity decreases with distance,

scientists can use the difference between an explosions observed and predicted brightness to determine how far away the blazing starstuff is. That characteristic has led to

type 1a supernovae being called cosmic mile markers and standard candles.

NO PLACE LIKE HOME: August 28, 2014

Christmas 2013 2015-01-23 Page 1 of 15

In the late 1990s, distance measurements based on type 1a supernovae revealed that the expanding universe is accelerating. In other words, its flying apart more quickly

now than it was billions of years ago. Scientists still dont know exactly whats going on, but they attribute the phenomenon to an enigmatic thing called dark energy. The

discovery represented a fundamental shift in cosmology and earned the Nobel Prize in physics in 2011.

But heres the thing: Despite their crucial cosmological importance, type 1a supernovae are still very much a mystery. As astronomers study more and more of them, its

becoming increasingly clear just how non-standard these explosions actually are and how little we really know about them.

Theyre standardizable candles, not standard candles, astrophysicist Brad Tucker told me a bit ago, while I was working on a feature describing type 1a supernovae for

the Proceedings of the National Academy of Sciences. Tucker splits his time between UC Berkeley and the Australian National University.

These are very powerful tools in cosmology, he said. But we really dont know whats going on with them.

Its true. The uncertainties swirling around these fascinating explosions are kind of astonishing. Here are a few.

1. Until now, there was no proof that white dwarfs were doing the exploding.

For starters, we didnt have solid observational evidence pointing to white dwarfs as the culprits behind type 1a supernovae until earlier this year, as reported yesterday in

the journal Nature. Decades of solid theoretical work (and circumstantial evidence) suggested as much, but the observations werent there to back it up.

But in January, a star exploded in the Cigar Galaxy. Essentially next door at only 11.5 million light-years away, it was the closest type 1a supernova to Earth in four

centuries. Chemical signatures in the billowing debris cloud revealed that supernova 2014J, as its called, is a type 1a supernova. Because the explosion was so nearby,

astronomers were able to detect gamma-rays coming from the debris, a type of radiation that hasnt been observable in other type 1a supernovae.

Simulations of type 1a supernovae predicted that exploding white dwarfs would produce gamma-rays, but the particles dont normally make it to Earth.

Yet earlier this year, a team of scientists observed gamma-rays coming from 2014J with the European Space Agencys INTEGRAL satellite.

Theres controversial evidence for the presence of an ex-companion star in Tychos supernova remnant. The explosion happened in 1572. (NASA/CXC/Chinese Academy of Sciences/F. Lu)

Supernova 2014 exploded in the Cigar Galaxy earlier this year, affording astronomers their closest look at a supernova in three decades. (NASA/ESA/Hubble Heritage Team)


Using pathways that explain how gamma-rays are formed from the synthesis of iron, cobalt, and nickel, the team worked backward from the detection and determined

what kind of star had exploded. It was a white dwarf star, scientists said, with about 1.4 solar masses of material stuffed into it.

This is the most solid observation to date that implicates detonating white dwarfs in the production of type 1a supernovae, and one that astronomers have been eagerly

awaiting, for a long time.

The importance of this discovery is not because something new/unknown was discovered, but we had an observation of a long-standing theory that had no real evidence,

Tucker wrote to me a few days ago. Knowing that our fundamental physics is correct is an important thing, especially given how all our other ideas (progenitors systems,

mass, donor stars) dont seem to be working!

(Its worth mentioning that another recent nearby supernova 2011fe, which exploded in the Pinwheel Galaxy in 2011 provided fairly good evidence for a white dwarf

being the progenitor star. Those constraints came from the PIRATE telescope in Mallorca, which serendipitously observed the supernova several hours after it exploded.

Because the billowing debris cloud wasnt yet visible, astronomers concluded the explosion must have been the work of a white dwarf. But there was still wiggle room. Its

the most compelling, by a reasonable margin, astronomer Alexei Filippenko of UC Berkeley said of the evidence late last year, when were discussing this point for the

PNAS feature. That was before 2014J exploded.)

2. The Chandrasekhar Limit sometimes isnt.

Secondly, theres a well-known fact that commonly appears when people write about type 1a supernovae: That white dwarfs explode when they get to be about 1.4 times as

massive as the sun a mass known as the Chandrasekhar Limit. All of this is true, but its not the whole story.

White dwarfs are incredibly dense, dead stars, formed from the collapse of stars that were once very much like the sun (and yes, our sun will become a white dwarf).

Theyre about the size of Earth, but with a suns mass of material squeezed in. Most of the time, white dwarfs can happily exist in this state for billions of years.

Thats because the white dwarfs intense, crushing gravity is counteracted by a quantum mechanical thing called electron degeneracy pressure, which basically prohibits

electrons from being shoved any closer together. In other words, degeneracy pressure prevents the star from collapsing further. (Degeneracy pressure is the reason why

white dwarfs are called degenerate stars.)

These two competing forces can keep a white dwarf stable forever, as long as it doesnt get too massive. But if the star gains enough material and exceeds about 1.4 solar

masses, gravity normally wins and degeneracy pressure fails and all hell breaks loose.

Thats the Chandrasekhar limit. And its important, but its not the only thing that lights up a supernova.

Something else happens at around 1.4 solar masses, and it turns out that this is a crucial part of exploding the star: At this point, the star is massive enough to begin fusing

carbon and that is what ignites the runaway thermonuclear reaction.

Fusion happens in a flash, astronomer Robert Kirshner of the Harvard Smithsonian Center for Astrophysics writes this week in Nature. A thermonuclear flame rips

through the white dwarf, fusing carbon into heavier elements with a sudden release of energy that tears the star apart.

Many white dwarfs are made from a ton of carbon and a lot of oxygen. So when carbon ignites, it just keeps going and the star is blown to bits. That this happens around

the Chandrasekhar limit is kind of a coincidence, says astronomer Ryan Foley of the University of Illinois, Urbana-Champaign. The Chandrasekhar limit is a red herring.

Its a physical thing thats very, very important, but for type 1a supernovae, its not the most important thing, he told me, when we were talking about supernova 1as for

the PNAS feature. You have to have the carbon ignition, somewhere near the center of the star.

AND ANOTHER THING ABOUT THAT: Astronomers have evidence for type 1a supernovae born from white dwarfs that exceeded the Chandrasekhar limit. By a lot. These

type 1a explosions, called super-Chandras, are so ridiculously, anomalously bright, and kick out so much radioactive nickel, that they could only come from a bulked up,

beefy dwarf star something with 1.6 or 1.8 or even more than two solar masses of material.

The remnant of a type 1a supernova that exploded in the year 1006. (NASA/CXC/et al. Click for full credit.


The first of these super-Chandras was discovered in 2003; several more have been seen since then, including supernova 2007if and supernova 2009dc.

So there are mechanisms that allow white dwarfs to bypass the Chandrasekhar limit (astronomers theorize that something like a very rapid spin rate might help the star

avoid catastrophic collapse) and produce explosions that are anything but standard.

Conversely, there are type 1a supernovae that are ridiculously, anomalously dim. These mini-supernovae, discovered in 2013, are called type 1ax explosions. Astronomers

have spotted about 30 of them.

Two weeks ago, Foley and his colleagues reported in Nature that theyd found the progenitor system for a type 1ax called SN 2012Z: A white dwarf, paired with a bright

blue helium star companion. The dwarf snagged some material from its gassy blue friend, the team says, but didnt gain enough mass to completely explode. So instead of

a bang, the star let out a whimper.

Scientists sorted this out by staring at old Hubble Space Telescope images of the supernovas home and determining which stars were involved in the eventual

conflagration. Its the first time anyone has been able to so precisely see a progenitor system before it blew up.

Astronomers affectionately refer to the collection of anomalous supernovae as weirdos. By studying the weirdos, they hope to better understand the normal type 1as, the

explosions that are useful for things like cosmology.

By identifying and studying the extreme cases, we can hope to learn not only about these interesting weirdos, but the weirdos might end up teaching us something about

the more normal ones, Filippenko said.

3. We dont know whos helping to kill the dwarf.

But is there a type of normal supernova 1a? Wellyes. Kind of.

For a long time, astronomers figured that since 1as were all so similar, there must be just one way to cook them up. A key step in the process of making a 1a is that the

white dwarf has to somehow grab enough material to ignite carbon fusion and explode.

This means that the white dwarf cant live on its own. It needs to live in close proximity to a stellar friend it can steal from. As the two stars orbit one another, the dwarfs

intense gravity will siphon material from its companion, and it will eventually gain enough mass to detonate.

For decades, astronomers thought that companion star was something like a red giant, something big and gassy thats easy to steal from. Many textbooks and diagrams

still depict this kind of supernova precursor, called a single-degenerate system (because theres only one degenerate star, the white dwarf).

And there is some recent observational evidence implicating this lethal pairing in type 1a supernovae.

The remnant of a supernova called PTF 11kx contains shells of gas that can only have come from a large, red giant companion, scientists reported in Science in 2012. And

the paper published this week about 2014J suggests that it came from a white dwarf-red giant pair.

But as with everything were learning about type 1a supernovae, those single-degenerate systems are not the only game in town. Now, many astronomers think those

systems produce a minority of type 1a supernovae and that most of the explosions we see come from a different deadly combination.

Less than 1 percent of all 1a supernovae could be from a companion red giant star, Tucker said. Its a dramatic shift in what were thinking. Its gone from the most

popular scenario to one no one wants to touch.

Supernova 2011fe exploded in the Pinwheel Galaxy, just 21 million light-years away. (Image by B. J. Fulton, Las Cumbres Observatory Global Telescope Network.)


More

Recently, evidence has been amassing that suggests type 1as result from a pair of two white dwarf stars (a double-degenerate system). So, instead of being locked in a

deadly dance with a red giant, the white dwarf is dancing with another white dwarf.

This kind of progenitor system was discounted years ago because explosion stimulations couldnt quite make it work scientists had trouble bringing the white dwarfs

close together quickly enough, or couldnt get the variety of elements produced by the explosion to come out right.

But in the last five or 10 years, thats changed. And a variety of new observations point toward double degenerate systems as producing type 1a supernovae.

These include studies of supernova remnants where astronomers have failed to find the large ex-companions that should still be visible (this kind of star would survive the

supernova and be detectable, whereas a second white dwarf would also be obliterated); surveys that look at the gas outflows produced by type 1a supernovae and find only

scarce evidence for the expected bits of red giant; and observations of exploding supernovae that show no evidence for what astronomers call a shock breakout, or a kind

of flare produced by a big star getting in the way of the billowing debris cloud. (And theres another recent paper that suggests 2014J came from a precursor system with

two white dwarfs.)

Now, it seems, instead of arguing about which of the two progenitors channels is The One, astronomers are debating how many different type 1a progenitor systems exist,

and how common each of them is. This bothers some scientists, who are having a tough time reconciling the observed similarities in normal type 1a explosions with the

fact that so many different ingredients could be involved.

Somehow, these completely different stellar systems that have evolved in completely different ways and are completely different at the end of their lives when the star

explodes make things that are almost identical, Foley said. That troubles me a bit.

Ultimately, it will be really important to sort this out because the more astronomers know about type 1a supernovae, the more accurately theyll be able to measure cosmic

distances. Even the normal type 1as the ones used for cosmology explode with varying brightness. Their explosions vary depending on what kind of galaxy theyre in,

where in the galaxy they are, and, potentially, which progenitor system is involved.

So that whole predictable brightness thing? Its not quite that simple.

4. Uh-oh. Does this mean dark energy is going away?

Nope. Dark energy and the accelerating universe are here to stay. But this is an era of precision cosmology, and tiny inaccuracies can have a big effect on what scientists

understand about the repulsive force that is dark energy.

This is especially true for the supernovae scientists are really hoping to detect the really, really ancient ones that can help nail down the behavior of dark energy in the

very early universe.

The question is, are those supernovae really the same as the ones nearby? This issue of progenitors really affects that, said Saurabh Jha, an astrophysicist at Rutgers

University.

Ten billion years ago, the universe was filled with a different population of stars than we see today. The matter making up those stars had different chemical compositions.

And if those ancient supernova 1as are behaving differently than the more recent ones, scientists need to know about it. Otherwise, imprecise distance measurements will

yield an inaccurate understanding of what was happening during these earlier time periods.

Long live the standard candle.

There are 8 Comments. Add Yours.

1.

Scientists couldnt find evidence for a large, red giant companion star in supernova remnant SNR0509-67.5, in the Large Magellanic Cloud. (NASA/CXC/SAO/J.Hughes/ESA/Hubble Heritage Team)

The heavier elements which arent form solid stuff of the star will come to disintegration when the pressure and temperature within its volume reach the following values: 2.10^(27 ) atm.[kg/cm^2 ];3.10^11K, see USM http://www.kanevuniverse.com The same is corresponds to the evolution of the neutrons stars. When the fuel of the some star comes to its end, which means that not only the thermonuclear synthesis of hydrogen atoms already is spend, but the thermo nuclear synthesis of more heavier atoms also is on its end, then the star begins to collapse (to shrinking) and when the above values of the pressure and temperature are reached, then these atoms come to disintegration finally again into hydrogen atoms, but old ones see USM http://www.kanevuniverse.com After that again according to part I of the theory the pressure and temperature begin to decreases and again is ignites the thermonuclear synthesis, the star begin to expand reaching the pressure and temperature above which this reaction is possible: 2.10^9 atm.;10^7K Below these parameters the thermonuclear reaction stops and the star disperse itself. Why the gravitation field cannot stop this process? Because the star is already on the periphery of its galaxy where it hasnt necessary resonance radius to spring up again, see page 80, 81, 82, 83 USM http://www.kanevuniverse.com where is calculated the radius of birth of the Sun and why the stars are birth on the center of their

George Kanev August 29, 2014


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own galaxies. So there is one exception if the observed star is very massive one, then if the resonance radius where the star is situated during its collapsing is enough short then there can be born some little star which its own centripetal acceleration correspond to their current parameters: velocity of bird and radius of bird. However, most probably the star springs up like supernova and after that die. What will happen with dispersed from such star old protons and old neutronsthey also come to disintegration into hypothetically substars and subplanets from the subspace see USM part I, part II and superconductivity http://www.kanevuniverse.com

so what happens to the companion star that doesnt explode? does it get ejected and hurtle through space?

(ND: Yup! That is pretty much the idea. The scientists looking for these stars sometimes describe their job as looking for funky stars.

Essentially, if the ex-companion star is big enough, it will survive the white dwarfs explosive death and be punted into space. Scientists suspect that these ex-companions will be traveling with peculiar speeds and spins, and could be marked with chemical signatures suggesting close proximity to a type 1a supernova.

The other star is pretty close to one of the most energetic events in our universe. It should look different. That probably wasnt very comfortable for the star, Wolfgang Kerzendorf, an astronomer at the University of Toronto, told me when I working on the PNAS feature.

Kerzendorf and his colleagues have searched for these ex-companions in the nearest type 1a supernova remnants and havent found anything that looks like a funky star.)

nuwan

August 29, 2014

So then, how common would white dwarf-red dwarf pairs be? Red dwarfs are the most common star are they not? Would that not make that kind of pairing common also? You wouldnt see a remnant since the RD would be destroyed, is there any evidence either way on this?

Kudzu September 1, 2014

Hi Nadia, in response to your answer to my previous question,

I guess there is the remote but scary possibility of an ex-companion running into another solar system.

Or is it more likely that if it does come into the proximity of another solar system it will feel the gravitational effects of the native star and form something like a new binary system?

nuwan

September 1, 2014

The red dwarf ex-companion to the white dwarf gets blown out, just like a candle in the wind. The resultant dark matter finds some place to cling to. Sounds very off the mark, but life (or death in theses cases) are full of surprises! Good article.

Patrick OConnor September 10, 2014

Im thinking that red dwarfs are going to be quite unlikely and rare to be the companion stars for Type IA supernova, for two reasons.

1. The average red dwarf is 10-20% of a solar mass. A white dwarf has to cross 1.4 solar masses before the Type Ia supernova happens. The average white dwarf on formation is about 0.8 solar masses. So it could consume the entire red dwarf companion and it wouldnt be enough material. You have to start out with the very biggest, and thus rarest, white dwarfs to even have a chance.

2. Red dwarfs are tiny, compact stars, and as far as we know, they dont evolve into a red giant phase. Even if were wrong about that and they do, the universe isnt old enough for any of them to have done so. The red giant phase, where the outer layers of the star puff out hugely, allows for a much wide range of orbits for the white dwarf star to be in to be able to siphon material many such hypothesized scenarios would actually have the white dwarf orbiting INSIDE the outer envelope of the companion star. That wont happen with a red dwarf, so youre restricted to just the tightest, most close-in orbits, and those too will be rarer.

It may well be that red dwarf-white dwarf pairs forming type Ia supernovae are morse likely to do so by colliding/merging than by the white dwarf siphoning material from the red dwarf.

But it does raise this interesting question if a white dwarf really was siphoning material from a red dwarf companion, since red dwarfs are so close to the hydrogen fusion limit already, is it possible for the red dwarf to drop BELOW the mass limit and stop hydrogen fusion, and be turned into a brown dwarf? Would this count as a mechanism of star death, since cessation of core fusion is what defines a star dying?

And, since the reason we think red dwarfs will not have a red giant phase is because they are wholly convective without a radiative zone in their atmospheres, meaning they will fuse all their hydrogen before they die, unlike larger stars which will only fuse their core hydrogen, would the abrupt termination of core fusion by mass loss while substantial hydrogen remains in the stars outer layer result in a core collapse followed by hydrogen shell fusion (not big enough for helium flash, I think) which will result in a mini-red giant outer shell expansion after all?

Amphiox September 17, 2014

Indeed red dwarfs shouldnt have a red giant phase unless they are the largest and rarest kind, so siphoning material is out. For red dwarfs there would be little difference between siphoning and merging, they would need to be quite close to the white dwarf either way.

However if white dwarf mergers are common then the entire mass of a star can be used at once instead of being siphoned. If I had been told about dwarf mergers being the possible major causes of type 1as without any scientific backing I would have dismissed the idea as wildly improbable.

Given this possibility, red dwarfs are massively common, even the heavier kinds and they certainly form a good number of binary systems red-white mergers must therefore not be a rare occurrence. The only question is if they pack enough oomph.

Kudzu September 17, 2014

Dear Caroline,I am a 55 year old astronomy buff who saw your story on the Rachel Maddow show tnioght. You are AWESOME! Wow! What a great piece of work.My daughter just graduated high school and was accepted into the US Naval Academy. That was phenomenal in itself. But to see another young person who can achieve so much at such a young age gives me great hope for our future. I congratulate you on your find. I also hope they call it Caroline. Good luck in your future endeavors and keep up the good work. Dan in AZ

Anita September 28, 2014

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Nadia Drake is a science journalist who grew up thinking about cosmic questions and staring at Saturn through giant telescopes. This is her space to talk about space -- from

other worlds to exploding stars to the fabric of the universe.

Her work has also appeared in Science News, Nature, New Scientist, the Proceedings of the National Academy of Sciences, and WIRED. She lives in beautiful, foggy San

Francisco.

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Nadia Drake, Type 1a Supernovae - Why Our Standard Candle Isnt Really Standard