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White dwarfs mean double trouble for supernovae
Posted: 13 May 2012

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RS Ophiuchi An artwork depicting the binary star system RS Ophiuchi, which comprises a white dwarf inside the outer atmosphere of a red giant. Will the white dwarf explode by accreting matter from the red giant? Or will it wait for a second white dwarf to emerge from the red giant before merging with it? Image: STFC/David Hardy.

There's more than one way to skin a cat and, now, it seems there's more than one way to create a supernova involving a white dwarf star, according to a new study by a multi-continental team of astronomers. Their results challenge not only traditional theory, but have implications for some of our most profound astronomical investigations - the nature of dark energy and the expansion of the Universe.

Supernovae - exploding stars - come in a variety of breeds, distinguished by the fine details of their spectral lines but, broadly, they've been split into two camps: massive stars that collapse and go boom, and white dwarfs that gather too much matter from a companion star and end up igniting in a thermonuclear explosion. Until now the nature of the companions has been unclear, but new analysis seeking to determine whether gas is present around these supernovae suggests that at least a quarter are caused by a Sun-like or red giant companion star accreting onto the white dwarf. Meanwhile, the remaining three-quarters may be caused by two white dwarfs merging.

White dwarf supernovae are described in astronomical nomenclature as type Ia supernovae, which are absent of any hydrogen in their spectra. This makes sense; white dwarfs are the remains of stars that were once like our Sun but which have evolved into old age, expanded into a red giant and died, leaving behind a cooling carcass in the form of their hot white dwarf core. All stars are giant natural fusion reactors, constantly churning hydrogen into helium and then into heavier elements such as carbon and oxygen. By the time of the star's red giant phase, the core is heavily processed through the fires of stellar nucleosynthesis, whereas the outer layers, which are gradually puffed away, are still dominated by hydrogen. This leaves behind cores that are deficient in hydrogen but rich in helium, carbon and oxygen.

SN2011fe-in-Pinwheel Type Ia supernova 2011fe, which exploded in M101, the Pinwheel Galaxy, last year. It was swiftly ruled out as being single degenerate. Image: Palomar Transient Factory/B J Fulton/Las Cumbres Observatory Global Telescope Network.

White dwarfs, after they emerge from the cocoon of their dying star, are typically less massive than the Sun (although this varies depending upon the mass of their progenitor star). At this stage, with their dearth of hydrogen, they are fairly inert as their densities are far too low for further nuclear fusion. Instead, they can look forward to cooling off from average temperatures of 100,000 degrees Celsius down to near absolute zero, a process that could take 15 billion years, longer than the current age of the Universe.

Mass is everything when it comes to stars and supernovae, and so it is with white dwarfs. Feed a white dwarf raw gas, fattening it up beyond 1.44 times the mass of the Sun, and you'll produce a supernova. Beyond this mass, known as the Chandrasekhar Limit, densities inside the white dwarf are high enough to ignite a chain reaction, a runaway process of nuclear fusion that utterly annihilates the white dwarf in a huge explosion - a type Ia supernova. Because this mass limit is ubiquitous, type Ia supernovae should all be of the same intrinsic brightness, making them perfect distance markers or 'standard candles' by which cosmologists can measure the expansion rate of the Universe at different epochs. Indeed, such a procedure uncovered the mysterious dark energy that is instigating an acceleration in the Universe's growth.

The trouble is, we've always been a little uncertain as to the process by which a white dwarf breaches the Chandrasekhar Limit and, in that context, it is somewhat surprising that we've placed so much faith in their role as standard candles. In recent years questions have been raised as to the veracity of the idea that type Ia supernovae all explode with the same luminosity. A new analysis of 23 type Ia supernovae by a team led by Ryan Foley of the Harvard-Smithsonian Center for Astrophysics and astronomers from as wide a field as Israel, Germany and Chile has now bolstered this uncertainty by linking the presence of gas outflows around type Ia supernovae to the properties of two different kinds of white dwarf explosions.

The first type, they call single degenerates. In other words, a single white dwarf accreting matter from a Sun-like companion star - the traditional view of type Ia supernovae. In these cases, gas outflows from the companion star's stellar winds should be present around the supernova and visible as lines in the supernova's spectrum.

The second type are the double degenerates, which have no circumstellar gas. These, argue Foley's group, are produced by two white dwarf stars merging.

They found that the fraction of type Ia supernovae with gas was at least 25 percent, which allows some broad constraints to be put into place. "What we can say with confidence right now is that no more than 75 percent of type Ia supernovae could come from double degenerates," says Josh Simon, a member of Foley's team from the Carnegie Observatories in Pasadena, California.

It is not unrealistic to expect two white dwarfs to be found orbiting one another; indeed, there is the statistic that the number of multiple stars systems in the Galaxy outnumbers the quantity of lone stars. Marten van Kerkwijk, a professor from the University of Toronto, has been championing white dwarf mergers for type Ia supernovae for some time, and sets out the recipe for making binary white dwarfs.

It starts with two Sun-stars of similar but not necessarily identical heft, separated by a good distance - perhaps one astronomical unit, the distance between Earth and the Sun. The more massive star will exhaust its hydrogen fuel first and expand into a red giant, ultimately leaving behind a white dwarf. Then, sometime thereafter, the second star treads the same evolutionary path, swelling into a red giant that encompasses the orbit of the white dwarf. This instigates a process of mass transfer between the two, reducing the white dwarf's angular momentum and causing it to begin spiralling toward the centre of the red giant. By the time the red giant's outer layers are cast off, leaving behind a second white dwarf, the two are on a collision course. Depending upon the comparative masses of the white dwarfs, one can either be torn apart by the gravitational tidal forces of the other to form an accretion disc from which matter falls onto the remaining white dwarf, or the two can collide and merge completely. Either way, the result is the same, says van Kerkwijk - a catastrophic explosion.

whitedwarfseq A series of illustrations showing two white dwarfs spiralling into one another, merging and then exploding. As they spiral around each other, they emit gravitational waves, hinting at a possible future detection method for before they explode. Image: GSFC/Dana Berry.

A white dwarf merger leads to a different set of circumstances than accretion from a normal star. For starters, double degenerate supernovae pack a slightly lesser punch, but nobody knows for sure why. Perhaps it is related to the composition of the material from the normal star in the single degenerate model, or perhaps merging white dwarfs produce supernovae that are even more asymmetric than normal, so we don't see all their power.

The physical processes by which they explode are also different in van Kerkwijk's picture, which is where his model really diverts from the traditional theory. After they have merged, whatever their combined mass, a supernova explodes, he proposes. "The argument is that in the phase after the merger, the interior becomes hot enough to trigger nuclear fusion, hence the Chandrasekhar mass is irrelevant," he says. Indeed, van Kerkwijk believes that almost all type Ia supernovae are produced like this, because the statistics just don't stack up otherwise. He argues that the number of white dwarfs born close enough to the Chandrasekhar Limit for accretion from a normal star to tip them over the limit is too few to match the observed rate of type Ia supernovae. There also appears to be a large range of energies, belying the type Ia's standard candle status, while evidence for supernova shockwaves smashing into companion stars are scant.

"The merger idea resolves these problems: there are enough progenitors and one now has a natural reason for the range in supernova properties as mergers of more massive white dwarfs will lead to more luminous explosions," says van Kerkwijk.

Although Foley and Simon's results indicate that single degenerates make up a significant portion of type Ia's, double degenerates also remain an important contributor. So does van Kerkwijk view the new results as a boon for his theories, or as an obstacle?

"In my picture the discovery of a circumstellar medium in a fair fraction of type Ia supernovae is surprising," he says. "I think their evidence for circumstellar material is strong, but it still constitutes only plausible indirect evidence for a normal companion. However, in the end, it may disprove my picture."

Which is how science works - evidence over supposition - but if nothing else it is looking increasingly like there is more than one way to make a type Ia supernova. As a consequence, could this spell trouble for the ways in which we utilise these phenomena?

In 1997 two competing teams - the High-z Supernova Search Team led by Brian Schmidt and Adam Riess, and the Supernova Cosmology Project championed by Saul Perlmutter - discovered that light from type Ia supernovae in faraway galaxies had been redshifted by a greater extent than they should have been for their distance. This implied that not only were the galaxies that housed these supernovae moving away from us as the Universe expanded as predicted by big bang cosmology, but that they were moving away at ever increasing rates. In other words, the expansion of the Universe is accelerating and the mysterious driving force behind this has been labelled dark energy. Nobody knows what dark energy is, but using type Ia supernovae as standard candles to measure the expansion at differing epochs over the history of the Universe enables astronomers to fathom more of dark energy's properties and hopefully, ultimately, converge on a satisfying explanation for it. Yet if there is more to type Ia supernovae than meets the eye, where does that leave dark energy studies?

eso0644a_400 Artwork depicting the asymmetric explosion of a type Ia supernova. Could double degenerates have greater asymmetry? Image: ESO.

Before we begin decrying dark energy, rest assured that uncertainties in white dwarf supernovae do not threaten cosmology in any major way. "Because we know that single degenerate and double degenerate explosions must be similar, this kind of change certainly does not affect the existence of dark energy or its present day energy density," says Simon.

Mark Sullivan, a cosmologist from the University of Oxford, concurs. "We've known for 20 years that fainter type Ia supernovae explode in older, or more massive, host galaxies," he says. "It's not clear why this is, but we correct for this during the cosmological analysis. After we correct for effects like this, we see no real evidence that the luminosity of the supernovae have any further dependence on any other variable we can measure. So, if the diversity of progenitor type does have an impact on the luminosity of supernovae, then we must already be correcting for it."

Van Kerkwijk's model even has an answer to the old galaxy/faint supernova conundrum. "In my picture, the reason is that older stars produce less massive white dwarfs and, on average, the mergers that occur will also involve less massive white dwarfs and thus produce less energetic explosions."

Either way, dark energy seems safe. Indeed, its presence was never very much in doubt anyway given an independent measure of its existence comes in the form of standard rulers, such as galaxy clusters, whose growth subject to the expansion of the Universe can be charted from epoch to epoch, dating back to the faint glow of the cosmic microwave background to the present day's immense conurbations of galaxies. However, caution must prevail as we move towards a future where one hopes we can solve the dark energy mystery. As Sullivan points out, over the coming decades dark energy surveys will grow ever more sensitive and subtle effects hitherto unnoticed in the data may make themselves apparent. For instance, points out Simon, suppose the fraction of type Ia supernovae that are single or double degenerate changes with time. "If ten billion years ago the mix was 50:50 as opposed to 25:75, then we wouldn't quite be comparing apples to apples, which might introduce subtle errors into measurements of the evolution of dark energy over the age of the Universe," he says.

A healthy area of research is one where ideas are plentiful, and that can certainly be said for the evolution of white dwarfs and type Ia supernovae. Single or double degenerate, the type Ia supernova story is not over by a long shot, although huge strides are nevertheless being made. "We've learnt more about the progenitors of type Ia supernova in the last year or so than we learnt in the previous few decades," says Sullivan. It's a very exciting time and I'm personally convinced that there is more than one progenitor system for making a type Ia supernova."

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