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Merging White Dwarfs Set Off Supernovae

Composite image of M31.  Inset shows central region as seen by Chandra. Credit: NASA/CXC/MPA/ M.Gilfanov & A.Bogdan;

New results from the Chandra X-Ray Observatory suggests that the majority of Type Ia supernovae occur due to the merger of two white dwarfs. This new finding provides a major advance in understanding the type of supernovae that astronomers use to measure the expansion of the Universe, which in turns allows astronomers to study dark energy which is believed to pervade the universe. “It was a major embarrassment that we still didn’t know the conditions and progenitor systems of some the most spectacular explosions in the universe,” said Marat Gilfanov of the Max Planck Institute for Astrophysics, at a press conference with reporters today. Gilfanov is the lead author of the study that appears in the Feb. 18 edition of the journal Nature.

Type Ia supernovae serve as cosmic mile markers to measure expansion of the universe. Because they can be seen at large distances, and they follow a reliable pattern of brightness. However, until now, scientists have been unsure what actually causes the explosions.

Most scientists agree a Type Ia supernova occurs when a white dwarf star — a collapsed remnant of an elderly star — exceeds its weight limit, becomes unstable and explodes. The two leading candidates for what pushes the white dwarf over the edge are the merging of two white dwarfs, or accretion, a process in which the white dwarf pulls material from a sun-like companion star until it exceeds its weight limit.

“Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging,” said co-author Akos Bogdan, also of Max Planck. “This is probably not what many astronomers would expect.”

The difference between these two scenarios may have implications for how these supernovae can be used as “standard candles” — objects of a known brightness — to track vast cosmic distances. Because white dwarfs can come in a range of masses, the merger of two could result in explosions that vary somewhat in brightness.

Because these two scenarios would generate different amounts of X-ray emission, Gilfanov and Bogdan used Chandra to observe five nearby elliptical galaxies and the central region of the Andromeda galaxy. A Type Ia supernova caused by accreting material produces significant X-ray emission prior to the explosion. A supernova from a merger of two white dwarfs, on the other hand, would create significantly less X-ray emission than the accretion scenario.

The scientists found the observed X-ray emission was a factor of 30 to 50 times smaller than expected from the accretion scenario, effectively ruling it out.

So, for example, the Chandra image above would be about 40 times brighter than observed if Type Ia supernova in the bulge of this galaxy were triggered by material from a normal star falling onto a white dwarf star. Similar results for five elliptical galaxies were found.

This implies that white dwarf mergers dominate in these galaxies.

An open question remains whether these white dwarf mergers are the primary catalyst for Type Ia supernovae in spiral galaxies. Further studies are required to know if supernovae in spiral galaxies are caused by mergers or a mixture of the two processes. Another intriguing consequence of this result is that a pair of white dwarfs is relatively hard to spot, even with the best telescopes.

“To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist,” said Gilfanov. “Now this path to supernovae will have to be investigated in more detail.”

Source: NASA


Nancy Atkinson is currently Universe Today's Contributing Editor. Previously she served as UT's Senior Editor and lead writer, and has worked with Astronomy Cast and 365 Days of Astronomy. Nancy is also a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Hon. Salacious B. Crumb February 19, 2010, 3:14 PM

    Aodhhan said;

    What I said is on. If you are using outdated information, this is your own ignorance.
    This is the problem between someone who actually has their own information, and someone who attempts to interpret others work. You get half of it and misinterpret the rest.

    Your own here ignorance is no excuse. If read what I said, I did say that “Much work has been advanced in the topic on close binaries and the restrictions.” Most of Iben’s 1985 paper, that I highlighted, actually still hold true.
    The only reason why I posted it is that it explains the basics on stellar evolution, and may guide others to understand the topic a little better.
    As for your continued lame attacks on what I present, well really who cares. Clearly it is your own comprehension on science and how it works need to learnt or possibly be brushed up on. After reading your responses here, your knowledge on general stellar evolution is obviously lacking somewhat. Just by sillily writing “1aSN” says it all!

    However, there is one other important point that probably really needs to be clarified here – stems from a misinterpretation from the previous brutal encounters with Anaconda and the EU lot. Here these individuals were all actually attacked and berated on producing old out of date papers from Perratt, Alfen, etc. The issue at the time was not really that of the content of these papers, but their ideas expressed by these authors haven’t been built upon, and more recent knowledge shows these views are mostly incorrect.
    In the case of the articles of Iben here, most of what he states is still part of theory, that has been greatly expanded upon in recent years. Much of what we know about SNI comes from observation of spectra and light-curves of many events. I.e. The actual accretion model, for example was formulated in 1973. The binary star one a few years later.
    As of right now both these models are still plausible and still do meet most expectations and predictions of standard adopted theory. A paper like this one just adds another likely chink in the story, but it does not just automatically supplant the accumulation of all previous knowledge.
    Clearly this work in this story still has holes – and some very big ones. I.e. The sample is only derived in elliptical galaxies, which were selected because they don’t have lots of hot gas. So the question remains: Is this a selection effect or common place in all galaxies? Even the authors admit more work has to be done; I.e. “Now this path to supernovae will have to be investigated in more detail.” In fact this avenue might lead to new understanding of these catastrophic events, it also might be a dead end.

    NOTE: So if you really think, say, Iben here is “out of date”, then you should explain why? (Just openly saying it does not make so.)

  • Hon. Salacious B. Crumb February 19, 2010, 5:14 PM

    Excalibur said;

    Agreed that it will be unlikely with 0.6+1.4Msun mergers, but at the same time it will be more unlikely with 1.1+1.1 mergers than it will be with 0.75+0.75Msun mergers, and possibly even with 0.65+0.8Msun mergers. Even while evolutionary mechanism favors similary weighted WD, that still does not have with the original article to do, that you where objecting about.

    You point here is quite valid. However, it is relevant to the discussion because of the stated misunderstanding of what “merging white dwarfs” actually means. Really I have only brought up the issue of the differences in he masses of the WD stars, because of the generally wrong assumption that they could be of any mass combinations.

    …but to clarify. The masses observed of course depend on the nature of the system itself. The reasoning behind the mass constraints is that the progenitor stars must be close enough so that when they convert into WDs the orbit can degrade enough so that they can merge together and form a SNIa. This maximum distance is calculated to be up to about 3 stellar radii, and this in turn means the stars will have a set maximum mass. (Any bigger, and the stars would have to be larger than the limit of 3 radii)

    The second reason why more massive WD are less likely is that massive stars are more rarer than less massive stars. Hence, 1.1 SolMass WD duos are less likely than 0.8 SolMas duos. (We can also know this from eclipsing binaries. There are many examples of the W UMa eclipsing binary variables, whose masses are >0.6 solar masses and if merged, probably forming the R Corona Borealis variables, and are never massive enough when combined to form a SNIa.

    A third point to raise is how WD merge together. They do not crash into each other, so to speak, like two solid objects I.e. like round Billiard or Pool balls. When they approach each other they form “The heavy disk phase” becoming into a Neptune-sized single-like object with heavy ring (something like an a double yoke egg or chocolate with a double cream centre, as my astronomy lecturer once described.) For me it is like a single ravioli, whose centre contains the two WD and the edges of the paste the outlying heavy ring. (Saturn on steroids!)
    The surfaces of the white dwarfs merge together until the 1.4 SolMass Chandrashekhar limit is reached, where is WDs collapses and forms into the SNIa.

    oxygen-neon-magnesium white dwarf would be the correct full term, sorry for being to lazy to type all that out. I didnt realise it would become an issue to you.

    I think you misconstrued my point, and I meany absolutely no criticism of you. (Sorry if I gave that impression) So actually it is no issue at all.

    Note: Several sources (including Wikipedia) have also made this same statement about magnesium too. In the literature most refer to ONe WD, mostly (I think) just to distinguish between CO (Carbon Oxygen) and He WD (Helium.) [I.e. See Figure 19 on Page 31 of Iben 1985 QJRAS paper, as I linked for you previously.]

    Afaik Deflagration versus Detonation discussion have not settled yet, but i can be wrong. Are you telling me it is definitely a Detonation

    Same word, similar meaning ;

    Deflagration, meaning; “combustion that propagates through a gas or across the surface of an explosive at subsonic speeds, driven by the transfer of heat”

    Detonation, meaning; “combustion of a substance that is initiated suddenly and propagates extremely rapidly, giving rise to a shock wave.”

    Deflagration is a term used in the US army to destroy munitions (especially mines), hence why I asked about your possible. military association. For most the term is quite interchangeable, however, the word detonation implies the expanding shock wave after the bounce that we see as the consequence of the visible exploding SNIa. The energy comes from the degeneration of the protons and electrons into neutrons.
    In the end, I suppose it comes down to whether you are taking about before or after the SNIa collapse.
    An interesting point, though!


  • Hon. Salacious B. Crumb February 20, 2010, 12:28 AM

    Clearly I must be a complete idiot.

    You make criticism on one article and then, lo and behold, you hear about the same issues elsewhere. I.e. My response was on February 17th, 2010 at 4:08 pm, and the a few others also begin criticising it too.

    Examples include; S&T Site “Supernova Mystery Remains Just That”, as written by Robert Naeye, February 19, 2010

    The first statement by Robert says;

    Whenever I hear a claimed discovery that overturns conventional wisdom on some important aspect of astronomy, my skepticism meter goes on high alert. Such was the case on Wednesday, when I listened to a NASA press conference in which two astronomers based in Germany presented evidence arguing that the most popular model for Type Ia supernovae is incorrect, at least for elliptical galaxies.

    Indeed there does something quite wrong here.

  • Excalibur February 20, 2010, 2:42 AM


    Yes my statement about likelyhood of certain mergers were based on the statistical likelyhood of suitable stars. In terms of SNIa being standard candles, worst case scenario there are still limits, and best case scenario the total mass of the merger will not have a significant difference in the total energy output anyway.

    My comment on Deflagration versus Detonation refers to the discussion weather there is a subsonic or a supersonic combustion. You didnt quite answer it, or atleast not in the way i expected.

    1) Deflagration would be the subsonic (but still very quick) combustion of the WD that eventuall lifts degeneracy and expands it violently. This was atleast earlier the proposed mechanism

    2) Detonation would mean the combustion is supersonic (faster than the speed of sound in the degenerate mass), but from earlier discussions this was still expected to halt the collapse.

    3) You talk about a collapse and rebound that sends a supersonic shockwave outwards (hence this would be a detonation aswell)

    Of these 3, are you saying alternative 3) is now the favored one ?

  • Hon. Salacious B. Crumb February 20, 2010, 4:43 AM


    Ah, the eternal Hillebrandt & Niemeyer problem of 2000 that is cited nearly everywhere. I.e. The definitive paper being “Type Ia Supernova explosion models” (2000)

    Actually both these solutions have there pro and cons and have been debated for many years. The mathematical proofs are really difficult problems.

    A more recent 2010 paper entitled (arXiv 2009 version) “Turbulence in a three-dimensional deflagration model for type Ia supernovae: II. Intermittency and the deflagration-to-detonation transition probability” by Schmidt, W. et.al. discusses this very issue.

    …delayed detonations (see Hillebrandt & Niemeyer 2000, for a review of explosion scenarios) seem to be the the most promising way of modeling the majority of the observed events.

    They conclude;

    Once all numerical challenges are met, quantitative theoretical arguments in favor or against delayed detonations as an explanation for type Ia supernovae will be within reach.”

    To answer your question the deflagration likely is caused during the collapse of the white dwarf in the seven to eight seconds it takes to reach its highest density during the collapse, after which the star detonates when a certain density is reached. ( The Schmidt (2010) paper say at ~10^7 10,000,000 g.cm^-3!!) Some debate has it that the detonation begins during the collapse itself. Other argue it is likely an intermittent process..

    In fact, all three points you give are likely possible in the SNI event, and vary depending on the circumstances inducing the explosion. I.e. This is why it is called the deflagration- to-detonation (DDT) transition. Whilst the differences may appear minor between the accretion and binary models, the way they destroy themselves is far more complicated.

    Note: My apologies for being so flippant to your posts. I was trying to keep the discussion fairly simple. The deeper explanation of supernova Ia is often best to avoid as it leaves to too many debates.
    Truly excellent comments and much food for thought. It is very much appreciated. Cheers!

  • Lawrence B. Crowell February 20, 2010, 6:10 AM

    I am not familiar with the astrophysics of coalescing white dwarfs, but as Crumb points out the scenario it appears to be a comparatively “gentle” process. In other words the sort of adiabatic approximation used for an accreting WD is possibly more or less valid. Hence these variants of SNIs should be at least similar to the standard model of SNIs. I can’t comment on the relative frequency of these events, but I suspect they are a minority.


  • Jon Hanford February 20, 2010, 12:20 PM

    Perhaps professional opinion should be sought in this matter.

    Individual Researchers: Supernovae

    * Dr. A. Alberdi (Valéncia)
    * Prof. J.M. Blondin (NCSU)
    * Prof. E. Baron (Oklahoma)
    * Prof. D. Branch (Oklahoma)
    * Prof. S. Bruenn (Florida Atlantic University)
    * Prof. A. Burrows (Arizona)
    * Prof. R. Chevalier (Virginia)
    * Prof. J. Cowan (Oklahoma)
    * Prof. Robert Fesen, and here, too (Dartmouth)
    * Dr. B. Gaensler (MIT)
    * Prof. Mike Guidry (Tennessee and ORNL)
    * M. Hamuy (Carnegie Observatories)
    * Prof. W. Herbst (Wesleyan/VVO)
    * Dr. M. Herant (LANL)
    * Prof. C. Hogan (Washington)
    * Dr. David Jeffery (ORNL)
    * Wayne Johnson’s Mr. Galaxy’s Supernovae
    * László Kiss (JATE University)
    * K. Krisciunas (Washington)
    * Prof. J.M. Lattimer (SUNY-SB)
    * Prof. R. McCray (JILA/Colorado)
    * Dr. Anthony Mezzacappa (ORNL)
    * Dr. P. Nugent (LBL)
    * Dr. Juha Peltoniemi’s Supernova Neutrino Page (U. of Helsinki)
    * Dr. M. Phillips (Las Campanas Observatory)
    * Prof. S. Pineault (Laval)
    * Dr. P. Plait (GSFC)
    # Dr. M. Richmond (Rochester Inst. of Technology) and his list of SNe since 1989
    # Dr. Eduardo Ros Ibarra (MPIfR, Bonn)
    # Dr. P. Ruiz-LaPuente (Barcelona)
    # Dr. S. Ryder (UKIRT)
    # Dr. B. Schmidt (MSSSO)

    * L. Germany (MSSSO)

    # Prof. C. Stubbs (Washington)
    # Prof. P.G. Sutherland (McMaster University)
    # Dr. N. Suntzeff (CTIO)
    # Prof. F.D. Swesty (Stonybrook)
    # Dr. M. Pérez Torres (València)
    # Dr. S. Van Dyk (IPAC)
    # Prof. R.V. Wagoner (Stanford)
    # Dr. K. Weiler (Naval Research Laboratory)
    # Prof. S. Woosley (UC Santa Cruz)

    among others……. :)

  • Hon. Salacious B. Crumb February 20, 2010, 4:23 PM

    @ Jon
    Nice list.

    However, if you had your chance, what would you want to ask them?

  • Jon Hanford February 20, 2010, 5:11 PM

    First off, how independent, peer-reviewed research by you and others compares to that of researchers with many years in the field (experimentalists, theorists, and alike).

  • DrFlimmer February 21, 2010, 3:14 AM

    @ Jon Hanford

    If I were on that list, I wouldn’t understand the question 😉