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

Merging White Dwarfs Set Off Supernovae

Article Updated: 24 Dec , 2015

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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

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The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 17, 2010 4:08 PM
Really the worst astrophysics article I’ve read on Universe Today. The information lies clearly in the detail and most if what we know about SN Ia (or SN I) here have been glossed over or just reprocessed as “new.” I.e. “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.” OK. Who disagrees? (except perhaps those EU twits?) Really this statement was true even in the 1950s! [Zwicky, no doubt, would be turning in his grave.] As for; “To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appeared to exist,” said… Read more »
ESA Exile
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ESA Exile
February 18, 2010 12:17 AM
I’m only an amateur but isn’t the difference between type I and type II down to the presence of visible hydrogen spectra (Balmer series) in the type II indicating the presence of stellar outer layers and thus implying it to be a core collapse of a massive star? Type I has no hydrogen and so should be a stellar remnant; a white dwarf (or maybe two?). There are of course sub-species of each general type. How a white dwarf actually manages to reach the Chandrasekhar limit without first losing mass in a simple nova has always been a bit of a mystery to me. The main point of the article though is that the two proposed forms of… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 1:36 AM
ESA Exile said; “How a white dwarf actually manages to reach the Chandrasekhar limit without first losing mass in a simple nova has always been a bit of a mystery to me.” This is easy to explain. All novae are caused by the ignition of the dense outer hydrogen atmosphere surrounding the WD which reaches thermonuclear temperatures and explodes the atmosphere into the surrounding space. This is likely from a close companion feeding hydrogen to the white dwarf or hydrogen filtering from inside the white dwarf. Most white dwarfs are in the approximate range of 0.6 up to 1.4 solar masses, so the scenario for reaching the Chandrasekhar Limit is really a special, and much rarer circumstance. As… Read more »
Excalibur
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Excalibur
February 18, 2010 4:40 AM
@Crumb The article says: “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.” Seems quite clear to me that they have modelled the x-ray emission _prior_ to any actual SNIa using the two different scenarios. 1) SNIa are caused by merging binary WD, and… 2) SNIa are caused by slow accretion pushing a single WD over the chandrasekhar limit. Accretion scenario (2) as modelled would (as i read it) show more overall x-rays by a factor about 30-50x than is… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 6:08 AM
@ Excalibur Yes. I read that too. My point is; It could also mean, for example, that SNIa of that scenario might not be as common in elliptical galaxies, or that the proportion of “merging binary WD” to “accretion WD” to for SNIa might be different for some reason. Open statements like; “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.” This is not true. BOTH scenarios are known to work, and we do know why! The other question of course follows… If the X-Rays… Read more »
Louise
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Louise
February 18, 2010 6:21 AM

According to Mario Livio, very few elliptical galaxy Type 1a SNs are used in Dark Energy studies, so this research doesn’t really say much about the accuracy of such. It will be very interesting to see their future results from the disks of spiral galaxies…

Excalibur
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Excalibur
February 18, 2010 7:21 AM
@Crumb “The other question of course follows… If the X-Rays are actually “30-50x than is actually observed”, then logically the difference between X-ray emissions in the two SNIa scenarios would be true for observed extragalactic supernova too. As far as I know, there is NO difference is energy output between SN Ia (or SNI) – hence holding them to standard candles.” This is where you loose me, the article doesnt talk about the x-ray emission from an actual SNIa, and you are correct in that there is no expected big difference between the two scenarios regarding the output of x-rays _after_ the explosion. But there is different expectations _prior_ to the explosion, as in how much x-rays would… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
February 18, 2010 10:48 AM

A supernova due to white dwarf merger would seem to be a physically different form. An SN1 has a fairly strict relative luminosity determined by luminosity and distance. This I have thought is set by the Chandreshanker limit. A merger would seem to negate that “rule.”

LC

RUF
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RUF
February 18, 2010 10:49 AM

I still don’t understand how Type Ia’s can be used as standard-candles when Type Ia’s are not standard at all.

The whole idea of accelerating expansion and dark matter is based on data that we think we understand, but we may be trying to “standardize” something that may be wildly variable.

Aodhhan
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Aodhhan
February 18, 2010 11:27 AM
The fact of Type1a SN being labeled as “standard candles” never meant each one gave off the ‘exact’ amount of energy as another, every time (One reason they are never used for really great distances now). Yet the 1aSN which gives off energy due to accretion from a star, were close enough to create a standard range which was good enough…so to speak. For looking back billions of years, redshifting is a better measurement. Before this study, it was assumed the majority of 1aSN were those which accreted energy from stars (possibly based on the fact, many stars are binary; and they can often go SN more than once… having a longer lifetime). So in many instances, when… Read more »
Jon Hanford
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Jon Hanford
February 18, 2010 2:28 PM

Some very interesting commentary on this story. I noticed a preprint of this paper by Bogdan and Gilfanov was posted today at arXiv.org: http://arxiv.org/PS_cache/arxiv/pdf/1002/1002.3353v1.pdf

Lawrence B. Crowell
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Lawrence B. Crowell
February 18, 2010 4:17 PM

As the article states there is significantly less X-ray production. This means there is a distinction between SN1 due to accretion of gas from a stellar partner and from a merger. Of course a spacecraft is required to monitor these to measure X-ray flux.

LC

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 8:14 PM
Some interesting comments and good points here; Excaliber said; “Even a 0.6+1.4 Msun merger would be a possibility according to the limits you stated.” Unlikely such a scenario would occur. Much of the problem is to do with stellar evolution, where two stars are formed in close proximity. (under about 3 radii) this constrains the masses somewhat allowing little variation between them. Proximity when the stars turn into white dwarfs can then have mass transfer of their mutual envelopes, than can change the overall mass of the core slightly You might like to consider reading a fairly general article on this subject being Iben’s 1985 paper; “The life and times of an intermediate mass star – In isolation/in… Read more »
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 8:18 PM

Note: The Iben paper above has a more expanded and more complex version ; entitled

<A HREF="http://adsabs.harvard.edu/abs/1985ApJS…58..661I" "On the evolution of close binaries with components of initial mass between 3 solar masses and 12 solar masses"

Again, some of this is already surpassed, but it gives a very good overview for more detailed study or understanding of the topic.

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 8:19 PM
The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 8:23 PM

“On the evolution of close binaries with components of initial mass between 3 solar masses and 12 solar masses.”

Gee, I wish there was some intermediate editor here to see how the submissions look!

The Eclectic Exterminator of Stupid Electricians
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The Eclectic Exterminator of Stupid Electricians
February 18, 2010 9:53 PM
Aodhhan said: “The fact of Type1a SN being labeled as “standard candles” never meant each one gave off the ‘exact’ amount of energy as another, every time (One reason they are never used for really great distances now). Yes, SNIa DO vary in output. The variation is not huge but is significant. The “standard candles” is actually based on the light-curve, which gives the luminosity of the event. Once the absolute magnitude is known, this of course gives the distance. In the end, knowing the mean maxima supernovae of all types, can be used to give distance with an error – being often much better than other means. Yet the 1aSN which gives off energy due to accretion… Read more »
Lawrence B. Crowell
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Lawrence B. Crowell
February 19, 2010 6:02 AM
@ Hon. Salacious B. Crumb: What you indicate appears to depend upon some fairly complicated astrophysics. A merger of two white dwarfs, say both M = 1M_sol, but where the fusion is initiated with the 1.4M_sol coalescence point seems to involve to complicated nonequilibrium conditions. After all the Chandrashekhar limit is computed for degenerate electron pressure in a stationary or equilibrium state. The accretion of matter by the white dwarf is treated as an adiabatic process, not some rapidly process with shock waves and so forth. If the fusion is initiated at the 1.4M_sol coalescence point for all conditions this would appear to represent considerable progress in this sort of astrophysics. If this is so then SNI from… Read more »
Aodhhan
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Aodhhan
February 19, 2010 6:41 AM

Crumb….

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.

Excalibur
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Excalibur
February 19, 2010 8:06 AM

@Crumb:

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.

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.

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

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