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What happened 400 years ago to create this stunningly beautiful supernova remnant – and were there two culprits or just one? This Hubble Space Telescope view of a Type Ia-created remnant has helped astronomers solve a longstanding mystery on the type of stars that cause some supernovae, known as a progenitor.
“Up until this point we haven’t really known where this type of supernova came from, despite studying them for decades,” said Ashley Pagnotta of Louisiana State University, speaking at a press briefing at the American Astronomical Society meeting on Wednesday. “But we now can say we have the first definitive identification of a Type 1a progenitor, and we know this one must have had a double degenerate progenitor – it is the only option.”
This supernova remnant that has a telephone number-like name of SNR 0509-67.5, lies 170,000 light-years away in the Large Magellanic Cloud galaxy.
Astronomers have long suspected that two stars were responsible for the explosion – as is the case with most type 1a supernovae — but weren’t sure what triggered the explosion. One explanation could be that it was caused by mass transfer from a companion star where a nearby star spills material onto a white dwarf companion, setting off a chain reaction that causes one of the most powerful explosions in the universe. This is known as the ‘single-degenerate’ path – which seems to be the most plausible, common and most preferred explanation for many Type 1a supernovae.
The other option is the collision of two white dwarfs, which is known as ‘double-degenerate, which seems to be the less common and not as widely accepted explanation for supernovae. To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appear to exist; indeed, there appear to be just handful that have been discovered so far.
The problem with SNR 0509-67.5 was that astronomers could not find any remnant of the companion star. That’s why the double degenerate scenario was considered, as in that case, there won’t be anything left as both white dwarfs are consumed in the explosion. In the case of a single progenitor, the non-white dwarf star will still be near the explosion site and will still look very much as it did before the explosion.
Therefore, a possible way to distinguish between the various progenitor models has been to look deep in the center of an old supernova remnant to search for the ex-companion star.
“We know Hubble has the sensitivity necessary to detect the faintest white dwarf remnants that could have caused such explosions,” said lead investigator Bradley Schaefer from LSU. “The logic here is the same as the famous quote from Sherlock Holmes: ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth.'”
In 2010, Schaefer and Pagnotta were preparing a proposal to look for any faint ex-companion stars in the center of four supernova remnants in the Large Magellanic Cloud when they saw an Astronomy Picture of the Day photo showing an image the Hubble Space Telescope had already had taken of one of their target remnants, SNR 0509-67.5.
(Note: the January 12, 2012 APOD image is of SNR 0509-67.5!)
Because the remnant appears as a nice symmetric shell or bubble, the geometric center can be determined accurately. In analyzing in more detail the central region, they found it to be completely empty of stars down to the limit of the faintest objects Hubble can detect in the photos. The young age also means that any surviving stars have not moved far from the site of the explosion. They were able to cross off the list all the possible single degenerate scenarios, and were left with the double degenerate model in which two white dwarfs collide.
“Since we can exclude all the possible single degenerates, we know it must be a double degenerate,” Pagnotta said. “The cause of SNR 0509-67.5 can be explained best by two tightly orbiting white dwarf stars spiraling closer and closer until they collided and exploded.”
Pagnotta also noted that this supernova is actually not a normal Type 1a supernova, but a subclass called 1991t, which is an extra bright supernova.
A paper in 2010 by Marat Gilfanov of the Max Planck Institute for Astrophysics indicated that perhaps many Type 1a supernova were caused by two white dwarf stars colliding, which was a surprise to many astronomers. Additionally, a review of the recent supernova SN 2011fe, which exploded in August of 2011, explores the possibility of the double degenerate progenitor. 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.
Schaefer and Pagnotta plan to look at other supernova remnants in the Large Magellenic Cloud to further test their observations.
Pagnotta confirmed that anyone with an internet connection could have made this discovery, as all the Hubble images used were available publicly, and the use of the Hubble data was sparked by APOD.
Sources: Science Paper by Bradley E. Schaefer and Ashley Pagnotta (PDF document), HubbleSite, AAS press briefing
Forgive my naivete in these matters, but i had a few thoughts about this.
Its my understanding that Type 1a supernovas are used as standard candles to measure extra galactic distances due to their constant brightness. If in-fact, as this article suggests, there are different types of 1a supernovae with different brightness, does that mean any distances measured with Type 1a supernova are thrown into question? Subsequently, if our measure of the size of the universe is now un-reliable that would have far reaching implications with the cosmological model, dark mater, inflation etc?
Am i blowing this out of proportion? (pun intended!) 🙂
Like Schrödinger’s cat you are right and wrong at the same time. 😉
I must confess that I am not 100% certain about the following, but it is not the overall brightness that is the same in Type 1a SNe.
However, the way the afterglow fades away afterwards is quite unique and can be compared. And this behavior is also somehow related to the brightness, which, then, can be inferred giving the SN’s distance.
As I said. I am not completely sure of that, but it is something like it.
Even in the double degenerate scenario, one or the other of the two white dwarfs will be the one that initiates a collapse and triggers the SNe wich blows it to pieces. That part of mass will still be roughly the same, and it is this part that produces the most important ingredient for the light-curve, radioactive iron, nickel and cobalt.
Although i suspect this will also ignite the progenitor, or what is left of it, that part will not suffer the extreme conditions that the collapsed part will, and will therefor not fuse through as complete before it disperses in the explosion.
This could be an indicator for the brighter SNe (1991t type), but since the amount of iron, nickel, cobalt should still be about the same, the lightcurve would then fall into a standard anyway. Brighter peak, standard decline.
I am still puzzled how the non-degenerate of the pair is able to survive the supernova explosion intact.
re: “In the case of a single progenitor, the non-white dwarf star will still be near the explosion site and will still look very much as it did before the explosion.”
Well, if you take into account the timescale over which such an explosion occurs, and that the companion star is effectively hit by a very bright, supped-up stellar wind shock wave, it isn’t hard to believe that a large part of the companion star isn’t too drastically affected. Make no mistake, the companion definitely feels the explosion, the timescale is just too short to destroy it… (I think; could be wrong :D)
The progenitor likely looses a significant amount of any lowdensity ‘atomsphere’ by the shockwave, but the core is much denser and will survive. And the core contains most of the mass of the companion star at that point.
But i am not convinced a single degenerate SNe can be ruled out, as there are a few situational possibilities aswell, albeit they would probably be extremely rare.
The paper by Shaefer and Pagnotta( http://hubblesite.org/pubinfo/pdf/2012/06/pdf.pdf Sec. 4 ) makes note of a faint nebula that is visible within the error circle at the center of the SNR. It can be seen faintly (circled) in the APOD image and in a B&W Hydrogen-alpha image in the linked paper(Fig 1).
The going assumption is that this is a background galaxy (as four other similar looking galaxies appear in the Hubble images), but the authors note that its central position is “suggestive of a connection”. One possibility they mention is that the nebula could the remnants of a short-lived accretion disk formed by the disrupted low mass white dwarf of the system before the explosion. Or it may be very low velocity ejecta left behind by the merging white dwarfs. A spectrum of the nebula should help to determine its true nature (though with a Vmag=23 this would likely require a 8-10m class scope).
As I was reading about this central nebula I was reminded of the nebulous object at the center of SN1987A: http://en.wikipedia.org/wiki/File:Sn87a.jpg
Seeing as both SNR are at similar distances (in the Large Magellanic Cloud) , I wonder if the faint nebula near the center of SNR 0509-67.5 might really be a 400 year-old version of the relatively young and bright plerion seen in SN1987A?
I’m right there with ya, Magnus. I’m sure many more types of Type 1a’s will be found, and this will end the useful effectiveness of 1a’s as standard candles.