Super Supernova: White Dwarf Star System Exceeds Mass Limit

Mar 15, 2010
Cosmologists use Type Ia supernovae, like the one visible in the lower left corner of this galaxy, to explore the past and future expansion of the universe and the nature of dark energy. (Photo: High-Z Supernova Search Team, HST, NASA)

(PhysOrg.com) -- An international team led by Yale University has, for the first time, measured the mass of a type of supernova thought to belong to a unique subclass and confirmed that it surpasses what was believed to be an upper mass limit. Their findings, which appear online and will be published in an upcoming issue of the Astrophysical Journal, could affect the way cosmologists measure the expansion of the universe.

Cosmologists use Type Ia —the violent explosions of dead cores of stars called —as a kind of cosmic ruler to measure distances to the supernovae's host galaxies and, as such, to understand the past and future expansion of the and explore the nature of . Until recently, it was thought that white dwarfs could not exceed what is known as the Chandrasekhar limit, a critical mass equaling about 1.4 times that of the Sun, before exploding in a supernova. This uniform limit is a key tool in measuring distances to supernovae.

Since 2003, four supernovae have been discovered that were so bright, cosmologists wondered whether their white dwarfs had surpassed the Chandrasekhar limit. These supernovae have been dubbed the "super-Chandrasekhar" supernovae.

Now Richard Scalzo of Yale, as part of a collaboration of American and French physicists called the Nearby Supernova Factory, has measured the mass of the white dwarf star that resulted in one of these rare supernovae, called SN 2007if, and confirmed that it exceeded the Chandrasekhar limit. They also discovered that the unusually bright supernova had not only a central mass, but a shell of material that was ejected during the explosion as well as a surrounding envelope of pre-existing material. The team hopes this discovery will provide a structural model with which to understand the other supermassive supernovae.

Using observations from telescopes in Chile, Hawaii and California, the team was able to measure the mass of the central star, the shell and the envelope individually, providing the first conclusive evidence that the star system itself did indeed surpass the Chandrasekhar limit. They found that the star itself appears to have had a mass of 2.1 times the mass of the Sun (plus or minus 10 percent), putting it well above the limit.

Being able to measure masses for all parts of the star system tells the physicists about how the system may have evolved—a process that is currently poorly understood. "We don't really know much about the stars that lead to these supernovae," Scalzo said. "We want to know more about what kind of stars they were, and how they formed and evolved over time."

Scalzo believes there's a good chance that SN 2007if resulted from the merging of two white dwarfs, rather than the explosion of a single white dwarf and hopes to study the other super-Chandrasekhar supernovae to determine whether they, too, could have involved a merger of two white dwarfs.

Theorists continue to explore how stars with masses above the Chandrasekhar limit, which is based on a simplified star model, could exist without collapsing under their own weight. Either way, a subclass of supernovae governed by different physics could have a dramatic effect on the way cosmologists use them to measure the expansion of the universe.

"Supernovae are being used to make statements about the fate of the universe and our theory of gravity," Scalzo said. "If our understanding of supernovae changes, it could significantly impact of our theories and predictions."

Explore further: New mass map of a distant galaxy cluster is the most precise yet

More information: Paper: arxiv.org/abs/1003.2217

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PinkElephant
2.8 / 5 (4) Mar 15, 2010
Here's one simple idea: what if the white dwarf is rapidly spinning? This would allow it to pack on a lot of mass at its equator, without triggering an explosion at 1.4 solar masses.

After all, theorists posit rapid rotation for millisecond pulsars, which they are supposed to acquire gradually over time as matter continues to accrete onto them from a co-spinning disk. Well, Type 1A supernovae are also supposed to be driven by accretion (presumably, from an accretion disk...) -- so they ought to be getting "spun up" by the selfsame process as ultimately brings them to the threshold of detonation.
jonnyboy
Mar 15, 2010
This comment has been removed by a moderator.
jamey
3 / 5 (2) Mar 15, 2010
That's actually been taken into account - as I recall the model, the 1.4 solar masses actually involves a white dwarf rotating just slower than the break-up point - part of the process leading to the formation of white dwarves involves it slinging said extra mass off due to the rotation.
PinkElephant
3.3 / 5 (3) Mar 15, 2010
@jamey,
That's actually been taken into account - as I recall the model, the 1.4 solar masses actually involves a white dwarf rotating just slower than the break-up point...
No. The 1.4 solar masses refers to the Chandrasekhar limit, which is derived for NON-rotating bodies. If you don't believe me, you can find the same thing being stated in the second sentence of the following article:

http://en.wikiped...ar_limit
...part of the process leading to the formation of white dwarves involves it slinging said extra mass off due to the rotation.
While rotation can play some role, it is not a necessary component for white dwarf origins. The key underlying process for the formation of white dwarfs, is depletion of hydrogen at the star's core, which leads to more energetic fusion reactions, which lead to a red giant phase, and then planetary nebula phase when the outgoing radiation pressure erodes away the outer layers of the star.
frajo
5 / 5 (4) Mar 16, 2010
Exciting news - the race is open again. If the theorists don't find plausible models to explain SN Ia luminosities in the light of these new observations then the quality of the most important cosmological standard candle has to be questioned.
And if they find such models the outcome might even be worse for the usability of that standard candle.
NotAsleep
not rated yet Mar 16, 2010
Thanks, frajo. I always get 1/5 stars when I make a comment like that, haha.

I hope astronomers constantly keep an open mind to the possibilities of "facts" not always being "facts"
jamey
3 / 5 (2) Mar 16, 2010
@PinkElephant - Wikipedia articles are *NOT* authoritative - unfortunately, the reference link is only to the abstract, and not the original paper. I find it difficult to believe that nobody has done the relatively minor extension to derive the value for a rotating white dwarf - and that would be the value used commonly. However, might be worth asking over on the BadAstronomy Blog - he might have references that *can* be found.
PinkElephant
not rated yet Mar 16, 2010
@jamey,

If I had the lecture notes online from the cosmology class I took 10 years ago, I'd post them. As it stands, Wikipedia is a pretty convenient reference, and it happens to be quite correct. Chandrasekhar's calculations addressed only a strictly non-rotating problem, just like Schwarzschild addressed only the metric of non-rotating black holes. Lack of rotation makes the math a lot more manageable...
mattytheory
5 / 5 (1) Mar 16, 2010
I am curious as to what effect this will have on dark energy theories? Do we still need dark energy to explain the expansion of space if it turns out the standard candles by which we used to measure this expansion were not actually standard?
PinkElephant
5 / 5 (3) Mar 16, 2010
@jamey,

Consider also, that for rotating objects there's no single solution. The dynamics depend entirely on how fast the object is spinning. One could, of course, take it to the absurd limit where the object's surface at the equator approaches light speed, but this wouldn't be remotely realistic, as no objects even close to such a limit would be likely to exist.

A nice thing about non-rotating objects, is that they produce a single, nice, final answer that only depends on mass as the single free parameter...

Anyway, to me rotation seems to just be a no-brainer when it comes to exceeding the Chandrasekhar limit. It is so simple, and so obvious, it's screaming for recognition. I find it actually rather odd, that it wasn't mentioned in the writeup to begin with. It seems to me much more likely, than a merging-of-two-dwarfs scenario. Not to say that the latter isn't possible, but it's just true that all stars spin, and most accretion disks spin up the target of accretion...
PinkElephant
5 / 5 (2) Mar 16, 2010
@mattytheory,
I am curious as to what effect this will have on dark energy theories? Do we still need dark energy to explain the expansion of space if it turns out the standard candles by which we used to measure this expansion were not actually standard?
I'd imagine there's some concern about this. However, keep in mind that the estimates of cosmic expansion are not based on any single supernova. Rather, they are based on statistical analysis of many events. If there is a stable population mean for 1A brightness, then even with variation around the mean, given enough events it's possible to extract a reliable signal (in this case, distance map.)

So ultimately, the question would be settled by a balance between how much variation there is, vs. how large is the population of measured events. But at any rate, the error bars just widened considerably...
jamey
not rated yet Mar 16, 2010
@PinkElephant - Yes, lack of rotation does make the Schwartzchild solutions of a black hole much simpler - but in the case of the white dwarf, structural issues are already being accounted for in prevention of the collapse, and so the oblation of a white dwarf is really not that much more complex than computing the oblation of Jupiter, Earth, or the Sun. The velocities involved are likely to be significantly less than relativistic, but possibly not. My guess is that the calculations have been done, and the addition of rotation does not significantly raise the mass limit.

I mean, seriously - Type 1 supernovae have been studied for at least a couple of decades now, and white dwarfs for another couple of decades before that, and *NOBODY* has bothered to compute what rotation does?

Now that I think about it, I am pretty sure Dr. Forward must have, when he was writing Dragon's Egg - though that concerns a neutron star, and not a white dwarf.
PinkElephant
5 / 5 (3) Mar 16, 2010
My guess is that the calculations have been done, and the addition of rotation does not significantly raise the mass limit.
Well, next time before putting forth false information with such bombastic confidence, perhaps you ought to first double-check your hunches. In particular, you should re-read the article above, and note that it specifically mentions the CHANDRASEKHAR LIMIT. Which is, BY DEFINITION, non-rotational.

As for mass limit not being significantly affected by rotation, even a simple Newtonian analysis ought to convince you that's false. The gravitational acceleration at the surface of a white dwarf can be easily compensated by kinetic energy of a rapidly rotating surface.