Astronomical fireworks: On the origins of Type Ia supernova

Feb 28, 2012
Fireworks
An ordinary-looking faint blue star (left) is actually a pair of white dwarfs that could someday light up the sky as a Type Ia supernova. We can't see the second white dwarf, but we know of its presence, and the future of both stars, from measuring multiple spectra of the star we can see. As the white dwarf orbits its unseen companion, it sometimes approaches us (blue in the drawing at the bottom) and sometimes receding from us (red). As it moves, lines in its spectrum shift to shorter and longer wavelengths due to the Doppler Effect (blue and red jagged lines, respectively). The amount of shift tells astronomers the star's velocity, which in turn tells them how soon the pair of white dwarfs will go supernova. Credit: Carles Badenes and the SDSS-III team

(PhysOrg.com) -- A little luck and a lot of hard work can really light up the sky.

Taking advantage of a little-known feature of the Sloan Digital Sky Survey, a team of astronomers led by Carles Badenes of the University of Pittsburgh has helped to clarify the origins of an important type of —using nothing but a few thousand small, faint stars in our own cosmic backyard.

The astronomical fireworks the team studied are so-called "Type Ia supernovae," exploding stars so incredibly bright that we can see them even in the most distant galaxies. In a paper published today on the arXiv preprint server, Badenes and colleagues compared the number of these supernovae in distant galaxies to the number of binary in our galaxy. The rate is similar, suggesting that merging white dwarfs are indeed a reasonable explanation for these giant explosions.

Type Ia supernovae, while rare, are important because of what they can tell us about the universe. "We know that all Type Ia supernovae should be about the same brightness, so if we see one that looks fainter, it must be farther away," Badenes says. "That means that if we happen to see a Type Ia supernova in a , we can figure out how far away that galaxy is." In fact, using Type Ia supernovae to measure galaxy distances is what led astronomers to discover that our universe is accelerating—a breakthrough discovery that was recognized with the 2011 Nobel Prize in Physics.

Although we know what happens during a Type Ia supernova explosion, amazingly, we don't know for sure what kinds of stars create them. "We know that there have to be two stars involved, and that one of them has to be a white dwarf," says Dan Maoz, an astronomer at Tel Aviv University in Israel and co-author of the paper released today. "But there are two possibilities for what the second star is, and we're not quite sure whether one or both possibilities is right."

The second star could be either a "normal" star like the Sun, or another white dwarf. If a star system contains two white dwarfs, then the white dwarfs revolve around one another at half a million miles an hour, speeding up and getting closer and closer—until one day they merge, most likely producing the fireworks of a .

"There are reasons to suspect that Type Ia supernovae come from the merging of a double white dwarf," Maoz says. "But the biggest question mark is—are there enough double white dwarfs out there to produce the number of Type Ia supernovae that we see?"

This series of artist's concepts shows two white dwarfs orbiting one another. In the future, their orbits will get smaller and smaller, and faster and faster, until someday they merge and explode. Credit: NASA/GSFC/D.Berry

Answering that question requires counting double white dwarfs, figuring out how often they merge, and comparing that rate to the supernova rate in distant galaxies. White dwarfs are so small and faint that there is no hope of seeing them in distant galaxies, so Badenes and Maoz turned to the only place we can see them—the part of our galaxy within about a thousand light-years of the Sun. And because their goal was to count double white dwarfs in the neighborhood, they turned to the only survey that could find enough of them to count.

To find a double star system, astronomers don't actually need to see both stars. Even if one of the stars is too faint, we can detect its presence by its effect on the star we do see. As the stars orbit each other at breakneck speeds, sometimes the visible star is moving towards us, sometimes away from us.

This back-and-forth motion can be detected in the star's spectrum (a measure of how much light the star gives off at different wavelengths), thanks to the Doppler Effect. The Doppler Effect is the reason that an ambulance siren changes in pitch from high to low as the ambulance passes. In the same way, light waves from an object that is moving towards us are shortened (blueshifted), while light waves from an object that is moving away from us are stretched (redshifted). Features in the spectrum called "spectral lines" provide landmarks to measure the shift, and hence the star's velocity.

But stars can move for lots of reasons, so just finding one velocity does not give enough evidence that the star has an unseen companion. To prove that, astronomers need at least two spectra. If the star's calculated velocity has changed between the time of the first and second spectrum, then know that the system is not one star, but two. Furthermore, they can deduce what kind of star the unseen companion must be. If both turn out to be white dwarfs, they can also use Einstein's General Theory of Relativity to calculate how much time remains until the two white dwarfs will merge and explode.

Using this technique requires at least two spectra of the white dwarf, and the SDSS had only one. So that approach wouldn't work for Badenes and Maoz. Or would it?

This image mosaic shows 99 of the nearly 4,000 white dwarfs that Badenes, Maoz, Bickerton, and their colleagues examined. Of the four thousand, they found fifteen double white dwarfs. Credit: Carles Badenes and the SDSS-III team

In 2008, Badenes was a postdoctoral researcher at Princeton University. Working in the same building was Robert Lupton, one of the "founding fathers" of the Sloan Digital Sky Survey. One day over coffee, Lupton told Badenes a little-known fact about SDSS data. When the SDSS telescope measures a spectrum, it is actually measuring three sub-spectra, then adding them together to make a complete spectrum. That way, if there was something wrong with one of the three, the survey could still use the other two. "Robert said that these spectra were there, and that someone should really do science with them," Badenes says. "I immediately realized that those sub-spectra were the missing piece. Now we could do exactly what we needed to do with SDSS data."

Then came the hard part. The three sub-spectra were never intended for public use, so they were hard to find, and in a format that was, as Badenes says, "unfriendly." Badenes and Maoz worked with Steve Bickerton, an astronomer at Princeton University, to gather and clean the sub-spectra. "We had to process the three sub-spectra separately, but process them in a way that we could directly compare them," says Bickerton. "It wasn't easy."

But persistence paid off, and within a year, the team had a list of more than 4,000 white dwarfs, each of which had two or more high-quality sub-spectra. Some of those four thousand, the team hoped, would show evidence that the star's velocity had changed between the three sub-spectra, indicating that what looked like one star was actually a double white dwarf.

From among the original four thousand, the team found fifteen stars certain to be double white dwarfs—and "if there were more than 15, we would have seen more than 15," Badenes says. Having found 15 double white dwarfs in the local neighborhood, the team could then use computer simulations to calculate the rate at which double white dwarfs merge. They could then compare the number of merging white dwarfs here to the number of Type Ia supernovae seen in distant galaxies that resemble the Milky Way.

The result? On average, one double white dwarf merger event occurs in the Milky Way about once a century. That number is remarkably close to the rate of Type Ia supernovae we observe in galaxies like our own, suggesting that the merger of a double white dwarf system is a plausible explanation for Type Ia supernovae.

In addition to providing a key clue about the nature of these important events, the team's discovery shows the potential of giant astronomical surveys like the SDSS. "Twenty years ago, we decided to take three sub-spectra for each spectrum. We did that for entirely practical reasons," says Robert Lupton, the Princeton astronomer who first told Badenes about the sub-spectra. "We had no idea that would someday give us an important clue to the mystery of Type Ia supernovae. That was a great insight by Carles and Dan, and it was a bit of a lucky break for us."

Who knows what other "lucky breaks" the universe might have in store?

Explore further: An unprecedented view of two hundred galaxies of the local universe

More information: Badenes, C. & Maoz, D. 2011, The merger rate of binary white dwarfs in the galactic disk, submitted to Astrophysical Journal Letters and available on the arXiv preprint server.

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Pressure2
1 / 5 (2) Feb 28, 2012
Quote from article: "That means that if we happen to see a Type Ia supernova in a distant galaxy, we can figure out how far away that galaxy is." In fact, using Type Ia supernovae to measure galaxy distances is what led astronomers to discover that our universe is accelerating a breakthrough discovery that was recognized with the 2011 Nobel Prize in Physics."

Read carefully, this is what the acceleration of the expansion of the universe is based on.

Lurker2358
1 / 5 (5) Feb 28, 2012
Quote from article: "That means that if we happen to see a Type Ia supernova in a distant galaxy, we can figure out how far away that galaxy is." In fact, using Type Ia supernovae to measure galaxy distances is what led astronomers to discover that our universe is accelerating a breakthrough discovery that was recognized with the 2011 Nobel Prize in Physics."

Read carefully, this is what the acceleration of the expansion of the universe is based on.



Yes, terrible fallacy, isn't it?

That whole, "All white dwarf pairs are the exact same mass, and explode in the exact same way," thing that follows from that failed assumption cracks me up.
SpiffyKavu
5 / 5 (7) Feb 28, 2012
And the other lines of evidence suggesting the same amount of dark energy predicted by the acceleration of the expansion of the universe don't do anything for you? Fitting the angular correlation spectrum of the cosmic microwave background and spatial correlations (baryon acoustic oscillation) hint at the same composition of the universe that the supernovae suggest. Spatial correlations essentially telling us the clumpiness of matter.

I'll grant you that all of these are model dependent things. And the spatial correlations also rely on distance measurements (I think they stick to cosmological redshifts though, but I'm not sure).

Most people aren't saying this picture is the absolute truth, but it is the best we've got right now so we work with it.
ccr5Delta32
5 / 5 (4) Feb 28, 2012
@ Lurker


Yes, terrible fallacy, isn't it?

That whole, "All white dwarf pairs are the exact same mass, and explode in the exact same way," thing that follows from that failed assumption cracks me up.

Who said that ?

Type 1a supernova occurs when a white dwarf star reaches the Chandrasekhar limit of about 1.4 solar masses
http://en.wikiped...ar_limit

They may reach this limit by accretion or as in mergers .But when they do they go bang . That's why they have similar brightness
Lurker2358
1 / 5 (5) Feb 28, 2012
They may reach this limit by accretion or as in mergers .But when they do they go bang . That's why they have similar brightness


Sorry, pal.

A merger is not a continuous event that simply works up to a limit and then stops...

Collisions and mergers can jump arbitrary finite limits by a significant amount. As in the case of all reactions, a change in mass of an interaction clearly changes the energy available to do work.

Example, if you merge two objects that are each 90% of the limit then you get 180% as much mass as the needed limit, implying the explosion could be 80% stronger than one at "barely" the limit.

thus a merger explosion which jumps the limit instantaneously by a large margin would be much more powerful than a gradual accretion explosion that "barely" hits the limit and then explodes.

Try again.
ccr5Delta32
5 / 5 (2) Feb 28, 2012
"A merger is not a continuous event that simply works up to a limit and then stops..."
And again who said that ?
It would be somewhat pointless to argue a point I haven't made
Lurker2358
1 / 5 (5) Feb 28, 2012
Well, I hope you see how my argument in the previous post makes a "standard candle" not a "standard" at all, if they are indeed caused by binary mergers, since you can't know the total mass of the original system...

These are used to calculate the "distance" by working the apparent brightness backwards, operating on the ASSUMPTION that they all have the same mass at the limit.

As I just showed in an example, the initial mass binary merger can actually vary by nearly 100% of the limit, even if both stars start out at just below the limit.

pathetic, really.
Lurker2358
1 / 5 (3) Feb 28, 2012
Anyway, I don't know why I make such a big deal about this, except that if you're going to do something and call it a standard, at least be honest about all of the hang-ups and exceptions involved.

It turns out that any error in excess mass increases the distance anyway, but it doesn't make it any less of a problem for correlating Type 1A to Red shift, because appearing 10% to 40% closer and "younger" than it should be is still a problem for the science either way.

In order to get "real" values for the correlation between the mass of these explosion and their power, we should not make assumptions. We should OBSERVE a white dwarf accreting matter, or colliding with another star, and measure the mass of the object(s) involved before time and afterwards, as well as the energy of the explosion. You need a distribution of at least 5 to 10 CLOSE explosions where the before-hand mass, distance, and brightness are both known to within about 10% margin, so you can relate mass to brightness.
ccr5Delta32
5 / 5 (5) Feb 28, 2012
" if they are indeed caused by binary mergers "
Here are some examples merges that might relate to your comment " SN 2006gz, SN 2007if and SN 2009dc " you'll heave to search yourself .
However while estimating the expansion of the universe there were many data points not all of which lay neatly on the line/curve ,there were some anomalies but in general they did .In other words ,while it's not an absolute fool proof standard candle ,with a large enough data set ,it's pretty good
Lurker2358
1 / 5 (3) Feb 28, 2012
then, once you have a good distribution, you could predict within standard deviations the probability of the explosion being of a total mass amount.

But again, that requires a baseline of absolute knowledge, i.e. a sample of 5 to 10 KNOWN explosions with absolute knowledge of the mass of the progenitor star(s) ahead of time, and absolute knowledge of the energy released.

Assuming doesn't cut it.

Unfortunately, taking a sample of such explosions entire life cycles in order to get a baseline sample with KNOWN mass to energy output correlations, would take at least several thousand years, since there's only 15 visible pairs with existing technology! Even if we could see every white dwarf in the galaxy and know it's mass and distance perfectly ahead of time for the explosion, it would take at least 500 to 1000 years to complete a baseline survey to establish mass-energy correlation within reasonable deviations...
ccr5Delta32
5 / 5 (6) Feb 28, 2012

Assuming doesn't cut it

The Chandrasekhar limit is based on equations of state of electron degenerate pressure relativistic effects .Observationally ,No white dwarf has been observed above that mass
Note ,if you're looking for absolute certainty's .This is science there are none
Lurker2358
1 / 5 (3) Feb 28, 2012

Assuming doesn't cut it

The Chandrasekhar limit is based on equations of state of electron degenerate pressure relativistic effects .Observationally ,No white dwarf has been observed above that mass


Of course you wouldn't "observe" a white dwarf above that mass.

That could only happen in a merger.

Unfortunately, that's what I was trying to explain above.

What you could observe is 2 or more white dwarfs merging into a bigger object, which immediately detonates.

A merger can produce an object significantly above the limit (which presumably immediately detonates,) thus defeating the assumption of all type 1a supernovas being the same size.

Note ,if you're looking for absolute certainty's .This is science there are none


Ah, but so many are so "sure" of theory which is taught as fact and obfuscated as fact.
Graeme
5 / 5 (3) Feb 28, 2012
I hope they follow up on observations of the binary spectra to determine the orbits. This should help show the life cycle and how fast the orbits decay, and which binary is about to go supernova next.
CardacianNeverid
5 / 5 (4) Feb 29, 2012
A merger can produce an object significantly above the limit (which presumably immediately detonates,) thus defeating the assumption of all type 1a supernovas being the same size.

Ah, but so many are so "sure" of theory which is taught as fact and obfuscated as fact -QCLurkerTard

Hey boy genius, if you're so sure about the failings of this research, why don't you demonstrate it with mathematics and observations and overturn accepted wisdom? There has already been one Nobel awarded in this field, why not got for two? Otherwise stop spamming nonsense!
Shinichi D_
5 / 5 (6) Feb 29, 2012

Ah, but so many are so "sure" of theory which is taught as fact and obfuscated as fact.


Like all anti-expansion dudes, you seem to forget about cepheids.
And many other observations. Simply stating that you don't belive type Ia-s are uniform is not enough to prove BB wrong.

stellar-demolitionist
5 / 5 (6) Mar 02, 2012
Lurker,

You have the inference backward, it was the uniformity of observed SN Ia that implied a progenitor with little variation. That plus the observed lack of hydrogen pointed to rapid combustion of a C O WD.

The use of SN Ia for distance determination is not based on the specifics of the theory, but on the calibration of observed brightness with other parameters. (In this case light curve shape.) All standard candles work this way. An intrinsic brightness is correlated to some other intrinsic property that is not distance dependent. For example, the brightness-period relation in Cepheids.

The peak luminosity and light curve shape are dependent on the amount of the WD that is burned to radioactive Ni-56, and not the total mass or kinetic energy. The ejected mass and KE are relatively poorly known and difficult to measure.
stellar-demolitionist
5 / 5 (6) Mar 02, 2012
WD/WD merger v. accretion to Chandrasekhar mass (Mch)

For the last 2 decades, accretion from a non-WD to Mch has been the favored model, but things have swung the other direction recently. This paper is part of that swing.

For the WD regular star models the triggering of burning has been relatively straight forward-- when the mass approaches Mch, the density increase at the center triggers C-burning and eventually an explosion. The difficulty has always been in reaching Mch without polluting the local environment with hydrogen that is not seen in the spectra of SN Ia.

The recent observations of SN 2001fe have placed tight constraints on the regular companion and interaction to the SN and donor companion. These constraints severely limit the progenitor systems and environment around regular SN Ia at explosion.

stellar-demolitionist
5 / 5 (6) Mar 02, 2012

For the WD-WD mergers the problem has been with the lack of systems narrow enough to merge in (0.3-1 B yr) and the previous models that showed a thick disk of material around one WD after the merger that burned slowly and did not explode.

The last decade or two has seen a great improvement in our knowledge of binary WD pairs. This study has identified several new candidate WD-WD binary pairs for follow up observation which could potentially satisfy the required SN Ia rate for our galaxy.

Recently the theoretical aspects of the WD-WD merger scenario have also improved. In new, better resolved, simulations, the WD that is ripped apart and accretes onto the other reaches the conditions necessary to trigger explosive burning and create a SN Ia that matches observation reasonably.