November 20, 2012
Supernovae, Bright And Dim, Can Teach Us A Lot
April Flowers for redOrbit.com - Your Universe Online
A type of oddly dim exploding star is probably a class of duds, a new study using supercomputer simulations finds, but one that could throw new light on the mysterious nature of dark energy.Thousands of exploding stars are classified as type Ia supernovae, and most of them look similar to each other. This is why astrophysicists use them as accurate cosmic distance indicators, they show that the expansion of the universe is accelerating under the influence of an unknown force called dark energy. Approximately 20 type Ia supernovae, however, look peculiar.
"They're all a little bit odd," said George Jordan, a research scientist at the University of Chicago's Flash Center for Computational Science.
Jordan and his colleague Hagai Perets, assistant professor of physics at Technion — Israel Institute of Technology, believe that comparing odd type Ia supernovae to normal type Ia supernovae may permit astrophysicists to more precisely define the nature of dark energy. They found that the peculiar ones are probably white dwarf stars that failed to detonate. The results of the study have been published in the Astrophysical Journal Letters.
"They ignite an ordinary flame and they burn, but that isn't followed by a triggering of a detonation wave that goes through the star," Jordan explains.
The team ran simulations that consumed approximately two million central processing unit hours on Intrepid, the Blue Gene/P supercomputer at Argonne National Laboratory.
In a normal type Ia supernovae, the triggering of a detonation wave incinerates white dwarf stars, which are stars that have shrunk to Earth size after burning up most or all of their nuclear fuel. Most white dwarfs are found in binary systems, as well.
Normal type Ia supernovae are easier to find than the peculiar ones, which can be anywhere from 10 to 100 times fainter. Scientists estimate that the peculiar ones may account for approximately 15 percent of all type Ia supernovae.
In 2002, the first in this class of exceptionally dim supernovae was discovered. Named SN2002cx, it is still considered the most peculiar type Ia supernova ever observed. It is not, however, the dimmest. That honor goes to a supernova found in 2008.
"If the brightness of a standard supernova could be thought of as a single 60-watt light bulb, the brightness of this 2008 supernova would be equivalent to a small fraction of a single candle or a few dozen fireflies," Robert Fisher, assistant professor of physics at the University of Massachusetts Dartmouth noted.
For years, the team has successfully simulated type Ia supernova explosions following the gravitationally confined detonation scenario, in which a white dwarf begins to burn near its center. Floating to the surface like a bubble, the ignition point burns outward with a cascade of hot ash flowing around the star and colliding with itself on the opposite end after it breaks the surface. This triggers the detonation.
"We took the normal GCD scenario and asked what would happen if we pushed this to the limits and see what happens when it breaks," Jordan said. In the failed detonation scenario, the white dwarf experiences more ignition points that are closer to the core, which fuels more burning than in the detonation scenario.
"The extra burning causes the star to expand more, preventing it from achieving temperatures and pressures high enough to trigger detonation," noted Daniel van Rossum of the Flash Center.
In this scenario, however, the white dwarf remains intact even though some of the star's mass burns up and is ejected from its surface, making it look quite similar to the peculiar type Ia. The simulations revealed phenomena that astronomers can look for, or can use to identify already observed events, including white dwarfs that display unusual compositions, asymmetric surface characteristics and a kick that sends the stars flying off at speeds of hundreds of miles per second.
"This was a completely new discovery," Perets said. "No one had ever suggested that white dwarfs could be kicked at such velocities."
Normal type Ia supernovae are relatively uniform in appearance. Their peculiar cousins, however, have asymmetric characteristics that often make them look much different from one another, depending on where on Earth the viewing scientist is located.
The asymmetrical explosion also produces a kick that is possibly powerful enough that it could release the white dwarf from the gravitational hold of its binary partner, producing a peculiar type of hyper-velocity white dwarf. The fastest of this type might even be able to escape its galaxy.
Smaller explosions might leave the original binary system intact but push the white dwarf into a tight, highly elliptical orbit around its companion star, unlike most white dwarfs which display a more circular orbit.
Some of the simulated white dwarfs that failed to detonate were also found to contain heavy elements such as calcium, titanium and iron. When the detonation fails, the ejected mass appears to fall back onto the surface of the star where the heavy elements become synthesized.
"I had never heard of such strange white dwarfs," Perets said. A literature search, however, revealed reports of white dwarfs with properties that an irregular composition could explain. "It is quite rare that a new model brings about so many novel predictions, and potentially solves several distinct, seemingly unrelated puzzles."
THE ORIGINS OF ULTRA-BRIGHT SUPERNOVAE
On the other end of the spectrum, a research group from Ohio State University has been using a unique new instrument on the world's largest optical telescope to reveal the likely origins of especially bright supernova. These types of supernovae are used by astronomers as easy-to-spot 'mile markers' to measure the expansion and acceleration of the universe.
The team used the Large Binocular Telescope (LBT) to capture observations of supernova 2011fe, using a tool created at Ohio State, the Multi-Object Double Spectrograph (MODS). The findings of this study are published in the Astrophysical Journal.
Stars shine at different frequencies depending on the chemical elements they are made of. A star like our sun, for example, is made of hydrogen, so it shines at a different frequency than a star that is made mostly of helium. MODS measures the frequencies and intensities shining from a star, allowing astronomers to use spectral data to determine what a particular star is made of.
The frequency of light being emitted by supernova 2011fe reveals that it is a type Ia supernova, most likely caused by the interaction between a pair of dead white dwarf stars. In the binary system, one star orbits the other and sheds material making the second star unstable until it explodes. This explosion shines billions of times brighter than our sun.
Different hypotheses have been proposed by astronomers around the world for confirming the origin of type Ia supernovae, including exotic scenarios involving white dwarfs that have paired with still "living" giant stars.
Kris Stanek, astronomy professor at Ohio State, says that settling this debate is important.
“We really want to know more about these supernovae, given their importance in our understanding of how the universe is expanding,” he said. “Many observations have been done over the years, and I think many astronomers are starting to accept one explanation — that two white dwarfs are probably responsible.”
Still, the alternative theories keep re-emerging “like zombies that won´t die,” he says.
“With this study, we were looking for a zombie ℠kill shot,´ and we think we found it.”
According to Rick Pogge, the lead designer for MODS, the spectrograph is the ideal tool for settling the debate.
“MODS is one of the most sensitive optical spectrometers in operation today, and being used on what is currently the world's largest optical telescope. If we couldn't kill this debate with MODS and the LBT, something would be dreadfully wrong,” he added.
Astronomers use type Ia supernova as mile markers for the universe because of their extreme brightness. Unless they are of the peculiar cousin variety mentioned above, they are typically about 5 billion times brighter than our sun, making them easy to see. They also have a distinctive pattern of brightening and dimming in the weeks after they appear that makes them easily identifiable. Astronomers use these characteristics to calculate the distance from Earth to the supernova, which in turn allows them to calculate how fast the universe is expanding. Understanding the composition of the originating stars could open up new ideas in understanding that expansion rate.
Nearly all astronomers agree that type Ia supernovae originate in binary systems. What is at issue, however, is the identity of the white dwarf's companion — is it another white dwarf or even a star like our own sun?
The team found what they consider to be the answer in the light spectrum emanating from supernova 2011fe. If the companion were a star like our own, or even a giant star, a large portion of the debris should contain atoms of hydrogen.
Supernova 2011fe is located in the Pinwheel Galaxy, some 21 million light-years from Earth, providing a good chance for researchers to test for the presence of hydrogen since it is the closest near-Earth type Ia supernova to occur in the last 20 years.
“If the companion were a star such as ours or even a red giant, we would expect to see a lot of hydrogen in the signal — maybe even half a solar mass´ worth, as the companion was blown away. But instead, we saw at most only one tenth of one percent of a solar mass´ worth of hydrogen. That suggests that the white dwarf´s companion had very little if any hydrogen in it, and is likely another white dwarf,” said Ben Shappee, doctoral student at Ohio State.
Pogge called the study “a beautiful demonstration of the kind of data we are able to get on a routine basis with the LBT and MODS. Our entire instrument team is very proud of how well MODS is working.”
This important study was completed with only one half of the MODS´ system operational: MODS1 — currently installed on one mirror of the LBT. MODS2, the twin to MODS1, is under construction and is scheduled to be installed in early 2013.