Chandra Supernova Resources:NASA will hold a teleconference with reporters at 1 p.m. EST on Wednesday, Feb. 17, to discuss the latest Chandra X-ray Observatory findings that advance our understanding of certain supernovae. This research is critical for studying dark energy, which astronomers believe pervades the universe.
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Audio of the teleconference will be streamed live on NASA's Web site at: http://www.nasa.gov/newsaudio
Type Ia supernovae, often used to calibrate cosmological measurements, may arise from merging white dwarfs, after all.
When stellar cataclysms known as type Ia supernovae flare up far across the universe, their brightness and consistency allow astronomers to use them as so-called standard candles to measure cosmological distances. Just over a decade ago, two teams used the supernovae to show that the universe is accelerating in its expansion due to the influence of dark energy, a shocking discovery that thrust type Ia supernovae into the astrophysical limelight. But how exactly did these cosmic mileposts come to be?
Ill-fated encounters between stellar couples may be responsible for the spectacular explosions used to measure the effects of dark energy, a new study suggests.
X-ray observations of the explosions could shift dark energy measurements.
New X-ray findings appear to have blown a hole in the leading model for the origin of stellar explosions called type 1a supernovas. Astronomers routinely use these bright supernovas to measure dark energy, a baffling entity thought to rev up the rate of expansion of the universe.
Type Ia supernovae are very important exploding stars. It’s thought that this particular type of supernova has a very special property: they all explode with about the same energy. This makes them very valuable, because it means that if you can simply measure how bright they appear to be, you can figure out how far away they are. It’s like seeing headlights on the highway; dim ones are far away, and bright ones are close.
Of course, in reality, it’s not that easy. But after a Herculean effort, astronomers in the late 1990s figured they had been able to account for any small differences in brightness and could use these stars as "standard candles", benchmarks to calculate cosmic distances. Because they’re so bright, they make great milestones because they can be seen pretty much all the way to the edge of the observable Universe.
he thing is, it’s not clear how a type Ia actually forms. There are two models, both involving white dwarfs. These are the ultradense remnants of dead stars, the exposed cores of stars after they shed their outer layers. The Sun will one day be a white dwarf (in about 6 – 7 billion years, so don’t hold your breath). Because of complicated quantum physics, it turns out that white dwarfs can only have so much mass; if they exceed about 1.4 times the mass of the Sun they can collapse, either forming an even denser neutron star, or exploding as a supernova.
Explosions that scientists have long used to measure the universe are finally explained as merging, dying stars.
- The "standard candles" used to measure cosmic distances are ignited by dead stars merging.
- Many scientists favored another scenario in which a dead white dwarf star steals from a sun-like star.
- The multi-million-year X-ray "fuses" that would accompany the stealing scenario was found lacking in six nearby galaxies.
Whenever I hear a claimed discovery that overturns conventional wisdom on some important aspect of astronomy, my skepticism meter goes on high alert. Such was the case on Wednesday, when I listened to a NASA press conference in which two astronomers based in Germany presented evidence arguing that the most popular model for Type Ia supernovae is incorrect, at least for elliptical galaxies.
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In the standard picture, the "accreting white dwarf" scenario occurs much more frequently than the merger scenario, because accreting binary systems should be more common. In addition, some theories predict that when two white dwarfs merge, they collapse to form a neutron star, with no supernova explosion whatsoever. As astrophysicist Mario Livio (Space Telescope Science Institute) points out, "Whether or not mergers can indeed produce Type Ia supernovae is not clear from a theoretical standpoint."
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"Our results suggest the supernovae in the galaxies we studied almost all come from two white dwarfs merging," says Bogdan in a Chandra press release. "This is probably not what many astronomers would expect."
To find out if this was a valid claim, I consulted a number of leading experts in the field. Although everyone seemed to agree that Gilfanov and Bogdan took a creative approach toward addressing the problem, the replies were all over the map, and it's clear that the origin of SN Ia remains very much an open question.
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Alex Filippenko (University of California, Berkeley) adds, "I'm not disagreeing with Gilfanov and Bogdan that some Type Ia supernovae come from white dwarf mergers. People have been saying this for a long time. It's just that we've had trouble finding actual binary white dwarfs that have the potential to merge in the age of the universe."
Brad Schaefer (Louisiana State University) points out that the Gilfanov/Bogdan study assumes that all accreting white dwarf binaries shine in X-rays at a relatively uniform and steady rate, but that doesn't hold true in many known systems that contain a white dwarf and a normal star. "Their conclusion is clearly and easily wrong," he says.
arXiv.org > astro-ph > arXiv:1003.2217 > 10 Mar 2010An 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.
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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.
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Using observations from telescopes in Chile, Hawaii and California, the team was able to measure the mass of the central star (SN 2007if), 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.
Just think...we were *THAT CLOSE* to having the Colbert X-ray Observatory.http://en.wikipedia.org/wiki/Subrahmanyan_Chandrasekhar wrote:
<<Chandrasekhar's most notable work was the astrophysical Chandrasekhar limit. The limit describes the maximum mass of a white dwarf star, ~1.44 solar masses, or equivalently, the minimum mass, above which a star will ultimately collapse into a neutron star or black hole (following a supernova). The limit was first calculated by Chandrasekhar in 1930 during his maiden voyage from India to Cambridge, England for his graduate studies. In 1999, NASA named the third of its four "Great Observatories'" after Chandrasekhar. This followed a naming contest which attracted 6,000 entries from fifty states and sixty-one countries. The Chandra X-ray Observatory was launched and deployed by Space Shuttle Columbia on July 23, 1999. The Chandrasekhar number, an important dimensionless number of magnetohydrodynamics, is named after him.>>
http://en.wikipedia.org/wiki/Champagne_Supernova_%28astronomy%29 wrote:
<<The SN 2003fg (sometimes called the "Champagne Supernova"), was an aberrant type Ia supernova discovered in 2003 and described in the journal Nature on September 21 of 2006. It was nicknamed after the 1995 song "Champagne Supernova" by English rock band Oasis.
It may potentially revolutionize thinking about the physics of supernovae because of its highly unusual nature, in particular the mass of its progenitor. According to the current understanding, white dwarf stars go supernova type Ia when they approach 1.4 solar masses, termed the Chandrasekhar limit; the explosion occurs when the central density grows to a critical 2 × 109 g/cm3. The mass added to the star is believed to be donated by a companion star, either from the companion's stellar wind or the overflow of its Roche lobe as it evolves. However, the progenitor of SN 2003fg reached two solar masses before exploding, more massive than thought possible.
The primary mechanism invoked to explain how a white dwarf can exceed the Chandrasekhar mass is unusually rapid rotation; the added support effectively increases the critical mass.
An alternative explanation is that the explosion resulted from the merger of two white dwarfs. The evidence indicating a higher than normal mass comes from the light curve and spectra of the supernova --- while it was particularly overluminous the kinetic energies measured from ejecta signatures in the spectra appeared smaller than usual. The explanation is that more of the total kinetic energy budget was expended climbing out of the deeper than usual potential well.
This is important because the brightness of type Ia supernovae was thought to be essentially uniform, making them useful "standard candles" in measuring distances in the universe. Such an aberrant type Ia supernova could throw distances and other scientific work into doubt; however, the light curve characteristics of SNLS-03D3bb were such that it would never have been mistaken for an ordinary high-redshift Type Ia supernova.
The discovery was made on the Canada-France-Hawaii Telescope and the Keck Telescope, both on Mauna Kea in Hawaii, and announced by researchers at the University of Toronto. The supernova occurred in a galaxy some 4 billion light-years from Earth.>>
The international Nearby Supernova Factory (SNfactory) based at Berkeley Lab has measured, for the first time, the mass of a kind of Type Ia supernova astronomers once thought would be so exceedingly rare they might never be found.
Richard Scalzo of Yale University, leading a team of his SNfactory colleagues, found that the progenitor star (or stars) of the extra-bright supernova 2007if had almost two and a half times the mass of our sun – a so-called super-Chandrasekhar-mass supernova.
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From the complementary spectral and photometric observations, it emerged that SN2007if was – except for the richness of its datasets – not quite unique. In fact it was remarkably similar to a supernova found by the Supernova Legacy Search (SNLS) in 2003, known as SNLS-03D3bb (SN2003fg), another extrabright Type Ia with slowly moving ejecta. SNLS-03D3bb was the first Type Ia supernova interpreted as having super-Chandrasekhar mass.
Two other Type Ia supernovae also seemed similar, having excess luminosity, low velocity ejecta, and other features such as evidence of unburned carbon. These clues pointed Scalzo to what he calls a “semi-analytical model,” an approximate but consistent and logical picture of what might account for these features.
Spectra and Light Curves of Six Type Ia Supernovae at 0.511 < z < 1.12 and the Union2 CompilationNarrower constraints from the newest analysis aren’t quite narrow enough
The Supernova Cosmology Project’s Union2 compilation and reanalysis of decades of supernova surveys from the world’s leading researchers, with the addition of six high-redshift supernovae, puts new bounds on possible values for the nature of dark energy. Einstein’s cosmological constant comfortably fits the data, but there’s still plenty of room at the top for dynamical theories.
New Cause for Supernova Explosion Identified
Supernovae, gigantic stellar explosions, are not only used as cosmic yardsticks by cosmologists, they are also important chemical element factories in our Universe. So far, astrophysicists know of two physical processes giving rise to these bursts: one is the core collapse of a massive star at the end of its lifetime, the other the thermonuclear detonation of an old white dwarf star. An international team of researchers, including scientists from the Max Planck Institute for Astrophysics, have now identified a third type of these stellar explosions, arising from a helium-rich, old stellar system. (Nature, 20 May 2010)
In the past decade, robotic telescopes have turned astronomers' attention to scads of strange exploding stars, one-offs that may or may not point to new and unusual physics.
But supernova (SN) 2005E, discovered five years ago by the University of California, Berkeley's Katzman Automatic Imaging Telescope (KAIT), is one of eight known "calcium-rich supernovae" that seem to stand out as horses of a different color.
"With the sheer numbers of supernovae we're detecting, we're discovering weird ones that may represent different physical mechanisms compared with the two well-known types, or may just be variations on the standard themes," said Alex Filippenko, KAIT director and UC Berkeley professor of astronomy. "But SN 2005E was a different kind of 'bang.' It and the other calcium-rich supernovae may be a true suborder, not just one of a kind."
An explosive pairDevoid of carbon, oxygen, but rich in helium, says U of T researcher
An international team of astronomers has uncovered a supernova whose origin cannot be explained by any previously known mechanism and which promises exciting new insights into stellar explosions.
SN2005E was first spotted on January 13, 2005 in the nearby galaxy NGC1032. Since then, scientists have carried out various observations of it using different telescopes including the Keck, the world's largest, at Mauna Kea, Hawaii. Analysis of the collected data, theoretical modeling and interpretation led to the conclusion that SN2005E wasn't a typical supernova. Supernovae result from the collapse of very massive stars or by thermonuclear detonation on the surface of white dwarf stars composed mainly of carbon and oxygen.
"But this one, although it appears to be from a white dwarf system, is devoid of carbon and oxygen. Instead it's rich in helium. It's surprisingly different," said Dae-Sik Moon of the University of Toronto's Department of Astronomy & Astrophysics, a member of the team publishing their findings in Nature on May 20.
A faint type of supernova from a white dwarf with a helium-rich companionThe discovery of a new type of supernova may shed light on some universal mysteries
Not all explosions are created equal: It's as true for film effects as it is for the stars. Yet, until now, scientists had only observed two basic kinds of exploding stars, known as supernovae. Now, scientists at the Weizmann Institute of science, in collaboration with others around the world, have identified a third type of supernova. Their findings appeared this week in Nature.
The first two types of supernova are either hot, young giants that go out in a violent display as they collapse under their own weight, or old, dense white dwarves that blow up in a thermonuclear explosion. The new supernova appeared in telescope images in early January, 2005 and scientists, seeing that it had recently begun the process of exploding, started collecting and combining data from different telescope sites around the world, measuring both the amount of material thrown off in the explosion and its chemical makeup. But Dr. Avishay Gal-Yam, Hagai Perets, (now at the Harvard-Smithsonian Center for Astrophysics), Iair Arcavi and Michael Kiewe of the Weizmann Institute's Faculty of Physics, together with Paolo Mazzali of the Max-Planck Institute for Astrophysics, Germany, and the Scuola Normale Superiore, Pisa, and INAF/Padova Observatory in Italy, Prof. David Arnett from the University of Arizona, and researchers from across the USA, Canada, Chile and the UK, soon found that the new supernova did not fit either of the known patterns.
An Unusual Supernova May Be a Missing Link in Stellar Evolution ResearchA research group led by Koji Kawabata of Hiroshima University, Keiichi Maeda, Ken’ichi Nomoto, Masaomi Tanaka of IPMU and their collaborators published their result in Nature (2010 May 20 issue). In the paper, they reported observations of a peculiar type Ib supernova 2005cz, and concluded that this is a supernova whose progenitor mass at its birth was about 10 times the Sun. Such a star represents a boundary between stars that end their lives with the gigantic supernova explosion and those without explosions. Supernovae from stars that were originally about 10 solar mass should occupy a large fraction of supernova explosions taking place in the whole universe. However a supernova whose progenitor mass lies just above the boundary has not been identified. This is a reason why astronomers have been seeking for an explosion in this mass range. This study thus finally provides a solid confirmation on the stellar evolution theory. Having identified properties of the resulting supernova explosion, this study also serves as an important step forward to understand roles of supernovae in evolution of the universe.
This SN was very peculiar: It was intrinsically faint, quickly faded, and showed much weaker [O I] 6300, 6364 line than [Ca II] 7291, 7323 in the late-phase spectrum (Fig. 2). The appearance in elliptical galaxy was also peculiear, since a genuine elliptical galaxy contains only old pupulation stars, that is, low-mass stars and cannot produce any core-collapse supernova.
The team proposed that these peculiarities are well explained by rather `normal' core-collapse supernova from a less massive progenitor star (main-sequence mass of approximately 10 solar mass) than other Type Ib SNe. This mass corresponds to the low-mass end of the range of massive stars that explode. Such stars die as supernova about a few ten milion years after their birth, which is consistent with the recent studies suggesting that the host galaxy probably underwent a galactic merger and would stimulate a global star formation 10-100 milion years ago. This is the first evidence of the theoretical prediction that stars in the 8-12 solar mass range can explode as supernovae. Such stars might have a very special abundance pattern in the ejecta and play important roles in the chemical evolution of galaxies.
There's more than one way to blow up a star—in fact, scientists know of two. But could there be a third? A team of astronomers has identified two supernovae that don't seem to fit into established categories, though another team claims that there's not much new about them. If the first team is right, it could solve a longstanding mystery about the origins of one of the basic elements needed for life.
The first type of supernova occurs when one star in a binary pair explodes. A white dwarf star, roughly the mass of our sun, keeps snatching excess gas from its close-orbiting stellar companion. At a certain point, the extra gas renders the consuming star unstable, like a balloon filling with too much air, and the accumulated gas triggers a thermonuclear explosion so powerful that the star blows itself completely out of existence. This class of supernovae include what are called Type Ia, the brightness and duration of which are so precise that astronomers use them to gauge distances to other galaxies and to track the rate of acceleration of the universe.
The second general variety occurs when a young, giant star greater than about 10 solar masses collapses under its own weight. The gravitational crunching of the star's core releases a tremendous amount of energy, propelling a good deal of the star's mass out into space as a blinding explosion and leaving behind a massive remnant, either a neutron star or a black hole. Such stars go out like candles in the wind, by stellar standards. Instead of burning for billions of years, like our sun, these giants can go supernova within 30 million years.
But there may be other supernovae that don't quite fit into these two tidy categories. Take the case of a pair of misfit supernovae called SN 2005cz and SN 2005E, discovered 5 years ago. Each is located in an elliptically shaped galaxy—the first in NGC 4589, about 80 million light-years from Earth, and the second is in NGC 1032, about 100 million light-years away. In both cases, astronomers thought at first that they were seeing the explosive collapse of giant stars. But the light was too faint and faded much too quickly to fit the core-collapse category. Adding to the mystery, elliptical galaxies are very old and contain few new stars. So how could a young, giant star blow up in such a place?
--------------------------------http://www.universetoday.com/2010/05/19/a-new-kind-of-supernova-explodes-in-unusual-way/#more-64655 wrote:
A New Kind of Supernova Explodes in Unusual Way
Written by Nancy Atkinson May 19th, 2010
SN 2005E, from a DSS image via Astrosurf.com.
<<Not all supernovae are created equal, astronomers are finding. A faint supernovae found by international teams of scientists is like nothing previously seen, and cannot be explained by conventional insights into these exploding stars. Until now, only two basic kinds of supernovae had been observed. But now there appears to be a third.
"The supernova explosion is the most energetic and brilliant event that happens in the universe," said Dae-Sik Moon from the University of Toronto, and part of a team publishing their findings this week in Nature. "It is rich with information, not only about how stars die, but to understanding the origin of life and the expansion of the universe. But this one is surprisingly different."
The first two types of supernova are either hot, young giants that go out in a violent display as they collapse under their own weight, or old, dense white dwarves that blow up in a thermonuclear explosion.
White dwarf stars are composed mainly of carbon and oxygen, and although the supernova, SN2005E, appears to be from a white dwarf system, it is devoid of carbon and oxygen and instead is rich in helium.
The environment of SN 2005E. The image to the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right. Note the remote location of the supernova (marked by the arrow) with respect to its host, about 750,000 light-years from the galaxy nucleus. Credit: Sloan Digital Sky Survey / Lick Observatory
SN2005E was first spotted on January 13, 2005 in the nearby galaxy NGC1032, and since then scientists have carried out various observations of it using different telescopes.
On the one hand, the amount of material hurled out from the supernova was too small for it to have come from an exploding giant. In addition, its location, distant from the busy hubs where new stars form, implied it was an older star that had had time to wander off from its birthplace. On the other hand, its chemical makeup didn't match that commonly seen in the second type.
"It was clear," said lead author Hagai Perets from the Weizmann Institute in Israel and the Harvard-Smithsonian Center for Astrophysics, "that we were seeing a new type of supernova."
SN 2005E had unusually high levels of the elements calcium and titanium, which are the products of a nuclear reaction involving helium, rather than carbon and oxygen.
"We've never before seen a spectrum like this one," said Paolo Mazzali of the Max-Planck Institute for Astrophysics. “Once thereceiving star has accumulated a certain amount, the helium starts to burn explosively. The unique processes producing certain chemical elements in these explosions could solve some of the puzzles related to chemical enrichment. This could, for example, be the main source of titanium.”
Computer simulations to see what kind of process could have produced such a result suggest that a pair of white dwarves are involved; one of them stealing helium from the other. When the thief star's helium load rises past a certain point, the explosion occurs.
"The donor star is probably completely destroyed in the process, but we're not quite sure about the fate of the thief star," said team member Avishay Gal-Yam. In fact, the astronomers say these relatively dim explosions might not be all that rare.
Alex Filippenko from UC Berkeley professor and colleague Dovi Poznanski, both part of the team studying SN 2005E reported last November another supernova, SN 2002bj, that they believe exploded by a similar mechanism: ignition of a helium layer on a white dwarf.
“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf accreting helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”
But this new type of supernova could explain some puzzling phenomena in the universe. For example, almost all the elements heavier than hydrogen and helium have been created in, and dispersed by supernovae; the new type could help explain the prevalence of calcium in both the universe and in our bodies.
It might also account for observed concentrations of particles called positrons in the center of our galaxy. Positrons are identical to electrons, but with an opposite charge, and some have hypothesized that the decay of yet unseen 'dark matter' particles may be responsible for their presence. But one of the products of the new supernova is a radioactive form of titanium that, as it decays, emits positrons.
"Dark matter may or may not exist," said Gal-Yam, "but these positrons are perhaps just as easily accounted for by the third type of supernova.">>
Carl Sagan's immortal words - "The Earth and every living thing are made of star stuff" - came to be this week. Astronomers have reported a whole new type of exploding star, or supernova, which seems to spew out calcium and titanium. So, besides carbon - gifted to us from another type of supernova - the calcium in our bones is definitely made of star stuff.
While most press reports have focused on the calcium, it's the titanium that's really interesting - the finding could throw a monkey wrench into ongoing efforts to find the signature of dark matter colliding at the centre of the Milky Way.
But first a bit about supernovae. Astronomers thought that supernovae came in two flavours. One is when massive, short-lived stars undergo gravitational collapse and eject a few solar masses' worth of material. This includes type Ib and Ic and type II supernovae. The other category - type Ia - is the thermonuclear explosion of a star that approaches a mass threshold beyond which it cannot support itself.
In Nature this week, Hagai Perets of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and colleagues propose a new type of supernova. New Scientist first reported on the claim last year.
Perets and colleagues describe a scenario with a pair of orbiting white dwarf stars, where one star was stealing helium from another. When its helium load increased to the point of instability, the white dwarf exploded. Because it was being fuelled by helium, the star was producing calcium and titanium.
The titanium is radioactive and emits positrons as it decays. Over the past couple of years, there have been reports from experiments such as ATIC and PAMELA of an excess of positrons coming from deep space. This excess, it has been argued, is a signature of dark matter particles colliding. But if the new supernova finding is anything to go by, these explosions could be quite commonplace and they could be the source of the excess positrons.
While this does not prove or disprove the existence of dark matter, it sure puts a damper on the euphoric interpretation that the excess positrons are coming from the annihilation of dark matter particles.
Stellar explosions provide the key to understanding the fate of the Universe
The mysteries of the Universe and how we came to be are set to be unlocked by a technique for modelling fluids, similar to one which is becoming increasingly popular within the film industry to improve the realism of special effects.
Theoretical Astrophysics student, Fergus Wilson from the University of Leicester, is currently utilising a fluid modelling technique within his doctoral research to enable investigation of the mass transfer from one star to another in a binary star system.
Smoothed Particle Hydrodynamics (SPH) is a computational method for modelling fluid as a set of moving particles and can be used to solve the equations of motion between two or more particles. A similar technique has been used to enhance the special effects in blockbuster Hollywood movies such as Tomb Raider and The Matrix Reloaded.
Mr Wilson uses the SPH method to model the explosive eruptions of dying stars to provide vital clues to the current accelerated expansion of the Universe. Preliminary results from the study will be showcased at the University of Leicester’s Festival of Postgraduate Research on 24 June. Mr Wilson’s research focuses on Type Ia supernovae, which occur when White Dwarf stars explode upon reaching a critical mass. His simulations model the formation of discs around accreting stars within a binary star system.
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His simulations investigate the different effects the wind speed and rotation of the ‘mass feeding’ stars will have on the disc size and how the energy in the blast waves effects the disc disruption to aid understanding of the process which will hopefully lead to future technological advancements.
Type Ia supernovae: maintaining the standardAn international collaboration of researchers led by Keiichi Maeda ... has solved a mystery regarding type Ia supernovae. Type Ia supernovae form a homogeneous class of stellar explosions and are used as “standard candles” to study the acceleration of the Universe. However, it has long been known that type Ia supernovae that otherwise look like twins can show diverse spectral features, questioning their accuracy as standard candles. The research group argues that type Ia supernovae explode asymmetrically, and that the different spectral appearance is merely a consequence of the random directions from which an SN is observed. This result is good news for cosmology - it suggests that the spectral diversity is no longer a major concern in using type Ia supernovae as cosmological standard candles (Nature, 1 July 2010 issue).
Type Ia supernovae have been playing a key role in cosmology and in the discovery of the Dark Energy, since they can be used to measure distances across the Universe. This relies on the well-developed relation between their brightness and the decline-rates – brighter supernovae decline slower.
This single parameter description of type Ia supernovae may well reflect the uniform nature of the progenitor system. Type Ia supernovae are explosions of a white dwarf consisting of carbon and oxygen. The explosion is triggered by sparks of thermonuclear flames ignited in the innermost part of the white dwarf, although how the explosion is initiated is still in debate. The explosion most likely takes place when the white dwarf reaches the so-called Chandrasekhar mass (about 1.4 times the Sun) through accreting materials from its binary companion or by merging with another white dwarf.
However, recent investigations have revealed that the true nature of type Ia supernovae is far more complicated. They indeed do not look like a uniform system in their spectral features – type Ia supernovae that look like twins concerning their luminosity evolution can demonstrate a quite different behavior in the speed with which their spectral features evolve (the so-called velocity gradient). This spectral evolution diversity was first noticed in the late 80’s, and quantified beyond the doubt in 2005. The origin of this diversity has not been clarified, raising a couple of concerns – Are they really good standard candles? Are they indeed from a uniform progenitor system?
Astrophysics: The supernova has two facesThe status of type Ia supernovae as cosmological 'standard candles' relies upon the assumption that they are very similar to one another and form a uniform class of objects. Recently, however, observational differences between type Ia supernovae have emerged. Maeda et al. propose a new model that takes account of recent theoretical and observational data and explains the observed spectral diversity as a consequence of random directions from which a theoretically proposed asymmetric explosion is viewed. On this basis, the spectral evolution diversity is no longer a concern in the continued use of type Ia supernovae as standard candles.
An asymmetric explosion as the origin of spectral evolution diversity in type Ia supernovaeThe variety of stellar deaths is less than we thought. A compilation of new and archival data shows that two previously distinct subtypes of supernova are actually two sides of the same lopsided coin.
Searching for the Elusive Type Ia Supernovae ProgenitorsWhen a star explodes as a supernova, it shines so brightly that it can be seen from millions of light-years away. One particular supernova variety - Type Ia - brightens and dims so predictably that astronomers use them to measure the universe's expansion. The resulting discovery of dark energy and the accelerating universe rewrote our understanding of the cosmos. Yet the origin of these supernovae, which have proved so useful, remains unknown.
"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy," says Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics (CfA).
Astronomers have very strong evidence that Type Ia supernovae come from exploding stellar remnants called white dwarfs. To detonate, the white dwarf must gain mass until it reaches a tipping point and can no longer support itself.
There are two leading scenarios for the intermediate step from stable white dwarf to supernova, both of which require a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomers look for evidence of these binary systems.
Given the average rate of supernovae, scientists can estimate how many pre-supernova white dwarfs should exist in a galaxy. But the search for these progenitors has turned up mostly empty-handed.
The Progenitors of Type Ia Supernovae. I. Are they Supersoft Sources?Astronomers have Type Ia supernovae pretty well figured out. The way these exploding stars brighten and then dim are so predictable that they have been used to measure the universe's expansion. This reliability led to the discovery that our universe was not only expanding but accelerating, which in turn led to the discovery of dark energy. There's just one minor detail: nobody knows for sure what causes a supernova.
"The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy," says Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics.
Astronomers are sure that for a Type Ia supernova, the energy for the explosion comes from the run-away fusion of carbon and oxygen in the core of a white dwarf. To detonate, the white dwarf must gain mass until it reaches a tipping point and can no longer support itself.
But how does a white dwarf get bigger? There are two leading scenarios for what leads a stable white dwarf to go ka-boom, and both include a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomers look for evidence of these binary systems.
