Posted on by Jean Tate

GRB Central Engines Observed in Nearby Supernovae?

SN 2009bb (Image Credit: NASA, Swift, Stefan Immler)

[/caption]
Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?

The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.

“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

SN 2007gr (Image Credit: Z. Paragi, Joint Institute for VLBI in Europe (JIVE))

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”

Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.

“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”

The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.

“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”

The 28 January, 2010 issue of Nature contains two papers reporting these discoveries: A relativistic type Ibc supernova without a detected γ-ray burst (arXiv:0908.2817 is the preprint), and A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr (arXiv:1001.5060 is the preprint).

Sources: Newborn Black Holes May Add Power to Many Exploding Stars, Newborn Black Holes Boost Explosive Power of Supernovae

Posted on by Jean Tate

Hypernova

Artist impression of the twin jets from a GRB. Credit: Dana Berry/SkyWorks Digital

[/caption]
Nova, “new star”; supernova, a “super” nova; hypernova, a super-duper, or super super, nova!

This word appeared in the astronomical literature at least as early as 1982, and refers to a kind of core-collapse supernova far brighter (>100 times) than usual; its meaning has changed somewhat, and today generally refers to the core collapse of particularly massive stars (>100 sols), whether or not they are spectacularly brighter than other core-collapse supernovae (though they are that too).

Most times you’ll come across hypernovae in material on gamma ray bursts (GRBs), many of which seem to involve emission of electromagnetic radiation with total energy many times that from ordinary supernovae (whether core collapse or Type Ia). Long-duration GRBs have jets, presumably from the poles of the temporary accretion disk which forms around the new black hole at the heart of the collapsed core of the progenitor (short-duration GRBs, which also produce jets, are thought to be the merger of two neutron stars, or a neutron star and a stellar-mass black hole), but even when viewed side-on (i.e. not looking into one of the jets), these GRBs are intrinsically much brighter than other core collapse supernovae.

If a supernova were to occur a few hundred light-years from us, we’d certainly notice it, and there might be some impact on our atmosphere; if there was a hypernova the same distance away, we’d suffer (not only from the increased incidence of cancer due to the far greater intensity of cosmic rays, but also from changes in weather and climate, and damage to ecosystems); if the jet were aimed directly at us, we’d be toast (while those on the other side of the world would survive the few seconds-long blast, they’d die from the consequences).

Fortunately, it seems there are no stars likely to go hypernova on us … at least not within a few tens of thousands of light-years. Whew!

Have I whet your appetite for more? Check these sites out! Brighter than an Exploding Star, It’s a Hypernova! (NASA’s Imagine the Universe), Face on Beauty (Phil Plait), and Hypernova (Swinburne University).

Like everyone else, Universe Today writers love a good story about explosions … so there are quite a few on hypernovae! Some examples: Gamma Ray Bursts and Hypernovae Linked, ESO Watches Burst Afterglow for Five Weeks, and Carbon/Oxygen Stars Could Explode as Gamma Ray Bursts.

No surprise that Astronomy Cast episode Gamma-Ray Bursts features hypermovae! Back in 2007, after attending the American Astronomical Society meeting, Pamela learning something new about hypernovae; what? Well, check out the episode, What We Learned from the American Astronomical Society and find out for yourself!

References:
NASA
ESO

Posted on by Nancy Atkinson

Supernova or GRB? Radio Observations Allow Astronomers to Find Unusual Object

Core-collapse supernova explosion expelling nearly-spherical debris shell. CREDIT: Bill Saxton, NRAO/AUI/NSF

[/caption]

For the first time, astronomers have found a supernova explosion with properties similar to a gamma-ray burst, but without seeing any gamma rays from it. Radio observations with the Very Large Array (VLA) showed material expelled from supernova explosion SN2009bb at speeds approaching the speed of light. The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. But astronomers don’t think this blast is one-of-a-kind, and say that more radio observations will point the way toward locating many more examples of these mysterious explosions.

“We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites,” said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.

Usually supernova explosions blasts the star’s material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.

Engine-driven

When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star’s material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a “core-collapse supernova” as the cause of one kind of gamma-ray burst.

The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.

“This is the only way we know that a supernova explosion could accelerate material to such speeds,” Soderberg said.

Until now, no such “engine-driven” supernova had been found any way other than by detecting gamma rays emitted by it.

“Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough. With the new capabilities of the Expanded VLA coming soon, we believe we’ll find more in the future through radio observations than with gamma-ray satellites,” Soderberg said.

Why didn’t anyone see gamma rays from this explosion? “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” Soderberg said. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.

“Another possibility,” Soderberg adds, “is that the gamma rays were ‘smothered’ as they tried to escape the star. This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.”

One important question the scientists hope to answer is just what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” Soderberg said. “We’d like to find out what that property is.”

One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen. However, Soderberg points out, that does not seem to be the case for this supernova.

This research will be published in January 28 issue of the journal Nature.

Source: NRAO

Posted on by Nicholos Wethington

Could A Faraway Supernova Threaten Earth?

Supernovae, just like any other explosions, are really cool. But, just like any other explosion, it’s preferable to have them happen at a good distance. The star T Pyxidis, which lies over 3,000 light-years away from the Earth in the constellation Pyxis, was previously thought to be far enough away that if anything happened in the way of a supernova, we’d be pretty safe.

According to Edward Sion, Professor of Astronomy and Physics at Villanova University, T Pyxidis may be in fact a “ticking time bomb,” and potential threat to the Earth if it were to go supernova, which it may do sometime in the future, though very, very far in the future on our timescale: by Scion’s calculations, at least 10 million years.

Sion presented his findings at the American Astronomical Society Meeting in Washington, D.C. earlier today. T Pyxidis, which lies in the constellation Pyxis, is what is called a recurring nova. The star, which is a white dwarf, accretes gas from a companion star. As the amount of matter increases in the white dwarf, it occasionally builds up to the point where there is a runaway thermonuclear reaction in the star, and it ejects large quantities of material.

T Pyxidis has had five different outbursts over the course of observations of the star. It was the American Association of Variable Star Observers’ variable star of the month in April, 2002.  The first was in 1890, followed by another outburst in 1902 (these two were discovered much later on photographic plates in the Harvard plate collection). The next three were in 1920, 1944 and 1967. Its average for outbursts is about 19 years, but there hasn’t been one since the 1966 brightening.

The distance estimate to T Pyxidis, revised to 3,260 light-years from the previously estimated distance of 6,000 light-years has prompted a reconsideration of the details about the white dwarf. Hubble images that have been taken of the star would then have to be re-examined so as to revise the amount of mass the star is expected to be ejecting.

If the recurring novae are ejecting enough material, then the white dwarf would stay small enough to continue to go through the phase of recurring novae. However, if the shells of gas repeatedly ejected by the star do not carry enough mass away, it would eventually build up to pass the Chandrasekhar limit – 1.4 times the mass of the Sun – and become a Type Ia supernova, one of the most destructive events in our Universe.

Sion concluded the presentation with the statement (shown here on his last powerpoint slide) that “A Type Ia supernova exploding within 1000 parsecs of Earth will greatly affect our planet”

A supernova within 100 light-years of the Earth would likely be a catastrophic event for our planet, but something as far out as T Pyxidis may or may not damage the Earth. One of the journalists in attendance pointed out this possibility during the questions session and Sion said that the main danger lies in the amount of X-rays and gamma rays that stream from such an event, which could destroy the protective ozone layer of the Earth and leave the planet vulnerable to the ultraviolet light streaming from the Sun.

There remains some doubt as to whether T Pyxidis will go supernova at all. There is a good treatment of this subject in “The Nova Shell and Evolution of the Recurrent Nova T Pyxidis” by Bradley E. Schaefer et al. on Arxiv.

If you’re worried about the dangers of exploding stars, you should check out this video by Phil Plait, the Bad Astronomer. He’ll calm you down.

Source: AAS Press Conference on USTREAM, Space.com

Posted on by Nancy Atkinson

Shapes Reveal Supernovae History

These two supernova remnants are part of a new study from NASA’s Chandra X-ray Observatory that shows how the shape of the remnant is connected to the way the progenitor star exploded. Credit: NASA/CXC/UCSC/L. Lopez et al.)

At a very early age, children learn how to classify objects according to their shape. Now, new research suggests studying the shape of the aftermath of supernovas may allow astronomers to do the same. Images of supernova remnants taken by the Chandra X-ray Observatory shows that the symmetry of the debris from exploded stars, or lack thereof, reveals how the star exploded. This is an important discovery because it shows that the remnants retain information about how the star exploded even though hundreds or thousands of years have passed.

“It’s almost like the supernova remnants have a ‘memory’ of the original explosion,” said Laura Lopez of the University of California at Santa Cruz, who led the study. “This is the first time anyone has systematically compared the shape of these remnants in X-rays in this way.”

Astronomers sort supernovas into several categories, or “types”, based on properties observed days after the explosion and which reflect very different physical mechanisms that cause stars to explode. But, since observed remnants of supernovas are leftover from explosions that occurred long ago, other methods are needed to accurately classify the original supernovas.

Lopez and colleagues focused on the relatively young supernova remnants that exhibited strong X-ray emission from silicon ejected by the explosion so as to rule out the effects of interstellar matter surrounding the explosion. Their analysis showed that the X-ray images of the ejecta can be used to identify the way the star exploded. The team studied 17 supernova remnants both in the Milky Way galaxy and a neighboring galaxy, the Large Magellanic Cloud.

Chandra X-ray Image of SNR 0548-70.4 (Credit: NASA/CXC/UCSC/L. Lopez et al.)
Chandra X-ray Image of SNR 0548-70.4 (Credit: NASA/CXC/UCSC/L. Lopez et al.)

For each of these remnants there is independent information about the type of supernova involved, based not on the shape of the remnant but, for example, on the elements observed in it. The researchers found that one type of supernova explosion – the so-called Type Ia – left behind relatively symmetric, circular remnants. This type of supernova is thought to be caused by a thermonuclear explosion of a white dwarf, and is often used by astronomers as “standard candles” for measuring cosmic distances.

On the other hand, the remnants tied to the “core-collapse” supernova explosions were distinctly more asymmetric. This type of supernova occurs when a very massive, young star collapses onto itself and then explodes.

“If we can link supernova remnants with the type of explosion”, said co-author Enrico Ramirez-Ruiz, also of University of California, Santa Cruz, “then we can use that information in theoretical models to really help us nail down the details of how the supernovas went off.”

Models of core-collapse supernovas must include a way to reproduce the asymmetries measured in this work and models of Type Ia supernovas must produce the symmetric, circular remnants that have been observed.

Out of the 17 supernova remnants sampled, ten were classified as the core-collapse variety, while the remaining seven of them were classified as Type Ia. One of these, a remnant known as SNR 0548-70.4, was a bit of an “oddball”. This one was considered a Type Ia based on its chemical abundances, but Lopez finds it has the asymmetry of a core-collapse remnant.

“We do have one mysterious object, but we think that is probably a Type Ia with an unusual orientation to our line of sight,” said Lopez. “But we’ll definitely be looking at that one again.”

While the supernova remnants in the Lopez sample were taken from the Milky Way and its close neighbor, it is possible this technique could be extended to remnants at even greater distances. For example, large, bright supernova remnants in the galaxy M33 could be included in future studies to determine the types of supernova that generated them.

The paper describing these results appeared in the November 20 issue of The Astrophysical Journal Letters.

Source: Chandra

Posted on by Nicholos Wethington

Superbright Supernova First Observed of Antimatter Variety

The supernova 2007bi, circled in the image above, might be the first confirmation of a pair-instability supernova. Image Credit: Nearby Supernova Factory

[/caption]

The supernova 2007bi wasn’t your typical supernova: it was 10 times brighter than a Type Ia supernova, making it one of the most energetic supernova events ever recorded. Astronomers from the University of California Berkeley have analyzed the explosion, which was recorded by a robotic survey in 2007, and found that it is likely the first confirmed observation ever made of a pair-instability supernova, a type of extremely energetic supernova that has been theorized but never directly confirmed.

The confirmed observation of a pair-instability supernova has been long-awaited – the theory that they exist has been around since the 1960’s – but it appears as if the wait is over. The supernova 2007bi, seen by the Nearby Supernova Factory in April of 2007, is the first observed supernova that fits the bill for the unfathomably huge proportions of pair-instability supernovae explosions. A team of astronomers led by Alex Filippenko of the University of California Berkeley published their analysis in in the December 3rd issue of Nature. The discovery was initially made by the Nearby Supernova Factory, and emission spectra of the event was taken with the Keck Telescope and Very Large Telescope in Chile 

These type of supernovae occur only in stars above 100 solar masses, and are incredibly bright. Energetic gamma rays are created by the intense heat in the core of the star. These gamma rays, in turn, create antimatter pairs of electrons and positrons. Because of this antimatter production, the outward pressure exerted by the nuclear reactions in the core of the star is lessened, and gravity takes over, quickly collapsing the massive core of the star and creating a supernova.

There are theorized to be two kinds: those that explode with just enough force to allow for the mass around the leftover core of the star to recombine, and those that explode completely with not a smidgen left to form a black hole or neutron star. The supernova 2006gy, which had a luminosity 10 times that of a Type Ia supernova, is thought to be of the first variety. Here’s our story on that one, Could Antimatter be Powering Super-Luminous Supernovae? and Eta Carinae may also fit the profile.hese types of pair-instability supernovae will eject the outer shells of the star’s matter, settle down into an equilibrium, and repeat that process until the mass is low enough for a normal supernova to occur.

But 2007bi was much too massive to settle back down and explode multiple times. With a mass of 200 suns, the runaway thermonuclear explosion that happened in its core was energetic enough to effectively vaporize the entire star. Pair-instability supernovae in stars above 130 solar masses leave nothing behind in the way of black holes or neutron stars, but because they are so energetic and luminous, the increasing light from the explosion peaks over a very long time – 70 days in the case of 2007bi.

Though the team detected the supernova almost a week after the peak, they were able to calculate the duration of the light curve. They then studied the remnants of the explosion over the next 555 days as it faded away.

Filippenko said, “The central part of the huge star had fused to oxygen near the end of its life, and was very hot. Then the most energetic photons of light turned into electron-positron pairs, robbing the core of pressure and causing it to collapse. This led to a nuclear runaway explosion that created a large amount of radioactive nickel, whose decay energized the ejected gas and kept the supernova visible for a long time.”

The star was unique in another way: it lies in a nearby dwarf galaxy, which contains little else but the elements hydrogen and helium. Because of this, 2007bi is much like the stars that existed near the beginning of the Universe, before the trillions of supernovae populated the Universe with heavier elements. Looking more closely at dwarf galaxies – the Universe has them in spades, but they are quite dim – may be the key to observing more supernovae of this kind. Being able to study its explosion and aftereffects will give scientists a look into what the earliest massive stars acted like.

Source: Berkeley Lab press release

Posted on by Fraser Cain

Supernova

Crab Nebula

[/caption]
A supernova is the explosion of a star. In an instant, a star with many times the mass of our own Sun can detonate with the energy of a billion suns. And then, within just a few hours or days, it dims down again. Some explode into a spray of gas and dust, while others become exotic objects like neutron stars or black holes.

Astronomers have classified supernovae into two broad classifications: Type I and Type II. Type I supernovae occur in binary systems, where one star pulls off mass from a second star until it reaches a certain amount of mass. This causes it to explode as a supernova. Type II supernovae are the explosions of massive stars which have reached the end of their lives.

All of the elements heavier than iron were created in supernova explosions. As a massive star runs out of hydrogen fuel, it starts to fuse together heavier and heavier elements. Helium into carbon and oxygen. And then oxygen into heavier elements. It goes up the periodic table this way, fusing heavier elements until it reaches iron. Once a star reaches iron, it’s no longer able to extract energy from the fusion process. The core collapses down into a black hole, and the material around it is fused together into the elements heavier than iron. If you’re wearing any gold jewelry, that was created in a supernova.

In 1054 Chinese astronomers saw a supernova explosion that was so bright it was visible in the middle of the day. The explosion of gas and dust is now visible as the Crab Nebula (that’s the picture at the top of this article). The most recent powerful supernova explosion occurred in 1987, when a star exploded in the Large Magellanic Cloud.

Astronomers use Type I supernovae to judge distances in the Universe. This is because they always explode with approximately the same amount of energy. When a white dwarf star collected approximately 1.4 times the mass of the Sun, it can’t support its mass and collapses. This amount is called the Chandrasekhar Limit. When an astronomer sees a Type I supernova, they know how bright it is, and so they can measure how far away it is.

We’ve written many articles about supernovae for Universe Today. Here’s an article about a slow motion supernova, and here’s an article about a theoretical supernova that was actually found to exist.

If you’d like to see a gallery of supernova photographs, check out this section of the Hubble Space Telescope site, and here’s NASA’s Photo Gallery of Nebulae.

We’ve also recorded several episodes of Astronomy Cast about supernovas. Check out this one, Episode 14: We’re All Made of Supernovae.

References:
http://www.cfa.harvard.edu/supernova//newdata/supernovae.html
http://en.wikipedia.org/wiki/Chandrasekhar_limit

Posted on by Jon Voisey

Slow-Motion Supernova

This artist’s impression of a supernova shows the layers of gas ejected prior to the final deathly explosion of a massive star. Credit: NASA/Swift/Skyworks Digital/Dana Berry

[/caption]

Supernovae are generally considered as fast and furious events. For the Type II, core collapse supernovae, the core implodes almost instantaneously although it takes some time for the shockwave to escape the star. As it does, the star brightens in what’s known as the “rise time” of the supernova. For most Type II supernovae, this takes about a week.

So what are astronomers to make of supernova 2008iy that had an unprecedented rise time of at least 400 days?

From the time it was discovered, SN 2008iy was an oddball. When its spectra was analyzed, it was placed in the rare IIn subclass. This subclass is reserved for supernovae that feature narrow emission lines. Most supernovae have broad emission lines, if they even have emission lines at all.

To learn more about the history of this unusual case, astronomers at the University of California, Berkeley turned to archival images from the Palomar Quest survey. They searched images of the region to trace back the supernova as far as July of 2007, before which, the star was too faint to appear in images. Thus, the supernova brightening started at least that early and continued until late October of 2008 giving it a rise time at least four times as long as any previously discovered supernova.

The main clue to explain this mystery stemmed from the unusual emission lines. Generally, stars and supernovae are characterized by their absorption spectra which are caused when relatively cool gas stands between a hotter source and our detection. To generate emission lines, there must been a relatively dense medium being excited by the supernova. Furthermore, the fact that the lines were narrow implied that it was fairly motionless.

Together, this pointed to the progenitor undergoing a heightened period of mass loss prior to the detonation. The idea is such that the progenitor had shed large amounts of material. When the supernova occurred, this shell initially obscured the event. But as the ejecta from the supernova overtook the relatively stationary earlier shells, the brighter material slowly seeped out giving rise to the 400 day rise time.

While all stars undergo a period of mass loss in their post main sequence life, such a dense shell would be uncommon. To explain this, the authors turned to a type of star known as a Luminous Blue Variable. These stars are typically near the theoretical limit for the mass of a star (150 times the mass of the sun). Due to their extreme mass, they have strong stellar winds which periodically blow off large amounts of material that could create shells similar to those necessary for SN 2008iy. Unfortunately, this event was so distant that it could not be resolved to search for such a nebula. Even the host galaxy proved difficult to distinguish due to its faintness, although it is believed to be an irregular dwarf galaxy. Eta Carinae is one such luminous blue variable star. If perhaps one day soon it decides to turn into a supernova, it too will unfold in slow-motion.

Posted on by Mike Simonsen

Eta Carinae- A Naked Eye Enigma

Credit: X-ray: NASA/CXC/GSFC/M.Corcoran et al.; Optical: NASA/STScI

[/caption]

Eta Carinae is a beast of a star. At more than 100 solar masses and 4 million times the luminosity of our Sun, eta Car balances dangerously on the edge of stellar stability and it’s ultimate fate: complete self-destruction as a supernova. Recently, Hubble Space Telescope observations of the central star in the eta Carinae Nebula have raised an alert on eta Car among the professional community. What they discovered was totally unexpected.

“It used to be, that if you looked at eta Car you saw a nebula and then a faint little core in the middle” said Dr. Kris Davidson, from the University of Minnesota. “Now when you look at it, it’s basically the star with a nebula. The appearance is completely different. The light from the star now accounts for more than half the total output of eta Car. I didn’t expect that to happen until the middle of this century. It’s decades ahead of schedule. We know so little about these very massive objects, that if eta Car becomes a supernova next Thursday we should not be very surprised.”

In 1843, eta Carinae underwent a spectacular eruption, making it the second brightest star in the sky behind Sirius. During this violent episode, eta Car ejected 2 to 3 solar masses of material from the star’s polar regions. This material, traveling at speeds close to 700 km/s, formed two large, bipolar lobes, now known as the Homunculus Nebula. After the great eruption, Eta Car faded, erupted again briefly fifty years later, then settled down, around 8th magnitude. Davidson picks up the story from there.

This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.
This light curve depicts the visual apparent brightness of Eta Car from 1822 to date. It contains visual estimates (big circles), photographic (squares), photoelectric (triangles) and CCD (small circles) observations. All of them have been fitted for consistency of the whole data. Red points are recent observations from La Plata (Feinstein 1967; Fernández-Lajús et al., 2009, 2010). Used by permission.

“Around 1940, Eta suddenly changed its state. The spectrum changed and the brightness started to increase. Unfortunately, all this happened at a time when almost no one was looking at it. So we don’t know exactly what happened. All we know is that by the 1950’s, the spectrum had high excitation Helium lines in it that it didn’t have before, and the whole object, the star plus the Homunculus, was gradually increasing in brightness. In the past we’ve seen three changes of state. I suspect we are seeing another one happening now.”

During this whole time eta Car has been shedding material via its ferocious stellar winds. This has resulted in an opaque cloud of dust in the immediate vicinity of the star. Normally, this much dust would block our view to the star. So how does Davidson explain this recent, sudden increase in the luminosity of eta Carinae?

“The direct brightening we see is probably the dust being cleared away, but it can’t be merely the expansion of the dust. If it’s clearing away that fast, either something is destroying the dust, or the stellar wind is not producing as much dust as it did before. Personally, I think the stellar wind is decreasing, and the star is returning to the state it was in more than three hundred years ago. In the 1670’s, it was a fourth magnitude, blue, hot star. I think it is returning to that state. Eta Carinae has just taken this long to readjust from its explosion in the 1840’s.”

After 150 years what do we really know about one of the great mysteries of stellar physics? “We don’t understand it, and don’t believe anyone who says they do,” said Davidson.  “The problem is we don’t have a real honest-to-God model, and one of the reasons for that is we don’t have a real honest-to-God explanation of what happened in 1843.”

Can amateur astronomers with modest equipment help untangle the mysteries of eta Carinae? Davidson think so, “The main thing is to make sure everyone in the southern hemisphere knows about it, and anyone with a telescope, CCD or spectrograph should have it pointed at eta Carinae every clear night.”

Posted on by Nancy Atkinson

Found: Theoretical Supernova Actually Exists

A new kind of supernova. Credit: Tony Piro

[/caption]

Astronomers have identified a type of supernova that appears to be a type predicted in theory but never actually observed before. Two years ago Lars Bildsten from UC Santa Barbara and his colleagues predicted a new type of supernova in distant galaxies which they dubbed the “.Ia” (point one a) mechanism, involving a helium detonation on a white dwarf, ejecting a small envelope of material. This theoretical explosion would be fainter than most other supernovae and its brightness would rise and fall in only a few weeks. Dovi Poznanski from Berkeley went back and looked at seven-year-old observations and found this unusual kind of supernova. Poznanski and colleagues say supernova 2002bj belongs in its own category, as its spectra suggest that it evolved extremely fast and produced an unusual combination of elements.

Supernovae are usually classified based on tell-tale lines in the spectrum of radiation they emit. The two main types are thought to develop from exploding white dwarfs and collapsing massive stars.

However, Bildsten’s theory said that in rare instances, there is a binary star system where helium flows from one white dwarf onto another and accumulates on the more massive white dwarf.

It is this rare occurrence that leads to unique conditions of the explosive thermonuclear ignition and complete ejection of the accumulated helium ocean. The plethora of unusual radioactive elements made in the rapid fusion leads to a bright light show from the freshly synthesized matter that lasts a few weeks.
The “usual” explosions of white dwarfs are referred to as “Type Ia supernova.” They are brighter than a whole galaxy for more than a month and are quite useful in cosmological studies. The predicted “.Ia” supernovae are only one-tenth as bright for one-tenth the time.

Poznanski and his team say 2002bj fits the bill for this never-seen-before type of supernova.

“This is the fastest evolving supernova we have ever seen,” said Poznanski. “It was three to four times faster than a standard supernova, basically disappearing within 20 days. Its brightness just dropped like a rock.”

Poznanski told Universe Today that he was actually looking at Type II supernovae for another purpose when he hit the spectrum of 2002bj. “My first reaction was great confusion,” he said. “My second reaction, after showing it to other experts was greater confusion. After matching it against every object we know of, and finding nothing the confusion was topped with a lot of excitement. This kept rising until the .Ia idea came up and matched pretty well.”

Then Poznanski and his team re-analyzed their data to make sure, and the rest is history.

This explosion was nothing like a regular Type Ia explosion, said team member Alex Filippenko, because the white dwarf survives the detonation of the helium shell. In fact, it has similarities to both a nova and a supernova. Novas occur when matter – primarily hydrogen – falls onto a star and accumulates in a shell that can flare up as brief thermonuclear explosions. SN 2002bj is a “super” nova, generating about 1,000 times the energy of a standard nova, he said.

“As we have talked about our work over the last years, most astronomers in the audience reminded us that they had never seen such an event,” said Bildsten. “We told them to keep looking! With the sky the limit, the observers are usually ahead of theory, so I am really happy that we were able to make a prediction that allowed for a rapid interpretation of a new phenomena. Even though the supernova was observed in 2002, it took the keen eye of Dovi Poznanski to appreciate its import and relevance.”

Source: Science