Posted on by Nancy Atkinson

Hubble Provides Evidence for ‘Double Degenerate Progenitor’ Supernova

Supernova remnant SNR 0509-67.5. Supernovae provided the heavier elements in the Sun. Image credit: NASA/ESA/CXC
Supernova remnant SNR 0509-67.5. Supernovae provided the heavier elements in the Sun. Image credit: NASA/ESA/CXC

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What happened 400 years ago to create this stunningly beautiful supernova remnant – and were there two culprits or just one? This Hubble Space Telescope view of a Type Ia-created remnant has helped astronomers solve a longstanding mystery on the type of stars that cause some supernovae, known as a progenitor.

“Up until this point we haven’t really known where this type of supernova came from, despite studying them for decades,” said Ashley Pagnotta of Louisiana State University, speaking at a press briefing at the American Astronomical Society meeting on Wednesday. “But we now can say we have the first definitive identification of a Type 1a progenitor, and we know this one must have had a double degenerate progenitor – it is the only option.”

This supernova remnant that has a telephone number-like name of SNR 0509-67.5, lies 170,000 light-years away in the Large Magellanic Cloud galaxy.

Astronomers have long suspected that two stars were responsible for the explosion – as is the case with most type 1a supernovae — but weren’t sure what triggered the explosion. One explanation could be that it was caused by mass transfer from a companion star where a nearby star spills material onto a white dwarf companion, setting off a chain reaction that causes one of the most powerful explosions in the universe. This is known as the ‘single-degenerate’ path – which seems to be the most plausible, common and most preferred explanation for many Type 1a supernovae.

The other option is the collision of two white dwarfs, which is known as ‘double-degenerate, which seems to be the less common and not as widely accepted explanation for supernovae. To many astrophysicists, the merger scenario seemed to be less likely because too few double-white-dwarf systems appear to exist; indeed, there appear to be just handful that have been discovered so far.

The problem with SNR 0509-67.5 was that astronomers could not find any remnant of the companion star. That’s why the double degenerate scenario was considered, as in that case, there won’t be anything left as both white dwarfs are consumed in the explosion. In the case of a single progenitor, the non-white dwarf star will still be near the explosion site and will still look very much as it did before the explosion.

Therefore, a possible way to distinguish between the various progenitor models has been to look deep in the center of an old supernova remnant to search for the ex-companion star.

“We know Hubble has the sensitivity necessary to detect the faintest white dwarf remnants that could have caused such explosions,” said lead investigator Bradley Schaefer from LSU. “The logic here is the same as the famous quote from Sherlock Holmes: ‘when you have eliminated the impossible, whatever remains, however improbable, must be the truth.'”

In 2010, Schaefer and Pagnotta were preparing a proposal to look for any faint ex-companion stars in the center of four supernova remnants in the Large Magellanic Cloud when they saw an Astronomy Picture of the Day photo showing an image the Hubble Space Telescope had already had taken of one of their target remnants, SNR 0509-67.5.

(Note: the January 12, 2012 APOD image is of SNR 0509-67.5!)

Because the remnant appears as a nice symmetric shell or bubble, the geometric center can be determined accurately. In analyzing in more detail the central region, they found it to be completely empty of stars down to the limit of the faintest objects Hubble can detect in the photos. The young age also means that any surviving stars have not moved far from the site of the explosion. They were able to cross off the list all the possible single degenerate scenarios, and were left with the double degenerate model in which two white dwarfs collide.

“Since we can exclude all the possible single degenerates, we know it must be a double degenerate,” Pagnotta said. “The cause of SNR 0509-67.5 can be explained best by two tightly orbiting white dwarf stars spiraling closer and closer until they collided and exploded.”

Pagnotta also noted that this supernova is actually not a normal Type 1a supernova, but a subclass called 1991t, which is an extra bright supernova.

A paper in 2010 by Marat Gilfanov of the Max Planck Institute for Astrophysics indicated that perhaps many Type 1a supernova were caused by two white dwarf stars colliding, which was a surprise to many astronomers. Additionally, a review of the recent supernova SN 2011fe, which exploded in August of 2011, explores the possibility of the double degenerate progenitor. An open question remains whether these white dwarf mergers are the primary catalyst for Type Ia supernovae in spiral galaxies. Further studies are required to know if supernovae in spiral galaxies are caused by mergers or a mixture of the two processes.

Schaefer and Pagnotta plan to look at other supernova remnants in the Large Magellenic Cloud to further test their observations.

Pagnotta confirmed that anyone with an internet connection could have made this discovery, as all the Hubble images used were available publicly, and the use of the Hubble data was sparked by APOD.

Sources: Science Paper by Bradley E. Schaefer and Ashley Pagnotta (PDF document), HubbleSite, AAS press briefing

Posted on by Jon Voisey

The Contributor to SN 2011fe

Astrophoto: Supernova PTF11kly in M101 by Rick Johnson
Supernova PTF11kly in M101. Credit: Rick Johnson

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When discovered on August 24, 2011, supernova 2011fe was the closest supernova since the famous SN 1987A. Located in the relatively nearby Pinwheel galaxy (M101), it was a prime target for scientists to study since the host galaxy has been well studied and many high resolution images exist from before the explosion, allowing astronomers to search them for information on the star that led to the eruption. But when astronomers, led by Weidong Li, at the University of California, Berkeley searched, what they found defied the typically accepted explanations for supernovae of the same type as 2011fe.

SN 2011fe was a type 1a supernova. This class of supernova is expected to be caused by a white dwarf which accumulates mass contributed by a companion star. The general expectation is that the companion star is a star evolving off the main sequence. As it does, it swells up, and matter spills onto the white dwarf. If this pushes the dwarf’s mass over the limit of 1.4 times the mass of the Sun, the star can no longer support the weight and it undergoes a runaway collapse and rebound, resulting in a supernova.

Fortunately, the swollen up stars, known as red giants, become exceptionally bright due to their large surface area. The eighth brightest star in our own sky, Betelgeuse, is one of these red giants. This high brightness means that these objects are visible from large distances, potentially even in galaxies as distant as the Pinwheel. If so, the astronomers from Berkeley would be able to search archival images and detect the brighter red giant to study the system prior to the explosion.

But when the team searched the images from the Hubble Space Telescope which had snapped pictures through eight different filters, no star was visible at the location of the supernova. This finding follows a quick report from September which announced the same results, but with a much lower threshold for detection. The team followed up by searching images from the Spitzer infrared telescope which also failed to find any source at the proper location.

While this doesn’t rule out the presence of the contributing star, it does place constraints on its properties. The limit on brightness means that the contributor star could not have been a luminous red giant. Instead, the result favors another model of mass donation known as a double-degenerate model

In this scenario, two white dwarfs (both supported by degenerate electrons) orbit one another in a tight orbit. Due to relativistic effects, the system will slowly lose energy and eventually the two stars will become close enough that one will become disrupted enough to spill mass onto the other. If this mass transfer pushes the primary over the 1.4 solar mass limit, it would trigger the same sort of explosion.

This double degenerate model does not exclusively rule out the possibility of red giants contributing to type Ia supernovae, but recently other evidence has revealed missing red giants in other cases.

Posted on by Vanessa Janek

Unlocking Cosmology With Type 1a Supernovae

New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these "time bombs" could be scattered throughout our Galaxy. In this artist's conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA)

[/caption]Let’s face it, cosmologists catch a lot of flack. It’s easy to see why. These are people who routinely publish papers that claim to ever more finely constrain the size of the visible Universe, the rate of its breakneck expansion, and the distance to galaxies that lie closer and closer to the edges of both time and space. Many skeptics scoff at scientists who seem to draw such grand conclusions without being able to directly measure the unbelievable cosmic distances involved. Well, it turns out cosmologists are a creative bunch. Enter our star (ha, ha): the Type 1a Supernova. These stellar fireballs are one of the main tools astronomers use in order to make such fantastic discoveries about our Universe. But how exactly do they do it?

First, let’s talk physics. Type 1a supernovae result from a mismatched marriage gone wrong. When a red giant and white dwarf (or, less commonly, two white dwarfs) become trapped in a gravitational standoff, the denser dwarf star begins to accrete material from its bloated companion. Eventually the white dwarf reaches a critical mass (about 1.4 times that of our own Sun) and the natural pressure exerted by its core can no longer support its weight. A runaway nuclear reaction occurs, resulting in a cataclysmic explosion so large, it can be seen billions of light years away. Since type 1a supernovae always result from the collapse of a white dwarf, and since the white dwarf always becomes unstable at exactly the same mass, astronomers can easily work out the precise luminosity of such an event. And they have. This is great news, because it means that type 1a supernovae can be used as so-called standard candles with which to probe distances in the Universe. After all, if you know how bright something is and you know how bright it appears from where you are, you can easily figure out how far away it must be.

A Type Ia supernova occurs when a white dwarf accretes material from a companion star until it exceeds the Chandrasekhar limit and explodes. By studying these exploding stars, astronomers can measure dark energy and the expansion of the universe. CfA scientists have found a way to correct for small variations in the appearance of these supernovae, so that they become even better standard candles. The key is to sort the supernovae based on their color. Credit: NASA/CXC/M. Weiss

Now here’s where cosmology comes in. Photons naturally lose energy as they travel across the expanding Universe, so the light astronomers observe coming from type 1a supernovae will always be redshifted. The magnitude of that redshift depends on the amount of dark energy that is causing the Universe to expand. It also means that the apparent brightness of a supernova (that is, how bright it looks from Earth) can be monitored to determine how quickly it is receding from our line of view. Observations of the night sky will always be a function of a specific cosmology; but because their distances can be so easily calculated, type 1a supernovae actually allow astronomers to draw a physical map of the expansion of the Universe.

Spotting a type 1a supernova in its early, explosive throes is a rare event; after all, the Universe is a pretty big place. But when it does happen, it offers observers an unparalleled opportunity to dissect the chaos that leads to such a massive explosion. Sometimes astronomers are even lucky enough to catch one right in our cosmic backyard, a feat that occurred last August when Caltech’s Palomar Transit Factory (PTF) detected a type 1a supernova in M101, a galaxy just 25 million light years away. By the way, it isn’t just professionals that got to have all the fun! Amateur and career astronomers alike were able to use this supernova (the romantically named PTF11kly) to probe the inner workings of these precious standard candles. Want to learn more about how you can get in on the action the next time around? Check out UT’s podcast, Getting Started in Amateur Astronomy for more information.

Posted on by Mike Simonsen

Supernova Alphabet Soup

SN 2011fe aka PTF11kly Image credit: Wikipedia

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The International Astronomical Union (IAU) is the sole body responsible for the official naming of astronomical objects. So if you have a problem with the way things in the Universe are named, you now know where to send your email and letters of protest.

Before we get into this, a quick grammar note. When we discuss more than one supernova, they are called supernovae (super- no- vee), not supernovas. The same holds true for more than one nova. They are novae (no- vee). Please don’t write and ask me about Novas. Those are old Chevrolets, not stars.

Fortunately, the naming convention used for supernovae is pretty simple and straightforward.

The name is formed by combining the prefix SN, for supernova, the year of discovery and a one- or two-letter designation. The first 26 supernovae of the year get an upper case letter from A to Z (SN 1987A). After that, we start over with pairs of lower-case letters are used, starting with aa, ab, and so on (SN 2005ap).

Of course there are exceptions, there are always exceptions. That’s one of the things about astronomical nomenclature that is maddening, but I digress…

Four important historical supernovae are known simply by the year they occurred- SN 1006, SN 1054, SN 1572 (more commonly referred to as Tycho’s Nova), and SN 1604 (also known as Kepler’s Star).

One reason I’m bringing this subject up now is that we are ending the year, so we are approaching the time where we reset the naming schema for 2012 and the first supernova of the new year will get named SN 2012A. With the annual number of discoveries rising each year to well over 500, it is always a bit surprising how long it takes for that first one of the year to get named. So each year we hold an unofficial contest to see who will discover the first SN of the new year.

One of the reasons it usually doesn’t occur on the first day of the year is that supernova discoveries have to be officially confirmed spectroscopically before they get an official IAU designation. When someone discovers a possible supernova it gets reported to the IAU and then listed on the CBAT Transient Objects Confirmation Page. If it is a possible SN it gets a temporary designation of PSN (possible supernova) followed by its coordinates (PSN J01560719+1738468).

Only after someone has taken a spectrum confirming it is a supernova does it get a name with the year and letter combination. This can take several days, so it is unlikely a SN discovered on January 1 will be named until later in the week or the second week of the month. If it were discovered on December 23rd and confirmed on the 1st of January it would still get a name from the previous year.

This time lag will not be acceptable in the near future, with surveys like LSST coming on line. Astronomers will want immediate notification of discoveries of all types of transient objects including supernovae, so what has happened is new groups searching for SNe have begun to make up their own names.

The Catalina Real Time Survey is one such group. They are discovering dozens of possible supernovae that don’t always get official IAU designations. Their discoveries are all named CSS (Catalina Sky Survey) followed by the date in yymmdd format and then the rough coordinates, like this CSS111227:104742+021815. Crazy, huh?

ROTSE, the Robotic Optical Transient Search Experiment, also discoveries SNe and gives them their own designation in the form of ROTSE3 (the third iteration of this experiment) followed by coordinates, such as ROTSE3 J133033.0-313427.

And there is the Palomar Transient Factory which names its discoveries with the prefix PTF of course, such as PTF11kly, the nearest supernovae in decades, visible with small telescopes in M101. This SN eventually received an IAU designation, SN 2011fe, but that just created more confusion, since now it is known variously by both names in the literature.

Somehow managing to keep it all together amidst the confusion, David Bishop maintains the Latest Supernova Website where you can see discovery images and keep track of your favorite supernovae and related news. There is an excellent article about David and how his website evolved from simple beginnings.

So if you’re asking WTF? about the latest SNe the on the WWW the URL that will lead you through the ABC’s is definitely http://www.rochesterastronomy.org/supernova.html.

Got that? Good, there will be a quiz later…

Posted on by Jason Major

X-rays Unwrap a Poky Little Pulsar

A pulsar within a supernova remnant in the Small Magellanic Cloud. X-rays are blue; optical data is red and green. (NASA/CXC/Univ.Potsdam/L.Oskinova et al.)

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For the first time astronomers have located a pulsar – the super-dense, spinning remains of a star – nestled within the remnants of a supernova in the Small Magellanic Cloud. The image above, a composite of x-ray  and optical light data acquired by NASA’s Chandra Observatory and ESA’s XMM-Newton, shows the pulsar shining brightly on the right surrounded by the ejected outer layers of its former stellar life.

The optically-bright area on the left is a large star-forming region of dust and gas nearby SXP 1062.

A pulsar is a neutron star that emits high-energy beams of radiation from its magnetic poles. These poles are not always aligned with its axis of rotation, and so the beams swing through space as the neutron star spins. If the Earth happens to be in direct line with the beams at some point along their path, we see them as rapidly flashing radiation sources… sort of like a cosmic lighthouse on overdrive.

What’s unusual about this pulsar – called SXP 1062 – is its slow rate of rotation. Its beams spin around at a rate of about once every 18 minutes, which is downright poky for a pulsar, most of which spin several times a second.

X-ray image of SXP 1062

This makes SXP 1062 one of the slowest known pulsars discovered within the Small Magellanic Cloud, a dwarf galaxy cruising alongside our own Milky Way about 200,000 light-years distant.

The supernova that presumably created the pulsar and its surrounding remnant wrapping is estimated to have taken place between 10,000 and 40,000 years ago – relatively recently, by cosmic standards. To see a young pulsar spinning so slowly is extra unusual since younger pulsars have typically been observed to have rapid rotation rates. Understanding the cause of its leisurely pace will be the next goal for SXP 1062 researchers.

Read more about SXP 1062on the Chandra photo album page.

 

Image credit: X-ray & Optical: NASA/CXC/Univ.Potsdam/L.Oskinova et al.

Posted on by Ray Sanders

A Psychedelic Guide to Tycho’s Supernova Remnant

Gamma-rays detected by Fermi's LAT show that the remnant of Tycho's supernova shines in the highest-energy form of light. This portrait of the shattered star includes gamma rays (magenta), X-rays (yellow, green, and blue), infrared (red) and optical data. Image Credit: Gamma ray, NASA/DOE/Fermi LAT Collaboration; X-ray, NASA/CXC/SAO; Infrared, NASA/JPL-Caltech; Optical, MPIA, Calar Alto, O. Krause et al. and DSS)

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By no means are we suggesting that NASA’s Fermi Gamma-Ray Space Telescope can induce altered states of awareness, but this ‘far-out’ image is akin to 1960’s era psychedelic art. However, the data depicted here provides a new and enlightened way of looking at an object that’s been observed for over 400 years. After years of study, data collected by Fermi has revealed Tycho’s Supernova Remnant shines brightly in high-energy gamma rays.

The discovery provides researchers with additional information on the origin of cosmic rays (subatomic particles that are on speed). The exact process that gives cosmic rays their energy isn’t well understood since charged particles are easily deflected by interstellar magnetic fields. The deflection by interstellar magnetic fields makes it impossible for researchers to track cosmic rays to their original sources.

“Fortunately, high-energy gamma rays are produced when cosmic rays strike interstellar gas and starlight. These gamma rays come to Fermi straight from their sources,” said Francesco Giordano at the University of Bari in Italy.

But here’s some not-so-psychedelic facts about supernova remnants in general and Tycho’s in particular:

When a massive star reaches the end of its lifetime, it can explode, leaving behind a supernova remnant consisting of an expanding shell of hot gas propelled by the blast shockwave. In many cases, a supernova explosion can be visible on Earth – even in broad daylight. In November of 1572, a new “star” was discovered in the constellation Cassiopeia. The discovery is now known to be the most visible supernova in the past 400 years. Often called “Tycho’s supernova”, the remnant shown above is named after Danish astronomer Tycho Brahe, who spent a great deal of time studying the supernova.

Tycho's map shows the supernova's position (largest symbol, at top) relative to the stars that form Cassiopeia. Image credit: University of Toronto
The 1572 supernova event occurred when the night sky was considered to be a fixed and unchanging part of the universe. Tycho’s account of the discovery gives a sense of just how profound his discovery was. Regarding his discovery, Tycho stated, “When I had satisfied myself that no star of that kind had ever shone forth before, I was led into such perplexity by the unbelievability of the thing that I began to doubt the faith of my own eyes, and so, turning to the servants who were accompanying me, I asked them whether they too could see a certain extremely bright star…. They immediately replied with one voice that they saw it completely and that it was extremely bright”

In 1949, physicist Enrico Fermi (the namesake for the Fermi Gamma-ray Space Telescope) theorized that high-energy cosmic rays were accelerated in the magnetic fields of interstellar gas clouds. Following up on Fermi’s work, astronomers learned that supernova remnants might be the best candidate sites for magnetic fields of such magnitude.

One of the main goals of the Fermi Gamma-ray Space Telescope is to better understand the origins of cosmic rays. Fermi’s Large Area Telescope (LAT) can survey the entire sky every three hours, which allows the instrument to build a deeper view of the gamma-ray sky. Since gamma rays are the most energetic form of light, studying gamma ray concentrations can help researchers detect the particle acceleration responsible for cosmic rays.

Co-author Stefan Funk (Kavli Institute for Particle Astrophysics and Cosmology) adds, “This detection gives us another piece of evidence supporting the notion that supernova remnants can accelerate cosmic rays.”

After scanning the sky for nearly three years, Fermi’s LAT data showed a region of gamma-ray emissions associated with the remnant of Tycho’s supernova. Keith Bechtol, (KIPAC graduate student) commented on the discovery, saying, “We knew that Tycho’s supernova remnant could be an important find for Fermi because this object has been so extensively studied in other parts of the electromagnetic spectrum. We thought it might be one of our best opportunities to identify a spectral signature indicating the presence of cosmic-ray protons”

The team’s model is based on LAT data, gamma-rays mapped by ground-based observatories and X-ray data. The conclusion the team has come to regarding their model is that a process called pion production is the best explanation for the emissions. The animation below depicts a proton moving at nearly the speed of light and striking a slower-moving proton. The protons survive the collision, but their interaction creates an unstable particle — a pion — with only 14 percent of the proton’s mass. In 10 millionths of a billionth of a second, the pion decays into a pair of gamma-ray photons.

If the team’s interpretation of the data is accurate, then within the remnant, protons are being accelerated to near the speed of light. After being accelerated to such tremendous speeds, the protons interact with slower particles and produce gamma rays. With all the amazing processes at work in the remnant of Tycho’s supernova, one could easily imagine how impressed Brahe would be.

And no tripping necessary.

Learn more about the Fermi Gamma-ray Space Telescope at: http://www.nasa.gov/mission_pages/GLAST/main/index.html

Source: Fermi Gamma-ray Space Telescope Mission News

Posted on by Tammy Plotner

Incredible Spinning Star Rotates At A Million Miles Per Hour!

This is an artist's concept of the fastest rotating star found to date. The massive, bright young star, called VFTS 102, rotates at a million miles per hour, or 100 times faster than our Sun does. Centrifugal forces from this dizzying spin rate have flattened the star into an oblate shape and spun off a disk of hot plasma, seen edge on in this view from a hypothetical planet. The star may have "spun up" by accreting material from a binary companion star. The rapidly evolving companion later exploded as a supernova. The whirling star lies 160,000 light-years away in the Large Magellanic Cloud, a satellite galaxy of our Milky Way. Credit: NASA, ESA, and G. Bacon (STScI)

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Located in the Large Magellanic Cloud, a star named VFTS 102 is spinning its heart out… Literally. Rotating at a mind-numbing speed of a million miles per hour (1.6 million kph), this hot blue giant has reached the edge where centrifugal forces could tear it apart. It’s the fastest ever recorded – 300 times faster than our Sun – and may have been split off from a double star system during a violent explosion.

Thanks to ESO’s Very Large Telescope at the Paranal Observatory in Chile, an international team of astronomers studying the heaviest and brightest stars in the Tarantula Nebula made quite a discovery – a huge blue star 25 times the mass of the Sun and about one hundred thousand times brighter was cruising through space at a speed which drew their attention.

“The remarkable rotation speed and the unusual motion compared to the surrounding stars led us to wonder if this star had an unusual early life. We were suspicious.” explains Philip Dufton (Queen’s University Belfast, Northern Ireland, UK), lead author of the paper presenting the results.

ESO's Very Large Telescope has picked up the fastest rotating star found so far. This massive bright young star lies in our neighbouring galaxy, the Large Magellanic Cloud, about 160 000 light-years from Earth. Astronomers think that it may have had a violent past and has been ejected from a double star system by its exploding companion. Credit: ESO

What they’ve discovered could possibly be a “runaway star” – one that began life as a binary, but may have been ejected during a supernova event. Further evidence which supports their theory also exists: the presence of a pulsar and a supernova remnant nearby. But what made this crazy star spin so fast? It’s possible that if the two stars were very close that streaming gases could have started the incredible rotation. Then the more massive of the pair blew its stack – expelling the star into space. So what would be left? It’s elementary, Watson… A supernova remnant, a pulsar and a runaway!

Even though this is a rather tidy conclusion, there’s always room for doubt. As Dufton concludes, “This is a compelling story because it explains each of the unusual features that we’ve seen. This star is certainly showing us unexpected sides of the short but dramatic lives of the heaviest stars.”

Original Story Source: HubbleSite News Release and ESO News Release. For Further Reading: he VLT-FLAMES Tarantula Survey I. Introduction and observational overview.

Posted on by Tammy Plotner

Supernova Candidate Stars May Signal “Impending Doom”

This Large Binocular Telescope image below of the Whirlpool Galaxy, otherwise known as M51, is part of a new galaxy survey by Ohio State University, where astronomers are searching for signs that stars are about to go supernova. The insets show one particular binary star system before (left) and after (right) one of its stars went supernova. Image by Dorota Szczygiel, courtesy of Ohio State University.

[/caption] This past year has given both backyard and professional astronomers a rare treat – a very visible supernova event. Hosted in the Whirlpool Galaxy (M51), these stellar death throes may have been clued to us by a rather ordinary binary star system. In a recent study done by researchers at Ohio State University, a galaxy survey may have captured evidence of a “stellar signal” just before it went supernova!

Employing the Large Binocular Telescope located in Arizona, the OSU team was undertaking a survey of 25 galaxies for stars that changed their magnitude in usual ways. Their goal was to find a star just before it ended its life – a three-year undertaking. As luck would have it, a binary star system located in M51 produced just the results they were looking for. One star dropped amplitude just a short period of time before the other exploded. While the probability factor of them getting the exact star might be slim, chances are still good they caught its brighter partner. Despite that, principal investigator Christopher Kochanek, professor of astronomy at Ohio State and the Ohio Eminent Scholar in Observational Cosmology, remains optimistic as their results prove a theory.

“Our underlying goal is to look for any kind of signature behavior that will enable us to identify stars before they explode,” he said. “It’s a speculative goal at this point, but at least now we know that it’s possible.”

“Maybe stars give off a clear signal of impending doom, maybe they don’t,” said study co-author Krzystof Stanek, professor of astronomy at Ohio State, “But we’ll learn something new about dying stars no matter the outcome.”

Postdoctoral researcher Dorota Szczygiel, the leader of the supernova study tells us why the galaxy survey remains paramount.

“The odds are extremely low that we would just happen to be observing a star for several years before it went supernova. We would have to be extremely lucky,” she said. “With this galaxy survey, we’re making our own luck. We’re studying all the variable stars in 25 galaxies, so that when one of them happens go supernova, we’ve already compiled data on it.”

On May 31, 2011, the whole astronomy world was abuzz when SN2011dh gave both amateurs and professionals a real thrill as an easily observable event. As luck would have it, it was a binary star system being studied by the OSU team, and consisted of both a blue and red star. At this point, the astronomers surmise the red star was the one that dimmed significantly over the three-year period while the blue one blew its top. When reviewing the LBT data, the Ohio team found that when compared with Hubble images, the red star dimmed at about 10% over the final three-year period at an estimated 3% per previous years. As a curiosity, the researchers surmise the red star may have actually survived the supernova event.

“After the light from the explosion fades away, we should be able to see the companion that did not explode,” Szczygiel said.

As the team continues to collect valuable information, they estimate they could also detect another candidate set of stars at a rate of about one per year. There is also a strong possibility these detections could act as a type of test bed to predict future supernova events… looking for signals of impending doom. However, according to the news release, the Sun won’t be one to bother with.

“There’ll be no supernova for the Sun – it’ll just fizzle out,” Kochanek said. “But that’s okay – you don’t want to live around an exciting star.”

Original Story Source: Ohio State Research News.

Posted on by Tammy Plotner

Cygnus X – A Cosmic-ray Cocoon

Cygnus X hosts many young stellar groupings, including the OB2 and OB9 associations and the cluster NGC 6910. The combined outflows and ultraviolet radiation from the region's numerous massive stars have heated and pushed gas away from the clusters, producing cavities of hot, lower-density gas. In this 8-micron infrared image, ridges of denser gas mark the boundaries of the cavities. Bright spots within these ridges show where stars are forming today. Credit: NASA/IPAC/MSX

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Situated about 4,500 light-years away in the constellation of Cygnus is a veritable star factory called Cygnus X… one estimated to have enough “raw materials” to create as many as two million suns. Caught in the womb are stellar clusters and OB associations. Of particular interest is one labeled Cygnus OB2 which is home to 65 of the hottest, largest and meanest O-type stars known – and close to 500 B members. The O boys blast out holes in the dust clouds in intense outflows, disrupting cosmic rays. Now, a study using data from NASA’s Fermi Gamma-ray Space Telescope is showing us this disturbance can be traced back to its source.

Discovered some 60 years ago in radio frequencies, the Cygnus X region has long been of interest, but dust-veiled at optical wavelengths. By employing NASA’s Fermi Gamma-ray Space Telescope, scientists are now able to peer behind the obscuration and take a look at the heart through gamma ray observations. In regions of star formation like Cygnus X, subatomic particles are produced and these cosmic rays shoot across our galaxy at light speed. When they collide with interstellar gas, they scatter – making it impossible to trace them to their point of origin. However, this same collision produces a gamma ray source… one that can be detected and pinpointed.

“The galaxy’s best candidate sites for cosmic-ray acceleration are the rapidly expanding shells of ionized gas and magnetic field associated with supernova explosions.” says the FERMI team. “For stars, mass is destiny, and the most massive ones — known as types O and B — live fast and die young.”

Because these star types aren’t very common, regions like Cygnus X become important star laboratories. Its intense outflows and huge amount of mass fills the prescription for study. Within its hollowed-out walls, stars reside in layers of thin, hot gas enveloped in ribbons of cool, dense gas. It is this specific area in which Fermi’s LAT instrumentation excels – detecting an incredible amount of gamma rays.

“We are seeing young cosmic rays, with energies comparable to those produced by the most powerful particle accelerators on Earth. They have just started their galactic voyage, zig-zagging away from their accelerator and producing gamma rays when striking gas or starlight in the cavities,” said co-author Luigi Tibaldo, a physicist at Padova University and the Italian National Institute of Nuclear Physics.

Clocked at up to 100 billion electron volts by the LAT, these highly accelerated particles are revealing the extreme origin of gamma-ray emission. For example, visible light is only two to three electron volts! But why is Cygnus X so special? It entangles its sources in complex magnetic fields and keeps the majority of them from escaping. All thanks to those high mass stars…

“These shockwaves stir the gas and twist and tangle the magnetic field in a cosmic-scale jacuzzi so the young cosmic rays, freshly ejected from their accelerators, remain trapped in this turmoil until they can leak into quieter interstellar regions, where they can stream more freely,” said co-author Isabelle Grenier, an astrophysicist at Paris Diderot University and the Atomic Energy Commission in Saclay, France.

However, there’s more to the story. The Gamma Cygni supernova remnant is also nearby and may impact the findings as well. At this point, the Fermi team considers it may have created the initial “cocoon” which holds the cosmic rays in place, but they also concede the accelerated particles may have originated through multiple interactions with stellar winds.

“Whether the particles further gain or lose energy inside this cocoon needs to be investigated, but its existence shows that cosmic-ray history is much more eventful than a random walk away from their sources,” Tibaldo added.

Original Story Source: NASA Fermi News.

Posted on by Steve Nerlich

Astronomy Without A Telescope – The Progenitor Problem

DEM L71 - a Type 1a supernova remnant. Analysis a the outer shockwave and inner ejecta indicate the explosion contains a mass not much exceeding 1 solar mass, which contains a high iron to silicon/ oxygen ratio. This all very suggestive that the progenitor star was a compact white dwarf. Apart from that the steps leading up to the explosion are a mystery (Credit: NASA/Chandra)

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With so much of our current understanding of the universe based on Type 1a supernovae data, a good deal of current research is focused upon just how standard these supposed standard candles are. To date, the weight of analysis seems reassuring – apart from a few outliers, the supernovae do all seem very standard and predictable.

However, some researchers have come at this issue from a different perspective by considering the characteristics of the progenitor stars that produce Type 1a supernovae. We know very little about these stars. Sure, they are white dwarfs that explode after accumulating extra mass – but just how this outcome is reached remains a mystery.

Indeed, the final stages preceding an explosion have never been definitively observed and we cannot readily point to any stars as likely candidates on a pathway towards Type Ia-ness. In comparison, identifying stars that are expected to explode as core collapse supernovae (Types Ib, Ic or II) is easy – core collapse should be the destiny of any star bigger than 9 solar masses.

Popular theory has it that a Type 1a progenitor is a white dwarf star in a binary system that draws material off its binary companion until the white dwarf reaches the Chandrasekhar limit of 1.4 solar masses. As the already compressed mass of predominantly carbon and oxygen is compressed further, carbon fusion is rapidly initiated throughout the star. This is such an energetic process that the comparatively small star’s self-gravity cannot contain it – and the star blows itself to bits.

Surprisingly, the white dwarf merger scenario seems the more likely cause of Type 1a supernovae, based on current (though largely circumstantial) evidence (Credit: Bad Astronomy/Discovery).

But when you try to model the processes leading up to a white dwarf achieving 1.4 solar masses, it seems to require a lot of ‘fine tuning’. The rate of accretion of extra mass has to be just right – too fast a flow will result in a red giant scenario. This is because adding extra mass quickly will give the star enough self-gravity so that it can partially contain the fusion energy – meaning that it will expand rather than explode.

Theorists get around this problem by proposing that a stellar wind arising from the white dwarf moderates the rate of infalling material. This sounds promising, although to date studies of Type 1a remnant material have found no evidence of the dispersed ions that would be expected from a pre-existing stellar wind.

Furthermore, a Type 1a explosion within a binary should have a substantial impact on its companion star. But all searches for candidate surviving companions – which would presumably possess anomalous characteristics of velocity, rotation, composition or appearance – have been inconclusive to date.

An alternative model for the events that lead up to a Type 1a are that two white dwarfs are drawn together, inexorably inspiralling until one or the other achieves 1.4 solar masses. This is not a traditionally favoured model as the time required for two such comparatively small stars to inspiral and merge could be billions of years.

However, Maoz and Mannucci review recent attempts to model the rate of Type 1a supernovae within a set volume of space and then align this with the expected frequency of different progenitor scenarios. Assuming that between 3 to 10 % of all 3-8 solar mass stars eventually explode as Type 1a supernovae – this rate does favour the ‘when white dwarfs collide’ model over the ‘white dwarf in a binary’ model.

There is no immediate concern that this alternate formation process would affect the ‘standardness’ of a Type 1a explosion – it’s just not the finding that most people were expecting.

Further reading:
Maoz and Mannucci Type-Ia supernova rates and the progenitor problem. A review.