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

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.

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.

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.

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.

Antique Stars Could Help Solve Mysteries Of Early Milky Way

The Milky Way is like NGC 4594 (pictured), a disc shaped spiral galaxy with around 200 billion stars. The three main features are the central bulge, the disk, and the halo. Credit: ESO
The Milky Way is like NGC 4594 (pictured), a disc shaped spiral galaxy with around 200 billion stars. The three main features are the central bulge, the disk, and the halo. Credit: ESO

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Utilizing ESO’s giant telescopes located in Chile, researchers at the Niels Bohr Institute have been examining “antique” stars. Located at the outer reaches of the Milky Way, these superannuated stellar specimens are unusual in the fact that they contain an over-abundance of gold, platinum and uranium. How they became heavy metal stars has always been a puzzle, but now astronomers are tracing their origins back to our galaxy’s beginning.

It is theorized that soon after the Big Bang event, the Universe was filled with hydrogen, helium and… dark matter. When the trio began compressing upon themselves, the very first stars were born. At the core of these neophyte suns, heavy elements such as carbon, nitrogen and oxygen were then created. A few hundred million years later? Hey! All of the elements are now accounted for. It’s a tidy solution, but there’s just one problem. It would appear the very first stars only had about 1/1000th of the heavy-elements found in sun-like stars of the present.

How does it happen? Each time a massive star reaches the end of its lifetime, it will either create a planetary nebula – where layers of elements gradually peel away from the core – or it will go supernova – and blast the freshly created elements out in a violent explosion. In this scenario, the clouds of material once again coalesce… collapse again and form more new stars. It’s just this pattern which gives birth to stars that become more and more “elementally” concentrated. It’s an accepted conjecture – and that’s what makes discovering heavy metal stars in the early Universe a surprise. And even more surprising…

Right here in the Milky Way.

“In the outer parts of the Milky Way there are old ‘stellar fossils’ from our own galaxy’s childhood. These old stars lie in a halo above and below the galaxy’s flat disc. In a small percentage – approximately one to two percent of these primitive stars, you find abnormal quantities of the heaviest elements relative to iron and other ‘normal’ heavy elements”, explains Terese Hansen, who is an astrophysicist in the research group Astrophysics and Planetary Science at the Niels Bohr Institute at the University of Copenhagen.

The 17 observed stars are all located in the northern sky and could therefore be observed with the Nordic Optical Telescope, NOT on La Palma. NOT is 2.5 meter telescope that is well suited for just this kind of observations, where continuous precise observations of stellar motions over several years can reveal what stars belong to binary star systems.
But the study of these antique stars just didn’t happen overnight. By employing ESO’s large telescopes based in Chile, the team took several years to come to their conclusions. It was based on the findings of 17 “abnormal” stars which appeared to have elemental concentrations – and then another four years of study using the Nordic Optical Telescope on La Palma. Terese Hansen used her master’s thesis to analyse the observations.

“After slaving away on these very difficult observations for a few years I suddenly realised that three of the stars had clear orbital motions that we could define, while the rest didn’t budge out of place and this was an important clue to explaining what kind of mechanism must have created the elements in the stars”, explains Terese Hansen, who calculated the velocities along with researchers from the Niels Bohr Institute and Michigan State University, USA.

What exactly accounts for these types of concentrations? Hansen explains their are two popular theories. The first places the origin as a close binary star system where one goes supernova, inundating its companion with layers of heavier elements. The second is a massive star also goes supernova, but spews the elements out in dispersing streams, impregnating gas clouds which then formed into the halo stars.

The research group has analysed 17 stellar fossils from the Milky Way’s childhood. The stars are small light stars and they live longer than large massive stars. They do not burn hydrogen longer, but swell up into red giants that will later cool and become white dwarves. The image shows the most famous of the stars CS31082-001, which was the first star that uranium was found in.
“My observations of the motions of the stars showed that the great majority of the 17 heavy-element rich stars are in fact single. Only three (20 percent) belong to binary star systems – this is completely normal, 20 percent of all stars belong to binary star systems. So the theory of the gold-plated neighbouring star cannot be the general explanation. The reason why some of the old stars became abnormally rich in heavy elements must therefore be that exploding supernovae sent jets out into space. In the supernova explosion the heavy elements like gold, platinum and uranium are formed and when the jets hit the surrounding gas clouds, they will be enriched with the elements and form stars that are incredibly rich in heavy elements”, says Terese Hansen, who immediately after her groundbreaking results was offered a PhD grant by one of the leading European research groups in astrophysics at the University of Heidelberg.

May all heavy metal stars go gold!

Original Story Source: Niels Bohr Institute News Release. For Further Reading: The Binary Frequency of r-Process-element-enhanced Metal-poor Stars and Its Implications: Chemical Tagging in the Primitive Halo of the Milky Way.

Different Supernovae; Different Neutron Stars

Artist concept of a neutron star. Credit: NASA
Artist concept of a neutron star. Credit: NASA

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Astronomers have recognized various ways that stars can collapse to undergo a supernova. In one situation, an iron core collapses. The second involves a lower mass star with oxygen, neon, and magnesium in the core which suddenly captures electrons when the conditions are just right, removing them as a support mechanism and causing the star to collapse. While these two mechanisms make good physical sense, there has never been any observational support showing that both types occur. Until now that is. Astronomers led yb Christian Knigge and Malcolm Coe at the University of Southampton in the UK announced that they have detected two distinct sub populations in the neutron stars that result from these supernova.

To make the discovery, the team studied a large number of a specific sub-class of neutron stars known as Be X-ray binaries (BeXs). These objects are a pair of stars formed by a hot B spectral class stars with hydrogen emission in their spectrum in a binary orbit with a neutron star. The neutron star orbits the more massive B star in an elliptical orbit, siphoning off material as it makes close approaches. As the accreted material strikes the neutron star’s surface it glows brightly in the X-rays, becoming, for a time, an X-ray pulsar allowing astronomers to measure the spin period of the neutron star.

Such systems are common in the Small Magellanic Cloud which appears to have a burst of star forming activity about 60 million years ago, allowing for the massive B stars to be in the prime of their stellar lives. It is estimated that the Small Magellanic Cloud alone has as many BeXs as the entire Milky Way galaxy, despite being 100 times smaller. By studying these systems as well the Large Magellanic Cloud and Milky Way, the team found that there are two overlapping but distinct populations of BeX neutron stars. The first had a short period, averaging around 10 seconds. A second group had an average of around 5 minutes. The team surmises that the two populations are a result of the different supernova formation mechanisms.

The two different formation mechanisms should also lead to another difference. The explosion is expected to give the star a “kick” that can change the orbital characteristics. The electron-captured supernovae are expected to give a kick velocity of less than 50 km/sec whereas the iron core collapse supernovae should be over 200 km/sec. This would mean the iron core collapse stars should have preferentially longer and more eccentric orbits. The team attempted to discern whether this too was supported by their evidence, but only a small fraction of the stars they examined had determined eccentricities. Although there was a small difference, it is too early to determine whether or not it was due to chance.

According to Knigge, “These findings take us back to the most fundamental processes of stellar evolution and lead us to question how supernovae actually work. This opens up numerous new research areas, both on the observational and theoretical fronts.

Astronomy Without A Telescope – Orphan Supernovae?

Supernova G292.0+1.8. Like most supernovae it detonated within a host galaxy - in fact ours. Credit: Chandra.

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For some years now astronomers have been scratching their heads over the appearance of supernovae that detonate out in the middle of nowhere – rather than within a host galaxy.

Various hypotheses have been proposed, notably that they might be hypervelocity stars – which are stars flung out of their host galaxy due to an unfortunate coincidence of gravitational interactions. It’s thought that such interactions may accelerate those stars up to a velocity of more than 100 kilometers a second – that is, more than the escape velocity of your average galaxy.

But Zinn et al suggest a more mundane suggestion for their particular orphan supernovae of interest, which is SN 2009z. They propose that it is in a galaxy, it’s just a galaxy that is very difficult to see.

They propose the supernova actually detonated within a low surface brightness galaxy, N271. From the images they have produced, this seems a reasonable claim – it’s just that low surface brightness galaxies (or LSBs) aren’t meant to have supernovae.

Left frame: Sloan Digital Sky Survey image showing the location of the Type IIB supernovae SN 2009z. Right frame: Close-up of the rectangular area taken by the New Technology Telescope (ESO), showing the location of SN 2009z at the cross marks. It seems closely associated with the small galaxy N271, even though such a galaxy is not usually thought capable of supporting massive star formation. Credit: Zinn et al.

Since galaxies can appear as extended objects, rather than as point-like stars, we refer to them as having ‘surface brightness’ – which can vary across the object’s apparent surface. LSB’s are generally isolated field galaxies, rather than being grouped in amongst dense galaxy clusters. They are most often dwarf galaxies as well, but at least one spiral LSB has been identified.

The dimness of LSB galaxies is suggestive of them having almost no active star formation – either being too old, with no free hydrogen remaining for new star formation – or just not dense enough for much star formation to ever have taken off.

But here you have supernova SN 2009z that was most likely was contained within LSB galaxy N271. And SN 2009z was a Type II supernova – a massive and short-lived star that underwent core collapse. Indeed, it was a Type IIb with only a small shell of hydrogen when it detonated. Type IIb supernovae are probably massive stars which lose most, but not all, of their hydrogen shell through having it stripped off by a companion star in a binary system.

This all seems quite unusual behaviour for a galaxy that does not support active star formation. Zinn et al propose that LSB galaxies must go through short bursts of active star formation followed by long quiescent phases of almost no activity. This then suggests that the progenitor star of supernova SN 2009z was formed in the previous starburst period, before N271 quietened down again.

Of course, none of this need suggest that hypervelocity stars don’t exist – indeed several have been discovered since the first confirmed finding in 2005. All those known are associated with the Milky Way, since finding a single isolated hypervelocity star ejected by a distant galaxy is probably beyond the detection of our current technology – unless of course they go supernovae.

But given what we know so far:
• a hypervelocity star arises from a binary system’s unfortunate interaction with a galaxy’s central supermassive black hole;
• one binary member is captured, the other flung violently outwards at escape velocity.
• but, massive stars that go supernovae only have a main sequence life span of the order of millions of years;
• so, even at more than 100 kilometers a second, it’s unlikely that any are going to make it across the many light years distance from the center of a galaxy to its outer boundary before they detonate.

Putting all this together… orphan supernovae? Busted (well, unless we find one anyway).

Further reading: Zinn et al. Supernovae without host galaxies? The low surface brightness host of SN 2009Z.

New NASA Mission Hunts Down Zombie Stars

This is an artist's concept of a pulsar (blue-white disk in center) pulling in matter from a nearby star (red disk at upper right). The stellar material forms a disk around the pulsar (multicolored ring) before falling on to the surface at the magnetic poles. The pulsar's intense magnetic field is represented by faint blue outlines surrounding the pulsar. Credit: NASA

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Neutron stars have been classed as “undead”… real zombie stars. Even though technically defunct, the neutron star continues to shine – and occasionally feed on a neighbor if it gets too close. They are born when a massive star collapses under its gravity and its outer layers are blown far and wide, outshining a billion suns, in a supernova event. What’s left is a stellar corpse… a core of inconceivable density… where one teaspoon would weigh about a billion tons on Earth. How would we study such a curiosity? NASA has proposed a mission called the Neutron Star Interior Composition Explorer (NICER) that would detect the zombie and allow us to see into the dark heart of a neutron star.

The core of a neutron star is pretty incredible. Despite the fact that it has blown away most of its exterior and stopped nuclear fusion, it still radiates heat from the explosion and exudes a magnetic field which tips the scales. This intense form of radiation caused by core collapse measures out at over a trillion times stronger than Earth’s magnetic field. If you don’t think that impressive, then think of the size. Originally the star could have been a trillion miles or more in diameter, yet now is compressed to the size of an average city. That makes a neutron star a tiny dynamo – capable of condensing matter into itself at more than 1.4 times the content of the Sun, or at least 460,000 Earths.

“A neutron star is right at the threshold of matter as it can exist – if it gets any denser, it becomes a black hole,” says Dr. Zaven Arzoumanian of NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We have no way of creating neutron star interiors on Earth, so what happens to matter under such incredible pressure is a mystery – there are many theories about how it behaves. The closest we come to simulating these conditions is in particle accelerators that smash atoms together at almost the speed of light. However, these collisions are not an exact substitute – they only last a split second, and they generate temperatures that are much higher than what’s inside neutron stars.”

If approved, the NICER mission will be launched by the summer of 2016 and attached robotically to the International Space Station. In September 2011, NASA selected NICER for study as a potential Explorer Mission of Opportunity. The mission will receive $250,000 to conduct an 11-month implementation concept study. Five Mission of Opportunity proposals were selected from 20 submissions. Following the detailed studies, NASA plans to select for development one or more of the five Mission of Opportunity proposals in February 2013.

This is an artist's concept of the NICER instrument on board the International Space Station. NICER is the cube in the foreground on the left. The circular objects protruding from the cube are telescopes that focus X-rays from the pulsar on to the detector. Credit: NASA

What will NICER do? First off, an array of 56 telescopes will gather X-ray information from a neutron stars magnetic poles and hotspots. It is from these areas that our zombie stars release X-rays, and as they rotate create a pulse of light – thereby the term “pulsar”. As the neutron star shrinks, it spins faster and the resultant intense gravity can pull in material from a closely orbiting star. Some of these pulsars spin so fast they can reach speeds of several hundred of rotations per second! What scientists are itching to understand is how matter behaves inside a neutron star and “pinning down the correct Equation Of State (EOS) that most accurately describes how matter responds to increasing pressure. Currently, there are many suggested EOSs, each proposing that matter can be compressed by different amounts inside neutron stars. Suppose you held two balls of the same size, but one was made of foam and the other was made of wood. You could squeeze the foam ball down to a smaller size than the wooden one. In the same way, an EOS that says matter is highly compressible will predict a smaller neutron star for a given mass than an EOS that says matter is less compressible.”

Now all NICER will need to do is help us to measure a pulsar’s mass. Once it is determined, we can get a correct EOS and unlock the mystery of how matter behaves under intense gravity. “The problem is that neutron stars are small, and much too far away to allow their sizes to be measured directly,” says NICER Principal Investigator Dr. Keith Gendreau of NASA Goddard. “However, NICER will be the first mission that has enough sensitivity and time-resolution to figure out a neutron star’s size indirectly. The key is to precisely measure how much the brightness of the X-rays changes as the neutron star rotates.”

So what else does our zombie star do that’s impressive? Because of their extreme gravity in such small volume, they distort space/time in accordance with Einstein’s theory of General Relativity. It is this space “warp” that allows astronomers to reveal the presence of a companion star. It also produces effects like an orbital shift called precession, allowing the pair to orbit around each other causing gravitational waves and producing measurable orbital energy. One of the goals of NICER is to detect these effects. The warp itself will allow the team to determine the neutron star’s size. How? Imagine pushing your finger into a stretchy material – then imagine pushing your whole hand against it. The smaller the neutron star, the more it will warp space and light.

Here light curves become very important. When a neutron star’s hotspots are aligned with our observations, the brightness increases as one rotates into view and dims as it rotates away. This results in a light curve with large waves. But, when space is distorted we’re allowed to view around the curve and see the second hotspot – resulting in a light curve with smoother, smaller waves. The team has models that produce “unique light curves for the various sizes predicted by different EOSs. By choosing the light curve that best matches the observed one, they will get the correct EOS and solve the riddle of matter on the edge of oblivion.”

And breathe life into zombie stars…

Original Story Source: NASA Mission News.

Did A Supernova Shape Our Solar System?

The time evolution of case I. Color coded is the density at t = 0 kyr, t = 4.16 kyr and t = 8.33 kyr. The length scale is given in units of the radius of the initial cold core (R0 = 0.21 pc). Credit: M. Gritschneder (et al)

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Away in space some 4.57 billion years ago, in a galaxy yet to be called the Milky Way, a hydrogen molecular cloud collapsed. From it was born a G-type main sequence star and around it swirled a solar nebula which eventually gelled into a solar system. But just what caused the collapse of the molecular cloud? Astronomers have theorized it may have been triggered by a nearby supernova event… And now new computer modeling confirms that our Solar System was born from the ashes a dead star.

While this may seem like a cold case file, there are still some very active clues – one of which is the study of isoptopes contained within the structure of meteorites. As we are well aware, many meteorites could very well be bits of our primordial solar nebula, left virtually untouched since they formed. This means their isotopic signature could spell out the conditions that existed within the molecular cloud at the time of its collapse. One strong factor in this composition is the amount of aluminium-26 – an element with a radioactive half-life of 700,000 years. In effect, this means it only takes a relatively minor period of time for the ratio between Al-26 and Al-24 to change.

“The time-scale for the formation events of our Solar System can be derived from the decay products of radioactive elements found in meteorites. Short lived radionuclides (SLRs) such as 26Al , 41Ca, 53Mn and 60Fe can be employed as high-precision and high-resolution chronometers due to their short half-lives.” says M. Gritschneder (et al). “These SLRs are found in a wide variety of Solar System materials, including calcium-aluminium-rich inclusions (CAIs) in primitive chondrites.”

However, it would seem that a class of carbonaceous chondrite meteorites known CV-chondrites, have a bit more than their fair share of Al-26 in their structure. Is it the smoking gun of an event which may have enriched the cloud that formed it? Isotope measurements are also indicative of time – and here we have two examples of meteorites which formed within 20,000 years of each other – yet are significantly different. What could have caused the abundance of Al-26 and caused fast formation?

“The general picture we adopt here is that a certain amount of Al-26 is injected in the nascent solar nebula and then gets incorporated into the earliest formed CAIs as soon as the temperature drops below the condensation temperature of CAI minerals. Therefore, the CAIs found in chondrites represent the first known solid objects that crystalized within our Solar System and can be used as an anchor point to determine the formation time-scale of our Solar System.” explains Gritschneder. “The extremely small time-span together with the highly homogeneous mixing of isotopes poses a severe challenge for theoretical models on the formation of our Solar System. Various theoretical scenarios for the formation of the Solar System have been discussed. Shortly after the discovery of SLRs, it was proposed that they were injected by a nearby massive star. This can happen either via a supernova explosion or by the strong winds of a Wolf-Rayet star.”

While these two theories are great, only one problem remains… Distinguishing the difference between the two events. So Matthias Gritschneder of Peking University in Beijing and his colleagues set to work designing a computer simulation. Biased towards the supernova event, the model demonstrates what happens when a shockwave encounters a molecular cloud. The results are an appropriate proportion of Al-26 – and a resultant solar system formation.

“After discussing various scenarios including X-winds, AGB stars and Wolf-Rayet stars, we come to the conclusion that triggering the collapse of a cold cloud core by a nearby supernova is the most promising scenario. We then narrow down the vast parameter space by considering the pre-explosion survivability of such a clump as well as the cross-section necessary for sufficient enrichment.” says Gritschneder. “We employ numerical simulations to address the mixing of the radioactively enriched SN gas with the pre-existing gas and the forced collapse within 20 kyr. We show that a cold clump at a distance of 5 pc can be sufficiently enriched in Al-26 and triggered into collapse fast enough – within 18 kyr after encountering the supernova shock – for a range of different metallicities and progenitor masses, even if the enriched material is assumed to be distributed homogeneously in the entire supernova bubble. In summary, we show that the triggered collapse and formation of the Solar System as well as the required enrichment with radioactive 26Al are possible in this scenario.”

While there are still other isotope ratios yet to be explained and further modeling done, it’s a step toward the future understanding of how solar systems form.

Original Story Source: MIT Technology Review News Release. For Further Reading: The Supernova Triggered Formation And Enrichment Of Our Solar System.