Fingerprint From a First-Generation Star?

The young universe was composed of a pristine mix of hydrogen, helium, and a tiny trace of lithium. But after hundreds of millions of years, it began to cool and giant clouds of the primordial elements collapsed to form the first stars.

The first “Population III” stars were extremely massive and bright, synthesizing the first batches of heavy elements, and erupting as supernovae after relatively short lifetimes of just a few million years. This cycle of star birth and death has steadily produced and dispersed more heavy elements throughout cosmic history.

Astronomers haven’t spotted any of the first stars still shining today. But now, a team using the 8.2-meter Subaru Telescope has discovered an ancient low-mass star that likely formed from the elements produced in the supernova explosion of a very massive first generation star.

Pop III stars with masses exceeding 100 times that of the Sun would have died in a peculiar explosion that theorists call a pair-instability supernova.

Like its lower-energy comrade, a pair-instability supernova occurs when a massive star no longer produces enough energy to counteract the inward pull of gravity. But with so much mass, the star’s core is squeezed to such a high temperature and pressure that runaway nuclear reactions power a devastating explosion. The whole star is obliterated and no compact remnant, such as a black hole or neutron star, is left behind.

Astronomers have seen hints of these rare events before. But now, Wako Aoiki from the National Astronomical Observatory of Japan and colleagues have approached the search in a different way, by finding a star that bears the chemical fingerprints of these ancient explosions.

The elements we see lacing a star’s surface provide a key to understanding the supernova that preceded the star’s birth. And the star, dubbed SDSS001820.5-093939.2, exhibits a peculiar set of chemical abundance ratios. It has high levels of heavy elements, such as nickel, calcium, and iron, but low levels of light elements, such as carbon, magnesium and cobalt.

Note that the star is still metal poor in the grand scheme of things. Its iron abundance is 1/100 of the solar level. But compared with most metal-poor stars, where the iron abundance can be 1/100,000 or less of the solar level, the star is metal rich.

The chemical abundance ratios (with respect to iron) of SDSS J0018-0939 (red circles) compared with model prediction for explosions of very-massive stars. The black line indicates the model of a pair-instability supernova by a star with 300 solar masses, whereas the blue line shows the model of an explosion caused by a core-collapse of a star with 1000 solar masses. The abundance ratios of sodium (Na) and aluminum (Al), which are not well-reproduced by these models, might be produced during the evolution of stars before the explosion, but that is not included in the current model. (Credit: NAOJ)
The chemical abundance ratios (with respect to iron) of SDSS J0018-0939 (red circles) compared with model prediction for explosions of very-massive stars. The black line indicates the model of a pair-instability supernova by a star with 300 solar masses, whereas the blue line shows the model of an explosion caused by a core-collapse of a star with 1000 solar masses. The abundance ratios of sodium (Na) and aluminum (Al), which are not well-reproduced by these models, might be produced during the evolution of stars before the explosion, but that is not included in the current model. (Credit: NAOJ)

These odd fingerprints suggest the star formed from material seeded by the death of a very massive Pop III star. In fact, the chemical composition of the star matches the elements that pair-instability supernovae are predicted to create.

The team notes that this is the only star of about 500 in the same low-metallicity range that has this peculiar makeup. It is — at the moment — our only window into the early universe and the first generation of stars.

The paper was published Aug. 22 in Science and is available online.

A Cosmic Collision: Our Best View Yet of Two Distant Galaxies Merging

An international team of astronomers has obtained the best view yet of two galaxies colliding when the universe was only half its current age.

The team relied heavily on space- and ground-based telescopes, including the Hubble Space Telescope, the Atacama Large Millimeter/submillimeter Array (ALMA), the Keck Observatory, and the Karl Jansky Very Large Array (VLA). But the greatest asset was a chance cosmic alignment.

“While astronomers are often limited by the power of their telescopes, in some cases our ability to see detail is hugely boosted by natural lenses created by the universe,” said lead author Hugo Messias of the Universidad de Concepción in Chile and the Centro de Astronomia e Astrofísica da Universidade de Lisboa in Portugal.

Such a rare cosmic alignment plays visual tricks, where the intervening lens (be it a galaxy or a galaxy cluster) appears to bend and even magnify the distant light. This effect, called gravitational lensing, allows astronomers to study objects which would not be visible otherwise and to directly compare local galaxies with much more remote galaxies, seen when the universe was significantly younger.

The distant object in question, dubbed H-ATLAS J142935.3-002836, was originally spotted in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). Although very faint in visible light pictures, it is among the brightest gravitationally lensed objects in the far-infrared regime found so far.

The Hubble and Keck images reveal that the foreground galaxy is a spiral galaxy, seen edge-on. Although the galaxy’s large dust clouds obscure part of the background light, both ALMA and VLA can observe the sky at longer wavelengths, which are unaffected by dust.

Using the combined data, the team discovered that the background system was actually an ongoing collision between two galaxies.

The Antennae galaxies. Credit: Hubble / ESA
The Antennae galaxies. Credit: Hubble / ESA

First, the team noticed that these two galaxies resembled a much closer system: the Antennae galaxies, two galaxies that have spent the past few hundred million years in a whirling embrace as they merge together. The similarity suggested a collision, but ALMA — with its high sensitivity and spatial resolution — was able to verify it.

ALMA has the unique ability to detect the emission from carbon monoxide, as opposed to other telescopes, which might only be able to probe the absorption along the line of sight. This allowed astronomers to measure the velocity of the gas in the more distant object. With this information, they were able to show that the lensed galaxy is indeed an ongoing galactic collision.

Such collisions naturally enhance star formation. Any gas within the galaxies will feel a headwind, much as a runner feels a wind even on the stillest day, and become compressed enough to spark star formation. Sure enough, ALMA shows that the two galaxies are forming hundreds of new stars each year.

“ALMA enabled us to solve this conundrum because it gives us information about the velocity of the gas in the galaxies, which makes it possible to disentangle the various components, revealing the classic signature of a galaxy merger,” said ESO’s Director of Science and coauthor of the new study, Rob Ivison. “This beautiful study catches a galaxy merger red handed as it triggers an extreme starburst.”

The findings have been published in the Aug. 26 issue of Astronomy & Astrophysics and is available online.

Extreme Weather is Linked to Global Warming, a New Study Suggests

Extreme weather is becoming much more common. Heat waves and heavy rains are escalating, food crops are being damaged, human beings are being displaced due to flooding and animals are migrating toward the poles or going extinct.

Although it has been postulated that these extreme weather events may be due to climate change, a new study has found much better evidence.

The research shows blocking patterns — high-pressure systems that become immobile for days or even weeks, causing extreme heat waves and torrential rain — may have doubled in summers over the last decade.

“Since 2000, we have seen a cluster of these events,” lead author Dim Doumou told The Gaurdian earlier this month. “When these high-altitude waves become quasi-stationary, then we see more extreme weather at the surface. It is especially noticeable for heat extremes.”

It was a blocking pattern that led to the heat wave in Alaska in 2013, and to the devastating floods in Colorado last summer.

These blocking patterns are associated with the jet stream, the fast flowing winds high in Earth’s atmosphere at latitudes between 30 and 60 degrees. Sometimes the flow weakens, and the winds can dip down into more southern latitudes. These excursions lead to blocking patterns.

And the jet stream is becoming “wavier,” with steeper troughs and higher ridges.

The climatologists analyzed 35 years of wind data amassed from satellites, ships, weather stations, and meteorological balloons. They found that a warming Arctic creates and amplifies the conditions that lead to jet stream excursions, therefore raising the chances for long-duration extreme events, like droughts, floods, and heat waves.

That said the climatologists were unable to see a direct causal link between climate change and extreme weather. Ordinarily we think about “cause” in a simple sense in which one thing fully brings about another. But the Colorado floods, for example, were partially caused by moisture from the tropics, a blocking pattern, and past wildfires that increased the risk of runoff.

So there is a difference between “direct causation” and “systematic causation.” The latter is not direct, but it is no less real. In this study, the team noticed that the rise in blocking patterns correlates closely with the extra heating being delivered to the Arctic by climate change. Statistically speaking, the two seem to go hand in hand.

But the team does hypothesize a direct causal link. The jet streams are driven by the difference in temperature between the poles and the equator. So because the Arctic is warming more quickly than lower latitudes, the temperature difference is declining, providing less energy for the jet stream and causing it to meander.

Although the study shows a correlation — not causation — between more frequent blocking patterns (and therefore extreme weather) and Arctic warming, it is a solid step forward in understanding how the two are related.

The article has been published in the journal Proceedings of the National Academies of Science (PNAS).

To see why Universe Today writes about climate change, please read a past article on the subject.

ESO’s Latest Dramatic Landscape

The universe is stunning. Images from even the most modest telescopes can unveil its brilliant beauty. But couple that with a profound reason — our ability to question and understand the physical laws that dominate that brilliant beauty — and the image transforms into something much more spectacular.

Take ESO’s latest image of two dramatic star formation regions in the southern Milky Way. John Herschel first observed the cluster on the left in 1834, during his three-year expedition to systematically survey the southern skies near Cape Town. He described it as a remarkable object and thought it might be a globular cluster. But future studies (and not to mention more dramatic images from larger telescopes) enriched our understanding, demonstrating that it was not an old globular but a young open cluster.

This chart shows the constellation of Carina (The Keel) and includes all the stars that can be seen with the unaided eye on a clear and dark night. This region of the sky includes some of the brightest star formation regions in the Milky Way. The location of the distant, but very bright and compact, open star cluster NGC 3603 is marked. This object is not spectacular in small telescopes, appearing as just a tight clump of stars surrounded by faint nebulosity. Credit: ESO
This chart shows the constellation of Carina and includes all the stars that can be seen with the unaided eye on a clear and dark night. The location of the open star cluster NGC 3603 is marked. This object is not spectacular in small telescopes, appearing as just a tight clump of stars surrounded by faint nebulosity. Credit: ESO

The Wide Field Imager at ESO’s La Silla Observatory in Chile recently captured the image again. The bright region on the left is the star cluster NGC 3603, located 20,000 light-years away in the Carina-Sagittarius spiral arm of the Milky Way galaxy. The bright region on the right is a collection of glowing gas clouds known as NGC 3576, located only 10,000 light-years away.

Stars are born in enormous clouds of gas and dust, largely hidden from view. But as small pockets in these clouds collapse under the pull of gravity, they become so hot they ignite nuclear fusion, and their light clears away — and brightens — the surrounding gas and dust.

Nearby regions of hydrogen gas are heated, and therefore partially ionized, by the ultraviolet radiation given off by the brilliant hot young stars. These regions, better known as HII regions, can measure several hundred light-years in diameter, and the one surrounding NGC 3603 has the distinction of being the most massive known in our galaxy.

Not only is NGC 3603 known for having the most massive HII region, it’s known for having the highest concentration of massive stars that have been discovered in our galaxy so far. At the center lies a Wolf-Rayet star system. These stars begin their lives at 20 times the mass of the Sun, but evolve quickly while shedding a considerable amount of their matter. Intense stellar winds blast the star’s surface into space at  several million kilometers per hour.

Where NGC 3603 is notable for its extremes, NGC 3576 is notable for its extremities — the two huge curved objects in the outreaches of the cluster. Often described as the curled horns of a ram, these odd filaments are the result of stellar winds from the hot, young stars within the central regions of the nebula. The stars have blown the dust and gas outwards across a hundred light-years.

Additionally, the two dark silhouetted areas near the top of the nebula are known as Bok globules, dusty regions found near star formation sights. These dark clouds absorb nearby light and offer potential sites for the future formation of stars. They may further sculpt the dramatic landscape above, which is the smallest slice of our stunning universe

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NASA’S NuSTAR Catches a Black Hole Bending Light, Space, and Time

NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) has captured a spectacular event: a supermassive black hole’s gravity tugging on nearby X-ray light.

In just a matter of days, the corona — a cloud of particles traveling near the speed of light — fell in toward the black hole. The observations are a powerful test of Einstein’s theory of general relativity, which says gravity can bend space-time, the fabric that shapes our universe, and the light that travels through it.

“The corona recently collapsed in toward the black hole, with the result that the black hole’s intense gravity pulled all the light down onto its surrounding disk, where material is spiraling inward,” said coauthor Michael Parker from the Institute of Astronomy in Cambridge, United Kingdom, in a press release.

The supermassive black hole, known as Markarian 335, is about 324 million light-years from Earth in the direction of the constellation Pegasus. Such an extreme system squeezes about 10 million times the mass of our Sun into a region only 30 times the diameter of the Sun. It spins so rapidly that space and time are dragged around with it.

NASA’s Swift satellite has monitored Mrk 335 for years, recently noting a dramatic change in its X-ray brightness. So NuSTAR was redirected to take a second look at the system.

NuSTAR has been collecting X-rays from black holes and dying stars for the past two years. Its specialty is analyzing high-energy X-rays in the range of 3 to 79 kiloelectron volts. Observations in lower-energy X-ray light show a black hole obscured by clouds of gas and dust. But NuSTAR can take a detailed look at what’s happening near the event horizon, the region around a black hole form which light can no longer escape gravity’s grasp.

Specifically, NuSTAR is able to see the corona’s direct light, and its reflected light off the accretion disk. But in this case, the light is blurred due to the combination of a few factors. First, the doppler shift is affecting the spinning disk. On the side spinning away from us, the light is shifted to redder wavelengths (and therefore lower energy), whereas on the side spinning toward us, the light is shifted to bluer wavelengths (and therefore higher energy). A second effect has to do with the enormous speeds of the spinning black hole. And a final effect is from the gravity of the black hole, which pulls on the light, causing it to lose energy.

All of these factors cause the light to smear.

Intriguingly, NuSTAR observations also revealed that the grip of the black hole’s gravity pulled the corona’s light onto the inner portion of the accretion disk, better illuminating it. NASA explains that as if somebody had shone a flashlight for the astronomers, the shifting corona lit up the precise region they wanted to study.

“We still don’t understand exactly how the corona is produced or why it changes its shape, but we see it lighting up material around the black hole, enabling us to study the regions so close in that effects described by Einstein’s theory of general relativity become prominent,” said NuSTAR Principal Investigator Fiona Harrison of the California Institute of Technology. “NuSTAR’s unprecedented capability for observing this and similar events allows us to study the most extreme light-bending effects of general relativity.”

The new data will likely shed light on these mysterious coronas, where the laws of physics are pushed to their limit.

The article has been published in the Monthly Notices of the Royal Astronomical Society and is available online.

Can Radio Waves Lead to Exomoons?

I firmly believe that our next greatest discovery will be detecting an exomoon in orbit around a distant exoplanet. Although no one has been able to confirm an exomoon — yet — the hunt is on.

Now, a research team thinks following a trail of radio wave emissions may lead astronomers to this groundbreaking discovery.

The difficulty comes in trying to spot an exomoon using existing methods. Some astronomers think that hidden deep within the wealth of data collected by NASA’s Kepler mission are miniscule signatures confirming the presence of exomoons.

If an exomoon transits the star immediately before or just after the planet does, there will be an added dip in the observed light. Although astronomers have searched through Kepler data, they’ve come up empty handed.

So the team, led by Ph.D. student Joaquin Noyola, from the University of Texas at Arlington, decided to look a little closer to home. Specifically, Noyola and colleagues analyzed the radio wave emissions that result from the interaction between Jupiter, and it’s closest moon, Io.

During its orbit, Io’s ionosphere interacts with Jupiter’s magnetosphere — a layer of charged plasma that protects the planet from radiation — to create a frictional current that emits radio waves. Finding similar emissions near known exoplanets could be the key to predicting where moons exist.

“This is a new way of looking at these things,” said Noyola’s thesis advisor, Zdzislaw Musielak, in a press release. “We said, ‘What if this mechanism happens outside of our Solar System?’ Then, we did the calculations and they show that actually there are some star systems that if they have moons, it could be discovered in this way.”

The team even pinpointed two exoplanets — Gliese 876b, which is about 15 light-years away, and Epsilon Eridani b, which is about 10.5 light-years away — that would be good targets to begin their search.

With such a promising discovery on the horizon, theoretical astronomers are beginning to address the factors that may deem these alien moons habitable.

“Most of the detected exoplanets are gas giants, many of which are in the habitable zone,” said coauthor Suman Satyal, another Ph.D. student at UT Arlington. “These gas giants cannot support life, but it is believed that the exomoons orbiting these planets could still be habitable.”

Of course one look at Io shows the drastic effects a nearby planet may have on its moon. The strong gravitational pull of Jupiter distorts Io, causing its shape to oscillate, which generates enormous tidal friction. This effect has led to over 400 active volcanoes.

But a moon at a slightly further distance could certainly be habitable. A second look at Europa — Jupiter’s second-most inner satellite — demonstrates this facet. It’s possible that life could very well exist under Europa’s icy crust.

Exomoons may be frequent, habitable abodes for life. But only time will tell.

The findings have been published in the Aug. 10 issues of the Astrophysical Journal and are available online.

How did Supermassive Black Holes Grow so Massive so Quickly?

Black holes one billion times the Sun’s mass or more lie at the heart of many galaxies, driving their evolution. Although common today, evidence of supermassive black holes existing since the infancy of the Universe, one billion years or so after the Big Bang, has puzzled astronomers for years.

How could these giants have grown so massive in the relatively short amount of time they had to form? A new study led by Tal Alexander from the Weizmann Institute of Science and Priyamvada Natarajn from Yale University, may provide a solution.

Black holes are often mistaken to be monstrous creatures that suck in dust and gas at an enormous rate. But this couldn’t be further from the truth (in fact the words “suck” and “black hole” in the same sentence makes me cringe). Although they typically accumulate bright accretion disks — swirling disks of gas and dust that make them visible across the observable Universe — these very disks actually limit the speed of growth.

First, as matter in an accretion disk gets close to the black hole, traffic jams occur that slow down any other infalling material. Second, as matter collides within these traffic jams, it heats up, generating energy radiation that actually drives gas and dust away from the black hole.

A star or a gas stream can actually be on a stable orbit around the black hole, much as a planet orbits around a star. So it is quite a challenge for astronomers to think of ways that would make a black hole grow to supermassive proportions.

Luckily, Alexander and Natarajan may have found a way to do this: by placing the black hole within a cluster of thousands of stars, they’re able to operate without the restrictions of an accretion disk.

Black holes are generally thought to form when massive stars, weighing tens of solar masses, explode after their nuclear fuel is spent. Without the nuclear furnace at its core pushing against gravity, the star collapses. While the inner layers fall inward to form a black hole of only about 10 solar masses, the outer layers fall faster, hitting the inner layers, and rebounding in a huge supernova explosion. At least that’s the simple version.

 A small black hole gains mass: Dense cold gas (green) flows toward the center of a stellar cluster (red cross in blue circle) with stars (yellow); the erratic path of the black hole through the gas (black line) is randomized by the surrounding stars Prof. Tal Alexander’s research is supported by the European Research Council.
The erratic path of the black hole through the gas (black line) is randomized by the surrounding stars (yellow circles). Meanwhile, dense cold gas (green arrows) flows toward the center of the cluster (red cross). Credit: Weizmann Institute of Science.

The team began with a model of a black hole, created from this stellar blast, embedded within a cluster of thousands of stars. A continuous flow of dense, cold, opaque gas fell into the black hole. But here’s the trick: the gravitational pull of many nearby stars caused it to zigzag randomly, preventing it from forming an accretion disk.

Without an accretion disk, not only is matter more able to fall into the black hole from all sides, but it isn’t slowed down in the accretion disk itself.

All in all, the model suggests that a black hole 10 times the mass of the Sun could grow to more than 10 billion times the mass of the Sun by one billion years after the Big Bang.

The paper was published Aug. 7 in Science and is available online.

Help a Universe Today Writer Share Stories About Our Search For Earth-like Planets

Since 1995, astronomers have detected thousands of worlds orbiting nearby stars, sparking a race to find the one that most resembles Earth. The discovery of habitable exoplanets and even extraterrestrial life is often referred to as the Holy Grail of science. So with the gold rush of exoplanet discoveries these days, it’s pretty tempting in news articles to lose readers in a fantastical narrative.

This month I’m launching a project on Beacon — a new independent platform for journalism — that will go behind the sensational headlines covering the search for Earth 2.0.

But I can’t do it without your help. In order to commit to writing about this on a regular basis, I need to raise $4,000 from subscribers who are willing to support my work over this month. Don’t worry, subscriptions are available for only $5 per month. This will supply the funding necessary to write for six months.

By Kepler’s definition, to be Earth-like a planet must be both Earth-size (less than 1.25 times Earth’s radius and less than twice Earth’s mass) and must circle its host star within the habitable zone: the band where liquid water can exist.

Image Credit: xkcd
Image Credit: xkcd

This simple, and yet variant, definition is a crucial starting point. But one glance at our Solar System (namely Venus and Mars) demonstrates that just because a planet is Earth-like doesn’t mean it’s an Earth twin.

So even if we do find Earth-like planets, we still don’t have the ability to know if they’re water worlds with luscious green planets and civilizations peering back at us.

But should we scale our definition of Earth-like planets up or down? Examples in the Solar System suggest that we should scale it down. Maybe planets located nearer to the center of the habitable zone are more congenial to life.

But can we base our definition on a single example — even if it’s the only example we know — alone? Theoretical astronomers suggest the picture is much more complicated. Life might arise on larger worlds, ones up to three times as massive as Earth, because they’re more likely to have an atmosphere due to more volcanic activity. Or life might arise on older worlds, where there’s simply more time for life to evolve.

It’s a crucial debate in astronomy research today, and it’s one that the media needs to handle with care. I am proud to be a part of Universe Today’s team, bringing readers up-to-date with the on goings in our local Universe. And Beacon will allow me to spend even more time, focusing on such a critical topic.

For each article, I will gather news, opinions and commentary from astronomers in the field. Not only do I have training as an astronomer, but my graduate school research focused on detecting exoplanet atmospheres from ground-based telescopes. With this deep-rooted understanding of the field at hand, I am able to parse complex information by directly reading peer-reviewed journal articles and interviewing astronomers I’ve met through my previous research.

But I really do need your help. Subscriptions are available for only $5 per month, and there are special rewards — such as gorgeous astronomy photos printed on canvas and gift subscriptions for friends — for people who subscribe at higher levels. You can directly subscribe here.

But here’s the best part: when you subscribe to my work, you’ll get access not only to all the stories I write, but the work of over 100 additional writers, based all over the world. This month Beacon is launching a series of astronomy projects, including one by Universe Today writer Elizabeth Howell.

Please help me write about our exciting search for Earth-like planets.

Hypervelocity Neutron Stars Crashing Into White Dwarfs — A Scenario for the Loneliest Supernovae?

It’s hard to comprehend the vast emptiness of space. Especially when we detect odd signatures, such as luminous explosions that are neither as bright nor as long as traditional supernovae, originating in the unfathomable emptiness.

But a team of astronomers is now beginning to understand these so-called calcium-rich transients, often referred to as the Universe’s loneliest supernovae, hypothesizing that they’re created by collisions between white dwarf stars and neutron stars — both of which have been thrown out of their galaxy.

“One of the weirdest aspects is that they seem to explode in unusual places. For example, if you look at a galaxy, you expect any explosions to roughly be in line with the underlying light you see from that galaxy, since that is where the stars are” said lead author Joseph Lyman from the University of Warwick in a press release. “However, a large fraction of these are exploding at huge distances from their galaxies, where the number of stellar systems is miniscule.”

The team guessed there could be very faint dwarf galaxies, hiding beneath the limit of detection, but found nothing with our best telescopes, namely the Very Large Telescope in Chile and the Hubble Space Telescope.

“So the question becomes, how did the get there?” pondered Lyman. Roughly a third of these events occur at least 65 thousand light-years away from a potential host galaxy.

We’ve discovered dozens of so-called hypervelocity stars — single stars that escape their home galaxy, traveling rapidly throughout intergalactic space — and even one runaway globular cluster. Nature clearly has a way of kicking systems out of an entire galaxy, likely by an interaction with the supermassive black hole lurking in the center of that galaxy.

So it’s viable that the source of these supernovae was first kicked out of its host galaxy. But the second puzzle wondered what type of system could have caused such an odd explosion.

Previous studies show that calcium comprises up to half of the material thrown off in these transients, compared to only a tiny fraction in normal supernovae. It remained unclear how to explain such a calcium-rich system.

So the research team compared their data to short-duration gamma ray bursts, which are also seen to explode in remote locations with no coincident galaxy detected. We think these enigmatic bursts occur when two neutron stars collide, or when a neutron star merges with a black hole.

Alas, the research team discovered that if a neutron star collided with a white dwarf, the explosion would not only provide enough energy to generate the low luminosity of the calcium rich transients, but it would also produce calcium rich material.

“What we therefore propose is these are systems that have been ejected from their galaxy,” said Lyman. “A good candidate in this scenario is a white dwarf and a neutron star in a binary system. The neutron star is formed when a massive star goes supernova. The mechanism of the supernova explosion causes the neutron star to be ‘kicked’ to very high velocities (100s of km/s). This high velocity system can then escape its galaxy, and if the binary system survives the kick, the white dwarf and neutron star will merge causing the explosive transient.”

Any merger should also produce high-energy gamma-ray bursts, motivating further observations of any new examples.

The paper has been published today in the journal Monthly Notices of the Royal Astronomical Society and is available online.

Hubble Archive Reveals Possible Culprit for Enigmatic Supernova

More than two decades of Hubble observations have produced more than 25 terabytes of data. Thanks to the wealth of information stored in the Hubble data archive, astronomers can easily revisit old images in an effort to better understand new discoveries.

Now, astronomers have used the archive to find the progenitor of a mysterious type of supernova, dubbed Type 1ax, which is less energetic and much fainter than its Type Ia cousin.

A Type 1a supernova occurs when a white dwarf siphons material off a companion star, building an additional layer of hydrogen on its surface that will eventually trigger a runaway reaction that detonates the accumulated gas.

The most popular explanation for Type 1ax supernovae is that they’re created in the same way, except the explosion doesn’t completely tear the white dwarf into pieces. Instead, the white dwarf ejects roughly half of its mass. It becomes battered and bruised, leaving behind a hot core composed of carbon and oxygen.

So far, astronomers have identified more than 30 of these mini-explosions, which occur at one-fifth the rate of Type 1a supernovae.

“Astronomers have been searching for decades for the progenitors of Type Ia’s,” said Saurabh Jha from Rutgers University in a NASA press release. “Type Ia’s are important because they’re used to measure vast cosmic distances and the expansion of the universe. But we have very few constraints on how any white dwarf explodes. The similarities between Type Iax’s and normal Type Ia’s make understanding Type Iax progenitors important, especially because no Type Ia progenitor has been conclusively identified.”

So after the team observed the weak supernova, dubbed SN 2012Z, in the Lick Observatory Supernova Search, they dug through Hubble’s archive. Fortuitously, Hubble had observed the supernova’s host galaxy, NGC 1309, in 2005, 2006, and 2010, before the supernova outburst.

Curtis McCully, a graduate student at Rutgers and lead author on the team’s paper, reprocessed the pre-explosion images to find an object at the supernova’s position.

“I was very surprised to see anything at the supernova’s location,” said McCully. “We expected that the progenitor system would be too faint to see, like in previous searches for normal Type Ia supernova progenitors. It is exciting when nature surprises us.”

The pre-supernova observations reveal a bright, blue source the team calls S1. McCully and colleagues concluded that they were most likely seeing a star that had lost its outer hydrogen envelope, revealing its helium core. But they don’t think it’s a type of star that was about to explode, rather it’s the companion that fed the white dwarf’s outburst.

The most likely explanation involves a binary star system where each star detonates mass to the other over time.

The team acknowledges that they can’t totally rule out other possibilities for the object’s identity, including that it was simply a single, massive star that exploded as a supernova. To settle any uncertainties the team plans to use Hubble again in 2015. Hopefully by then the supernova should fade enough to get a better look at what remains.

The team’s results will appear in the journal Nature tomorrow.