Of White Dwarfs, “Zombie” Stars and Supernovae Explosions

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Artistic view of the aftermath of a supernova explosion, with an unexpected white dwarf remnant. These super-dense but no longer active stars are thought to play a key role in many supernovae explosion. (Copyright Russell Kightley)
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White dwarf stars, the remnant cores of low-mass stars that have exhausted all their nuclear fuel, are among the most dense objects in the sky.
 
Their mass is comparable to that of the sun, while their volume is comparable to that of Earth. Very roughly, this means the average density of matter in a white dwarf would be on the order of 1,000,000 times greater than the average density of the sun.
 
Thought to be the final evolutionary state of stars whose mass is not high enough to become a neutron star — a category that includes the sun and over 97% of the other stars in the Milky Way — they are dim objects first identified a century ago but only in the last decade the subject of broad study.
 
In recent years the white dwarfs have become more and more closely associated with supernovae explosions, though the processes involved remained hotly debated.  A team using the Hubble Space Telescope even captured  before and after images of what is hypothesized to be an incomplete white dwarf supernova.  What was left behind has been described by some as a “zombie star.”
 
Now a team of astronomers led by Stephane Vennes of the Czech Academy of Sciences has detected another zombie white dwarf, LP-40-365 , that they put forward as a far-flung remnant of a long-ago supernova explosion.  This is considered important and unusual because it would represent a first detection of such a remnant long after the supernova conflagration.
 
This dynamic is well captured in an animation accompanying the Science paper that describes the possible remnant.  Here’s the animation and a second-by-second description of what is theorized to have occurred:
 
 
00.0 sec: Initial binary star outside the disk of the Milky Way galaxy. A massive white dwarf accreting
material through an accretion disk from its red giant companion star. The stars orbit around the center of
mass of the binary system.
 
14.6 sec: The white dwarf reaches the Chandrasekhar mass limit and explodes as a bright Type Ia
supernova. However, the explosion is not perfect; a fraction of the white dwarf shoots out like a shrapnel to the left. The binary system disrupts.
 
18.0 sec: The supernova explosion again, at an edge – on view. The shrapnel comes at the viewer and passes by.
 
20.0 sec: After passing by, the remnant flies off towards the disk of the Milky Way towards the spiral arm with the Solar System.
 
24.0 sec : The fast moving remnant from the solar neighborhood as it passes by the stars in our galactic arm, including the Sun. The remnant gets in the reach of our telescopes. (Copyright Sardonicus Pax)

 

A supernova — among the most powerful forces in the universe — occurs when there is a change in the core of a star. A change can occur in two different ways, with both resulting in a thermonuclear explosion.

Type Ia supernova occurs at the end of a single star’s lifetime. As the star runs out of nuclear fuel, some of its mass flows into its core. Eventually, the core is so heavy that it cannot withstand its own gravitational force. The core collapses, which results in the giant explosion of a supernova. The sun is a single star, but it does not have enough mass to become a supernova.

The second type takes place only in binary star systems. Binary stars are two stars that orbit the same point. One of the stars, a carbon-oxygen white dwarf, steals matter from its companion star. Eventually, the white dwarf accumulates too much matter. Having too much matter causes the star to explode, resulting in a supernova.

Type Ia supernovae, which are the result of the complete destruction of the star in a thermonuclear explosion, have a fairly uniform brightness that makes them useful for cosmology. The light emitted by the supernova explosion can be, for a short while at least, as bright as the whole of the Milky Way.

Recently, astronomers have discovered a related form of supernova, called Type Iax, which look like Type Ia, but are much fainter. Type Iax supernovae may be caused by the partial destruction of a white dwarf star in such an explosion. If that interpretation is correct, part of the white dwarf should survive as a leftover object.

And that leftover object is precisely what Vennes et al claim to have found.

They have identified LP 40-365 as an unusual white dwarf with a low mass, high velocity and strange composition of oxygen, sodium and magnesium  – exactly as might be expected for the leftover star from a Type Iax event. Vennes describes the white dwarf remnant his team has detected as a “compact star,” and perhaps the first of its kind in terms of the elements it contains.

The team calculate that the explosion must have occurred between five and 50 million years ago.

 

The two inset images show before-and-after images captured by NASA’s Hubble Space Telescope of Supernova 2012Z in the spiral galaxy NGC 1309, what some call a “zombie star.”. The white X at the top of the main image marks the location of the supernova in the galaxy. A supernova typically obliterates the exploding white dwarf, or dying star.  In 2014, scientists found that this faint supernova may have left behind a surviving portion of the white dwarf star.(NASA,ESA)

In an email exchange, Vennes told me that he has been studying the local white dwarf population for thirty years.

“These compact, dead stars tell us a lot about the “old” Milky Way, how stars were born and how they died,” he wrote.

“Tens of thousands of these white dwarfs have been catalogued over this past century, most of them in the last decade, but we keep an eye on outliers, objects that are out of the norm. We look for exceedingly large velocity, peculiar chemical composition or abnormal mass or radii.

Stephane Vennes, a longtime specialist in white dwarf stars at the Czech Academy of Science.

“The strange case of LP40-365 came unexpectedly, but this was a classic case of serendipity in astronomy. Out of hundreds of targets we observed at the telescope, this one was uniquely peculiar. Fortunately, theorists are very imaginative and the model we adopted to interpret the observed properties of this object were only recently published. Our research on this object was certainly inspired and directed by their theory.”

Vennes says the team was surprised to learn that the white dwarf LP40-365 is relatively bright among its peers and that similar objects did not show up in large-scale surveys such as the Sloan Digital Sky Survey.

“This fact has convinced us that many more similarly peculiar white dwarfs await discovery. We should search among fainter, more distant samples of white dwarfs,” he wrote.

And that search can be done by the European Space Agency’s Gaia astrometric space telescope, with follow-up observations at large telescopes such as the European Southern Observatory’s Very Large Telescope and the Gemini observatory in Chile.

“It is also likely that our adopted model involving a subluminous {faint} Type Ia supernova will be modified or even superseded by teams of theorists coming up with new ideas. But we remain confident that these new ideas would still involve a cataclysmic event on the scale of a supernova.”

Here is another animated version of the cataclysm described in the paper: 

An ultra-massive and compact dead star, or white dwarf, (shown as a small white star) is accreeting matter from its giant companion (the larger red star). The material escapes from the giant and forms an accretion disk around the white dwarf.
Once enough material is accreted onto the white dwarf, a violent thermonuclear runaway tears it apart and destroys the entire system. The giant star and the surviving fragment of the white dwarf are flung into space at tremendous speeds. The surviving white dwarf shrapnel hurtles towards our region of the galaxy, where its radiation is detected by ground based telescopes. (Copyright Russell Kightley)

 

A supernova burns for only a short period of time, but it can tell scientists a lot about the universe.

One kind of supernova has shown scientists that we live in an expanding universe, one that is growing at an ever increasing rate.

Scientists also have determined that supernovas play a key role in distributing elements throughout the universe. When the star explodes, it shoots elements and debris into space. Many of the elements we find here on Earth are made in the core of stars.

These elements travel on to form new stars, planets and everything else in the universe — making white dwarfs and supernovae essential to the process that ultimately led to life.

 

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Supernovae Give, And Can Take Away

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What is likely the brightest supernova in recorded human history, SN 1006 lit up planet Earth’s sky in the year 1006 AD. The expanding debris cloud from the stellar explosion, still puts on a cosmic light show across the electromagnetic spectrum. The supernova is located about 7,000 light-years from Earth, meaning that its thermonuclear explosion actually happened 7,000 years before the present day.  Shockwaves in the remnant accelerate particles to extreme energies and are thought to be a source of the mysterious cosmic rays. NASA, ESA, Zolt Levay (STScI)

We live in a dangerous universe. We know about meteor and comets, about harmful radiation that could extinguish life without an electromagnetic shield, about major changes in climate that are both natural and man-made.

There’s another risk out there that some scientists assert could cause large-scale extinctions even though it would occur scores of light-years away.  These are supernovae – explosions of massive stars that both create and spread the heavy elements needed for life and send out high energy cosmic rays that can travel far and cause enormous damage.

As with most of these potential threats, they fortunately occur on geological or astronomical time scales rather than human ones. But that doesn’t mean they don’t happen.

At the recent Astrobiology Science Conference (AbSciCon) a series of talks focused on that last threat – starting with a talk on “When Stars Attack.”

And together five different presenters made a persuasive case that Earth was on the receiving end of a distant supernova explosion some two to three million years ago, and probably around 7 or 8 million years ago as well. The effects of the cosmic ray bombardment have been debated and disputed, but the evidence for the occurrences is based on the rock record and is now strong.

“The evidence is there on the ocean floor:  in rocky crusts, nodules and sediment,” said Brian Fields, professor of astronomy at University of Illinois.  “We’ve been able to date it and provide some idea of how far away the star blew up.”  The answer is between about 90 and 300 light-years.

Supernova 1994D exploded on the outskirts of disk galaxy, and outshines even the center of the galaxy. Supernovae may expel much, if not all, of the material away from a star,  at velocities up to 30,000 km/s or 10% of the speed of light. This drives an expanding and fast-moving shock wave into the surrounding interstellar medium that, if close to Earth (or any other planet) can have dire consequences.  Supernovae also create, fuse and eject the bulk of the chemical elements produced by nucleosynthesis, the heavier elements needed to form planets and later make possible life.  ( High-Z Supernova Search Team, HST, NASA)

“Supernova explosions happen all the time– on average every 30 years in our galaxy, though they are most often distant and obscured from view,” Fields said.  “They generate cosmic rays that can spread through the galaxy for 30 million years.  These are the cosmic rays that make carbon-14 and can threaten astronauts in space.  But that’s not what we’re focused on — we look at the ones that are close to us and could have a far more dramatic effect, and they are pretty rare.”

What is deemed to be the “kill zone” for a planet nearby a supernova is 30 light-years; the high energy particles from an explosion that close would, he said, likely end all or most life on Earth by setting into motion a variety of atmospheric and surface changes. Fields there is no evidence of such a close and damaging supernove within the past 10 million years, the period that has been studied with some rigor.

But because a close supernova explosion hasn’t happened recently doesn’t mean that it didn’t happened during earlier times.  Or that it couldn’t happen in the far future.

“By nailing the signal of a close but not ‘kill zone’ supernova two to three million years ago, and most likely another at 7 to 8 million years ago, we make the case that supernova can and do have significant effects on Earth.”

The community of scientists who study supernovae and their effects on Earth, both potential and known, is small, and has been most active in the past decade.  There was an earlier time when scientists focused on supernovae as the potential cause for the massive dinosaur extinction, but the field shrank with confirmation in 1990 that a six-mile wide meteor landed on Mexico’s Yucatan Peninsula about 65 million years ago and was the likely cause of the global extinction.

Brian Fields, chair of the astronomy department at the University of Illinois and a professor of physics, focuses on cosmology, nuclear and particle astrophysics and astrobiology as well as supernovae — especially those of the near-Earth variety. (University of Illinois)

But now, with the advent of new theories and some very high tech and precise measuring the field and subject has come to life, with research nodes in Germany, Australia and the American Midwest.

The key to understanding the effects of distant supernovae on Earth involves a radioactive isotope of iron, iron-60.  It’s one of the many elements known to be sent into the cosmos by the massive thermonuclear blasts that define a supernova, that send out shock waves capable of spurring the formation of new stars as well as providing the universe with the heavier chemical elements needed to form everything from planets to genes.

It was the young Fields and colleagues who theorized some two decades ago that iron-60 could be a telltale sign of a relatively nearby supernova.  He told me that no other significant sources of iron-60 are known to exist, and so if it were found on Earth scientists would know where it came from.

With a half-life of some three million years, the iron-60 would be a potentially strong signal for that length of time and and then a weaker but potentially detectable signal after that.

The question was how do you find iron-60 on Earth? The answer came from the bottom of the ocean.

First in 1999 a group from the Technical University of Munich in Germany identified some iron-60 in iron-manganese crustal rocks at the bottom of the Pacific, and then in 2013 reported finding the telltale isotope in not only rocks but also in nodules and most important in fossil bacteria and sea-floor sediments.  They used ultra-sensitive accelerator mass spectrometry to isolate and identify the iron-60, which they reported was deposited some 1.6 to 3 million years ago.

These are transmission electron microscope images showing tiny magnetofossils containing iron-60, a form of iron produced during the violent explosion and death of a massive star in a supernova. They were deposited by bacteria in sediments found on the floor of the Pacific Ocean.© Marianne Hanzlik, Chemie Department, FG Elektronenmikroskopie, Technische Universität München

Last year as well the Australian group, led by Anton Wallner of the Australian National University, found the iron-60 to be deposited globally and to have arrived within the same general time frame.  And Gunther Korschinek, a physicist at the Technical University of Munich involved in the initial German iron-60 detections, led a team that found elevated amounts of iron-60 in lunar soil samples brought from to moon back to Earth during the Apollo program.

As Fields put it, the studies together gave a clear signal of a supernova explosion, or series of explosions, at 2 to 3 million years ago, and a less clear but likely signal of the same at 7 to 8 million years ago.

Since Fields and other scientists were presenting during the AbSciCon conference, the talks not surprisingly focused on potential biological implications of supernova explosions.  And while supernova impacts on the biosphere are not particularly well understood, a number of intriguing theories were presented.

Brian Thomas of Washburn University described how cosmic rays from close supernova would significantly increase levels of electrically charged elements and molecules in the atmosphere, lasting thousands of years.  In the upper atmosphere this would have the effect of setting into motion a chemical cascade that would deplete stratospheric ozone. In the lower atmosphere, the effect would likely be changes in climate and minor mass extinctions.

The “holy grail” of their supernova work is matching a detected one with a dramatic event in the Earth biosphere, most especially a mass extinction.  The 2 to 3 million years ago period includes the boundary between the Pleistocene and Pliocene epochs, when Earth climate changed and major glaciations periods began — possibly supernova-related changes but not the extreme change a close supernova could produce.

Another potential effect of the supernova event of 2 to 3 million years ago is increased rates of mutation and of lightning, and thus forest fires on Earth.

Adrian Melott of the University of Kansas suggested that expected mutations from radiation sources such as supernovae could explain evolutionary changes in a variety of groups of organisms and creatures during that period — as a result of increased deadly cancers in some species and increased positive mutations in others.

He also said that evidence of more widespread wildfires during that long period — as measured in charcoal deposits — could be the result of increased cloud to ground lightning induced by the additional high-energy particle environment created by a relatively close supernova explosion.

The Crab nebula – one of the most glorious images produced by the Hubble Space Telescope — is the remnant of supernovae explosions that occurred at a distance of some  6,700 light-years.  The very bright light of the explosion was noted in 1054 and remained visible for around two years. The event was recorded in contemporary Chinese astronomy, and references to it are also found in a later (13th-century) Japanese document,  perhaps in pictograph associated with the Anasazi people of the Southwest.  The supernova, SN 1054 has been widely studied and is often considered the best known supernova in astronomy.  (NASA).

The iron-60 signatures of a close supernova have been a great boon to the field, but they do not go back beyond that almost 10 million year period when the radioactivity was present.  To go back further than that, Fields said different radioactive signatures would be needed — and not those that go back to the formation of the planet.

“It’s a hard problem because nature has been unkind,”  he said.  “The early mass extinctions – 100 million and more years ago – need radioactivity that lasts that long.  And the only element we’ve found is plutonium-244, which is not stable in any form.”

Plutonium-244 has a half life of 80 million years, and so could potentially be used to identify close supernova explosions in a manner similar to iron-60, but during that much longer time frame.  And as Fields explained it, plutonium-244 is produced in a few dramatic ways:  during the explosion of a nuclear bomb, the explosion of a supernova, or the merging of a pair of neutron stars.”

Although the science around the formation and detection of plutonium-244 in nature is immature, he said it remains the best pathway to find that “holy grail” — a known mass extinction directly associated with a close supernova explosion.

 

Supernovae can burn with a luminosity of ten billion suns. This show a before and after for supernova 1987A, which exploded in 1987 in the Large Magellanic Cloud (LMC), a nearby galaxy. (Australian Astronomical Observatory/ David Malin)

 

 

 

 

 

 

 

 

 

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