Posted on by Mike Simonsen

Do Puny White Dwarfs Make Wimpy Supernovae?

The binary star system J0923+3028 consists of two white dwarfs: a visible star 23 percent as massive as our Sun and about four times the diameter of Earth, and an unseen companion 44 percent of the Sun's mass and about one Earth-diameter in size. The stars will spiral in toward each other and merge in about 100 million years. (Credit: Clayton Ellis (CfA))

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Based on results from a radial velocity survey, Warren Brown, (Smithsonian Astrophysical Observatory) and his team have placed a few more pieces into the supernova puzzle.

Supernovae come in many flavors. There are Type Ia, the “standard candles” everyone has heard of; and there are Type Ib and Ic, which also involve binary systems. We also have Type II supernovae that are believed to be the core collapse of single, super-massive stars. There are also super-luminous supernovae, which may be the explosive conversion of a neutron star into a quark star, and finally the weak-kneed cousins of the bunch, the under-performing underluminous supernovae.

Underluminous supernovae are a rare type of supernova explosion 10–100 times less luminous than a normal SN Type Ia and eject only 20% as much matter. Brown and his team have been investigating the connection between underluminous supernovae and merging pairs of white dwarfs.

In the 1980s, on the basis of our theoretical understanding of stellar and binary evolution it was predicted that many close double white dwarfs would exist. However, it was not until 1988 that the first one was actually discovered.

The way to find close double white dwarfs is to take high resolution spectra of the H-alpha absorption line of a white dwarf at several different times and look for variation that is caused by the orbital motion of the white dwarf around an unseen (dimmer) companion. The first systematic searches were not very unsuccessful. Only one system was found. Then, during the 1990s, Tom Marsh and collaborators concentrated their search on low-mass white dwarfs, which, based on current theories, could _only_ be formed in a binary system. In this way a dozen more systems were found.

Extremely low mass (ELM) white dwarfs (WDs) with less than 0.3 solar masses are the remnants of stars that never ignited helium in their cores. The Universe is not old enough to have produce ELM WDs by single star evolution. Therefore, ELM WDs must undergo significant mass loss sometime in their evolution. Producing WDs with 0.2 solar masses most likely requires compact binary systems.

“These white dwarfs have gone through a dramatic weight loss program,” said Carlos Allende Prieto, an astronomer at the Instituto de Astrofisica de Canarias in Spain and a co-author of the study. “These stars are in such close orbits that tidal forces, like those swaying the oceans on Earth, led to huge mass losses.”

Observational data for ELM WDs is pretty hard to come by because of their rarity. For example, of the 9316 WDs identified in the Sloan Digital Sky Survey, less than 0.2% have masses below 0.3 solar.

Half of the pairs discovered by Brown and collaborators are merging and might explode as supernovae in 100 million years or more.

“We have tripled the number of known, merging white-dwarf systems,” said Smithsonian astronomer and co-author Mukremin Kilic. “Now, we can begin to understand how these systems form and what they may become in the near future.” Unlike normal white dwarfs made of carbon and oxygen, these are made almost entirely of helium.

“The rate at which our white dwarfs are merging is the same as the rate of under-luminous supernovae – about one every 2,000 years,” explained Brown. “While we can’t know for sure whether our merging white dwarfs will explode as under-luminous supernovae, the fact that the rates are the same is highly suggestive.”

At least 25% of these ELM WDs belong to the old thick disk and halo components of the Milky Way. This helps astronomers know where to look for underluminous SNe and where they are unlikely to find them, if the models are correct. If merging ELM WD systems are the progenitors of underluminous SNe, the next generation of surveys such as the Palomar Transient Factory, Pan-STARRS, Skymapper, and the Large Synoptic Survey Telescope should find them amongst the older populations of stars in both elliptical and spiral galaxies.

The papers announcing their find are available online at: http://arxiv.org/abs/1011.3047 and http://arxiv.org/abs/1011.3050.

Posted on by Jon Voisey

Galaxy Zoo Searches for Supernovae

Aside from categorizing galaxies, another component of the Galaxy Zoo project has been asking participants to identify potential supernovae (SNe). The first results are out and have identified “nearly 14,000 supernova candidates from [Palomar Transient Factory, (PTF)] were classified by more than 2,500 individuals within a few hours of data collection.”

Although the Galaxy Zoo project is the first to employ citizens as supernova spotters, the background programs have long been in place but were generating vast amounts of data to be processed. “The Supernova Legacy Survey used the MegaCam instrument on the 3.6m Canada-France-Hawaii Telescope to survey 4 deg2” every few days, in which “each square degree would typically generate ~200 candidates for each night of observation.” Additionallly, “[t]he Sloan Digital Sky Survey-II Supernova Survey used the SDSS 2.5m telescope to survey a larger area of 300 deg2” and “human scanners viewed 3000-5000 objects each night spread over six scanners”.

To ease this burden, the highly successful Galaxy Zoo implemented a Supernova Search in which users would be directed through a decision tree to help them determine what computer algorithms were proposing as transient events. Each image would be viewed and decided on by several participants increasing the likelihood of a correct appraisal. Also, “with a large number of people scanning candidates, more candidates can be examined in a shorter amount of time – and with the global Zooniverse (the parent project of Galaxy Zoo) user base this can be done around the clock, regardless of the local time zone the science team happens to be based in” allowing for “interesting candidates to be followed up on the same night as that of the SNe discovery, of particular interest to quickly evolving SNe or transient sources.”

To identify candidates for viewing, images are taken using the 48 inch Samuel Oschin telescope at the
Palomar Observatory. Images are then calibrated to correct instrumental noise and compared automatically to reference images. Those in which an object appears with a change greater than five standard deviations from the general noise are flagged for inspection. While it may seem that this high threshold would eliminate other events, the Supernova Legacy Survey, starting with 200 candidates per night, would only end up identifying ~20 strong candidates. As such, nearly 90% of these computer generated identifications were spurious, likely generated by cosmic rays striking the detector, objects within our own solar system, or other such nuisances and demonstrating the need for human analysis.

Still, the PTF identifies between 300 and 500 candidates each night of operation. When exported to the Galaxy Zoo interface, users are presented with three images: The first is the old, reference image. The second is the recent image, and the third is the difference between the two, with brightness values subtracted pixel for pixel. Stars which didn’t change brightness would be subtracted to nothing, but those with a large change (such as a supernova), would register as a still noticeable star.

Of course, this method is not flawless, which also contributes to the false positives from the computer system that the decision tree helps weed out. The first question (Is there a candidate centered in the crosshairs of the right-hand [subtracted] image?) eliminates misprocessing by the algorithm due to misalignment. The second question (Has the candidate itself subtracted correctly?) serves to drop stars that were so bright, they saturated the CCD, causing odd errors often resulting in a “bullseye” pattern. Third (Is the candidate star-like and approximately circular?), users eliminate cosmic ray strikes which generally only fill one or two pixels or leave long trails (depending on the angle at which they strike the CCD). Lastly, users are asked if “the candidate centered in a circular host galaxy?” This sets aside identifications of variable stars within our own galaxy that are not events in other galaxies as well as supernovae that appear in the outskirts of their host galaxies.

Each of these questions is assigned a number of positive or negative “points” to give an overall score for the identification. The higher the score, the more likely it is to be a true supernova. With the way the structure is set up, “candidates can only end up with a score of -1, 1 or 3 from each classification, with the most promising SN candidates scored 3.” If enough users rank an event with the appropriate score, the event is added to a daily subscription sent out to interested parties.

To confirm the reliability of identifications, the top 20 candidates were followed up spectroscopically with the 4.2m William Herschel Telescope. Of them, 15 were confirmed as SNe, with 1 cataclysmic variable, and 4 remain unknown. When compared to followup observations from the PTF team, the Galaxy Zoo correctly identified 93% of supernova that were confirmed spectroscopically from them. Thus, the identification is strong and this large volume of known events will certainly help astronomers learn more about these events in the future.

If you’d like to join, head over to their website and register. Presently, all supernovae candidates have been processed, but the next observing run is coming up soon!

Posted on by Tammy Plotner

New Supernova Lights Up Leo

A new supernova? Darn right. Lighting up Leo? Well… not without some serious visual aid, but the fact that someone out there is watching and has invited us along for the ride is mighty important. And just who might that someone be? None other than Tim Puckett.

Less than 24 hours ago, the American Association of Variable Star Observer’s Report #222 stated:

“Bright Supernova in UGC 5189A: SN 2010jl
November 5, 2010

We have been informed by Tim Puckett and by the Central Bureau for Astronomical Telegrams (CBET 2532, Daniel W. E. Green, Ed.) of the discovery of a bright supernova in UGC 5189A by J. Newton and Puckett, Portal, AZ, on November 3.52 UT at unfiltered magnitude 13.5. Confirming images (limiting magnitude 19.1) by Puckett on Nov. 4.50 UT showed the object at magnitude 12.9.

Spectroscopic observations (CBET 2536, Daniel W. E. Green, Ed.) by S. Benetti and F. Bufano, Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Padova, on behalf of a larger collaboration, and by J. Vinko, University of Szeged, G. H. Marion, Harvard-Smithsonian Center for Astrophysics and University of Texas, T. Pritchard, Pennsylvania State University, and J. C. Wheeler and E. Chatzopoulos, University of Texas, show that SN 2010jl is a type-IIn supernova. Vinko et al. also report that simultaneous measurements with Swift/UVOT in the ultraviolet bands confirm that the transient is ultraviolet-bright, as expected for young, interacting supernovae.

Coordinates: 09 42 53.33 +09 29 41.8 (J2000.0) This position is 2.4″ east and 7.7″ north of the center of UGC 5189A. This AAVSO Special Notice was prepared by Elizabeth O. Waagen.”


While magnitude 12-12.9 isn’t unaided eye bright by a long shot, it’s well within the reach of most of today’s backyard telescopes. The image you see here on the right is of UGC 5189A before the event and the lefthand image was taken at the time of the supernova report. Visually the SN event outshines the galaxy! While chasing a faint supernova event might not be everyone’s cup of tea, Mr. Puckett’s devotion is absolutely legendary and I strongly encourage you to have a look if you have the the tools and talent.

So many supernovae… So little time!

Posted on by Jon Voisey

What was SN 1961V?

NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF
NGC 1058. Image credit: Bob Ferguson and Richard Desruisseau/Adam Block/NOAO/AURA/NSF

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Up there in the sky! It’s a supernova! It’s a Luminous Blue Variable eruption! It’s…. well, we’re not sure….

In July of 1961, a star in the spiral galaxy NGC 1058 blew up, but in a very odd fashion. The time to reach its peak brightness was several months as well as a slow decline including a three year plateau. Narrow spectral lines revealed a slow expansion velocity of 2,000 km sec-1. Some proposed it was an unusual supernova. Others claimed it was an especially energetic eruption of a Luminous Blue Variable (LBV) star like Eta Carinae. The infamous Fritz Zwicky called it a “Type V Supernova” which meant a supernova in name only, but could be anything as it was simply an “impostor”. For nearly 50 years, astronomers have been trying to sort out what this supernova impostor truly was.

One front on which much of the effort has focused is on the nature of the star before the explosion. The host galaxy is a beautiful face on spiral galaxy and was a tempting target for many observations well before the eruption. This has allowed astronomers to use archival images to determine properties of the parent star. And what a whopper it was. The star had an absolute magnitude near -12! Even Eta Carinae, one of the most massive stars currently known, only has an absolute magnitude of around -5.5. This extreme luminosity led astronomers towards early estimates for its mass to be as much as a staggering 2,000 M☉! While this is certainly incorrect, it still reveals just how massive SN 1961V’s progenitor truly was. Most estimates now put it in the range of 100 – 200 M☉.

A key difference between a supernova and an eruption is the remnant. In the case of a supernova, it is expected that the result would be a neutron star or black hole. If the object were an eruption, even a large one, the star would remain intact. In this vein, many astronomers have also attempted to inspect the remnant. However, due to the shell of gas and dust created in either scenario, imaging the objects has proven to be a challenge. While prior to the event, the culprit stuck out like a sore thumb, the remnant is lost in a haze of other stars.

Numerous telescopes have been aimed at the region to attempt to ferret out the leftovers including the powerful Hubble, but many attempts have remained inconclusive. Recently, the Spitzer Space telescope was employed to study the region, and although not intended for studying individual stars, its infrared vision can allow it to pierce the veil of dust and potentially find the source responsible. If there is still an intense IR source, it would mean the star survived, and the supernova truly was an impostor.

This attempt at identification was recently undertaken by a team of astronomers from Ohio State University, led by Christopher Kochanek. Upon inspection, the team was unable to conclusively identify a source with sufficient intensity as to be a survivor of the SN 1961V event. As such, the team concluded that the event Zwicky defined as a “supernova impostor” was a “‘supernova impostor’ impostor”.

The team compared it to another recent supernova, SN 2005gl, which also had a supermassive progenitor and was observed prior to detonation. Previous studies of this supernova suggested that, just prior to the explosion itself, the star underwent a heavy phase of mass loss. If a similar scenario occurred in 1961V, it could explain the unusual expansion velocity. During this time, the star may quake ferociously, imitating LBV eruptions which could explain the pre-nova plateau.

While this comparison relies on a single strongly similar case, it underscores the need “that studies of SN progenitors should evolve from simple attempts to obtain a single snapshot of the star to monitoring their behavior over their final years.” Hopefully, future studies and observations will provide better theoretical simulations and the numerous large surveys will provide sufficient data on stars prior to eruption to better constrain the behavior of these monsters.

Posted on by Jon Voisey

Poor in one, Rich in another

Tycho's Supernova Remnant. Credit: Spitzer, Chandra and Calar Alto Telescopes.

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Just over three years ago, I wrote a blog post commemorating the 50th anniversary of one of the most notable papers in the history of astronomy. In this paper, Burbidge, Burbidge, Fowler, and Hoyle laid out the groundwork for our understanding of how the universe builds up heavy elements.

The short version of the story is that there are two main processes identified: The slow (s) process and the rapid (r) process. The s-process is the one we often think about in which atoms are slowly bombarded with protons and neutrons, building up their atomic mass. But as the paper pointed out, this often happens too slowly to pass roadblocks to this process posed by unstable isotopes which don’t last long enough to catch another one before falling back down to lower atomic number. In this case, the r-process is needed in which the flux of nucleons is much higher in order to overcome the barrier.

The combination of these two processes has done remarkably well in matching observations of what we see in the universe at large. But astronomers can never rest easily. The universe always has its oddities. One example is stars with very odd relative amounts of the elements built up by these processes. Since the s-process is far more common, they’re what we should see primarily, but in some stars, such as SDSS J2357-0052, there exists an exceptionally high concentration of the rare r-process elements. A recent paper explores this elemental enigma.

As the designation implies, SDSS J2357-0052’s uniqueness was discovered by the Sloan Digital Sky Survey (SDSS). The survey uses several filters to image fields of stars at different wavelengths. Some of the filters are chosen to lie in wavelength ranges in which there are well known absorption lines for elements known to be tracers of overall metallicity. This photometric system allowed an international team of astronomers, led by Wako Aoki of the National Astronomical Observatory in Tokyo, to get a quick and dirty view of the metal content of the stars and choose interesting ones for followup study.

These followup observations were done with high resolution spectroscopy and showed that the star had less than one one-thousandth the amount of iron that the Sun does ([Fe/H] = -3.4), placing it among the most metal poor stars ever discovered. However, iron is the end of the elements produced by the s-process. When going beyond that atomic number, the relative abundances drop off very quickly. While the drop off in SDSS J2357-0052 was still steep, it wasn’t near as dramatic as in most other stars. This star had a dramatic enhancement of the r-process elements.

Yet this wasn’t exceptional in and of itself. Several metal poor stars have been discovered with such r-process enhancements. But none coupled with such an extreme deficiency of iron. The implication of this combination is that this star was very close to a supernova. The authors suggest two scenarios that can explain the observations. In the first, the supernova occurred before the star formed, and SDSS J2357-0052 was formed in the immediate vicinity before the enhanced material would be able to disperse and mix into the interstellar medium. The second is that SDSS J2357-0052 was an already formed star in a binary orbit with a star that became a supernova. If the latter case is true, it would likely give the smaller star a large “kick” as the mass holding the system would change dramatically. Although no exceptional radial velocity was detected for SDSS J2357-0052, the motion (if it exists) could be in the plane of the sky requiring proper motion studies to either confirm or refute this possibility.

The authors also note that the first star with somewhat similar characteristics (although not as extreme), was discovered first in the outer halo where the likelihood of the necessary supernova occurring is low. As such, it is more likely that that star was ejected in such a process establishing some credibility for the scenario in general, even if not the case for SDSS J2357-0052.

Posted on by Jon Voisey

Type II-P Supernovae as a New Standard Candle

Artist illustration of a supernova. Image credit: ESO

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Much of astronomical knowledge is built on the cosmic distance ladder. This ladder is built to determine distances to objects in our sky. Low lying rungs for nearby objects are used to calibrate the methodology for more distant objects which are, in turn, used to calibrate for more distant objects and so on. One of the reason so many runs need to be added is that techniques often become difficult to impossible to used past a certain distance. Cepheid Variables are a fantastic object to allow us to measure distances, but their luminosity is only sufficient to allow us to detect them to a few tens of millions of parsecs. As such, new techniques, based on brighter objects must be developed.

The most famous of these is the use of Type Ia Supernovae (ones that collapse just pass the Chandrasekhar limit) as “standard candles”. This class of objects has a well defined standard luminosity and by comparing its apparent brightness to the actual brightness, astronomers can determine distance via the distance modulus. But this relies on the fortuitous circumstance of having such an event occur when you want to know the distance! Obviously, astronomers need some other tricks up their sleeve for cosmological distances, and a new study discusses the possibility of using another type of supernova (SN II-P) as another form of standard candles.

Type II-P supernovae are classical, core-collapse supernovae that occur when the core of a star has passed the critical limit and can no longer support the mass of the star. But unlike other supernovae, the II-P decays more slowly, leveling off for some time creating a “plateau” in the light curve (which is where the “P” comes from). Although their plateaus are not all at the same brightness, making them initially useless as a standard candle, studies over the past decade have shown that observing other properties may allow astronomers to determine what the brightness of the plateau actually is and making these supernovae “standardizable”.

In particular, discussion has been centering recently around possible connections between the velocity of ejecta and the brightness of the plateau. A study published by D’Andrea et al. earlier this year attempted to link the absolute brightness to the velocities of the Fe II line at 5169 Angstroms. However, this method left large experimental uncertainties which translated to an error of up to 15% of the distance.

A new paper, to be published in October’s issue of the Astrophysical Journal, a new team, led by Dovi Poznanski of the Lawrence Berkley National Laboratory attempts to reduce these errors by utilizing the hydrogen beta line. One of the primary advantages to this is that hydrogen is much more plentiful allowing the hydrogen beta line to stand out whereas the Fe II lines tend to be weak. This improves the signal to noise (S/N) ratio and improves overall data.

Using data from the Sloan Digital Sky Survey (SDSS), the team was able to decrease the error in distance determination to 11%. Although this was an improvement over the D’Andrea et al. study, it is still significantly higher than many other methods for distance determination at similar distances. Poznanski suggests that this data is likely skewed due to a natural bias towards brighter supernovae. This systematic error stems from the fact that the SDSS data is supplemented up with follow-up data which the team employed, but the follow-ups are only conducted if the supernova meets certain brightness criteria. As such, their method is not fully representative of all supernovae of this type.

To improve their calibration and hopefully improve the method, the team plans to continue their study with expanded data from other studies that would be free of such biases. In particular the team intends to use the Palomar Transient Factory to supplement their results.

As the statistics improve, astronomers will gain another rung on the cosmological distance ladder, but only if they’re lucky enough to find one of this type of supernova.

Posted on by Nancy Atkinson

Supernova Spews Its Guts Across Space

Supernova 1987A and a glowing gas ring encircling the supernova remnant known as the "String of Pearls." Credit: NASA

The recently refurbished Hubble Space Telescope has taken a new look at Supernova 1987A and its famous “String of Pearls,” a glowing ring 6 trillion miles in diameter encircling the supernova remnant. The sharper and clearer images are allowing astronomers to see the “innards” of the star being ejected into space following the explosion, and comparing the new images with ones taken previously provides a unique glimpse of a young supernova remnant as it evolves. They found significant brightening of the object over time, and also evident is how the shock wave from the star’s explosion has expanded and rebounded.

Kevin France from the University of Colorado Boulder and colleagues compared the new Hubble data on the SN1987A taken in 2010 with older images, and observed the supernova in optical, ultraviolet and near-infrared light. They were able to look at the interplay between the stellar explosion and the ‘String of Pearls’ that encircles the supernova remnant. The gas ring — energized by X-rays — likely was spewed out about 20,000 years before the supernova exploded, and shock waves rushing out from the remnant have been brightening some 30 to 40 pearl-like “hot spots” in the ring — objects that likely will grow and merge together in the coming years to form a continuous, glowing circle.

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“The new observations allow us to accurately measure the velocity and composition of the ejected ‘star guts,’ which tell us about the deposition of energy and heavy elements into the host galaxy,” said France, lead author of the study which was published in Science. “The new observations not only tell us what elements are being recycled into the Large Magellanic Cloud, but how it changes its environment on human time scales.”

The significant brightening that was detected is consistent with some theoretical predictions about how supernovae interact with the galactic environment surrounding them. Discovered in 1987, Supernova 1987A is the closest exploding star to Earth to be detected since 1604 and is located in the nearby Large Magellanic Cloud, a dwarf galaxy adjacent to our own Milky Way Galaxy.

In addition to ejecting massive amounts of hydrogen, 1987A has spewed helium, oxygen, nitrogen and rarer heavy elements like sulfur, silicon and iron. Supernovae are responsible for a large fraction of biologically important elements, including oxygen, carbon and iron found in plants and animals on Earth today, France said. The iron in a person’s blood, for example, is believed to have been made by supernovae explosions.

The team compared STIS observations in January 2010 with Hubble observations made over the past 15 years on 1987A’s evolution. STIS has provided the team with detailed images of the exploding star, as well as spectrographic data — essentially wavelengths of light broken down into colors like a prism that produce unique fingerprints of gaseous matter. The results revealed temperatures, chemical composition, density and motion of 1987A and its surrounding environment, said France.

Since the supernova is roughly 163,000 light-years away, the explosion occurred in roughly 161,000 B.C., said France. One light year is about 6 trillion miles.

“To see a supernova go off in our backyard and to watch its evolution and interactions with the environment in human time scales is unprecedented,” he said. “The massive stars that produce explosions like Supernova 1987A are like rock stars — they live fast, flashy lives and die young.”

France said the energy input from supernovae regulates the physical state and the long-term evolution of galaxies like the Milky Way. Many astronomers believe a supernova explosion near our forming sun some 4 to 5 billion years ago is responsible for a significant fraction of radioactive elements in our solar system today, he said.

“In the big picture, we are seeing the effect a supernova can have in the surrounding galaxy, including how the energy deposited by these stellar explosions changes the dynamics and chemistry of the environment,” said France. “We can use this new data to understand how supernova processes regulate the evolution of galaxies.”

France and his team will be looking at Supernova 1987A again with Hubble’s Cosmic Origins Spectrograph, an instrument which scientists hope will help them better understand the “cosmic web” of of material permeating the cosmos and learn more about the conditions and evolution of the early universe.

Source: ScienceExpress

Posted on by Steve Nerlich

Astronomy Without A Telescope – Alchemy By Supernova

(Caption) Supernova remnant G1.9+0.3 (Combined image from Chandra Xray data and radio data from NRAO's Very Large Array). Credit: http://chandra.harvard.edu

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The production of elements in supernova explosions is something we take for granted these days. But exactly where and when this nucleosynthesis takes place is still unclear – and attempts to computer model core collapse scenarios still pushes current computing power to its limits.

Stellar fusion in main sequence stars can build some elements up to, and including, iron. Further production of heavier elements can also take place by certain seed elements capturing neutrons to form isotopes. Those captured neutrons may then undergo beta decay leaving behind one or more protons which essentially means you have a new element with a higher atomic number (where atomic number is the number of protons in a nucleus).

This ‘slow’ process or s-process of building heavier elements from, say, iron (26 protons) takes place most commonly in red giants (making elements like copper with 29 protons and even thallium with 81 protons).

But there’s also the rapid or r-process, which takes place in a matter of seconds in core collapse supernovae (being supernova types 1b, 1c and 2). Rather than the steady, step-wise building over thousands of years seen in the s-process – seed elements in a supernova explosion have multiple neutrons jammed in to them, while at the same time being exposed to disintegrating gamma rays. This combination of forces can build a wide range of light and heavy elements, notably very heavy elements from lead (82 protons) up to plutonium (94 protons), which cannot be produced by the s-process.

How stuff gets made in our universe. The white elements (above plutonium) can be formed in a laboratory, but it is unclear whether they form naturally - and, in any case, they decay quickly after they are formed. Credit: North Arizona University

Prior to a supernova explosion, the fusion reactions in a massive star progressively run through first hydrogen, then helium, carbon, neon, oxygen and finally silicon  – from which point an iron core develops which can’t undergo further fusion. As soon as that iron core grows to 1.4 solar masses (the Chandrasekhar limit) it collapses inwards at nearly a quarter of the speed of light as the iron nuclei themselves collapse.

The rest of the star collapses inwards to fill the space created but the inner core ‘bounces’ back outwards as the heat produced by the initial collapse makes it ‘boil’. This creates a shockwave – a bit like a thunderclap multiplied by many orders of magnitude, which is the beginning of the supernova explosion. The shock wave blows out the surrounding layers of the star – although as soon as this material expands outwards it also begins cooling. So, it’s unclear if r-process nucleosynthesis happens at this point.

But the collapsed iron core isn’t finished yet. The energy generated as the core compressed inwards disintegrates many iron nuclei into helium nuclei and neutrons. Furthermore, electrons begin to combine with protons to form neutrons so that the star’s core, after that initial bounce, settles into a new ground state of compressed neutrons – essentially a proto-neutron star. It is able to ‘settle’ due to the release of a huge burst of neutrinos which carries heat away from the core.

It’s this neutrino wind burst that drives the rest of the explosion. It catches up with, and slams into, the already blown-out ejecta of the progenitor star’s outer layers, reheating this material and adding momentum to it. Researchers (below) have proposed that it is this neutrino wind impact event (the ‘reverse shock’) that is the location of the r-process.

It’s thought that the r-process is probably over within a couple of seconds, but it could still take an hour or more before the supersonic explosion front bursts through the surface of the star, delivering some fresh contributions to the periodic table.

Further reading: Arcones A. and Janka H. Nucleosynthesis-relevant conditions in neutrino-driven supernova outflows. II. The reverse shock in two-dimensional simulations.

And, for historical context, the seminal paper on the subject (also known as the B2FH paper) E. M. Burbidge, G. R. Burbidge, W. A. Fowler, and F. Hoyle. (1957). Synthesis of the Elements in Stars. Rev Mod Phy 29 (4): 547. (Before this nearly everyone thought all the elements formed in the Big Bang – well, everyone except Fred Hoyle anyway).

Posted on by Nancy Atkinson

Searching for the Elusive Type Ia Supernovae Progenitors

This Hubble image reveals the gigantic Pinwheel Galaxy (M101), one of the best known examples of "grand design spirals," and its supergiant star-forming regions in unprecedented detail. Astronomers have searched galaxies like this in a hunt for the progenitors of Type Ia supernovae, but their search has turned up mostly empty-handed. Credit: NASA/ESA
This Hubble image reveals the gigantic Pinwheel Galaxy (M101), one of the best known examples of "grand design spirals". Credit: NASA/ESA

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Astronomers have Type Ia supernovae pretty well figured out. The way these exploding stars brighten and then dim are so predictable that they have been used to measure the universe’s expansion. This reliability led to the discovery that our universe was not only expanding but accelerating, which in turn led to the discovery of dark energy. There’s just one minor detail: nobody knows for sure what causes a supernova.

“The question of what causes a Type Ia supernova is one of the great unsolved mysteries in astronomy,” says Rosanne Di Stefano of the Harvard-Smithsonian Center for Astrophysics.

Astronomers are sure that for a Type Ia supernova, the energy for the explosion comes from the run-away fusion of carbon and oxygen in the core of a white dwarf. To detonate, the white dwarf must gain mass until it reaches a tipping point and can no longer support itself.

But how does a white dwarf get bigger? There are two leading scenarios for what leads a stable white dwarf to go ka-boom, and both include a companion star. In the first possibility, a white dwarf swallows gas blowing from a neighboring giant star. In the second possibility, two white dwarfs collide and merge. To establish which option is correct (or at least more common), astronomers look for evidence of these binary systems.

In this negative image of the Pinwheel Galaxy (M101), red squares mark the positions of 'super-soft' X-ray sources. The Pinwheel should contain hundreds of accreting white dwarfs on which nuclear fusion is occurring, which should produce prodigious X-rays. Yet we only detect a few dozen super-soft X-ray sources. This means that we must devise new methods to search for the elusive progenitors of Type Ia supernovae. Credit: R. Di Stefano (CfA)

To find evidence of the first scenario, astronomers looked for accreting white dwarfs by seeking out so- called “super-soft” X-rays, which are produced when gas hitting the star’s surface undergoes nuclear fusion. Given the average rate of supernovae, a typical galaxy should contain hundreds of this type of X-ray sources. However, they are few and far between.

This led astronomers to believe that perhaps the merger scenario was the source of Type Ia supernovae, at least in many galaxies. That conclusion relies on the assumption that accreting white dwarfs will appear as super-soft X-ray sources when the incoming matter experiences nuclear fusion.

But a new paper by Di Stefano and her colleagues argues that the data do not support this hypothesis. The paper argues that a merger-induced supernova would also be preceded by an epoch during which a white dwarf accretes matter that should undergo nuclear fusion. White dwarfs are produced when stars age, and different stars age at different rates. Any close double white-dwarf system will pass through a phase in which the first-formed white dwarf gains and burns matter from its slower-aging companion. If these white dwarfs produce X-rays, then we should find roughly a hundred times as many super-soft X-ray sources as we do.

This means that super-soft X-rays aren’t providing evidence for either scenarios – an accretion-driven explosion and a merger-driven explosion – since they both involve accretion and fusion at some point The alternative proposed by Di Stefano is that the white dwarfs are not luminous at X-ray wavelengths for long stretches of time. Perhaps material surrounding a white dwarf can absorb X-rays, or accreting white dwarfs might emit most of their energy at other wavelengths.

If this is the correct explanation, says Di Stefano, “we must devise new methods to search for the elusive progenitors of Type Ia supernovae.”

Read Di Stefano’s paper in The Astrophysical Journal.

Source: CfA

Posted on by Nancy Atkinson

A New Kind of Supernova Explodes in Unusual Way

SN 2005E, from a DSS image via Astrosurf.com.

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Not all supernovae are created equal, astronomers are finding. A faint supernovae found by international teams of scientists is like nothing previously seen, and cannot be explained by conventional insights into these exploding stars. Until now, only two basic kinds of supernovae had been observed. But now there appears to be a third.

“The supernova explosion is the most energetic and brilliant event that happens in the universe,” said Dae-Sik Moon from the University of Toronto, and part of a team publishing their findings this week in Nature. “It is rich with information, not only about how stars die, but to understanding the origin of life and the expansion of the universe. But this one is surprisingly different.”

The first two types of supernova are either hot, young giants that go out in a violent display as they collapse under their own weight, or old, dense white dwarves that blow up in a thermonuclear explosion.

White dwarf stars are composed mainly of carbon and oxygen, and although the supernova, SN2005E, appears to be from a white dwarf system, it is devoid of carbon and oxygen and instead is rich in helium.

The environment of SN 2005E. The image to the left shows NGC 1032, the host galaxy of the supernova, before the supernova explosion. The discovery of the supernova SN 2005E is shown on the right. Note the remote location of the supernova (marked by the arrow) with respect to its host, about 750,000 light-years from the galaxy nucleus. Credit: Sloan Digital Sky Survey / Lick Observatory

SN2005E was first spotted on January 13, 2005 in the nearby galaxy NGC1032, and since then scientists have carried out various observations of it using different telescopes.

On the one hand, the amount of material hurled out from the supernova was too small for it to have come from an exploding giant. In addition, its location, distant from the busy hubs where new stars form, implied it was an older star that had had time to wander off from its birthplace. On the other hand, its chemical makeup didn’t match that commonly seen in the second type.

“It was clear,” said lead author Hagai Perets from the Weizmann Institute in Israel and the Harvard-Smithsonian Center for Astrophysics, “that we were seeing a new type of supernova.”

SN 2005E had unusually high levels of the elements calcium and titanium, which are the products of a nuclear reaction involving helium, rather than carbon and oxygen.

“We’ve never before seen a spectrum like this one,” said Paolo Mazzali of the Max-Planck Institute for Astrophysics. “Once thereceiving star has accumulated a certain amount, the helium starts to burn explosively. The unique processes producing certain chemical elements in these explosions could solve some of the puzzles related to chemical enrichment. This could, for example, be the main source of titanium.”

Computer simulations to see what kind of process could have produced such a result suggest that a pair of white dwarves are involved; one of them stealing helium from the other. When the thief star’s helium load rises past a certain point, the explosion occurs.

“The donor star is probably completely destroyed in the process, but we’re not quite sure about the fate of the thief star,” said team member Avishay Gal-Yam.

In fact, the astronomers say these relatively dim explosions might not be all that rare.

Alex Filippenko from UC Berkeley professor and colleague Dovi Poznanski, both part of the team studying SN 2005E reported last November another supernova, SN 2002bj, that they believe exploded by a similar mechanism: ignition of a helium layer on a white dwarf.

“SN 2002bj is arguably similar to SN 2005E, but has some clear observational differences as well,” Filippenko said. “It was likely a white dwarf accreting helium from a companion star, though the details of the explosion seem to have been different because the spectra and light curves differ.”

But this new type of supernova could explain some puzzling phenomena in the universe. For example, almost all the elements heavier than hydrogen and helium have been created in, and dispersed by supernovae; the new type could help explain the prevalence of calcium in both the universe and in our bodies.

It might also account for observed concentrations of particles called positrons in the center of our galaxy. Positrons are identical to electrons, but with an opposite charge, and some have hypothesized that the decay of yet unseen ‘dark matter’ particles may be responsible for their presence. But one of the products of the new supernova is a radioactive form of titanium that, as it decays, emits positrons.

“Dark matter may or may not exist,” said Gal-Yam, “but these positrons are perhaps just as easily accounted for by the third type of supernova.”

Other researchers include: Iair Arcavi and Michael Kiewe of the Weizmann Institute’s Faculty of Physics, astronomers from the Scuola Normale Superiore, Pisa, and INAF/Padova Observatory in Italy, Prof. David Arnett from the University of Arizona, and researchers from across the USA, Canada, Chile and the UK.

Original publications:

H.B. Perets, A. Gal-Yam, P. Mazzali et al., “A new type of stellar explosion from a helium rich progenitor,” Nature, 20 May 2010.

A. Gal-Yam, P. Mazzali, E. O. Ofek, et al., “Supernova 2007bi was a pair-instability supernova explosion,” Nature, Vol. 462, p. 624-627, 3 December 2009.

Sources: Max Planck Institute, EurekAlert, Weisman Institute EurekAlert