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 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 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 Jean Tate

Cosmological Constant

Seven Year Microwave Sky (Credit: NASA/WMAP Science Team)

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The cosmological constant, symbol Λ (Greek capital lambda), was ‘invented’ by Einstein, not long after he published his theory of general relativity (GR). It appears on the left-hand side of the Einstein field equations.

Einstein added this term because he – along with all other astronomers and physicists of the time – thought the universe was static (the cosmological constant can make a universe filled with mass-energy static, neither expanding nor contracting). However, he very quickly realized that this wouldn’t work, because such a universe would be unstable … and quickly turn into one either expanding or contracting! Not long afterwards, Hubble (actually Vesto Slipher) discovered that the universe is, in fact, expanding, so the need for a cosmological constant went away.

Until 1998.

In that year, two teams of astronomers independently announced that distant Type Ia supernovae did not have the apparent luminosity they should, in a universe composed almost entirely of mass-energy in the form of baryons (ordinary matter) and cold dark matter.

Dark Energy had been discovered: dark energy is a form of mass-energy that has a constant density throughout the universe, and perhaps throughout time as well; counter-intuitively, it causes the expansion of the universe to accelerate (i.e. it acts kinda like anti-gravity). The most natural form of dark energy is the cosmological constant.

A great deal of research has gone into trying to discover if dark energy is, in fact, just the cosmological constant, or if it is quintessence, or something else. So far, results from observations of the CMB (by WMAP, mainly), of BAO (baryon acoustic oscillations, by extensive surveys of galaxies), and of high-redshift supernovae (by many teams) are consistent with dark energy being the cosmological constant.

So if the cosmological constant is (a) mass-energy (density), it can be expressed as kilograms (per cubic meter), can’t it? Yes, and the best estimate today is 7.3 x 10-27 kg m 3.

Ned Wright’s Cosmology Tutorial (UCLA) and Sean Carroll’s Cosmology Primer (California Institute of Technology) between them cover not only the cosmological constant, but also cosmology! NASA’s What Is A Cosmological Constant? is a great one-page intro.

Universe Today has many, many stories featuring the cosmological constant! Here are a few to whet your appetite: Universe to WMAP: LCDM Rules, OK?, Einstein’s Cosmological Constant Predicts Dark Energy, and No “Big Rip” in our Future: Chandra Provides Insights Into Dark Energy.

There are many Astronomy Cast episodes which include discussion of the cosmological constant … these are among the best: The Big Bang and Cosmic Microwave Background, The Important Numbers in the Universe, and the March 18th, 2009 Questions Show.

Sources:
http://map.gsfc.nasa.gov/universe/uni_accel.html
http://super.colorado.edu/~michaele/Lambda/lambda.html
http://en.wikipedia.org/wiki/Cosmological_constant

Posted on by Jean Tate

GRB Central Engines Observed in Nearby Supernovae?

SN 2009bb (Image Credit: NASA, Swift, Stefan Immler)

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Are the relativistic jets of long gamma ray bursts (GRBs) produced by brand new black holes? Do some core-collapse supernovae result in black holes and relativistic jets?

The answer to both questions is ‘very likely, yes’! And what recent research points to those answers? Study of an Ic supernova (SN 2007gr), and an Ibc one (SN 2009bb), by two different teams, using archived Gamma-Ray Burst Coordination Network data, and trans-continental Very Long Baseline Interferometry (VLBI) radio observations.

“In every respect, these objects look like gamma-ray bursts – except that they produced no gamma rays,” said Alicia Soderberg at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Soderberg led a team that studied SN 2009bb, a supernova discovered in March 2009. It exploded in the spiral galaxy NGC 3278, located about 130 million light-years away.

SN 2007gr (Image Credit: Z. Paragi, Joint Institute for VLBI in Europe (JIVE))

The other object is SN 2007gr, which was first detected in August 2007 in the spiral galaxy NGC 1058, some 35 million light-years away (it’s one of the closest Ic supernovae detected in the radio waveband). The team which studied this supernova using VLBI was led by Zsolt Paragi at the Netherlands-based Joint Institute for Very Long Baseline Interferometry in Europe, and included Chryssa Kouveliotou, an astrophysicist at NASA’s Marshall Space Flight Center in Huntsville, Alabama.

The researchers searched for gamma-rays associated with the supernovae using archived records in the Gamma-Ray Burst Coordination Network located at NASA’s Goddard Space Flight Center in Greenbelt, Md. This project distributes and archives observations of gamma-ray bursts by NASA’s SWIFT spacecraft, the Fermi Gamma-ray Space Telescope and many others. However, no bursts coincided with the supernovae.

“The explosion dynamics in typical supernovae limit the speed of the expanding matter to about three percent the speed of light,” explained Kouveliotou, co-author of one of the new studies. “Yet, in these new objects, we’re tracking gas moving some 20 times faster than this.”

Unlike typical core-collapse supernovae, the stars that produce long gamma-ray bursts possess a “central engine” – likely a nascent black hole – that drives particle jets clocked at more than 99 percent the speed of light (short GRBs are likely produced by the collision/merger of two neutron stars, or a neutron star and a stellar mass black hole).

By contrast, the fastest outflows detected from SN 2009bb reached 85 percent of the speed of light and SN 2007gr reached more than 60 percent of light speed; this is “mildly relativistic”.

“These observations are the first to show some supernovae are powered by a central engine,” Soderberg said. “These new radio techniques now give us a way to find explosions that resemble gamma-ray bursts without relying on detections from gamma-ray satellites.”

The VLBI radio observations showcase how the new electronic capabilities of the European VLBI Network empower astronomers to react quickly when transient events occur. The team led by Paragi included 14 members from 12 institutions spread over seven countries, the United States, the Netherlands, Hungary, the United Kingdom, Canada, Australia and South Africa.

“Using the electronic VLBI technique eliminates some of the major issues,” said Huib Jan van Langevelde, the director of JIVE “Moreover it allows us to produce immediate results necessary for the planning of additional measurements.”

Perhaps as few as one out of every 10,000 supernovae produce gamma rays that we detect as a long gamma-ray burst. In some cases, the star’s jets may not be angled in a way to produce a detectable burst; in others, the energy of the jets may not be enough to allow them to blast through the overlying bulk of the dying star.

“We’ve now found evidence for the unsung crowd of supernovae – those with relatively dim and mildly relativistic jets that only can be detected nearby,” Kouveliotou said. “These likely represent most of the population.”

The 28 January, 2010 issue of Nature contains two papers reporting these discoveries: A relativistic type Ibc supernova without a detected γ-ray burst (arXiv:0908.2817 is the preprint), and A mildly relativistic radio jet from the otherwise normal type Ic supernova 2007gr (arXiv:1001.5060 is the preprint).

Sources: Newborn Black Holes May Add Power to Many Exploding Stars, Newborn Black Holes Boost Explosive Power of Supernovae

Posted on by Jean Tate

Hypernova

Artist impression of the twin jets from a GRB. Credit: Dana Berry/SkyWorks Digital

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Nova, “new star”; supernova, a “super” nova; hypernova, a super-duper, or super super, nova!

This word appeared in the astronomical literature at least as early as 1982, and refers to a kind of core-collapse supernova far brighter (>100 times) than usual; its meaning has changed somewhat, and today generally refers to the core collapse of particularly massive stars (>100 sols), whether or not they are spectacularly brighter than other core-collapse supernovae (though they are that too).

Most times you’ll come across hypernovae in material on gamma ray bursts (GRBs), many of which seem to involve emission of electromagnetic radiation with total energy many times that from ordinary supernovae (whether core collapse or Type Ia). Long-duration GRBs have jets, presumably from the poles of the temporary accretion disk which forms around the new black hole at the heart of the collapsed core of the progenitor (short-duration GRBs, which also produce jets, are thought to be the merger of two neutron stars, or a neutron star and a stellar-mass black hole), but even when viewed side-on (i.e. not looking into one of the jets), these GRBs are intrinsically much brighter than other core collapse supernovae.

If a supernova were to occur a few hundred light-years from us, we’d certainly notice it, and there might be some impact on our atmosphere; if there was a hypernova the same distance away, we’d suffer (not only from the increased incidence of cancer due to the far greater intensity of cosmic rays, but also from changes in weather and climate, and damage to ecosystems); if the jet were aimed directly at us, we’d be toast (while those on the other side of the world would survive the few seconds-long blast, they’d die from the consequences).

Fortunately, it seems there are no stars likely to go hypernova on us … at least not within a few tens of thousands of light-years. Whew!

Have I whet your appetite for more? Check these sites out! Brighter than an Exploding Star, It’s a Hypernova! (NASA’s Imagine the Universe), Face on Beauty (Phil Plait), and Hypernova (Swinburne University).

Like everyone else, Universe Today writers love a good story about explosions … so there are quite a few on hypernovae! Some examples: Gamma Ray Bursts and Hypernovae Linked, ESO Watches Burst Afterglow for Five Weeks, and Carbon/Oxygen Stars Could Explode as Gamma Ray Bursts.

No surprise that Astronomy Cast episode Gamma-Ray Bursts features hypermovae! Back in 2007, after attending the American Astronomical Society meeting, Pamela learning something new about hypernovae; what? Well, check out the episode, What We Learned from the American Astronomical Society and find out for yourself!

References:
NASA
ESO

Posted on by Nancy Atkinson

Supernova or GRB? Radio Observations Allow Astronomers to Find Unusual Object

Core-collapse supernova explosion expelling nearly-spherical debris shell. CREDIT: Bill Saxton, NRAO/AUI/NSF

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For the first time, astronomers have found a supernova explosion with properties similar to a gamma-ray burst, but without seeing any gamma rays from it. Radio observations with the Very Large Array (VLA) showed material expelled from supernova explosion SN2009bb at speeds approaching the speed of light. The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. But astronomers don’t think this blast is one-of-a-kind, and say that more radio observations will point the way toward locating many more examples of these mysterious explosions.

“We think that radio observations will soon be a more powerful tool for finding this kind of supernova in the nearby Universe than gamma-ray satellites,” said Alicia Soderberg, of the Harvard-Smithsonian Center for Astrophysics.

Usually supernova explosions blasts the star’s material outward in a roughly-spherical pattern at speeds that, while fast, are only about 3 percent of the speed of light. In the supernovae that produce gamma-ray bursts, some, but not all, of the ejected material is accelerated to nearly the speed of light.

Engine-driven

When the nuclear fusion reactions at the cores of very massive stars no longer can provide the energy needed to hold the core up against the weight of the rest of the star, the core collapses catastrophically into a superdense neutron star or black hole. The rest of the star’s material is blasted into space in a supernova explosion. For the past decade or so, astronomers have identified one particular type of such a “core-collapse supernova” as the cause of one kind of gamma-ray burst.

The superfast speeds in these rare blasts, astronomers say, are caused by an “engine” in the center of the supernova explosion that resembles a scaled-down version of a quasar. Material falling toward the core enters a swirling disk surrounding the new neutron star or black hole. This accretion disk produces jets of material boosted at tremendous speeds from the poles of the disk.

“This is the only way we know that a supernova explosion could accelerate material to such speeds,” Soderberg said.

Until now, no such “engine-driven” supernova had been found any way other than by detecting gamma rays emitted by it.

“Discovering such a supernova by observing its radio emission, rather than through gamma rays, is a breakthrough. With the new capabilities of the Expanded VLA coming soon, we believe we’ll find more in the future through radio observations than with gamma-ray satellites,” Soderberg said.

Why didn’t anyone see gamma rays from this explosion? “We know that the gamma-ray emission is beamed in such blasts, and this one may have been pointed away from Earth and thus not seen,” Soderberg said. In that case, finding such blasts through radio observations will allow scientists to discover a much larger percentage of them in the future.

“Another possibility,” Soderberg adds, “is that the gamma rays were ‘smothered’ as they tried to escape the star. This is perhaps the more exciting possibility since it implies that we can find and identify engine-driven supernovae that lack detectable gamma rays and thus go unseen by gamma-ray satellites.”

One important question the scientists hope to answer is just what causes the difference between the “ordinary” and the “engine-driven” core-collapse supernovae. “There must be some rare physical property that separates the stars that produce the ‘engine-driven’ blasts from their more-normal cousins,” Soderberg said. “We’d like to find out what that property is.”

One popular idea is that such stars have an unusually low concentration of elements heavier than hydrogen. However, Soderberg points out, that does not seem to be the case for this supernova.

This research will be published in January 28 issue of the journal Nature.

Source: NRAO