Astronomy Without A Telescope – Alchemy By Supernova

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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).

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

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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

A New Kind of Supernova Explodes in Unusual Way

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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