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A New Mechanism for Supernovae

Are dozens of Type Ia supernovae waiting to happen within a few thousand light years of the Earth? New research from the Harvard-Smithsonian Center for Astrophysics suggests the answer is yes. Type Ia events are thought to occur when a white dwarf accretes material from a companion star. The idea is that the white dwarf — a stellar remnant that is no longer capable of fusion — eventually exceeds the so-called Chandrasekhar mass, roughly 1.4 times the mass of the Sun. When a star pushes past the limit, gravity compacts the dwarf to the point of runaway nuclear fusion, and a spectacular stellar event appears in the heavens.

Type Ia supernovae are a well studied phenomenon, but a continuing problem with these events is that the scenario doesn’t quite explain everything we see, or don’t see. Most Type Ia supernovae show none of the hydrogen and helium near the explosion that we would expect. As this news release from the CfA notes, the gas should be there, in the form of remnant materials that had not yet been accreted by the white dwarf at the time of the explosion, or possibly left behind by the disruption of the companion star during it. Where is the missing gas?

Image: New research shows that some old stars known as white dwarfs might be held up by their rapid spins, and when they slow down, they explode as Type Ia supernovae. Thousands of these “time bombs” could be scattered throughout our Galaxy. In this artist’s conception, a supernova explosion is about to obliterate an orbiting Saturn-like planet. Credit: David A. Aguilar (CfA).

Rosanne Di Stefano and colleagues are now examining a new mechanism for such supernovae, one in which the spin of the white dwarf would explain the lack of hydrogen and helium. The work notes that a white dwarf gains angular momentum as it gains mass. Get it rotating fast enough and the additional spin can help to support it, so that it crosses the barrier of 1.4 solar masses and becomes a super-Chandrasekhar-mass star.

Eventually accretion will stop and the white dwarf will begin to slow down. When the spin is no longer sufficient to counteract the effects of gravity, the Type Ia supernova occurs. It’s an ingenious solution, and because the spin-down process could create an interval of as much as a billion years between the 1.4 solar mass point and the ensuing explosion, it would explain the dissipation of materials, and allow time for the companion star to evolve into a second white dwarf.

From the paper:

Conservation of angular momentum plays an important role in astrophysics. It allows NSs and black holes to be spun up to near maximal rotation. It seems almost certain that WDs can be similarly spun up. Indeed, given the variety of donors and accretion geometries exhibited in nature, spin-up can fail only if there is a fundamental physical principle that disallows it. As long as spin-up to near-maximal rotation occurs, some of the effects we discussed will occur. Although theoretical uncertainties make predictions difficult, we have shown that spin-up/spin-down has testable consequences. The measurements we propose can therefore provide input for theoretical work.

Take the estimate of three Type Ia supernovae every thousand years in our galaxy and the calculations of Di Stefano’s team are that there are probably dozens of such pre-explosion systems within several thousand light years of the Sun. “We don’t know of any super-Chandrasekhar-mass white dwarfs in the Milky Way yet, but we’re looking forward to hunting them out,” says Di Stefano’s colleague and co-author Rasmus Voss (Radboud University, Nijmegen, The Netherlands). Wide-field surveys like Pan-STARRS and the Large Synoptic Survey Telescope should help us detect supernova precursors like these for study in depth.

The paper is Di Stefano and Voss, “Spin-Up/Spin-Down models for Type Ia Supernovae,” Astrophysical Journal Letters, (preprint).

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Comments on this entry are closed.

  • Dan Ibekwe September 8, 2011, 19:45

    An alarming thought – standard candles close enough to set fire to our eyebrows.

    But also, these candles would no longer be standardised – each white dwarf could exceed the Chandrasekhar limit by an arbitrary amount, giving a mix of candles, menorahs, candelabras and the odd bonfire.

    Pardon my ignorance, but have any of the posited rapidly-spinning white dwarves actually been observed? And if any are found, and this theory holds true, where does that leave cosmic acceleration and Dark Energy?

  • Rob Henry September 8, 2011, 20:26

    While reading this I couldn’t help but wonder if there were any factors that varied according to the time since the big bang, and that could bias the average factor that these objects exceeded the Chandrasekhar limit by. Surely galaxies can’t be born with their strong magnetic fields, and could this do the trick (even if indirectly by influencing the formation of pairs of stars). Is our expansion really accelerating?

  • Mike September 8, 2011, 21:47

    If observations support this assertion it may raise some questions regarding cosmic distance scaling. If it’s shown that some Type 1A supernovae are developing brighter intrinsic magnitudes due to larger pre-collapse masses.

    Hopefully there aren’t any surprises slowly spinning down nearby or we might have to break out the sunblock. The SPF 10000 sunblock.

  • Eniac September 11, 2011, 11:50

    Or we could wear hats or stay inside. Silliness aside, I think the danger of nearby supernova explosions have been greatly exaggerated. Apparently the direct radiation is so harmless after it crosses the light years that fear-mongers have had to rely on more or less speculative secondary effects such as gamma ray induced chemical reactions that then proceed to attack that time-tested old fear-monger’s standby, the ozone layer. What happened to that hole, anyway, that was going to kill us all? Did it close up again?

  • amphiox September 12, 2011, 15:24

    Eniac, AFAIK, the hole is still there, fluctuating in size year by year. And no one credible has ever claimed that it was going “kill us all”.

  • Eniac September 12, 2011, 20:13

    Thanks, amphiox. Sorry about the overly facetious post…. Late night and too many uncontrolled substances.

  • Rob Henry September 12, 2011, 23:21

    Strange as it may seem, life, especially complex life, can be destroyed without killing a single individual. If it is possible for some factor to sufficiently elevate mutation rates for a hundred generations or so, there would eventually be a lowering of fertility rates of viable offspring below replacement rates. This makes it hard to know whether some suggested mechanisms for mass extinction are incredible fantasy or credible possibilities.

  • ljk September 20, 2012, 23:58

    Sept. 20, 2012

    Study: White dwarfs’ tidal effects may create novae

    Jim Fuller

    The white dwarfs J0651A and J0651B have the shortest orbital period of any known pair of stars that are completely detached from each other. They orbit each other every 12.75 minutes.

    By Susan Kelley

    Theoretical physicists at Cornell may have found a new way to explain the formation of novae, stars that suddenly become very bright then quickly fade.

    At the heart of the theory is a pair of old, dense stars called white dwarfs, orbiting each other so closely that their gravitational forces create violent tidal waves of plasma that break near the surfaces of the stars. The phenomenon is what the researchers have dubbed a tidally induced nova.

    If their theory is correct, it would represent a big step forward for astrophysics, said lead researcher Jim Fuller, a Ph.D. candidate in the field of astronomy and space sciences.

    “It’s an important problem because there are a lot of white dwarfs in tight binaries,” said Fuller, who co-wrote the study with Dong Lai, professor of astronomy. “They’re spiraling in toward each other, so tidal effects are going to be very important, but no one has really studied that.”

    Full article here:

    http://www.news.cornell.edu/stories/Sept12/TidalNovae.html