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

Searching for the Elusive Type Ia Supernovae Progenitors

Article written: 12 Jul , 2010
Updated: 24 Dec , 2015
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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



4 Responses

  1. Lawrence B. Crowell says

    Given SNI’s have a consistent luminosity, it makes sence to me the process was due to the accretion of material from a companion star onto the white dwarf. In this way the process which initiate the explosion is a sort of adiabatic perturbation that has little effect on the detals of the explosion. I have a hard time seeing that as the case with a collision.

    LC

  2. Richard Kirk says

    I would have thought the frequency of type 1a supernovae should be linked to the total soft X-ray emission, and not the count of sources.

    Suppose a type 1a supernova comes a white dwarf that is gathering matter; and a steady gathering of matter will mean the white dwarf was a super-soft X-ray emitter. If we only spot 1% of the super-soft X-ray emitters we expect for the frequency of type 1a supernovae, then it means the objects are not emitting the X-rays in our direction.

    If the white dwarf was gathering mass from all directions, we might expect the X-ray emissions to be symmetric. If the white dwarf is sucking a tail of matter from a companion star, then we might expect the tail to hit a small area on the surface of the white dwarf. We might then expect to see fewer sources because only a few of them are aligned so we can see them, but the sources we see ought to seem brighter because we are looking at them from the right direction.

    I must say my instincts go with Lawrence here. It seems logical that stars or a particular type that steadily fatten up will tend to go ‘bang’ at the same point, and give off the same amount of energy. I can’t see us getting the same consistency from collisions. However, there seems to be dark matter and water on Mars, so my instincts are not to be relied upon.

  3. Lawrence B. Crowell says

    This does raise an interesting possible question. The collision of two white dwarf stars would probably be pretty calamitous. The range of conditions in such collisions would seem highly variable, where mass differences in each case certainly come to mind. So this might be a source of supernova. Yet I think the adiabatic variation in the mass of a white dwarf to its Chandresekhar limit would limit the role the initial conditions can have in the SNI.

    Finding a SNI precursor would be a bit tough. The variation in X-rays with the orbit of the white dwarf and the other star is one possible way. Of course it would be tough to know if the white dwarf is near the 1.3M_{sol} limit without it being close enough to do some precise orbital measurements. We don’t want SNIs to be very close!

    LC.

  4. jimhenson says

    Gilfanov and Bogdan discovered almost 97% of type la supernovas in 6 early ellipitcal galaxies that emitted enough x-rays for analysis PROVE white dwarf merger collisions are the cause, not a binary star system pulling mass by transfer accretion. Howell says scientists believe wrong theories about type la supernovas. Standard candle supernova observations reveal time dilation slow down effects that increase noticeably with increasing acceleration and distance due to dark energy expansion. This is how they measure the rate of accelration of the universe. However, this only works for stars of known mass reaching the chandresekhar limit in computer model simulations. Widely varying masses between two colliding stars will produce variable supernova energy outputs, and should not really be standard candles measurements for the expansion of the universe.

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