Data details dynamically driven double-degenerate double-detonation supernova theory

Identification of hyper-fast stars suggests a reassessment of how astronomy’s “standard candles” are formed. Andrew Masterson reports.

An image of G299.2-2.9, a Type 1a supernova remnant caught on camera by NASA's Chandra X-ray Observatory.

An image of G299.2-2.9, a Type 1a supernova remnant caught on camera by NASA's Chandra X-ray Observatory.

X-ray: NASA/CXC/U. Texas at Arlington/S.Park et al, ROSAT; Infrared: 2MASS/UMass/IPAC-Caltech/NASA/NSF

The identification of three white dwarf stars zooming at hyper-fast velocities might provide evidence to support an alternative theory explaining the formation of Type 1a supernovae, researchers say.

Type 1a supernovae, orthodoxy holds, arise when two white dwarf stars locked in a binary orbit merge with each other, kick-starting a runaway nuclear fusion reaction and consequent massive explosion.

Understanding precisely how these phenomena arise is extremely important, because they are very useful to the practice of astronomy. As far back as the early 1990s it was noted that Type 1a supernovae all exhibit very similar brightness. Thus, they can be regarded as “standard candles” – essentially, known quantities that can be used to accurately calculate distances.

Now, a team of researchers led by Ken Shen from the University of California, Berkeley, in the US has used data gathered by Gaia, the orbiting observatory operated by the European Space Agency, to test a second theory of Type 1a supernova formation – known by the impressively alliterative name, the “dynamically driven double-degenerate double-detonation model”.

In this model, two white dwarfs are locked in a binary dance, as before. However, rather than merge in an orgy of mutual destruction, mass transfer between the two triggers only a single explosion, the force of which sends the remaining star flying off at extreme high speed.

Evidence to support this model should, at least in theory, be relatively easy to find. To do so, Shen and colleagues turned to Gaia’s data, which includes the positions of more than a billion stars across the sky.

Based on the numbers of supernovae recorded, they estimated that about 30 hyper-velocity white dwarfs should be whizzing around within 3000 light years of Earth.

Combing through the mass of information, the researchers initially identified seven likely targets, and followed them up with ground-based observations. Four turned out to be ordinary stars, but the remaining three were revealed to be moving at speeds up to 3000 kilometres a second.

That makes them some of the fastest known stars ever observed – and prime candidates for white dwarf supernovae escapees.

In a paper published in The Astrophysics Journal, Shen and colleagues concede that their findings do not constitute binding proof of their alternative model for Type 1a supernova formation – but they do, at the very least, mean that more research needs to be done.

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