Study raises questions about supernova origins

X-ray observations of the explosions could shift dark energy measurements

New X-ray findings appear to have blown a hole in the leading model for the origin of stellar explosions called type 1a supernovas. Astronomers routinely use these bright supernovas to measure dark energy, a baffling entity thought to rev up the rate of expansion of the universe.

CONTACT Two white dwarfs merge and are about to become a type 1a supernova in this simulation. Red hot spots indicate the initial explosion site. E. Erastov, M. Rampp

Although the new study, published in the Feb. 18 Nature, is unlikely to change the interpretation of previous dark energy studies, a new understanding of how type 1a supernovas form (SN: 8/15/09, p. 22) may be critical for future, more precise dark energy measurements, says study coauthor Marat Gilfanov of the Max Planck Institute for Astrophysics in Garching, Germany. Because 1a supernovas are all similarly luminous and can be seen from afar, the explosions serve as ideal cosmic mileposts for measuring the universe’s expansion and deducing the presence of dark energy, which accelerates that expansion.

In the prevailing model for the origin of type 1a supernovas, a white dwarf — the dense remains of an elderly, sunlike star — siphons or accretes matter from an ordinary companion star until the dwarf reaches a critical mass and explodes. In an alternative model, a white dwarf merges with a closely orbiting companion to reach that critical mass.

To determine which scenario is correct, Gilfanov and Ákos Bogdán, also of the Max Planck Institute for Astrophysics, employed the Chandra X-ray Observatory. If the accretion model is right, the white dwarf ought to emit an abundance of X-rays for some 10 million years before it explodes. In the merger model, in contrast, X-rays aren’t emitted until just before the explosion.

Gilfanov and Bogdán report that the X-ray emissions from five nearby elliptical galaxies and the central region of the Andromeda spiral galaxy are one-thirtieth to one-fiftieth the amount expected in the accretion model. The study “shows that the accretion scenario, believed by many to be the most likely one, does not contribute more than a few percent to the observed type 1a supernova rate in [elliptical] galaxies, notes Gilfanov. “At present, the only viable alternative is the merger of two white dwarfs; therefore they are the likely cause of the majority of type 1a supernovae.”

In the Jan. 7 Nature, however, Rüdiger Pakmor and his colleagues at the Max Planck Institute for Astrophysics reported simulations showing that merging white dwarfs can indeed make type 1a supernovas, but only the rarest, least luminous ones. Future simulations will have to reconcile these observations.

Gilfanov and Bogdán’s study leaves open the possibility that accretion could still be the leading way to make type 1a supernovas in spiral galaxies. To maximize the detection of X-rays from white dwarfs, the researchers focused their study on elliptical galaxies and the center of one spiral galaxy, because these areas contain only small amounts of the gas and dust that can block X-rays from reaching detectors.

Even if most type 1a supernovas in elliptical galaxies result from white dwarf mergers, “this does not make them any worse tools for determining the properties of dark energy,” says theorist Mario Livio of the Space Telescope Science Institute in Baltimore. Understanding exactly how a white dwarf triggers an explosion is critical for determining if type 1a supernovas might somehow have been intrinsically fainter or brighter in the past than they are now.

Astronomers generally assume that supernovas that exploded when the cosmos was about half its current age have the same luminosity as those of today. If that assumption turns out not to be true, it could affect researchers’ ability to more precisely measure dark energy and determine whether it will remain constant or grow stronger or weaker over time. If dark energy grows much stronger, for example, the universe could end in a Big Rip with every atom ultimately torn asunder.

The new findings suggest that cosmologists ought to take into account the type of galaxy — elliptical or spiral — in which each stellar explosion arises, says supernova observer Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Because only a small fraction of type 1a supernovas used by cosmologists come from elliptical galaxies, any difference “is just barely detectable with today’s samples,” he adds. “Still, we should get this straightened out.”

The study highlights a bigger issue for understanding why such supernovas are not all created equally, says theorist Adam Burrows of Princeton University. Cosmologists working on future missions to scout these explosions, such as the space-based Joint Dark Energy Mission, may require an even larger number of observations than anticipated to sort out the variations and deduce the true nature of dark energy, Burrows says.  

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