The image of supernova remnant G299.2-2.9 is more than just beautiful. Its unusual asymmetry may also force scientists to rethink the way Type Ia supernovae explode.
Observed with the Chandra X-ray telescope—and with the lightwaves shifted into the visible range—G299.2-2.9 looks like a prized piece of jewelery, an opal set in gold perhaps. To the casual observer, however, it might not look at all exceptional among the other nebulae that dot the sky.
G299, as it is shortened, is the remnant of a Type Ia supernova explosion that occurred more than 4,500 years ago. While Type II and Ib supernovae are known to give rise to asymmetric explosions, which are reflected in the spreading remnants we see thereafter, Type Ia are thought to be far more even.
Traces of asymmetry are visible in G299 even at first glance, with indentations at the upper left and lower right. These can be explained by pockets of existing gas interfering with the spread of material from the explosion. The colors, however, are harder to explain.
The blue in the image above represents high-energy X-rays from agitated silicon and sulfur. The green, on the other hand, is lower in frequency and is mostly emitted by iron. Looking closely, we can see the blue and green are not evenly distributed – there is far more iron to the north of the origin star and silicon to the south, with another iron-rich area stretching off to the right-hand side of the image.
These observations have been brought to light in The Astrophysical Journal where the authors, led by graduate student Seth Post of the University of Texas at Arlington, propose two explanations. One is that the region around the exploding star already had patches rich in either iron or silicon, enriching the material that flowed in those directions. The more interesting hypothesis is that Type Ia supernovae are not always as symmetrical as we thought, requiring a rewrite of the ways in which these events occur.
Although G299 is the clearest example yet observed of an asymmetric Type Ia supernova remnant, it is not the only one, which lends support to the idea that these events are more complex than previously believed.
While other types of supernovae occur through the collapse of a very large star that cannot sustain itself, Type Ias are thought to be the result of white dwarf stars drawing material off a companion star until they reach the mass limit where they become unstable. It is proposed that some Type Ia supernovae occur when two white dwarf stars merge.
Newer models of Type Ia explosions suggest it's possible for detonations to occur away from the center of the progenitor star, particularly in the merging dwarf star model.
The authors conclude that G299 is probably the result of a delayed-detonation supernova, which occurs when an initial explosion lowers the stellar density, and allows a more powerful one to follow.
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