North Carolina State University astronomers led by Dr John Blondin have used a very long Chandra observation of the remnant of Kepler’s supernova to deduce that the supernova was triggered by an interaction between a white dwarf and a red giant star. This is significant because another team has already shown that a so-called Type Ia supernova caused the Kepler supernova remnant.
The thermonuclear explosion of a white dwarf star produces such supernovas. Because they explode with nearly uniform brightness, astronomers have used them as cosmic distance markers to track the accelerated expansion of the Universe.
However, there is an ongoing controversy about Type Ia supernovas. Are they caused by a white dwarf pulling so much material from a companion star that it becomes unstable and explodes? Or do they result from the merger of two white dwarfs?
“While we can’t speak to all Type Ia supernovas, our evidence points to Kepler being caused by a white dwarf pulling material from a companion star, and not the merger of two white dwarfs. To continue improving distance measurements with these supernovas, it is crucial to understand how they are triggered,” explained Dr Mary Burkey, first author of the paper reporting the findings in the Astrophysical Journal (arXiv.org version).
SN 1604’s remnant is one of only a few Type Ia supernovas known to have exploded in our galaxy. Its proximity and its identifiable explosion date make it an excellent object to study.
The new Chandra data reveal a disk-shaped structure near the center of the remnant. The team interprets this X-ray emission to be caused by the collision between supernova debris and disk-shaped material that the giant star expelled before the explosion. Another possibility is that the structure is just debris from the explosion.
The evidence that this disk-shaped structure was left behind by the giant star is two-fold: first, a substantial amount of magnesium – an element not produced in great amounts in Type Ia supernovas – was found in the Kepler remnant. This suggests the magnesium came from the giant companion star. Secondly, the disk structure bears a remarkable resemblance in both shape and location to one observed by Spitzer Space Telescope. These infrared-emitting disks are thought to be dusty bands expelled by stars in a wind, rather than material ejected in a supernova.
The researchers found a remarkably large and puzzling concentration of iron on one side of the center of the remnant but not the other. The authors speculate that the cause of this asymmetry might be the ‘shadow’ in iron that was cast by the companion star, which blocked the ejection of material. Previously, theoretical work has suggested this shadowing is possible for Type Ia supernova remnants.
“One remaining challenge is to find the damaged and fast-moving leftovers of the giant star that was pummeled by the explosion at close quarters,” said co-author Dr Kazimierz Borkowski.
Bibliographic information: Mary T. Burkey et al. 2013. X-Ray Emission from Strongly Asymmetric Circumstellar Material in the Remnant of Kepler’s Supernova. ApJ 764, 63; doi: 10.1088/0004-637X/764/1/63