Astronomers Map a Dandelion Supernova

Astronomers have, for the first time, mapped the 3D filamentary structure of a famous dandelion-shaped supernova remnant. The debris from the explosion is still speeding outwards at more than 1,000 kilometers per second (2 million mph) almost a millennium after the star at the center of it all detonated.

In AD 1118, Chinese and Japanese astronomers recorded the arrival of a “guest star” in the constellation Cassiopeia. Today, we know that what they saw was a supernova, the cataclysmic demise of a star. Yet it took until 2013 for an amateur astronomer, Dana Patchick, to find the remnant of the explosion in archived images from NASA’s Wide-field Infrared Survey Explorer (WISE). Thinking it was a planetary nebula, he added it to his catalog as Pa 30.

Now we know that Pa 30 is the remnant of a Type Iax supernova, triggered by the explosion of a white dwarf star. The “x” denotes the fact that some of the star survived this ordeal, making the supernova fainter than usual and potentially explaining why it took astronomers so long to find its remnant.

Last year, a separate team of astronomers discovered strange filaments within Pa 30, again using WISE. Now, a team led by Tim Cunningham (Center for Astrophysics, Harvard & Smithsonian) and Ilaria Caiazzo (Institute of Science and Technology, Austria) has mapped the distribution of these filaments in three dimensions using the Keck Cosmic Web Imager (KCWI) at the W. M. Keck Observatory in Hawai‘i. Their results are published in The Astrophysical Journal Letters.

“A standard image of the supernova remnant would be like a static photo of a fireworks display,” says team member Christopher Martin (Caltech). “KCWI gives us something more like a ‘movie’ since we can measure the motion of the explosion’s embers as they streak outward from the central explosion.” These streaks resemble the petals of a dandelion flower.

Thanks to KCWI, each pixel within the image of Pa 30 came with information about its brightness at a range of wavelengths. The team looked for pixels whose light was either redshifted or blueshifted, telling them whether the material that produced that emission is moving toward or away from us.

“We find the material in the filaments is expanding ballistically,” says Cunningham. “This means that the material has not been slowed down nor sped up since the explosion. From the measured velocities, looking back in time, you can pinpoint the explosion to almost exactly the year 1181.”

The analysis also reveals a large cavity inside the spindly filaments. How this overall structure formed is still a mystery. One potential explanation is a reverse shock wave, which travels more slowly than the initial shock wave. Some of the shocked material is collapsing back towards the center. According to Cunningham, it “may  be condensing surrounding dust into filaments, but we don’t know yet.” He adds: “The morphology of this object is very strange and fascinating.”

“One of the main points of the paper that I find it particularly interesting is the flux asymmetry in the ejecta that may hint at an asymmetric explosion,” says Georgios Dimitriadis(Lancaster University, UK), who was not involved in the research. The most widely accepted trigger mechanism for SNe Iax — a white dwarf accreting helium from a stellar companion — should lead to a symmetrical explosion. “If [this] explosion was asymmetric, then possibly more work on the progenitor system of SNe Iax should be done,” Dimitriadis says.

More generally, astronomers use SN Ia as standard candles to measure cosmic distances and in turn the expansion rate of the universe. Even though SNe Iax aren’t used in this way, they can still shed light on more typical white dwarf detonations. “One of the main sources of uncertainty in cosmological measurements from SN Ia is the unknown physics of the explosion,” Dimitriadis says. “Understanding how the extreme/peculiar SNe Ia, such as SNe Iax, explode may help us understand how the normal ones explode, too.” Future 3D mapping of similar supernova remnants could well prove crucial in deciphering some of the universe’s biggest mysteries.