This Ancient Supernova Remnant Looks Like a Stellar Dandelion

In 1181, Japanese and Chinese astronomers saw a bright light appear in the constellation Cassiopeia. It shone for six months, and those ancient observers couldn’t have known it was an exploding star. To them, it looked like some type of temporary star that shone for 185 days.

In the modern astronomical age, we’ve learned a lot more about the object. It was a supernova called SN 1181 AD, and we know that it left behind a remnant “zombie” star. New research examines the supernova’s aftermath and the strange filaments of gas it left behind.

Though it was seen almost 850 years ago, only modern astronomers have been able to explain SN 1181. For a long time, it was an orphan. While astronomers were able to identify the modern remnants of many other historical supernovae, SN 1181 was stubborn. Finally, in 2013, amateur astronomer Dana Patchick discovered a nebula with a central star and named it Pa 30. Research in 2021 showed that Pa 30 is the remnant of SN 1181. The SN exploded when two white dwarfs merged and created a Type 1ax supernova.

SN 1181 is unusual. When supernovae explode, there’s usually only a black hole or a neutron star left as a remnant. But SN 1181 left part of a white dwarf behind, an intriguing object astronomers like to call a zombie star. Strange filaments resembling dandelion petals extend from this strange star, adding to the object’s mystery.

Researchers have gotten a new, close-up look at Pa 30 and published their results in The Astrophysical Journal Letters. The research is titled “Expansion Properties of the Young Supernova Type Iax Remnant Pa 30 Revealed.” The lead author is Tim Cunningham, a NASA Hubble Fellow at the Center for Astrophysics, Harvard & Smithsonian.

“The recently discovered Pa 30 nebula, the putative type Iax supernova remnant associated with the historical supernova of 1181 AD, shows puzzling characteristics that make it unique among known supernova remnants,” the authors write. Pa 30 has a complex morphology, including a “unique radial and filamentary structure.”

The hot stellar remnant at Pa 30’s center is also unique. Its presence, as well as the lack of hydrogen and helium in its filaments, indicates that it’s the result of a rare Type1ax supernova. Since hydrogen and helium make up 90% of the chemicals in the Universe, objects without either of them are immediately interesting.

In this research, the astronomers used the Keck Cosmic Imager Spectrograph (KCIS) to examine the 3D structure and the velocities of the filaments. The KCIS was built to observe the cosmic web, the intricate arrangement of gas, dust, and dark matter that makes up the large-scale structure of the Universe. The gas and dust are extremely dim, and the KCIS was made to perform spectroscopy on these types of low surface brightness phenomena. That makes it a powerful tool for observing the strange filaments coming from Pa 30.

KCIS is a powerful spectrograph that can capture spectral information for each pixel in an image. It can also measure the redshift and blueshift of objects it observes, meaning it can determine their velocity and direction of movement. The researchers were able to show that material in the filaments travelled ballistically at approximately 1,000 kilometres per second.

“This means that the ejected material has not been slowed down, or sped up, since the explosion,” said lead author Cunningham. “Thus, from the measured velocities, looking back in time allowed us to pinpoint the explosion to almost exactly the year 1181.”

Pa 30 has some unusual features. It’s unusually asymmetrical, while most SN remnants are more spherical. Its filamentary structure displays significant variation in ejecta distribution along the line of sight. Some filaments are more prominent than others and extend further, creating an irregular and lopsided appearance. Some parts of the nebula are travelling at different speeds and in different directions. Elements in the nebula are also distributed unevenly. Iron, for example, is far more concentrated in some regions than others. All of these features suggest that the initial explosion mechanism was asymmetric and that the ejecta in the filaments stem from the initial explosion observed in 1181. Pa 30 also has a very sharp inner edge with an inner gap that surrounds the zombie star.

Many of Pa 30’s features suggest an asymmetric explosion as the cause. “The ejecta show a strong asymmetry in flux along the line of sight, which may hint at an asymmetric explosion,” the authors explain. The researchers found that the total flux from redshifted filaments is 40% higher than from blueshifted filaments. “This is tantalizing evidence for asymmetry in the explosion,” they write.

An asymmetric supernova explosion suggests that the underlying physics are complex. Rotation, complex magnetic fields, and the presence of a stellar companion can all contribute to asymmetry. Coupled with the unusually hot white dwarf left behind and its high-velocity stellar wind, the evidence suggests that it was a Type 1ax supernova.

That means the zombie star is likely the remnant of a failed thermonuclear explosion in a white dwarf. The white dwarf could have been just below the Chandrasekhar mass and not exploded completely. Or it could’ve been one of the theoretically possible but elusive super-Chandrasekhar mass white dwarfs. These objects are of great interest because they could be the cause of unusually bright supernovae. If Pa 30’s progenitor was a super-Chandrasekhar mass white dwarf, it could explain some of the remnant’s unusual characteristics.

“Our first detailed 3D characterization of the velocity and spatial structure of a supernova remnant tells us a lot about a unique cosmic event that our ancestors observed centuries ago. But it also raises new questions and sets new challenges for astronomers to tackle next,” said co-author Ilaria Caiazzo.

Some of the questions could be answered with more Keck Cosmic Imager Spectrograph IFU observations.

“Further IFU spectroscopic observations with wider coverage of the nebula will confirm if there exists a global asymmetry in the nebula ejecta, providing important constraints on dynamical models of the ejecta,” the authors conclude.