In August 1181, astronomers in China and Japan witnessed a bright “guest star” in the night sky that we now know to have been a supernova—one of just a handful of recorded supernovae in our Milky Way that were visible to the naked eye. It shone brightly for a full six months before it disappeared. Astronomers haven’t been able to identify the remnant of the source for SN 1181 for centuries, and that detail is crucial to determine which class the supernova belongs to. Now, an international team of astronomers think they have pinpointed that source as one of the hottest stars in the galaxy within the Pa30 nebula, according to a new paper published in the Astrophysical Journal Letters.
As we’ve written previously, there are two types of known supernova, depending on the mass of the original star. An iron-core collapse supernova occurs with massive stars (greater than ten solar masses), which collapse so violently that it causes a huge, catastrophic explosion. The temperatures and pressures become so high that the carbon in the star’s core begins to fuse. This halts the core’s collapse, at least temporarily, and this process continues, over and over, with progressively heavier atomic nuclei. When the fuel finally runs out entirely, the (by then) iron core collapses into a black hole or a neutron star.
Then there is a thermonuclear supernova. Smaller stars (up to about eight solar masses) gradually cool to become dense cores of ash known as white dwarfs. If a white dwarf that has run out of nuclear fuel is part of a binary system, it can siphon off matter from its partner, adding to its mass until its core reaches high enough temperatures for carbon fusion to occur.
There are also rarer types of supernovae. Among the earliest and most famous “guest stars” was recorded by Chinese astronomers around July 4, 1054. It was visible in broad daylight for 23 days. The remnants now form the Crab Nebula. Some have hypothesized that SN 1054 was a so-called “electron-capture” supernova, first described some 40 years ago.
If that is indeed the case, SN 1054 has a 21st-century cousin. Back in June, we reported that a team of astronomers had identified a second recent supernova—dubbed SN 2018zd—that meets all the criteria for an electron-capture supernova. In this scenario, a star isn’t heavy enough to produce an iron core collapse supernova, yet it’s not light enough to prevent its core from collapsing entirely. Instead, such stars stop the fusion process when their cores are composed of oxygen, neon, and magnesium. In this scenario, electrons get gobbled up by the neon and magnesium in the core, thereby causing the core to buckle under its own weight. The end result is a supernova.
According to this new analysis, SN 1181 appears to belong to another relatively rare category known as Type Iax. It’s related to the Type Ia category, in which the supernova is the result of a binary star system where one of the two stars is a white dwarf. Typically, the white dwarf siphons off hydrogen and helium from its companion star, eventually hitting a critical mass and exploding, destroying the white dwarf in the process. But there are cases, such as with SN 2012Z, where the white dwarf only loses half its mass and leaves behind a zombie star as a remnant.
“SN 1181 was until now the only remaining historical supernova of the last millennium without a certain counterpart,” the authors wrote. For years, the most likely candidate remnant was a radio and x-ray pulsar known as 3C-58, which currently rotates about 15 times per second. That would mean the pulsar hasn’t lost much rotational energy over the last 900 years. SN 1054’s remnant, the Crab Nebula, in contrast, has lost roughly two-thirds of its rotational energy. And according to recent radio surveys of 3C-58, the pulsar is likely much older than SN 1181 and hence could not be the remnant.
Enter the disk-like nebula Pa30, first discovered by astronomers in 2013. Pa30 surrounds a rare, massive Wolf-Rayet star known as Parker’s Star. The authors determined that the dust and gas in Pa30 is expanding at more than 1100 km/sec, and the team used that velocity to derive the nebula’s age: about 1,000 years. This makes it an excellent candidate for the remnant of SN 1181.
“The historical reports place the guest star between two Chinese constellations, Chuanshe and Huagai. Parker’s Star fits the position well,” said co-author Albert Zijlstra of the University of Manchester. “That means both the age and location fit with the events of 1181.”
Astronomers had previously hypothesized that Pa30 and Parker’s Star resulted from the collision and ensuing merger of two white dwarf stars, producing a Type Iax supernova, and Zijlstra et al.‘s findings are in keeping with that hypothesis. “Only around 10 percent of supernovae are of this type and they are not well understood,” said Zijlstra. “The fact that SN1181 was faint but faded very slowly fits this type. It is the only such event where we can study both the remnant nebula and the merged star and also have a description of the explosion itself. It is nice to be able to solve both a historical and an astronomical mystery.”