Astronomers solve the age-old mystery of the supernova observed in 1181

In August 1181, astronomers from China and Japan observed a bright “guest star” in the night sky that we now know to be a supernova, one of the few recorded supernovas in our Milky Way that were visible to the naked eye. . He shone for a full six months before he passed away. Astronomers have not been able to identify the rest of the source of SN 1181 for centuries, and this detail is crucial in determining which class the supernova belongs to. Now, an international team of astronomers believe they have identified this source as one of the hottest stars in the galaxy within the Pa30 nebula, according to a new article published in the Astrophysical Journal Letters.

As we wrote earlier, there are two known types of supernovae, 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 they cause a huge catastrophic explosion. The temperatures and pressures get so high that the carbon in the star’s core begins to fuse. This stops the nucleus from collapsing, at least temporarily, and this process continues, over and over again, with increasingly heavy atomic nuclei. When the fuel finally runs out completely, the iron core (by then) collapses into a black hole or neutron star.

Then there is a thermonuclear supernova. Smaller stars (up to about eight solar masses) gradually cool down to become dense ash cores known as white dwarfs. If a white dwarf running out of nuclear fuel is part of a binary system, it can siphon matter from its partner, adding to its mass until its nucleus reaches temperatures high enough for carbon fusion to occur. .

There are also rarer types of supernovae. One of the oldest and most famous “guest stars” was recorded by Chinese astronomers around July 4, 1054. It was visible in broad daylight for 23 days. The remains now form the Crab Nebula. Some have speculated that SN 1054 was a so-called “electron-capturing” supernova, first described about 40 years ago.

If so, SN 1054 has a 21st century cousin. Last June, we reported that a team of astronomers had identified a second recent supernova, dubbed SN 2018zd, which meets all the criteria for an electron-capture supernova. In this scenario, a star is not heavy enough to produce an iron core collapsing supernova, but it is not light enough to prevent its core from completely collapsing. Instead, these stars stop the fusion process when their nuclei are made up of oxygen, neon, and magnesium. In this scenario, the electrons are engulfed by the neon and magnesium in the nucleus, causing the nucleus to deform 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 is 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 hydrogen and helium from its companion star, eventually reaching critical mass and exploding, thus destroying the white dwarf. But there are cases, like with SN 2012Z, where the white dwarf only loses half of its mass and leaves behind a zombie star as a remnant.

“SN 1181 was so far the only historical supernova of the last millennium without some quid pro quo,” the authors wrote. For years, the most likely remaining candidate was a radio and x-ray pulsar known as 3C-58, which currently rotates about 15 times per second. This would mean that the pulsar has not lost much rotational energy over the past 900 years. The remnant of SN 1054, the Crab Nebula, on the other hand, lost about two-thirds of its rotational energy. And according to recent radio readings from 3C-58, the pulsar is probably much older than SN 1181 and therefore could not be the vestige.

Enter the disc-shaped Pa30 nebula, first discovered by astronomers in 2013. Pa30 surrounds a rare and massive Wolf-Rayet star known as Parker’s Star. The authors determined that the dust and gas in Pa30 extend at over 1,100 km / s, and the team used this speed to derive the age of the nebula: around 1,000 years. This makes it a great candidate for the rest of SN 1181.

“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. “This means that the age and location correspond to the events of 1181.”

Astronomers previously hypothesized that Pa30 and Parker’s star resulted from the collision and fusion of two white dwarf stars, producing an Iax-like supernova, and Zijlstra et al.the conclusions of s agree with this hypothesis. “Only about 10 percent of supernovae are of this type and they are not well understood,” Zijlstra said. “The fact that SN1181 was weak but faded very slowly fits this type. It is the only such event where we can study both the remaining nebula and the merged star and also have a description of it. The explosion itself It’s nice to be able to solve both a historical mystery and an astronomical mystery.

DOI: Astrophysical Journal Letters, 2021. 10.3847 / 2041-8213 / ac2253 (About DOIs).