Key Takeaways:
- Pa 30 nebula is the result of a collision between two white dwarfs that merged instead of exploding in a supernova.
- Pa 30 is a strong candidate for a Type Iax supernova, a rare type of “failed” supernova that leaves a surviving star.
- The age and location of Pa 30 aligns with a supernova recorded by Chinese astronomers in 1181 AD, potentially solving a longstanding astronomical mystery.
- The key feature of Pa 30, glowing sulfur gas filaments, was initially missed by professional telescopes due to broad filters. Amateur astronomers using specialized filters were the first to identify it.
- Studying Pa 30 will provide valuable insights into Type Iax supernovae and the process of stellar mergers.
In binary star systems, when two white dwarfs eventually spiral in toward one another and collide, the outcome is typically mutually assured destruction, a thermonuclear explosion that devours both stars and disperses their remnants throughout space.
Astronomers have discovered a single case, though, in which the collision produced fireworks of a different kind.
Recent observations of the faint nebula Pa 30 have shown that filaments of glowing sulfur gas surround it; these filaments resemble the trails of sparks ejected from an exploding fireworks shell. Scientists believe that this particular scene resulted from a collision between two white dwarfs that avoided destroying one another. Instead, they apparently merged and formed a magnetic monster of a star that blows its own material into space, whisking debris from the merger outward to form the sulfuric, streaming contrails.
The nebula and its central star, according to researchers, are a singular object with very few observational precedents. During his thirty years of studying supernova remnants, Robert Fesen of Dartmouth College in Hanover, New Hampshire, said, “I’ve never seen anything like this.” At a press conference, Fesen presented his team’s findings at the American Astronomical Society’s (AAS) winter meeting in Seattle. “There’s nothing like this in our galaxy.” The Astrophysical Journal Letters has accepted their report for publication; a draft can be found on the arXiv preprint server.
Benson Guest, an X-ray astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study, described the object as “a really interesting.” “These things are very hard to detect because they’re not very bright compared to a normal supernova, so you’re looking for a very faint transient [object].”
The latest data supports the theory that Pa 30 is a Type Iax supernova, a kind of “failed” supernova that leaves behind a surviving star and produces a comparatively moderate outburst of light. These have been observed in distant galaxies, but “this would be the first one we’ve ever found” in the Milky Way that we can easily study, said Guest. “That’s something really cool in astronomy anytime you can say that.”
The object’s age is also determined by the new observations, which also make a compelling case for it being the answer to a 900-year-old astronomical mystery.
Gem overlooked
In Cassiopeia, Pa 30 is located at a distance of only 7,500 light-years and has a diameter of about 3′, or roughly one-tenth that of the Full Moon. Amateur astronomer Dana Patchik found it in 2013 while looking through Wide Infrared Survey Satellite (WISE) archived data. According to the data, the object had a typical round, doughnut-like appearance. It looked like a planetary nebula, which is created when an aging star releases its outer layers of gas into space, excites that gas, and causes it to glow when exposed to radiation.
Follow-up observations were carried out over the next few years by several professional observatories, including the 10-meter Gran Telescopio Canarias (GTC) on La Palma. However, they hardly noticed any gas emissions from nitrogen, oxygen, or hydrogen. The researchers never thoroughly examined the data because there didn’t seem to be anything to study.
When some French amateur astronomers discovered in 2018 that the nebula was home to a very blue star at its center, interest in the object surged once more through the use of an 8-inch scope. They informed a team at the University of Hong Kong (HKU) that had observed Pa 30, and they started reanalyzing their data.
But a Russian team that had noticed the same thing beat them to publication in 2019. They acquired a spectrum with the Russian 6-meter telescope in the Caucasus Mountains, which showed some astounding characteristics of this star: With a surface temperature of roughly 200,000 degrees Celsius (360,000 degrees Fahrenheit), a wind of material shoots into space at a speed of 35 million mph (57,000 km/h), giving it the brightness of 36,000 Suns. The Russian team suggested in a paper published in Nature that the star was the remains of a Type Iax double-white dwarf supernova, spinning quickly and possessing a magnetic field powerful enough to quicken the winds.
Fresh evidence for a cold case
The HKU team added new information when they published their findings in The Astrophysical Journal Letters in 2021. Initially, they discovered that the nebula contained some sulfur but not a lot of glowing oxygen gas. According to their spectra, this sulfur is leaving the central star at a speed of roughly 2.5 million miles per hour (4 million kilometers per hour). They calculated the supernova’s approximate age to be roughly 1,000 years ago, give or take 250 years, assuming that this is debris from the Type Iax supernova and using the WISE image as a guide for the gas’s journey.
The HKU team saw that this suggested Pa 30 might have the answer to a long-standing supernova mystery. Astronomers from China and Japan discovered a “guest star” in this area of the sky in 1181. It was relatively low for a supernova in our galaxy, perhaps only magnitude -1 brighter than the brightest star.
Scientists had believed since the 1970s that the result of this event was a pulsar, a kind of rapidly rotating neutron star left over from a supernova, located in a nebula known as 3C 58. But that was called into question by observations in the past few decades that estimated 3C 58 was more like 2,500 years old. To be associated with the demise of a star as recent as SN 1181, 3C 58 had to be too cool and too static in the sky. The HKU team’s estimate of Pa 30’s age fit him better, though there was still a lot of uncertainty.
Fesen saw an opportunity to do one better, as did Louisiana State University colleague Bradley Schaefer. The majority of professional observatories are only made to use filters that permit a relatively wide range of wavelengths to pass through; this includes all of the observatories that have observed Pa 30 thus far. For the same reason, the massive oxygen emission feature that was recently discovered by a group of amateur astronomers using narrowband filters next to the Andromeda Galaxy went undetected by professional astronomers. (Fesen co-authored that study as well.)
A narrow passband Sulfur-II filter was usable on Kitt Peak’s 2.4-meter Hiltner telescope by Fesen, Schaefer, and Patchik. It was “low-hanging fruit,” Fesen told Astronomy at the AAS meeting in Seattle. “In fact, it was so low, it was on the ground,” he joked. Guest agrees that it is a standard approach and methodology. “It’s just a nonstandard object.”
The outcome was astounding: Using a filter that could eliminate significantly more background noise, what had first appeared to be a fuzzy blob turned out to be tiny gas contrails being pushed outward by the central star’s strong winds. Fesen remarked, “It’s beautifully symmetric.”
Because of these features’ clarity, the team was able to calculate the sulfur gas’s distance from its central star much more precisely. By combining that measurement with the velocity of 2.5 million mph (4 million km/hr) derived from sulfur, they were able to determine the approximate age of PA 30 to be 844 years, give or take 55 years, which is almost exactly the same as the 842-year age of SN 1181. “Well, that’s ridiculously good … almost too good,” said Fesen. “Finally, we have really nailed down the remains of the star that the Chinese and Japanese saw in early August of 1181 A.D.”
Fesen stated that it “should be amazing” that he and his colleagues will be able to get additional observations of the object using the Hubble or James Webb space telescopes. He says the object will provide more information about the physics of Type Iax supernovae, which astronomers know very little about, and the specifics of how a star survives.
“There’s beauty, science, and history in the story,” Fesen said. “An Iax has never been observed in our galaxy. This one is located only a few thousand light years away.
