Supernovae are some of the most powerful events in the Universe. They are extremely energetic and luminous explosions that can light up the sky. Astrophysicists have a pretty good idea of how they work, and they’ve arranged supernovae into two broad categories: they’re the end state of massive stars exploding near the end of their lives, or they’re white dwarfs that draw gas from a companion that triggers merging out of control.
Now there may be a third type.
Scientists have discovered a white dwarf star that is accelerating through the Milky Way after a ‘partial supernova’. Evidence of the star was found at the Hubble Space Telescope by a team of researchers led by astronomers at the University of Warwick.
Their findings are presented in an article titled “The partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?” The lead author is Professor Boris Gaensicke of the Physics Department at the University of Warwick. The article is published in The Monthly of the Royal Astronomical Society.
The discovery of this phenomenon is based in part on unusual spectroscopic measurements of a white dwarf with Hubble.
Most stars end their lives as white dwarfs. It is the fate that awaits our own Sun. After it leaves the main sequence, it will become a red giant and eventually a white dwarf.
Image credit: by Lithopsian – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48486177 “class =” wp-image-147094 “width =” 580 “height =” 532 “srcset =” https://www.universetoday.com/wp-content/uploads/2020/07/Star-evolution.png 912w, https://www.universetoday.com/wp-content/ uploads / 2020/07 / Star-evolution-580×533.png 580w, https://www.universetoday.com/wp-content/uploads/2020/07/Star-evolution-250×230.png 250w, https: // www. universetoday. com / wp-content / uploads / 2020/07 / Star-evolution-768×706.png 768w “sizes =” (maximum width: 580px) 100vw, 580px “/>But the newly discovered white dwarf star is spectroscopically different from most other white dwarfs.
White dwarfs have left the merger behind. They are the nuclei of stars that have exhausted their fuel, and contain mainly degenerate matter of electrons. They have atmospheres that are mostly hydrogen or helium, with some heavier occasional elements that have surfaced from the white dwarf’s core.
The star at the center of this study was discovered a couple of years ago. It’s called SDSS J1240 + 6710, and it was first observed in 2015. It’s unusual because its atmosphere contained neither hydrogen nor helium, and because follow-up observations with Hubble showed that the atmosphere also contained carbon, sodium, and aluminum.
Those three elements are all produced in supernova explosions, during the first phase. But that’s not all Hubble discovered. Measurements also showed a lack of elements from the iron group. The elements of the iron group are iron, cobalt, nickel, chromium and manganese. A full-fledged supernova creates these elements near the end of the supernova process. But this white dwarf had none.
In their article, the team wrote: “We did not detect any element of the iron group, with strict limits on the abundance of Ti, Fe, Co and Ni, and conclude that the star burned oxygen, but did not reach ignition conditions for burn silicon. ”
What gives?
There is something more unusual in SDSS J1240 + 6710. It is accelerating through the Milky Way to approximately 900,000 km / h (560,000 mp / h.) Lastly, the white dwarf is much less massive than other white dwarfs, with only 40 % of the mass of our Sun.
All of the star’s properties point to a partial supernova explosion as its source.
“The low mass of the white dwarf and its moderately high velocity in the resting frame suggest an origin involving a thermonuclear supernova in a compact binary,” the researchers wrote in their article.
“This star is unique because it has all the key characteristics of a white dwarf, but it has this very high speed and unusual abundances that are meaningless when combined with its low mass,” lead author Gaensicke said in a press release.
“It has a chemical composition that is the fingerprint of nuclear combustion, a low mass and a very high speed: all these facts imply that it must have come from some type of nearby binary system and that it must have undergone thermonuclear ignition. It would have been a type of supernova, but a type we haven’t seen before. “
This white dwarf must have had a companion star. In these scenarios, a white dwarf orbits a common center of gravity with a larger companion star. As the companion star ages and becomes a giant, the white dwarf’s gravity draws gas from the companion star to its own surface. The mass of the white dwarf grows to the point where a supernova explosion is fired.
In this case, the initial stages of the supernova disrupted the white dwarf’s orbit. The two stars would have been launched on separate, opposite paths through space. That would explain the high speed of SDSS J1240 + 6710 through space.
“If it was a narrow binary and it underwent thermonuclear ignition, expelling much of its mass, you have the conditions to produce a low-mass white dwarf and make it fly at orbital speed,” explained Professor Gaensicke.
This study highlights some of the challenges in observing supernovae. Scientists generally only receive alerts when they explode. The details prior to the explosions are difficult to decipher.
The researchers wonder if this is one of our first examples of a new type of supernova. In this case, the supernova explosion that sent this star at full speed through the galaxy was very short-lived, and there would only have been a brief flash to signal it. Normally, a Type 1A supernova like this, which completed its supernova explosion, would be visible for months. The explosion produces a large amount of radioactive nickel (Ni) that generates a long-lasting, persistent glow.
But this did not produce much Ni. As the authors write in the conclusion of their article, “The very low mass of Ni produced and expelled in such events would make detection extremely difficult within current time domain surveys.”
“The study of thermonuclear supernovae is a huge field and there is a great deal of observational effort to find supernovae in other galaxies,” said Professor Gaensicke. “The difficulty is that you see the star when it explodes, but it is very difficult to know the properties of the star before it explodes.”
“We are now discovering that there are different types of white dwarfs that survive supernovae under different conditions and, using the compositions, masses, and velocities they have, we can determine what type of supernova they have suffered from,” Gaensicke explained. “Clearly there is a complete zoo out there. Studying supernova survivors in our Milky Way Galaxy will help us understand the myriad supernovae we see in other galaxies. “
