A star discovered at full speed by the Milky Way is so strange that astronomers can only conclude that it was ejected during a very unusual supernova event.
The star, a white dwarf about 1,430 light-years away called SDSS J1240 + 6710, and nicknamed Dox, advances at 250 kilometers per second (155 miles per second), in the direction of the galaxy’s rotation. But that is not what is strange.
Dox’s chemical composition is extremely unusual, so unusual, it suggests that the star was kicked at high speed by a type of supernova explosion that we’ve never seen before.
White dwarfs are what remains when a low-mass star, up to about eight times the mass of the Sun, reaches the end of its useful life, running out of material to merge into its core.
Solitary white dwarfs, as our Sun will be, will fly most of their mass before the nucleus collapses on itself into an object of less than 1.4 solar masses. This is the maximum mass for a stable white dwarf.
These lone white dwarf stars will not become supernovae, they will simply continue to be white dwarfs, slowly cooling down over billions of years. Its chemistry is fairly well understood, and most have atmospheres made up mostly of hydrogen and helium, along with a little carbon and oxygen.
In 2015, however, astronomers discovered Dox, an unusually low-mass white dwarf star, about 40 percent of the Sun’s mass. Spectroscopic analysis revealed that its atmosphere was almost pure oxygen, with traces of magnesium, neon, and silicon, not a trace of hydrogen or helium.
This obviously warranted a closer look, so a team of astronomers led by physicist Boris Gänsicke from the University of Warwick in the UK took their own spectroscopic observations using the Hubble Space Telescope.
What they found was even more peculiar. Dox’s atmosphere also contained traces of carbon, sodium, and aluminum. All of these elements are produced during the initial thermonuclear reactions of a supernova explosion.
But the heavier elements that are forged from these lighter elements during the later stages of a supernova (the “iron group” elements, iron, nickel, chromium, and manganese) were also completely missing. As if the supernova started, then it died out.
When the team calculated the star’s velocity, they discovered how fast it was buzzing through the galaxy and the pieces fell into place.
“This star is unique because it has all the key characteristics of a white dwarf, but it has very high speed and unusual abundances that are meaningless when combined with its low mass,” said Gänsicke.
“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 come from some kind of nearby binary system and that it must have undergone thermonuclear ignition. It would have been a kind of supernova, but kind we haven’t seen before. “
Many stars in the Universe are in binary pairs, locked in a close mutual orbit, and this is how we can get a white dwarf supernova. If at least one of the stars is a white dwarf, and it extracts material from its companion star, it can accumulate too much material to remain stable, resulting in a supernova explosion.
The team believes this process began, but the thermonuclear ignition and subsequent mass ejection were enough to disrupt the binary orbit, sending both stars flying in opposite directions.
“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,” said Gänsicke.
White dwarf binary supernovae are some of the most studied in the Universe. They are called Type Ia supernovae, and their well-characterized absolute brightness makes them an incredibly useful tool for measuring cosmic distances.
They remain in the sky for some time, first lighting up for a few months as the star explodes, then gradually fading over a few years. That residual light runs on radioactive nickel, and the lack of it in the failed Dox kaboom could explain how we might have lost it.
The partial supernova would have been just a brief flash, an event that is easy to miss if we’re not looking at it, and in this case, one that highlights how little we know about how stars die.
“We are now discovering that there are different types of white dwarfs that survive supernovae under different conditions and, using the compositions, masses, and speeds they have, we can determine what type of supernova they have suffered from,” said Gänsicke.
“Clearly there is a complete zoo out there. Studying supernova survivors in our Milky Way will help us understand the myriad supernovae that we see shooting in other galaxies.”
The research has been published in the Monthly notices from the Royal Astronomical Society.
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