A Star had a Partial Supernova and Kicked Itself Into a High-Speed Journey Across the Milky Way

Supernovae are some of the most powerful events in the Universe. They’re extremely energetic, luminous explosions that can light up the sky. Astrophysicists have a pretty good idea how they work, and they’ve organized supernovae into two broad categories: they’re the end state for massive stars that explode near the end of their lives, or they’re white dwarfs that draw gas from a companion which triggers runaway fusion.

Now there might be a third type.

Scientists have discovered a white dwarf star that is speeding through the Milky Way after a ‘partial supernova.’ Evidence for the star was found in Hubble Space Telescope by a team of researchers led by astronomers at the University of Warwick.

Their findings are presented in a paper titled “The partially burned remnant of a low-mass white dwarf that underwent thermonuclear ignition?” Lead author is Professor Boris Gaensicke from the Department of Physics at the University of Warwick. The paper’s published in The Monthly of the Royal Astronomical Society.

The discovery of this phenomenon is based partly on unusual spectroscopic measurements of a white dwarf with the Hubble.

Most stars end their lives as white dwarfs. It’s the fate that awaits our own Sun. After it leaves the main sequence it’ll become a red giant, and then finally a white dwarf.

Our Sun, and any star with the same mass, will follow a common evolutionary path. Once it leaves the main sequence, after hydrogen burning is complete, it becomes a red giant, then a white dwarf. <Click to enlarge.> Image Credit: By Lithopsian – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=48486177

But the newly discovered white dwarf star is spectroscopically different than most other white dwarfs.

White dwarfs have left fusion behind. They’re the cores of stars that have depleted their fuel, and they contain mostly electron-degenerate matter. They have atmospheres that are mostly hydrogen or helium, with some occasional heavier elements that have risen to the surface from the white dwarf’s core.

The star at the center of this study was discovered a couple years ago. It’s named SDSS J1240+6710 and was first observed in 2015. It’s unusual because its atmosphere contained neither hydrogen nor helium, and because follow-up observations with the Hubble showed that the atmosphere also contained carbon, sodium, and aluminium.

Artist’s rendition of a white dwarf from the surface of an orbiting exoplanet. Image Credit: Madden/Cornell University

Those three elements are all produced in supernovae explosions, during the first phase. But that’s not all that Hubble found out. Measurements also showed a lack of iron group elements. The iron group elements are iron, cobalt, nickel, chromium and manganese. A full-blown supernova creates these elements near the end of the supernova process. But this white dwarf had none.

In their paper, the team wrote “We do not detect any iron-group element, with tight limits on the abundances of Ti, Fe, Co, and Ni, and conclude that the star underwent oxygen burning, but did not reach the ignition conditions for silicon burning.”

What gives?

There’s something else unusual about SDSS J1240+6710. It’s speeding through the Milky Way at about 900,000 km/h (560,000 mp/h.) Lastly, the white dwarf is much less massive than other white dwarfs, at only 40% the mass of our Sun.

All of the star’s properties point to a partial supernova explosion as their source.

“The low mass of the white dwarf and its moderately high rest-frame velocity suggest an origin involving a thermonuclear supernova in a compact binary,” the researchers wrote in their paper.

“This star is unique because it has all the key features of a white dwarf but it has this very high velocity and unusual abundances that make no sense when combined with its low mass,” said lead author Gaensicke in a press release.

“It has a chemical composition which is the fingerprint of nuclear burning, a low mass and a very high velocity: all of these facts imply that it must have come from some kind of close binary system and it must have undergone thermonuclear ignition. It would have been a type of supernova, but of a kind that that 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 white dwarf’s mass grows to the point where a supernova explosion is triggered.

An artist’s image of a white dwarf drawing material away from its companion. Image Credit: NASA

In this case, the initial stages of the supernova disrupted the white dwarf’s orbit. Both stars would’ve been flung into separate, opposite, trajectories through space. That would explain SDSS J1240+6710’s high velocity through space.

“If it was a tight binary and it underwent thermonuclear ignition, ejecting quite a lot of its mass, you have the conditions to produce a low mass white dwarf and have it fly away with its orbital velocity,” Professor Gaensicke explained.

This study brings to the fore some of the challenges in observing supernovae. Typically, scientists are only alerted to them once they explode. The details prior to the explosions are difficult to tease out.

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 careening through the galaxy was very short-lived, and there would’ve been only a brief flash to signal it. Normally, a Type 1A supernova like this, that completed its supernova explosion, would be visible for months. The explosion produces lots of radioactive nickel (Ni) that powers a long-lasting afterglow.

But this one didn’t produce much Ni. As the authors write in the conclusion of their paper, “The very low mass of Ni produced and ejected in such events would make their detection extremely challenging within the current time-domain surveys.”

Supernova 1994D in Galaxy NGC 4526. Normally, a supernova explosion is visible for months. The afterglow is caused by abundant, radioactive Nickel. But SDSS J1240+6710 produced very little nickel. Image Credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team

“The study of thermonuclear supernovae is a huge field and there’s a vast amount of observational effort into finding supernovae in other galaxies,” Professor Gaensicke said. “The difficulty is that you see the star when it explodes but it’s very difficult to know the properties of the star before it exploded.”

“We are now discovering that there are different types of white dwarf that survive supernovae under different conditions and using the compositions, masses and velocities that they have, we can figure out what type of supernova they have undergone,” Gaensicke explained. “There is clearly a whole zoo out there. Studying the survivors of supernovae in our Milky Way will help us to understand the myriads of supernovae that we see going off in other galaxies.”

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Evan Gough

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