White dwarf

Science

This rare Frankenstein star wears the skin of its dead companion

This kind of "merger" is so rare that only two stars of this type are known.

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Astronomers say they’ve found a unique new kind of star in the universe, one born from a merger between two white dwarves. The resulting star, which consists of a core of helium surrounded by an outer layer of carbon and oxygen, is extremely rare — they’ve found just two so far out of the hundreds of millions of cataloged stars.

Even weirder, the merger that creates this new type of star involves tearing one of the white dwarves apart and layering it on top of the other white dwarf, in the process kickstarting a new round of nuclear fusion inside the star. At least, that’s the hypothesis put forward in a new paper by a team of researchers from Argentina and Germany, who say they’ve finally come up with an explanation for these exotic stars. The team researched two odd, small white dwarves called PG 1654+322 and PG 1528+025 emitting unique wavelengths of light corresponding to elements that shouldn’t have been there.

In a new paper, a team of astronomers say they think they know how these exotic stars form. It’s published in the Monthly Notices of the Royal Astronomical Society alongside another paper describing the stars themselves.

Here’s the background — Stars are basically giant balls of hydrogen and helium that are undergoing nuclear fusion, or the process of putting atoms together to make new, larger atoms. For most of a normal star’s life, that means smashing hydrogen atoms together to make helium. Later on, that helium can also undergo fusion to make things like carbon and oxygen.

When stars like our Sun run out of hydrogen fuel in their cores, they start to expand into what’s called a red giant. White dwarfs are the super-dense cores left over after a red giant expands and dissipates. Some contain lots of helium, and others have a core of carbon and oxygen. But this new star has a ton of carbon and oxygen on the outside instead of in the core. Its shell is about 40 percent carbon and oxygen, split evenly between the two elements. Until now, none of our models of how stars evolve and die could explain how that might happen.

“This is very strange,” Marcelo Miller Bertolami, an astronomer with the Instituto de Astrofísica de La Plata in Argentina and coauthor of a paper on the new stars, tells Inverse. Normally, stars are about three-fourths hydrogen at their surface, about one-fourth helium, and around two percent other elements, he says.

“Stars that have a lot of helium at their surface (more than 40 percent) are already rare, but stars with almost 40 percent of carbon and oxygen at the surface are extremely exotic,” he says.

What’s new — The hypothesis is that the ingredients for the new kind of star, officially called a carbon-oxygen-rich subdwarf O star, are very specific, Bertolami says. You need two different kinds of white dwarfs of very particular sizes, he says, and they need to crash into each other.

These kinds of stellar collisions aren’t all that rare — they often happen between pairs of stars, called binaries, that gradually circle closer in to each other before becoming a single star. But the unique ingredients make this new star special.

First, you need a white dwarf made of just helium and that’s bigger than about .4 times the mass of the Sun, Bertolami says. Then, it needs to have a companion with a lot of carbon and oxygen and which is around .35 times the mass of the Sun. Finally, merge them together violently to create a new star.

Artist’s depiction of the violent event that birthed this new freak star. Nicole Reindl / University of Potsdam

When the smaller carbon-oxygen white dwarf gets close to its helium companion, the immense gravitational pull starts literally tearing the star apart and pulling its mass into the more massive star. The final Franken-star ends up being a core of helium covered in a layer of carbon and oxygen. That explains why the new type of star has so much carbon and oxygen on its outside — it’s actually one star covered in the remains of a completely different star.

The energy and extra material from the merger is also enough to essentially restart the new star. While white dwarfs are inert, meaning they don’t have enough material to create fusion, their collision injects a huge amount of energy into the stellar material, enough to kickstart nuclear fusion.

“The stars are burning helium in the core and no longer white dwarfs,” Bertolami says. “Just weird helium-burning objects covered in carbon and oxygen.”

What’s next — While just two of these born-again stars have been found so far, Bertolami thinks there are probably a few more out there. He says his team is also going to keep gathering data to get a better idea of whether their hypothesis for how these stars form is right.

And with other stellar ingredients, who knows what kind of weird stars might be forming out there in the universe. Carbon and oxygen-covered Frankenstars might be far from the weirdest things we find one day.

Abstract — Helium-rich subdwarf O stars (sdOs) are hot compact stars in a pre-white dwarf evolutionary state. Most of them have effective temperatures and surface gravities in the range Teff = 40 000–50 000 K and log g = 5.5–6.0. Their atmospheres are helium dominated. If present at all, C, N, and O are trace elements. The abundance patterns are explained in terms of nucleosynthesis during single star evolution (late helium core flash) or a binary He-core white dwarf merger. Here we announce the discovery of two hot hydrogen-deficient sdOs (PG1654+322 and PG1528+025) that exhibit unusually strong carbon and oxygen lines. A non-LTE model atmosphere analysis of spectra obtained with the Large Binocular Telescope and by the LAMOST survey reveals astonishingly high abundances of C (⁠≈20 per cent⁠) and O (⁠≈20 per cent⁠) and that the two stars are located close to the helium main sequence. Both establish a new spectroscopic class of hot H-deficient subdwarfs (CO-sdO) and can be identified as the remnants of a He-core white dwarf that accreted matter of a merging low-mass CO-core white dwarf. We conclude that the CO-sdOs represent an alternative evolutionary channel creating PG1159 stars besides the evolution of single stars that experience a late helium-shell flash.
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