Astronomers have discovered a new type of binary star, that was missing for decades, which represents a "missing link" in the evolution of so-called extremely low mass (ELM) white dwarfs.
When 97 percent of the Universe's stars run out of nuclear fuel and can longer support themselves against collapse, they will become a compact stellar core called a white dwarf. But, in some rare instances stars with a third of the mass of the sun die and form ELMs.
This presents a problem because if stellar evolution calculations are correct, all ELM white dwarfs should be more than 13.8 billion years old. That is older than the age of the universe itself and thus seemingly impossible. This has led to the conclusion that ELMs could only form if they have binary partners lending a hand in their creation by stripping material away from them.
Astronomers have spotted normal stars like the sun in binary pairings accreting onto white dwarfs, a system called cataclysmic variables, and ELM white dwarfs with normal white dwarf companions.
That means they had seen the "before" and "after" picture of ELMs but not the transitional phase of the evolution that links the two when the star has lost most of its mass and has nearly contracted to an ELM white dwarf. That was until now.
"We've discovered a new kind of binary star consisting of a white dwarf being orbited by a bloated, stripped stellar core," Center for Astrophysics postdoctoral fellow, Kareem El-Badry, told Newsweek.
"These binaries are products of mass transfer, formed when the white dwarf's gravity pulls off the outer layers of a normal star, leaving the helium core exposed. Soon, the helium core will contract to form a low-mass white dwarf."
El-Badry and his team's discovery consists of 21 stars that are egg-shaped due to the gravitational influence of a binary partner pulling them. The Harvard researcher said that the stars were more puffed and bloated than normal ELMs. This is because they are in the process of transforming into low mass white dwarfs, making them a "missing link" between sun-like stars in cataclysmic variables and ELM white dwarfs.
Of the stars discovered by the team, thirteen appeared as if they were still losing mass to their companion stars, while the process seemed to have halted for the other eight stars. These detached stars were also hotter than cataclysmic variables observed in the past.
El-Badry said that while astronomers have theories about how binary stars evolve, and which kind of binaries turn into which kind of stellar objects, because this evolution often occurs on timescales of millions or billions of years, they can't just wait and watch individual binaries evolve.
"Instead, we have to find binaries at different evolutionary stages and try to piece together how the evolution occurred," the Havard post-doc said. "It's a bit like paleontologists looking to find transitional species that fill 'gaps' in the fossil record."
Despite being speculated about for decades, this type of binary star has proved elusive. El-Badry explained to Newsweek why astronomers have had a difficult time spotting these stellar objects.
"In the past, astronomers have often found unusual binaries by their colors. But these objects have pretty much identical colors to normal stars," he said. "In order to distinguish them from normal stars, we needed to know their sizes and masses. To do that, we needed to know their distances. It only became possible to measure their distances after the launch of the Gaia satellite."
The team discovered a total of 50 candidates for this pre-ELM binary system, and El-Badry now plans to follow up on the other 29 candidates. He said: "We need to get more data to measure their chemical compositions. I'm actually going to Hawaii next week to use a telescope at Keck observatory on Mauna Kea to do exactly that."
The findings were published in The Astrophysical Journal.
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December 02, 2021 at 11:11PM
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