Вадим Дудченко
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Using new data from ESA’s Gaia star-mapping satellite, the Zwicky Transient Facility, and the Shane Telescope at Lick Observatory, astronomers have observed the first physical proof of a new population of transitional binary stars.

An artist’s impression of a cataclysmic variable. Image credit: David Hardy, www.astroart.org.

“When a star dies, there’s a 97% chance it will become a white dwarf, a small dense object that has contracted and dimmed after burning through all its fuel,” explained Dr. Kareem El-Badry, a postdoctoral researcher at the Harvard & Smithsonian’s Center for Astrophysics, and his colleagues.

“But in rare instances, a star can become an extremely low mass (ELM) white dwarf.”

“Less than one-third the mass of the Sun, these stars present a conundrum: if stellar evolution calculations are correct, all ELM white dwarfs would seem to be more than 13.8 billion years old — older than the age of the Universe itself and thus, physically impossible.”

Over the years, astronomers have concluded that the only way for an ELM white dwarf to form is with the help of a binary companion.

The gravitational pull from a nearby companion star could quickly eat away at a star until it became an ELM white dwarf.

Dr. El-Badry and colleagues aimed to find a star that had long eluded astronomers: the pre-ELM white dwarf, also referred to as an evolved cataclysmic variable.

They identified 50 potential candidates and then followed-up with close observations of 21 of the stars.

“100% of the candidates were these pre-ELMs we’d been looking for. They were more puffed up and bloated than ELMs. They also were egg-shaped because the gravitational pull of the other star distorts their spherical shape,” Dr. El-Badry said.

“We found the evolutionary link between two classes of binary stars — cataclysmic variables and ELM white dwarfs — and we found a decent number of them,” he added.

“Thirteen of the stars showed signs that they were still losing mass to their companion, while eight of the stars seemed to no longer be losing mass.”

“Each of them was also hotter in temperature than previously observed cataclysmic variables.”

The findings appear in the Monthly Notices of the Royal Astronomical Society.


Kareem El-Badry et al. 2021. Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs. MNRAS 508 (3): 4106-4139; doi: 10.1093/mnras/stab2583


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