Astronomers Confirm a New Class of Binary Stars


ELM white dwarf star


In about 5 billion years, our Sun will expand to become a red giant, lose its hydrogen-rich envelope, and leave behind a white dwarf star, which is the compact remnant core with about half the original mass of our Sun. 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. 

Astronomers believe that ELM white dwarfs are the result of binary star interactions, whereby a stellar companion strips the hydrogen envelope of a giant before the core can grow sufficiently in mass. Until now, the formation of an ELM white dwarf had not been observed. According to a new paper by researchers at the Harvard/Smithsonian Center for Astrophysics, the progenitors of ELM white dwarfs have now been identified. The discovery finally confirms how a rare type of star in the universe forms and evolves.

New Population of Binary Stars

The new class of stars was discovered by Kareem El-Badry, an astrophysicist at the Institute for Theory and Computation with the Harvard/Smithsonian Center for Astrophysics, using the Shane Telescope at Lick Observatory in California and data from several astronomical surveys.

“We have observed the first physical proof of a new population of transitional binary stars,” says El-Badry. “This is exciting; it’s a missing evolutionary link in binary star formation models that we’ve been looking for.”

New Type of Star Not Made by Norman Stellar Evolution

When a star dies, there’s a 97 percent chance it will become a white dwarf, a small dense object that has contracted and dimmed after burning through all its fuel.

“The universe is just not old enough to make these stars by normal evolution,” says El-Badry, a member of the Institute for Theory and Computation at the Center for Astrophysics.

Evidence is Not Foolproof

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 (at least, in less than 13.8B years) eat away at a star until it became an ELM white dwarf. But evidence for this picture is not foolproof.

Astronomers have observed normal, massive stars like our Sun accreting onto white dwarfs—something called cataclysmic variables. They have also observed ELM white dwarfs with normal white dwarf companions. They had not, however, observed the transitional phase of evolution, or the transformation in between: when the star has lost most of its mass and has nearly contracted to an ELM white dwarf.

Missing Evolutionary Link

El-Badry often compares stellar astronomy to 19th century zoology: “You go out into the jungle and find an organism. You describe how big it is, how much it weighs—and then you go on to some other organism,” he explains. “You see all these different types of objects and need to piece together how they are all connected.”

In 2020, El-Badry decided to go back into the “jungle” in search of the star that had long eluded scientists: the pre-ELM white dwarf (also referred to as an evolved cataclysmic variable).

Gaia and Zwicky Transient Data

Using new data from the Gaia space-based observatory and the Zwicky Transient Facility (ZTF) at Caltech, El-Badry narrowed down one billion stars to 50 potential candidates. The luminosities, colors, and photometric light curves of those 50 candidates were unique compared to the millions of other variable stars discovered by ZTF. 

“If it weren’t for projects like the Zwicky Transient Facility and Gaia, which represent huge amounts of work behind the scenes from hundreds of people—this work just wouldn’t be possible,” he says.

El-Badry then followed-up with close observations of 21 of the stars: The selection strategy worked. “100 percent of the candidates were these pre-ELMs we’d been looking for,” he says. “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.”

“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,” El-Badry adds. 

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 Last Word 

“The ages probably range from 3 to 10 billion years,”  El-Badry wrote in an email to The Daily Galaxy. “There were some misinterpretations of the press release implying these stars are older than the age of the Universe, but that obviously can’t be right. The point is that *if low-mass white dwarfs formed by single-star evolution* stellar evolution would imply their ages are older than the age of the Universe. This just means they didn’t form via single-star evolution. 

“The selection process to get from a billion to 50 has many steps,” he explains, “which are described in the paper. In brief, we looked for stars that are smaller than main-sequence stars but have similar temperatures and surface gravities and show evidence of tidal distortion in their light curves. We did this using distance measurements from Gaia and light curves from the ZTF survey.”

El-Badry plans to continue studying the pre-ELM white dwarfs and may follow-up on the 29 other candidate stars he previously discovered.

Source: Kareem El-Badry et al, Birth of the ELMs: a ZTF survey for evolved cataclysmic variables turning into extremely low-mass white dwarfs, Monthly Notices of the Royal Astronomical Society (2021). DOI: 10.1093/mnras/stab2583

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Harvard CfA

Image credit: M.Weiss/Center for Astrophysics | Harvard & Smithsonian

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