Researchers from the Center for Astrophysics, Harvard and Smithsonian Institution have finally
discovered a binary star that has long been thought to exist.
The research, published in the Monthly Notices of the Royal Astronomical Society, demonstrates
how unusual types of stars form and evolve in the universe.
Kareem El Badry, a member of the Institute for Theory and Computation at the Center for
Astrophysics, identified the new stars by combining data from the Shane Telescope at
Lick Observatory in California and numerous astronomical surveys.
El Badry described the discovery as interesting, saying it is a missing stellar evolutionary
link in the binary star formation models that they've been looking for.
Usually, when a star dies, there's a 97% chance they become a white dwarf, a small dense object
that has contracted and dimmed after burning through all its fuels.
In rare cases, stars can potentially become extremely low mass (ELM) white dwarfs.
According to stellar evolution calculations, these ELM white dwarfs, which are one-third
the mass of the Sun, are more than 13.8 billion years old, which is physically impossible.
The universe is not just old enough to make these stars by normal evolution.
Over the years, astronomical studies have shown that the only way to form ELM white
dwarf is through a binary companion. While some studies suggest that the gravitational
pull of a nearby companion star could eat away at a star more quickly, there's no full-proof
evidence to it yet. Astronomers have seen normal, massive stars
like the Sun accreting onto white dwarfs, a system known as cataclysmic variables.
Also, ELM white dwarfs have been observed as companions to white dwarfs. But they did
not observe the transitional phase of evolution that links the two, or the transformation
in between: when the star has lost most of its mass and is on the verge of contracting
to an ELM white dwarf. The team discovered a new type of binary star
consisting of a white dwarf being orbited by a bloated, stripped stellar core.
These binaries are products of mass transfer, which form when the gravity of a white dwarf
pulls off the outer layers of an ordinary star, exposing the helium core. The helium
core will contract soon, forming a low-mass white dwarf.
El-Badry frequently compares stellar astronomy to zoology in the 19th Century. You describe
how big it is, how much it weighs, and then you go on to some other organism.
You see all these different types of objects and need to piece together how they are all
connected. In 2020, he decided to go back into the jungle
to find this missing evolutionary link. He began his search for a star that had long
alluded scientists: the pre-ELM white dwarf, also known as an evolved cataclysmic variable.
He narrowed down his search from a billion stars to 50 probable candidates using data
from Gaia, the space-based observatory launched by the European Space Agency, and the Zwicky
Transient Facility at Caltech. For his research, the astronomer highlights
the importance of public data from astronomical surveys.
This work would not be possible if it weren't for projects like the Zwicky Transient Facility
and Gaia, which represent a huge amount of work behind the scenes from hundreds of individuals.
El-Badry then went on to do detailed observations of 21 of the stars. He began to see results
in his mission because 100% of the candidates were the pre-ELMs which he was looking for.
The stars were puffier and more bloated than ELMs, and they were also egg-shaped because
the gravitational pull of the other star distorts their spherical shape.
The researchers found the evolutionary link between two classes of binary stars; cataclysmic
variables and ELM white dwarfs, and they found a decent number of them.
During the research, 13 of the stars exhibited evidence of losing mass to their companion,
while the other eight did not lose mass. However, each of them was hotter than previously recorded
cataclysmic factors. Following his current success, EL-Badry intends
to continue studying the pre-ELM white dwarfs, as well as the other 29 stars he discovered
earlier. Like modern-day anthropologists filling in
the gaps in human evolution, he is astounded by the vast range of stars that may arise
from simple science.