Astronomers have finally found the satisfactory solution to a perplexing cosmic problem: the apparent mystery of stars fundamentally too small to exist.
White dwarfs considered too tiny to exists in the current life of the universe now have, however, they were spotted with mass slurped-off by binary companions – a long suspected mechanism that can explain their dimensions, but never before proven in wild.
These “missing link” binaries are called evolved cataclysmic variables, and their discovery helps us understand one of stages on evolutionary path of dead stars.
“We have seen the first physical evidence of a new population of transitional binary stars,” said astronomer Kareem El Badry of Harvard and Smithsonian Center for Astrophysics.
“It is exciting ; this is an evolutionary link missing in the binary star formation models that we were looking for.
White dwarfs are what becomes a star less than about 8 times the mass of the Sun when it runs out of fuel for nuclear fusion process.
The dying star ejects most of its mass & the core collapses-down into an extremely dense object, up to about 1.4 times the mass of Sun, packed into a sphere the size of Earth.
On rare occasions, however, they can have such a low mass that, according to our models of stellar evolution, they should not exist. These extremely low mass white dwarfs, or ELMs, only about a third of the Sun’s mass. Such mass loss is expected to take much longer than the current lifespan of the Universe, which is only about 13.8 billion years. .
But this is only if the star exists in isolation. If something else, like a binary companion, were to absorb the mass off the white dwarf, it would speed up the process considerably, clearly explaining the presence of ELM in the Universe. The only problem that remained until now was that the process had never been observed.
According to theory, the process should take place after a phase called the cataclysmic variable phase. It is then that a white dwarf is in a binary system with another star, so close that the white dwarf accreting material from its companion.
Periodically, the white dwarf erupts as the accreted material triggers the uncontrolled fusion of hydrogen in its atmosphere. These cataclysms cause variations in luminosity, hence the name.
Sometimes a white dwarf star accrete so much material that it becomes unstable and explodes in a Type Ia supernova, but there is another option as well. The balance can tip so other star stealing mass from white dwarf instead.
El Badry & his colleagues used data from astronomical survey data from Gaia Space Observatory & Zwicky Transition Facility, identifying 50 stars that may be in this intermediate stage. They then used Shane Telescope at Lick Observatory to get detailed observations of 21 of these binaries.
They hit gold.
“100% of the candidates were those pre-ELMs that we were looking for. They were more puffed-up & bloated than the ELMs. Plus, they were shaped like an egg because the gravitational pull of the other star distorts their spherical shape, ”says El Badry.
“We found evolutionary link between 2 classes of binary stars – the cataclysmic variables & ELM white dwarfs – and we found a decent number of them.
Most binaries consisted of a white dwarf about 0.15 times the mass of the Sun, with companions about 0.8 times the mass of the Sun.
All white dwarfs have shown signs of mass loss to their companion stars; for 13 of the stars, the process was still in progress, while the other eight were no longer losing mass, but were swollen, as if the mass loss was recent. The 21 stars were all hotter & brighter than what is normally seen in a cataclysmic variable white dwarf.
Further work is needed to fully understand the population of evolved cataclysmic variables, including more detailed observations of the 21 binaries. The team also hopes to go back & take a closer look at the remaining 29 binaries from initial 50 candidates.
The team’s research has been published in the Monthly Notices of the Royal Astronomical Society.