The case of the shrinking white dwarf

For the first time, astronomers have found observational evidence of a contracting white-dwarf star.
By | Published: November 16, 2017
In this artist’s concept, the white dwarf (left) in the binary system HD 49798/RX J0648.0-4418 is shown spinning while surrounded by an accretion disk of matter taken from its larger binary companion (right). Astronomers found the white dwarf is spinning faster and faster over time, which could be because it is contracting.
Francesco Mereghetti
Consider a Sun-like star, a red giant, and a white dwarf. They all seem pretty different. But really, one star can be all three of these throughout its life. In about 5 billion years, the Sun will turn into a red giant, bloating until it swallows the Earth. Then about a billion years after that, it will expand too far and lose its outer layers, leaving just its hot, dense core behind. This core will be a white dwarf.

A lot of white dwarfs have been spotted over the years, but a study published this week in Monthly Notices of the Royal Astronomical Society presented the first observational evidence of a shrinking white dwarf, which they found has been consistently contracting for the past 2 million years.

According to theory, a typical white dwarf can shrink its radius by several hundred kilometers during its first million years, but astronomers have never actually witnessed this behavior before. “For decades it has been theoretically clear that young white dwarfs are contracting,” said astrophysicist and lead author of the study, Sergei Popov, in a press release. “Yet, that very phase of contraction has never been observed in ‘real time.’”

This is partly because many white dwarfs observed thus far are extremely old, so they finished shrinking a long time ago. But it’s also incredibly difficult for astronomers to measure miniscule changes in a white dwarf’s radius since the stellar core is both very distant and very compact. (A white dwarf roughly the mass of the Sun would only be about the size of Earth).

A solar mass white dwarf is only about the size of Earth; however, the more massive a white dwarf, the smaller it gets.
The shrinking star in this study is actually part of a binary system succinctly named HD 49798/RX J0648.0-4418, located some 2,000 light-years away in the constellation Puppis (the Stern). According to Popov, the team was able to accurately measure changes in the white dwarf because “the uniqueness of the binary system [meant] the white dwarf was literally illuminated (due to the accretion of matter from the neighboring star).”

“In other similar systems, accretion is much more powerful,” Popov said, “it determines how the white dwarf rotates, which makes it impossible to notice the beauty of contraction.” Since the spin of the white dwarf in HD 49798/RX J0648.0-4418 was not significantly influenced by infalling gas from its companion, the team realized that any changes in the dwarf’s spin rate would likely be a result of it changing in size.

Sandro Mereghetti, astronomer and co-author of the study, discovered that the white dwarf’s rotational velocity was not only the fastest ever observed for such a remnant, but it also has been speeding up over the past 20 years. He found that the white dwarf’s original 13-second spin period — the time it takes to complete one full rotation — is decreasing by about seven nanoseconds each year.

Though a few nanoseconds per year may not seem like much, for an object as massive and compressed as a white dwarf, this corresponds to a significant shift in angular momentum — something that could not be accomplished through the accretion of matter. Instead, the researchers demonstrated that the white dwarf’s faster spin could easily be explained if the star were contracting, much like the way a spinning figure skater rotates faster as she pulls her arms in.

This simplified graphic displays the different evolutionary pathways stars can take depending on their mass. Sun-like stars (top) typically end up going through a red giant phase before eventually forming a white dwarf.
Based on evolutionary calculations, the researchers determined the white dwarf is about 2 million years old. And at this age, theory predicts it should be shrinking by about one centimeter per year, which perfectly fits with the increase in spin rate the team observed.

“Thanks to this discovery, astrophysicists will be able to study and evaluate the evolution patterns of young white dwarfs — and successfully look for similar systems in the galaxy,” Popov said.

If astronomers can locate other systems like HD49798/RX J0648.0-4418, they will not only learn more about how young white dwarfs evolve, but they will also be able to further explore the role accretion plays in such systems.