Professional astronomers and volunteers working with the Backyard Worlds: Planet 9 citizen science project have spotted the oldest and coldest known white dwarf ringed by dust and debris. The discovery is described in a paper in the Astrophysical Journal Letters.
In this illustration, an asteroid (bottom left) breaks apart under the powerful gravity of LSPM J0207+3331, the oldest, coldest white dwarf known to be surrounded by a ring of dusty debris. Image credit: NASA’s Goddard Space Flight Center / Scott Wiessinger.
The newly-found white dwarf, named LSPM J0207+3331, is located around 145 light-years away in the constellation Capricornus.
White dwarfs slowly cool as they age, and the astronomers calculated the object is about 3 billion years old based on a temperature just over 10,500 degrees Fahrenheit (5,800 degrees Celsius).
A strong infrared signal picked up by NASA’s Wide-field Infrared Survey Explorer (WISE) suggested the presence of dust, making LSPM J0207+3331 the oldest and coldest white dwarf with dust yet known.
Previously, dust disks and rings had only been observed surrounding white dwarfs about one-third LSPM J0207+3331’s age.
“This white dwarf is so old that whatever process is feeding material into its rings must operate on billion-year timescales,” said Dr. John Debes, an astronomer at the Space Telescope Science Institute.
“Most of the models scientists have created to explain rings around white dwarfs only work well up to around 100 million years, so this star is really challenging our assumptions of how planetary systems evolve.”
When a Sun-like star runs out of fuel, it swells into a red giant, ejects at least half of its mass, and leaves behind a very hot white dwarf. Over the course of the star’s giant phase, planets and asteroids close to the star become engulfed and incinerated.
Planets and asteroids farther away survive, but move outward as their orbits expand. That’s because when the star loses mass, its gravitational influence on surrounding objects is greatly reduced.
Some white dwarfs — between 1 and 4% — show infrared emission indicating they’re surrounded by dusty disks or rings. Astronomers think the dust may arise from distant asteroids and comets kicked closer to the star by gravitational interactions with displaced planets.
As these small bodies approach the white dwarf, the star’s strong gravity tears them apart in a process called tidal disruption. The debris forms a ring of dust that will slowly spiral down onto the surface of the star.
LSPM J0207+3331 was first spotted by Melina Thévenot, a citizen scientist working with the Backyard Worlds: Planet 9 project.
“That is a really motivating aspect of the search,” Thévenot said.
“The researchers will move their telescopes to look at worlds you have discovered. What I especially enjoy, though, is the interaction with the awesome research team. Everyone is very kind, and they are always trying to make the best out of our discoveries.”
According to the astronomers, LSPM J0207+3331’s ring may even be multiple rings.
They suggest there could be two distinct components, one thin ring just at the point where the star’s tides break up the asteroids and a wider ring closer to the white dwarf.
Follow-up with future missions like NASA’s James Webb Space Telescope may help them tease apart the ring’s constituent parts.
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John H. Debes et al. 2019. A 3 Gyr White Dwarf with Warm Dust Discovered via the Backyard Worlds: Planet 9 Citizen Science Project. ApJL 872, L25; doi: 10.3847/2041-8213/ab0426
