Astronomers recently discovered that LSPM J0207+3331, an ancient white dwarf star about 145 light-years away from Earth, continues to accrete planetary debris even though it has cooled for about 3 billion years. This star in the constellation Triangulum is considered to be one of the oldest and coolest white dwarfs known to have a dust disk, and it also challenges the traditional understanding of the evolution of stellar systems.
The research was led by Érika Le Bourdais, a doctoral student at the University of Montreal. The research team initially discovered the star through the "Backyard Worlds: Planet 9" project in 2019, and later confirmed with the Keck Telescope in Hawaii that the infrared signal it emits is consistent with the dust ring, indicating that strong gravity is tearing apart asteroids to form a dust disk surrounding the star.
Researchers say this discovery shows that even if a white dwarf has existed for billions of years, planetary fragments, comets and even planets may still remain in orbit for a long time and be disturbed and fall into the star much later.
Through spectral analysis, the team identified 13 heavy elements in the white dwarf's atmosphere, including sodium, magnesium, aluminum, silicon, calcium, titanium, chromium, manganese, iron, cobalt, nickel, copper and strontium. Normally, heavy elements settle rapidly in hydrogen-rich white dwarfs, making them difficult to detect, but this time the results far exceeded expectations.
The study further pointed out that these fragments are likely to come from a rock-rich celestial body that has undergone layered evolution. Its structure is similar to the Earth or Vesta, with a metallic core and rocky mantle. In terms of chemical composition, it is quite close to earth materials, except that magnesium and silicon are slightly deficient in iron, and the lack of carbon element characteristics indicates that its source material does not contain obvious volatile carbon components.
Co-author Patrick Dufour, a professor at the University of Montreal, said white dwarfs are almost one of the few windows through which humans can directly measure the composition of exoplanets. When planetary fragments get too close to a star, they are torn apart by tidal forces and contaminate the star's atmosphere, leaving a clear chemical "fingerprint."
The team also detected weak Ca II H and K-line core emission, making it only the second isolated contaminated white dwarf known to exhibit this signature. The study believes that this may mean that additional physical processes are occurring in the upper atmosphere of the star, so it is particularly important to include heavy elements in model calculations, otherwise it will affect the inference of the structure and parameters of the white dwarf star.
Previously, it was believed that the infrared excess of this star comes from two dust rings, but the new analysis shows that only one dust disk composed of silicate can explain the observed signal at 11.6 microns, which also makes the explanation of the system structure simpler.
As for why these fragments fell into the star at such a late stage, research is still not conclusive. One possibility is that the giant planets in the system have gradually disturbed the orbits of small celestial bodies over the long years; another possibility is that nearby stars passing by have changed the paths of these debris. The research team believes that in the future, the two explanations can be further distinguished with the help of the James Webb Space Telescope or combined with archival data from the European Space Agency's Gaia mission.
The researchers pointed out that the most common type of white dwarf is a hydrogen-rich white dwarf, and the coldest of these tend to be the oldest stars in the Milky Way. Precisely because less attention has been paid to whether such stars are still accreting matter in the past, this case prompts the astronomical community to expand the search scope and re-examine more similar celestial bodies.
The discovery suggests that planetary systems may remain active and complex billions of years after stars die. Studying these late accretion events can not only help humans understand the composition of distant worlds, but may also provide an important reference for understanding the fate of the solar system in the future.