When stars like our sun burn out, they turn into small, pale relics - and develop a cannibalistic appetite, as another study shows: Astronomers have identified a metallic trace on a white dwarf that apparently comes from an "eaten" celestial body of the former planetary system . The results show that the stellar magnetic fields directed the metallic material towards the star. The scientists explain that the trace of the stellar meal was apparently created specifically at its magnetic poles.
Celestial bodies also have “life histories” that are characterized by birth, youth, old age and death. As far as stars are concerned, their fate depends heavily on their mass. Specimens the size of our Sun are expected to expand into red giants when their supply of fuel runs out at the end of their stellar lives. The dying stars then shed their outer shells until in the end all that is left is a compact, faintly glowing star remnant in which hydrogen nuclear fusion no longer takes place. These hot, but only faintly glowing relics are called white dwarfs.
Burned out but still hungry
It is assumed that the inflation and the further development processes up to the formation of the stellar remnant also influence the surrounding planetary system. Nearby planets are devoured by the red giant or are later attracted and torn apart by the white dwarf. In this case, the remains of planets or asteroids form debris disks around white dwarfs. The fragments can then be pulled onto its surface. Ultimately, the extinct star gradually eats its former “children”. Evidence of this process has already been found in some white dwarfs: their radiation signature shows elements on the surface that come from rock or metallic components of the incorporated celestial bodies.
However, the team of astronomers led by Stefano Bagnulo from the Armagh Observatory and Planetarium in Northern Ireland is now reporting on a case in which a special trace of the “meal” of a white dwarf is emerging. The target was the white dwarf WD 0816-310. It is the roughly Earth-sized remnant of a star that once resembled our sun. The team studied it using the FORS2 instrument and data from the X-Shooter unit at the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Chile. The instruments are used for spectroscopic analyzes of starlight, which enable conclusions to be drawn about certain substances on the star's surface. FORS2 can also measure the polarization of light, which in turn is related to a star's magnetic field. “This gives us a combination of capabilities that are required for the observation of faint objects such as white dwarfs and the sensitive measurement of stellar magnetic fields,” says Bagnulo.
Directed to the star by the magnetic field
The astronomers report that they detected the spectral signature of metals on the star's surface, which appear to come from incorporated material. What was special, however, was that the strength of the metallic signal changed as the star rotated. This suggests that the material is concentrated in a specific area on the white dwarf's surface. “Surprisingly, the material was not evenly distributed across the surface of the star, as theoretically predicted. Instead, it forms a concentrated patch of planetary material,” says co-author John Landstreet of the University of Western Ontario in London, Canada.
The team also found that this abnormality is associated with fluctuations in the white dwarf's magnetic field. As the astronomers explain, this suggests that the metallic spot is located at one of the celestial body's magnetic poles. Taken together, these results suggest that the stellar magnetic field directed the metallic material on the star to specific areas. Specifically, the astronomers suspect that when a planetary body in the system was destroyed, the material evaporated and was ionized. It was then directed to the magnetic poles by the white dwarf's magnetic field.
Modeling has also provided clues as to what the white dwarf absorbed in this way: "The metals could have come from a planetary fragment that was at least as large as the asteroid Vesta - the second largest asteroid in the solar system with a diameter of around 500 kilometers." says co-author Jay Farihi from University College London. Bagnulo continues: “We knew that some white dwarfs were absorbing parts of their planetary systems. “We have now discovered that the star’s magnetic field may have played a key role in this process,” said the astronomer. Ultimately, the study also makes it clear that even after a star “death”, its system can remain dynamically active.
Source: ESO, specialist article: The Astrophysical Journal Letters, doi: 10.3847/2041-8213/ad2619