Astronomers discover dirty spot on white dwarf star for the first time It is associated with the accretion of planetary material caused by magnetism

Astronomers have traced for the first time the influence of a white dwarf’s magnetic field on the accretion process of planetary material. WD 0816â310 is thought to have absorbed previously evaporated material from an object with a mass close to or comparable to the asteroid Vesta, but the accretion flow through the magnetic field was directed toward the dwarf’s magnetic pole. This article was published in “The Astrophysical Journal Letters”. The high surface gravity of a white dwarf’s atmosphere allows a mechanism to deposit elements heavier than hydrogen or helium into deeper layers. However, scientists already know of many white dwarfs with metals in their atmospheres, which are associated with the accumulation of material from planetesimals and destroyed planets. Such observations will allow us to better understand the geochemistry of exoplanet systems and the planet formation process. In particular, astronomers have already investigated the water and lithium content of exoplanetes, and have also discovered samples of the mantle from exoplanets. It is also interesting to track the accretion process of material onto white dwarfs and understand the influence of the dwarf’s magnetic field, which can increase the accretion rate and change the direction of the flow. A team of astronomers led by Stefano Vanullo from Armagh Observatory in Northern Ireland used the FORS2 instrument installed at the ground-based VLT telescope complex to detect dirty white dwarf star WD 0816-310 in February and March 2023. The results of spectroscopic polarization observations were announced. . WD 0816â310 is located at a distance of 63.2 light-years from the Sun and belongs to the DZ spectral class, which has absorption lines in its spectrum mainly of metals such as Ca, Mg, and Fe. Furthermore, it has a variable longitudinal magnetic field, and speculation has been made about the relationship between this field and the dynamics of the metal content in the dwarf’s atmosphere, based on early spectroscopic observations. The dwarf observations can be explained by a model of a dipolar magnetic field tilted at 70 degrees to the white dwarf’s axis of rotation and then tilted at an additional angle with a near-polar magnetic field strength of 140 kilogauss. An angle of 70 degrees relative to the direction of the observer’s Earth. The dwarf’s rotation period is 10.8 days and its effective temperature is 6250 Kelvin. The scientists found that the dynamics of the metal content in the dwarf star’s atmosphere, expressed by changes in the intensity of absorption lines in the spectrum, has a time scale of several days and is significantly correlated with the longitudinal component of the magnetic field. discovered. rotation of stars. In the case of WD 0816-310, the magnetic field strength is thought to be sufficient to detach the accretion disk and drive the flow of material along magnetic field lines to the magnetic poles on the white dwarf’s surface. Horizontal mixing by the field promotes metal accumulation in these zones. However, in this case, it is not entirely clear whether the evaporated planetary material is sufficiently ionized against the magnetic field to actually control the accretion flow. The researchers found that even if there was not enough ultraviolet radiation from the dwarf star to fully ionize the gas, the presence of a small number of atoms ionized by the radiation could reduce the degree to which the collisions ionized the material in the disk. It was concluded that it increases gradually. Another important point is the nature of the planetary material that fell into the dwarf star. Scientists concluded that this was the destruction of an object with a mass equal to or comparable to that of the asteroid Vesta, but the metal has a 310,000-year period of time to reach depth and distribution after an atmospheric accretion episode. I had extra time. horizontal direction.

source: https://iopscience.iop.org/article/10.3847/2041-8213/ad2619