A new insight into the magnetism of white dwarfs

Dr. Stefano Bagnulo
Armagh Observatory and Planetarium, UK
18 Novembre 2021 ore 11:00 – 12:00

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Many stars evolve into magnetic white dwarfs, but we do not know when the magnetic field appears at their surface, if and how it evolves during the cooling phase, and, above all, what are the mechanisms that generate the field, and why they act on some but not all degenerate stars.
Observations may help to find an answer to these questions, but their interpretation is dramatically affected by biases due to target selection and a non-homogeneous field detection threshold.
To minimise these biases we have carried out a volume-limited spectropolarimetric survey, and observed more than two third of the about 150 white dwarfs that lie within 20 pc from the Sun.
After combining our new data with previous literature data, we have checked for magnetic field almost the entire population of white dwarfs of the local 20 pc volume, with a sensitivity that ranges from better than 1 kG for most of the stars of spectral type DA, to about 1 MG for the featureless white dwarfs of spectral type DC.
In this talk I will present our survey and show the correlations that we have found between the frequency of the occurrence of magnetic field and other stellar features.
I will finally discuss the implications of our findings in relation to some of the proposals that have been put forward to explain the origin and evolution of magnetic fields in degenerate stars.


Local contact:

Alberto Cellino

artist’s impression shows the magnetar

This artist’s impression shows the magnetar in the very rich and young star cluster Westerlund 1. This remarkable cluster contains hundreds of very massive stars, some shining with a brilliance of almost one million suns. European astronomers have for the first time demonstrated that this magnetar — an unusual type of neutron star with an extremely strong magnetic field — was formed from a star with at least 40 times as much mass as the Sun. The result presents great challenges to current theories of how stars evolve, as a star as massive as this was expected to become a black hole, not a magnetar.