Analysis of a sample of volume-limited white dwarfs has provided the best evidence yet of how the frequency of magnetism in these stars correlates with age.
This could help explain the origin and evolution of magnetic fields in these dying stars. More than 90% of the stars in our galaxy end their lives as white dwarfs. Although many have a magnetic field, it is still unknown when it appears on the surface, if it evolves during the cooling phase of the and, above all, what are the mechanisms that generate it.
Astronomical observations are frequently subject to strong biases. Because white dwarfs are dying stars, they get colder and therefore weaker and weaker over time. As a consequence, observations tend to favor the study of the brightest, which are hot and young. There is also a more subtle and contradictory effect. Due to their degenerate state, the most massive white dwarfs are smaller than the lower mass ones. Because the smaller white dwarfs are also fainter, the observations tend to favor the less massive stars as well.
In summary, observations of targets selected according to their brightness (for example, observing all WDs brighter than a certain magnitude) tend to focus on younger, less massive stars, totally neglecting older ones.
Another problem is that most WD observations are made with spectroscopic techniques that are sensitive only to the strongest magnetic fields, thus failing to identify a substantial fraction of magnetic white dwarfs. The sensitivity of spectropolarimetry to magnetic fields can be more than two orders of magnitude better than spectroscopy. Spectropolarimetry has shown that weak fields, which escape detection by spectroscopic techniques, are quite common in white dwarfs.
To conduct a full spectropolarimetric study, astronomers at the Armagh Observatory and the University of Western Ontario selected all the white dwarfs from the Gaia catalog in a volume 20 parsecs from the Sun, according to a statement from the Isaac Newton Group of Telescopes.
About two-thirds of this sample, or about 100, had not been observed before and therefore no data were available in the literature. Consequently, the team observed them using the ISIS spectrograph and polarimeter on the William Herschel Telescope (WHT), along with similar instruments on other telescopes.
They discovered that magnetic fields are rare at the beginning of a white dwarf’s life, when the star no longer produces energy within it and begins its cooling phase. Therefore, a magnetic field does not appear to be a characteristic of a white dwarf from its “birth”. Most often, it is generated or brought to the stellar surface during the cooling phase.
They also found that the magnetic fields of the white dwarfs show no obvious signs of ohmic decay, again an indication that these fields are generated during the cooling phase, or at least continue to emerge on the stellar surface as the white dwarf ages.
This image is totally different from what is observed, for example, in the upper main sequence Ap and Bp magnetic stars, where it is found that not only are magnetic fields present as soon as the star reaches the zero age main sequence, but also that the intensity of the field decreases rapidly with time. Therefore, the magnetism in white dwarfs appears to be a totally different phenomenon from the magnetism of the stars Ap and Bp.
Not only does the frequency of the magnetic field increase with the age of the white dwarf, but it is known that the frequency is correlated with the stellar mass and that the fields appear more frequently after the carbon-oxygen core of the star has started to crystallize. A dynamo mechanism can explain the weakest fields among those observed in a white dwarf, and recent work suggests that the same mechanism might be capable of producing stronger fields than originally predicted.
For comparison, the strength of the Earth’s magnetic field, produced by a dynamo mechanism, is about one Gauss. A dynamo mechanism can explain fields of up to 0.1 million Gauss in force, but fields of up to several hundred million Gauss have been observed in white dwarfs. Also, a dynamo mechanism needs rapid rotation, but this is not generally seen in white dwarfs. More theoretical and observational research is needed to distance this situation.