Monitoring and modelling of white dwarfs with extremely weak magnetic fields

Abstract

Magnetic fields are detected in a few percent of white dwarfs. The number of such magnetic white dwarfs known is now some hundreds. Fields range in strength from a few kG to several hundred MG. Almost all the known magnetic white dwarfs have a mean field modulus ≥ 1 MG. We are trying to fill a major gap in observational knowledge at the low field limit (≤ 200 kG) using circular spectro-polarimetry. In this paper we report the discovery and monitoring of strong, periodic magnetic variability in two previously discovered "super-weak field" magnetic white dwarfs, WD 2047+372 and WD 2359-434. WD 2047+372 has a mean longitudinal field that reverses between about −12 and +15 kG, with a period of 0.243 d, while its mean field modulus appears nearly constant at 60 kG. The observations can be intepreted in terms of a dipolar field tilted with respect to the stellar rotation axis. WD 2359-434 always shows a weak positive longitudinal field with values between about 0 and +12 kG, varying only weakly with stellar rotation, while the mean field modulus varies between about 50 and 100 kG. The rotation period is found to be 0.112 d using the variable shape of the Hα line core, consistent with available photometry. The field of this star appears to be much more complex than a dipole, and is probably not axisymmetric. Available photometry shows that WD 2359-434 is a light variable with an amplitude of only 0.005 mag; our own photometry shows that if WD 2047+372 is photometrically variable, the amplitude is below about 0.01 mag. These are the first models for magnetic white dwarfs with fields below about 100 kG based on magnetic measurements through the full stellar rotation. They reveal two very different magnetic surface configurations, and that, contrary to simple ohmic decay theory, WD 2359-434 has a much more complex surface field than the much younger WD 2047+372.

Magnetic fields in isolated and interacting white dwarfs

“…WD1953-011 Maxted et al, 2000) MWDs. Furthermore, there are examples of both high field (Euchner et al, 2005), and low field (Maxted et al, 2000;Landstreet et al, 2017) MWDs that appear to exhibit a similar level of field complexity. The complexity of the field could be linked to its origin.…”

“…It has been known for some time that MWDs tend to exhibit complex field structures and that off-centred dipole models generally produce a better fit to spectroscopic observations than centred dipole models (see Wickramasinghe and Ferrario, 2000). Structures with dominant quadrupolar components or evidence for signif-3 icant contributions from higher order multipoles have also emerged from detailed modelling of isolated and accreting MWDs (Maxted et al, 2000;Euchner et al, 2005Euchner et al, , 2006Beuermann et al, 2007;Landstreet et al, 2017), although not all MWDs require complex field structures to understand their spectra. This suggests that there may be a complex interplay between fossil and dynamo generated fields during stellar evolution and/or stellar merging events which are still poorly understood phenomena despite some extensive studies conducted over many decades (Tout et al, 2004;Brun et al, 2005;Featherstone et al, 2009;Potter and Tout, 2010;Quentin and Tout, 2018).…”

Evidence of enhanced magnetism in cool, polluted white dwarfs

“…The hot hydrogen-rich white dwarf EUVE J0317-85.5 also appears to have an underlying field of ∼ 185 MG with a high field spot of ∼ 425 MG (Vennes et al 2003). More recently, Landstreet et al (2017) reported on the field topology of two magnetic white dwarfs with fields less than 200 kG, with one star (WD 2047+372) having a field topology corresponding to a simple dipole and the other (WD 2359−434) having a more complex topology with an underlying dipolar field and an overlaying quadrupolar field.…”

Discovery of kilogauss magnetic fields on the nearby white dwarfs WD 1105–340 and WD 2150+591

“…In part this difference between WD 1105-340 and WD 2150+591 may be due to the much stronger field present on WD 2150+591, which leads to significant variation in the wavelengths of the σ components due to even moderate variations in the local value of |B|. However, this difference between the two stars in the width of the σ components relative to the π component is similar to the difference noted between WD 2047+372 (with |B| = 60 kG and very well-defined σ components) and WD 2359-434 (with |B| ∼ 50 − 100 kG and very diffuse σ components) (Landstreet et al 2017). For that pair of stars, the different appearance of the σ components was apparently a symptom of rather different global magnetic field structures, with that of the young star WD 2047+372 being "simpler" than that of the considerably older WD 2359-434.…”