A DZ white dwarf with a 30 MG magnetic field

TL;DR

This paper reports the discovery and analysis of a magnetic DZ white dwarf with a surprisingly low magnetic field of 30 MG, using advanced quantum-chemical calculations to identify spectral lines from metals in a complex spectrum.

Contribution

It introduces the application of finite-field coupled-cluster methods to identify metal lines in a strongly magnetic white dwarf spectrum at 30 MG, a novel approach for such high-field systems.

Findings

Identified metal lines from Na, Mg, and Ca in the spectrum.

Determined the magnetic field strength to be 30 MG.

Modeled the magnetic field as an offset dipole.

Abstract

Magnetic white dwarfs with field strengths below 10 MG are easy to recognise since the Zeeman splitting of spectral lines appears proportional to the magnetic field strength. For fields $\geq 100$ MG, however, transition wavelengths become chaotic, requiring quantum-chemical predictions of wavelengths and oscillator strengths with a non-perturbative treatment of the magnetic field. While highly accurate calculations have previously been performed for hydrogen and helium, the variational techniques employed become computationally intractable for systems with more than three to four electrons. Modern computational techniques, such as finite-field coupled-cluster theory, allow the calculation of many-electron systems in arbitrarily strong magnetic fields. Because around 25 percent of white dwarfs have metal lines in their spectra, and some of those are also magnetic, the possibility arises for some metals to be observed in very strong magnetic fields, resulting in unrecognisable spectra. We have identified SDSSJ114333.48+661531.83 as a magnetic DZ white dwarf, with a spectrum exhibiting many unusually shaped lines at unknown wavelengths. Using atomic data calculated from computational finite-field coupled-cluster methods, we have identified some of these lines arising from Na, Mg, and Ca. Surprisingly, we find a relatively low field strength of 30 MG, where the large number of overlapping lines from different elements make the spectrum challenging to interpret at a much lower field strength than for DAs and DBs. Finally we model the field structure of SDSSJ1143+6615 finding the data are consistent with an offset dipole.