Analysis of hydrogen-rich magnetic white dwarfs detected in the Sloan Digital Sky Survey

TL;DR

This study models the magnetic field structures of hydrogen-rich white dwarfs from SDSS, revealing diverse magnetic geometries and suggesting different progenitor or evolutionary scenarios than previously thought.

Contribution

It provides a comprehensive analysis of magnetic field geometries in white dwarfs using advanced spectral modeling and a large SDSS sample, including newly discovered objects.

Findings

Magnetic fields range from ~1 to 900 MG.

Approximately 50% of objects show non-centered dipole fields.

The observed magnetic properties challenge the sufficiency of Ap/Bp progenitors.

Abstract

We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS. We have calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs with magnetic field strengths between 1 MG and 1200 MG for different angles, and for effective temperatures between 7000 K and 50000 K. We used a least-squares minimization scheme with an evolutionary algorithm in order to find the magnetic field geometry best fitting the observed data. We used simple centered dipoles or dipoles which were shifted along the dipole axis to model the coadded SDSS fiber spectrum of each object. We have analysed the spectra of all known magnetic DAs from the SDSS (97 previously published plus 44 newly discovered) and also investigated the statistical properties of magnetic field geometries of this sample. The total number of known magnetic white dwarfs already more than tripled by the SDSS and more objects are expected from a more systematic search. The magnetic fields span a range between ~1 and 900 MG. Our results further support the claim that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and either another source of progenitor types or binary evolution is needed. Moreover clear indications for non-centered dipoles exist in about ~50% of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars.