Magnetic field tomography of hydrogen-rich white dwarfs

Abstract

White dwarfs are among the oldest objects in the Universe, and hold the key to understand past, present and future of nearly 95 % of all stars in the Milky Way. About 10 % of those are born surrounded by gigantic magnets with eld intensities vastly ranging between low and high limits, far larger than even the elds produced in the most so sticated devices at laboratories on Earth. Most part of the mysteries concerning these stars remain unsolved. In this work, I used a numerical tting routing based on 2 minimization with pre-calculated synthetic spectra of white dwarfs with hydrogen-rich atmospheres for nding the best solutions to the geometrical con guration of their magnetic eld with basically three tted parameters: magnetic eld strength (Bp), dipole o -set in the z direction (zo ) and inclination of the dipole axis. This attempt was made for around 156 spectra from the data releases of the Sloan Digital Sky Survey (SDSS) with signal-to-noise ratio between 10-90 through a computational technique for calculating the magnetic distribution over a visible stellar hemisphere by superposing elds with a basic input geometry, represented by spherical harmonics, and with di erent intensities along the grided stellar surface. With the aid of a statistical hypothesis test, I analysed the solutions with distinct input geometries so rst-order approximations to the magnetic topologies could be found.