A numerical code for the analysis of magnetic white dwarf spectra that includes field effects on the chemical equilibrium

Abstract

We present a new magnetic atmosphere model code for obtaining synthetic spectral fluxes of hydrogen-rich magnetic white dwarfs. To date, observed spectra have been analyzed with models that neglet the magnetic field effects on the atomic populations. In this work, we incorporate to state-of-art theory into the evaluation of numerical densities of atoms, free electrons and ions in local thermodynamical equilibrium under the action of a magnetic field. The energy distribution of atoms is rigorously evaluated for arbitrary field strength. This energy pattern includes from tightly bound states to metastable or truly bound, highly excited states embedded in the continuum, i.e., over the first Landau level. Finite nuclear mass effects and the coupling between the internal atomic structure and the motion of the atom across the magnetic field are also considered. Synthetic fluxes are generated with integrations of numerical solutions of polarized radiative transfer over the visible stellar disk using a spherical t-design method. The atmosphere code is tested with observations from the Sloan Digital Sky Survey for a group of known magnetic white dwarfs. Physical stellar parameters are obtained from least-squares fits to the observed energy distribution and compared with results of previous works. We show that the use of zerofield ionization equilibrium in spectral analysis can lead to underestimated effective temperatures for high magnetic white dwarfs.