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

摘要

Context. A large number of magnetic white dwarfs discovered in the SDSS have so far only been analyzed by visual comparison ofthe observations with relatively simple models of the radiation transport in a magnetised stellar atmosphere.Aims. We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS.Methods. We calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs (WDs) withmagnetic field strengths of between 1 MG and 1200 MG for different angles between the magnetic field vector and the line of sight,and for effective temperatures between 7000 K and 50 000 K. We used a least squares minimization scheme with an evolutionaryalgorithm to find the best-fit magnetic field geometry of the observed data. We used centered dipoles or dipoles that had been shiftedalong the dipole axis to model the coadded SDSS fiber spectrum of each object.Results. We analyzed the spectra of all known magnetic hydrogen-rich (DA) WDs from the SDSS (97 previously published, plus44 newly discovered) and also investigated the statistical properties of the magnetic field geometries of this sample.Conclusions. The total number of known magnetic white dwarfs has already been more than tripled by the SDSS and more objects areexpected after more systematic searches. The magnetic fields have strengths of between r1 and 900 MG. Our results further supportthe claims that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, andthat of either another source of progenitor types or binary evolution is needed. Clear indications of non-centered dipoles exist in about~50%, of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars.