Browsing by Author "Yu, Sicheng"

Aims. In this thesis, we review the history in studying the evolution path of magnetic white dwarfs and explain the longstanding questions and debates over their magnetic origin. We intend to find magnetic white dwarfs in various forms (isolated or with companions) from the spectral database of LAMOST DR7 to complement the current magnetic white dwarf catalogue. We then move on to compare our results with some commonly accepted scenarios regarding their magnetic origin. Methods. Low-resolution spectra is the main source in this study, we intend to locate signs of Zeeman-splitting in spectra of isolated white dwarfs, measure separations of substructures due to Zeeman-splitting and estimate magnetic field strength. Magnetic white dwarfs in binary or multiple systems are found by seeking signs of cyclotron radiation due to mass transfer and particle movement in magnetic fields. Photometric survey from Transiting Exoplanet Survey Satellite (TESS) was used to fold periodic light curves for targets of interest, in order to further study the nature of our candidates, especially the ones that are believed to coexist with companions. Results. We identified 31 isolated magnetic white dwarfs in the LAMOST DR7 database by the discovery of Zeeman-splitting components. Their estimated magnetic field strength ranging from below 1 mega gauss (MG) to a scale ten times larger. Two Polars/Intermediate Polars were found with both Zeeman-splitting components and broad Balmer emissions usually seen in cataclysmic variables. We also discovered two candidates of detached magnetic binaries. These systems are believed to be the progenitors of polars or intermediate polars. Despite their rarity, these candidates serve as vital hints in clarifying the ongoing debates concerning the magnetic field origins in white dwarfs.