Quasi-Periodic Variations of the Pulsation Frequencies of Three White Dwarfs

摘要

We report on variations of the normal mode frequencies of the pulsating DB white dwarfs EC 20058-5234 and KIC 8626021 and the pulsating DA white dwarf GD 66. We discover periodic and quasi-periodic variations of multiple normal mode frequencies that cannot be due to the presence of planetary companions. We note the possible signature of a planetary companion to EC 20058-5234 and now know that GD 66 cannot have a planet in a several AU orbit down to half a Jupiter mass. We also announce secular variations of the normal mode frequencies of all three stars that are inconsistent with cooling alone. Periodic variations in the amplitudes of many of these modes is also noted. We hypothesize that these frequency variations are caused by complex variations of the magnetic field strength and geometry, analogous to behavior observed in the Sun.