Discovery of Magnetically Guided Metal Accretion Onto a Polluted White Dwarf

Dynamically active planetary systems orbit a significant fraction of whitedwarf stars. These stars often exhibit surface metals accreted from debrisdisks, which are detected through infrared excess or transiting structures.However, the full journey of a planetesimal from star-grazing orbit to finaldissolution in the host star is poorly understood. Here, we report thediscovery that the cool metal polluted star WD0816-310 has cannibalized heavyelements from a planetary body similar in size to Vesta, and where accretionand horizontal mixing processes have clearly been controlled by the stellarmagnetic field. Our observations unveil periodic and synchronized variations inmetal line strength and magnetic field intensity, implying a correlationbetween the local surface density of metals and the magnetic field structure.Specifically, the data point to a likely persistent concentration of metalsnear a magnetic pole. These findings demonstrate that magnetic fields may playa fundamental role in the final stages of exoplanetary bodies that are recycledinto their white dwarf hosts.