  Dynamically active planetary systems orbit a significant fraction of white
dwarf stars. These stars often exhibit surface metals accreted from debris
disks, which are detected through infrared excess or transiting structures.
However, the full journey of a planetesimal from star-grazing orbit to final
dissolution in the host star is poorly understood. Here, we report the
discovery that the cool metal polluted star WD0816-310 has cannibalized heavy
elements from a planetary body similar in size to Vesta, and where accretion
and horizontal mixing processes have clearly been controlled by the stellar
magnetic field. Our observations unveil periodic and synchronized variations in
metal line strength and magnetic field intensity, implying a correlation
between the local surface density of metals and the magnetic field structure.
Specifically, the data point to a likely persistent concentration of metals
near a magnetic pole. These findings demonstrate that magnetic fields may play
a fundamental role in the final stages of exoplanetary bodies that are recycled
into their white dwarf hosts.
