From their paper (link in my other message): "(White dwarfs) WDs are the remnant cores left behind when a star ejects its hydrogen-rich outer layers after the

From their paper (link in my other message): "(White dwarfs) WDs are the remnant cores left behind when a star ejects its hydrogen-rich outer layers after the red giant phase. These remnant cores are ~0.5 M (solar masses) and about the same radius as Earth, are no longer powered by fusion, and slowly cool over time. Because of their high densities, and thus strong gravitational fields, elements heavier than helium rapidly sink below their surfaces, becoming unobservable. Nonetheless, spectroscopic studies show that the atmospheres of up to half of WDs with effective temperatures <25,000 K are “polluted” by elements heavier than He (3–5). The source of these heavy elements is exogenous, coming from accretion of debris from rocky bodies that previously orbited the WDs (6–9). We exploit this pollution to measure the elemental constituents of extrasolar rocky bodies." So the answer is "the later."