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The origin of trace hydrogen in white dwarfs (WDs) with He-dominatedatmospheres is a long-standing problem, one that cannot satisfactorily beexplained by the historically-favoured hypothesis of accretion from theinterstellar medium. Here we explore the possibility that the gradual accretionof exo-Oort cloud comets, which are a rich source of H, contributes to theapparent increase of trace H with WD cooling age. We determine how oftenremnant exo-Oort clouds, freshly excited from post-main-sequence stellar massloss, dynamically inject comets inside the WD's Roche radius. We improve uponprevious studies by considering a representative range of single WD masses(0.52-1.00 Solar masses) and incorporating different cloud architectures, giantbranch stellar mass loss, stellar flybys, Galactic tides and a realistic escapeellipsoid in self-consistent numerical simulations that integrate beyond 8 Gyrages of WD cooling. We find that about 10^{-5} of the material in an exo-Oortcloud is typically amassed onto the WD, and that the H deposits accumulate evenas the cloud dissipates. This accumulation may account for the relatively largeamount of trace H, 10^{22}-10^{25} g, that is determined frequently among WDswith cooling ages >= 1 Gyr. Our results also reaffirm the notion that exo-Oortcloud comets are not the primary agents of the metal budgets observed inpolluted WD atmospheres.

Hydrogen delivery onto white dwarfs from remnant exo-Oort cloud comets