These specific stars are the empirical anchor of the white-dwarf bound: each is simultaneously (i) massive enough to stop cosmic-ray-produced neutral black holes, (ii) weakly magnetized enough that cosmic rays reach its surface unscreened, and (iii) old enough that catalyzed decay on <~100 Myr timescales would already have destroyed it. Sirius B (1.0 Msun, ~120 Myr) plays the same role for the interstellar-medium production channel, which is magnetic-field-independent. Quoted mass/age/field values carry ~10% column-density systematics (temperature, composition).