  Upon formation, degenerate He core white dwarfs are surrounded by a radiative
H-rich layer primarily supported by ideal gas pressure. In this Letter, we
examine the effect of this H-rich layer on mass transfer in He+C/O double white
dwarf binaries that will eventually merge and possibly yield a Type Ia
supernova (SN Ia) in the double detonation scenario. Because its thermal
profile and equation of state differ from the underlying He core, the H-rich
layer is transferred stably onto the C/O white dwarf prior to the He core's
tidal disruption. We find that this material is ejected from the binary system
and sweeps up the surrounding interstellar medium hundreds to thousands of
years before the SN Ia. The close match between the resulting circumstellar
medium profiles and values inferred from recent observations of circumstellar
absorption in SNe Ia gives further credence to the resurgent double detonation
scenario.
