Flash Mixing on the White Dwarf Cooling Curve: Far Ultraviolet Spectroscopic Explorer Observations of Three He-rich sdB Stars^*

We present Far-Ultraviolet Spectroscopic Explorer (FUSE) spectra of three He-rich sdB stars. Two of these stars, PG 1544+488 and JL 87, reveal extremely strong C III lines at 977 and 1176 Å, while the carbon lines are quite weak in the third star, LB 1766. We have analyzed the FUSE data using TLUSTY non-LTE line-blanketed model atmospheres and find that PG 1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL 87 shows a similar surface enrichment of carbon and nitrogen, but some significant fraction of hydrogen still remains in its atmosphere. Finally, LB 1766 has a surface composition devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions are the progeny of stars which underwent a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the hydrogen envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical (i.e., unmixed) EHB stars. Two types of flash mixing are possible: "deep" and "shallow," depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG 1544+488 and JL 87 are examples of "deep" and "shallow" flash mixing, respectively. Flash mixing may therefore represent a new evolutionary channel for producing the hottest EHB stars. However, flash mixing cannot explain the abundance pattern in LB 1766, which remains a challenge to current evolutionary models.