2020ApJ...891...40B

The main objective of this paper is to explore abundances of fluorine in hot extreme helium stars (EHes). Overabundance of fluorine is a characteristic feature for cool EHes and R Coronae Borealis stars and further enforces their close connection. For hot EHes this relationship with the cooler EHes, based on their fluorine abundance is unexplored. We present in this paper the first abundance estimates of fluorine determined from singly ionized fluorine lines (F II) for 10 hot EHe stars from optical spectra. Fluorine abundances were determined using the F II lines in two windows centered at 3505 Å and 3850 Å. Six of the 10 stars show significant enhancement of fluorine similar to the cool EHes. Two carbon-poor hot EHes show no signature of fluorine and have a significant low upper limit for the F abundance. These fluorine abundances are compared with the other elemental abundances observed in these stars, which provide an idea about the formation and evolution of these stars. The trends of fluorine with C, O, and Ne show that significant helium burning after a CO-He white dwarf merger can account for a majority of the observed abundances. Predictions from simulations of white dwarf mergers are discussed in light of the observed abundances.