Abstract:
The main objective of this paper is to explore abundances of fluorine
in hot
Extreme Helium Stars (EHes). Overabundance of fluorine is a chara
cteristic
feature for cool EHes and R Coronae Borealis (RCB) stars and fur
ther enforces
their close connection. For hot EHes this relationship with the cooler
EHes,
based on their fluorine abundance is unexplored. We present in this p
aper the first
abundance estimates of fluorine determined from singly ionised fluor
ine lines (F
ii
)
for 10 hot EHe stars from optical spectra. Fluorine abundances w
ere determined
using the F
ii
lines in two windows centered at 3505
̊
A and 3850
̊
A. 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 sign
ificant low
upper limit for the F abundance. These fluorine abundances are com
pared with
the other elemental abundances observed in these stars which pr
ovide an idea
about the formation and evolution of these stars. The trends of fl
uorine with
C, O, and Ne show that significant helium burning after a CO-He white d
warf
merger can account for a majority of the observed abundances.
Predictions
from simulations of white dwarf mergers are discussed in light of the o
bserved
abundances.
