Detection of Fluorine in Hot Extreme Helium Stars
A study led by Indian Institute of Astrophysics (IIA) which detected the presence of singly ionised ﬂuorine for the first time in the atmospheres of hot Extreme Helium Stars makes a strong case that the main formation of these objects involves a merger of a carbon-oxygen (CO) and a Helium (He) white dwarf.
• An extreme helium star or EHe is a low-mass supergiant that is almost devoid of hydrogen, the most common chemical element of the universe.
• There are 21 of them detected so far in our galaxy.
• The origin and evolution of these Hydrogen deficient objects have been shrouded in mystery. Their severe chemical peculiarities challenge the theory of well-accepted stellar evolution as the observed chemical composition of these stars do not match with that predicted for low mass evolved stars.
The research, which showed ﬂuorine abundances, determined from singly ionized fluorine (F II) lines suggest a very high enrichment of ﬂuorine, about a factor of 100 to 10000 times higher than normal stars.
Clues to evolution of extreme helium stars require accurate determinations of their chemical composition, and the peculiarities, if any, become very important.
Fluorine plays a very crucial role in this regard to determine the actual evolutionary sequence of these hydrogen deﬁcient objects.
The scientists explored the relationship of hot EHes (EHes having effective temperature ≥ 14000K), with the cooler EHes, based on their ﬂuorine abundance and spotted it in the former, thus establishing an evolutionary connection across a wide range of effective temperature.
The detection of enhanced ﬂuorine abundances in the atmospheres of hot EHes solves a decade-old mystery about their formation. It ﬁrmly places hot EHes in an evolutionary sequence with cool EHes and other hydrogen-deficient stars and zeros in the evolutionary scenario, which involves the merger of two double degenerate white dwarfs (WDs).