News Excerpt
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.

Pre-Connect
•    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.

Highlights
    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).