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(The Gist of PIB) Extreme helium star

[July-2020]

A study by the 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.
Since there are no known conditions where stars devoid of hydrogen can be formed from molecular clouds, it is theorized that they are the product of the mergers of helium-core and carbon-oxygen core white dwarfs.

Extreme helium stars form a sub-group within the broader category of hydrogen-deficient stars.
The latter includes cool carbon stars like R Coronae Borealis variables, helium-rich spectral class O or B stars, population I Wolf–Rayet stars, AM CVn stars, white dwarfs of spectral type WC, and transition stars like PG 1159.