Double white dwarfs and the connections between hot subdwarfs, R CrB stars and extreme helium stars

Simon Jeffery
The proposal that the merger of two white dwarfs would form a major channel for the formation of hot subdwarfs has been current for over two decades. Naturally, such a merger would result in a single star, rather than a binary. The
consequences for frequency, mass distribution, rotation velocity, surface composition, and so on are less obvious.

This seminar will review the general picture of double white dwarf mergers, including links between white dwarf mergers and various classes of evolved star. It will present recent work on:
a) links between main-sequence binaries and double white dwarf
merger progenitors,
b) stellar evolution calculations following the merger of two white
dwarfs, and
c) the correlation between predicted and observed surface abundance
of post white-dwarf mergers.

It will conclude with a summary of current challenges and
unsolved problems.