Merged Red-Giant Stars

Red giants are the biggest and brightest stars, and make many common chemical elements, like carbon and nitrogen.
Because of their great size, and because most stars are in binary systems, interaction with a companion star is very likely.
This proceeds through common-envelope evolution and forms exotic binary phenomena like thermonuclear novae, type-Ia
supernovae and neutron-star and black-hole binaries which merge to make the recently-detected gravitational waves.
Unfortunately, common-envelope evolution is the most uncertain of all stellar-evolution phases. Quantitative predictions of
rates and properties of stars that survive it are notoriously unreliable. This project will reverse the problem. Many binary
systems do not survive common-envelope evolution: instead they merge to form single, peculiar giants, the ?red
stragglers?. Detailed models of these giants will be constructed to predict stellar-structure, rotation and chemistry, and
then be compared to the latest observations to understand the mysterious, but hugely important, common-envelope
mergers.