The Century-old R-Star Mystery

The R stars are carbon-rich K-type giants. For more than a century their origin has remained a mystery. The warmest of them, the early-R stars, have luminosities similar to core helium burning stars yet canonical stellar evolution theory suggests they should not be carbon rich. The early-R stars are chemically peculiar, being enhanced in ¹²C, ¹³C and ¹⁴N but not in s-process elements, and are all single stars. Binary mergers have been suggested as the evolutionary channel which leads to the early-R stars: we test this scenario with a state-of-the-art binary population synthesis.