Thermohaline Mixing in Extremely Metal-poor Stars

Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well studied, they make up less than half of all EMP stars with [Fe/H] ∼ -3.0. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offsets are derived) is very important; however, the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coefficient. Our stellar evolution models utilize a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] = -3 and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.