  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] $\sim -3.0$. 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 offfsets 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
coeffcient. Our stellar evolution models utilise 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.
