We examine the effects of thermohaline mixing on the composition of the envelopes of low-metallicity asymptotic giant branch ( AGB ) stars .
We have evolved models of 1 , 1.5 and 2 M _ { \mathrm { \odot } } from the pre-main sequence to the end of the thermally pulsing asymptotic giant branch with thermohaline mixing applied throughout the simulations .
In agreement with other authors , we find that thermohaline mixing substantially reduces the abundance of ^ { 3 } \mathrm { He } on the upper part of the red giant branch in our lowest mass model .
However , the small amount of ^ { 3 } \mathrm { He } that remains is enough to drive thermohaline mixing on the AGB .
We find that thermohaline mixing is most efficient in the early thermal pulses and its efficiency drops from pulse to pulse .
Nitrogen is not substantially affected by the process , but we do see substantial changes in ^ { 13 } \mathrm { C } .
The ^ { 12 } \mathrm { C } / ^ { 13 } \mathrm { C } ratio is substantially lowered during the early thermal pulses but the efficacy of the process is seen to diminish rapidly .
As the process stops after a few pulses , the ^ { 12 } \mathrm { C } / ^ { 13 } \mathrm { C } ratio is still able to reach values of 10 ^ { 3 } -10 ^ { 4 } , which is inconsistent with the values measured in carbon-enhanced metal-poor stars .
We also note a surprising increase in the ^ { 7 } \mathrm { Li } abundance , with \log _ { 10 } \epsilon \mathrm { ( } ^ { 7 } \mathrm { Li } ) reaching values of over 2.5 in the 1.5 M _ { \mathrm { \odot } } model .
It is thus possible to get stars which are both C- and Li-rich at the same time .
We compare our models to measurements of carbon and lithium in carbon-enhanced metal-poor stars which have not yet reached the giant branch .
These models can simultaneously reproduced the observed C and Li abundances of carbon-enhanced metal-poor turn-off stars that are Li-rich , but the observed nitrogen abundances still can not be matched .