The depletion of carbon by extra mixing in metal-poor giants

Abstract

There is an apparent dichotomy between the metal-poor ([Fe/H] <= -2) yet\ncarbon-normal giants and their carbon-rich counterparts. The former undergo\nsignificant depletion of carbon on the red giant branch after they have\nundergone first dredge-up, whereas the latter do not appear to experience\nsignificant depletion. We investigate this in the context that the extra mixing\noccurs via the thermohaline instability that arises due to the burning of\nhelium-3. We present the evolution of [C/Fe], [N/Fe] and carbon-12/carbon-13\nfor three models: a carbon-normal metal-poor star, and two stars that have\naccreted material from a 1.5 solar mass AGB companion, one having received 0.01\nsolar masses of material and the other having received 0.1 solar masses. We\nfind the behaviour of the carbon-normal metal-poor stars is well reproduced by\nthis mechanism. In addition, our models also show that the efficiency of\ncarbon-depletion is significantly reduced in carbon-rich stars. This\nextra-mixing mechanism is able to reproduce the observed properties of both\ncarbon-normal and carbon-rich stars.\n