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

TL;DR

This paper investigates how extra mixing via thermohaline instability causes carbon depletion in metal-poor giants, explaining differences between carbon-normal and carbon-rich stars through stellar evolution models.

Contribution

It introduces models that incorporate thermohaline mixing to explain observed carbon depletion patterns in metal-poor giants, including accretion effects from AGB companions.

Findings

Models reproduce observed carbon depletion in normal stars.

Carbon-rich stars show reduced depletion efficiency.

Thermohaline mixing explains the dichotomy in carbon abundances.

Abstract

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