Red clump stars of the Milky Way - laboratories of extra-mixing

TL;DR

This study analyzes atmospheric parameters and isotope ratios in 28 Galactic clump stars to understand their chemical composition and test models of stellar extra-mixing processes.

Contribution

It provides detailed measurements of C, N, O abundances and 12C/13C ratios in clump stars, comparing them to stellar evolution models of extra-mixing.

Findings

Carbon is depleted by about 0.2 dex compared to the Sun.

Nitrogen is enhanced by about 0.2 dex.

The observed 12C/13C ratios support the thermohaline mixing model over cool bottom processing.

Abstract

In this work we present the main atmospheric parameters, carbon, nitrogen and oxygen abundances, and 12C/13C ratios determined in a sample of 28 Galactic clump stars. Abundances of carbon were studied using the C2 band at 5086.2 {\AA}. The wavelength interval 7980-8130 {\AA} with strong CN features was analysed in order to determine nitrogen abundances and 12C/13C isotope ratios. The oxygen abundances were determined from the [O I] line at 6300 {\AA}. The mean abundances of C, N and O abundances in the investigated clump stars support our previous estimations that, compared to the Sun and dwarf stars of the Galactic disc, carbon is depleted by about 0.2 dex, nitrogen is enhanced by 0.2 dex and oxygen is close to abundances in dwarfs. The 12C/13C and C/N ratios for galactic red clump stars analysed were compared to the evolutionary models of extra-mixing. The steeper drop of 12C/13C ratio in the model of thermohaline mixing by Charbonnel & Lagarde better reflects the observational data at low stellar masses than the more shallow model of cool bottom processing by Boothroyd & Sackmann. For stars of about 2 Msun masses a modelling of rotationally induced mixing should be considered with rotation of about 250 km s-1 at the time when a star was at the hydrogen-core-burning stage.