NLTE CNO abundances in a sample of nine field RR lyr type stars

TL;DR

This study presents the first NLTE analysis of carbon, nitrogen, and oxygen in nine RR Lyrae stars, revealing increased nitrogen due to dredge-up and mixing processes, with results varying with metallicity.

Contribution

It provides the first direct NLTE abundances of CNO elements in RR Lyrae stars, highlighting the effects of dredge-up and thermohaline mixing on surface compositions.

Findings

Nitrogen abundance increases with decreasing metallicity.

C/N ratio decreases as metallicity decreases.

Oxygen abundance correlates with metallicity similarly to Population II stars.

Abstract

For the first time, a direct NLTE analysis of carbon and nitrogen lines in the spectra of nine RR Lyrae stars was carried out. We have determined the abundances of these elements together with oxygen, and have shown that the nitrogen content is increased in metallicity deficient program stars. We conclude that this is a sign of the first dredge up, which occurred at the previous stage of the red giant branch, and brought material processed in an incomplete CNO cycle to the surface of the star. This effect is significantly enhanced by thermohaline (extra-) mixing, which is more effective for metal-poor RR Lyrae stars. This is clearly seen in the plot showing that C/N ration in our sample of stars gradually decreases as metallicity decreases from about --0.2 to --2. Oxygen abundance depends on metallicity in a similar way to what we see in the Population II stars.