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

Abstract

AbstractFor the first time, a direct non‐local thermodynamic equilibrium (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 carbon, nitrogen, oxygen (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.