NLTE C and O abundances in RR lyrae stars

Abstract

AbstractThe high excitation levels that form the O i and C i lines seen in RR Lyrae stars have long been known not to obey local thermodynamic equilibrium (LTE) statistics. We have observed 21 RR Lyrae and related stars with the Apache Point Observatory echelle spectrograph on the 3.5‐m telescope and, for the first time, have derived abundances using non‐local thermodynamic equilibrium (NLTE) analyses in conjunction with spectral synthesis. In our previous investigation, we determined carbon to be overabundant using an LTE analysis. We now find the carbon abundance to be underabundant relative to solar abundance for stars with [Fe/H] < 0.0 ([C/Fe] < 0 and by implication [C/H] < 0). Abundances derived for C using an NLTE analysis are consistent with both main‐sequence stars and red giants. This supports the predictions by He flash hydrodynamic simulations, which predict no convective mixing that might bring carbon produced in the He flash to the atmosphere. For oxygen, we find the dependence of [O/Fe] on [Fe/H] to be similar to other stars with [Fe/H] from 0.0 to −2.8.