Non-LTE Nitrogen Abundances in the Sun and Selected A–F-Type Stars

Abstract

A new N I model atom has been constructed using the energy levels known from laboratory measurements and predicted in N I atomic structure calculations and up-to-date atomic data for calculating the radiative and collisional transition rates. The solar abundance \(\log\varepsilon_{\odot,\textrm{N}}\text{(1D NLTE)}=7.92\pm 0.03\) has been determined from N I lines by the synthetic spectrum method with a classical one-dimensional (1D, MARCS) solar model atmosphere and by taking into account the departures from local thermodynamic equilibrium (non-LTE effects). By applying the 3D corrections of Amarsi et al. (2020), we have obtained \(\log\varepsilon_{\odot,\textrm{N}}(\textrm{NLTE}+\textrm{3D})=7.88\pm 0.03\) for the Sun. Based on high-resolution spectra, we have derived the non-LTE nitrogen abundances for 11 unevolved A–F-type stars with reliably determined atmospheric parameters. Non-LTE leads to a strengthening of N I lines, and the non-LTE effects grow with increasing effective temperature. For each of the stars the departures from LTE lead to a decrease in the root-mean-square (rms) abundance error compared to the LTE case. For superficially normal A stars non-LTE removes the enhancements relative to the solar nitrogen abundance obtained in an LTE analysis. The \(\lambda\) Boo-type star HD 172167 (Vega) also has a nearly solar nitrogen abundance. Four Am stars exhibit a scatter, from a nitrogen underabundance with \(\textrm{[N/H]}=-0.44\) to a nitrogen overabundance with \(\textrm{[N/H]}=0.39\). The nitrogen abundances for the Sun and superficially normal A stars are consistent within the error limits with the nitrogen abundance in the interstellar gas and early B-type stars.