Non-LTE abundances of nitrogen in the Sun and reference A-F type stars

TL;DR

This study develops a new nitrogen model atom and determines non-LTE nitrogen abundances in the Sun and A-F type stars, revealing the importance of non-LTE effects for accurate stellar nitrogen measurements.

Contribution

The paper introduces a new nitrogen model atom and applies non-LTE and 3D corrections to improve nitrogen abundance determinations in stars.

Findings

Solar nitrogen abundance is 7.92±0.03 in 1D NLTE and 7.88±0.03 with 3D corrections.

Non-LTE effects strengthen N I lines and reduce abundance errors.

Nitrogen abundances in stars are consistent with interstellar and early B-type star values.

Abstract

A new model atom of nitrogen was developed using the energy levels of N I from laboratory measurements and also predicted in atomic structure calculations and the most up-to-date atomic data for computing radiative and collisional rates of the transitions. Solar abundance $\log\varepsilon_{\odot,N}$(1D NLTE) = 7.92$\pm$0.03 was determined from lines of N I by applying the synthetic spectrum method with the plane-parallel (1D) MARCS model atmosphere and taking into account the departures from local thermodynamic equilibrium (non-LTE = NLTE effects). Having implemented the 3D-corrections of Amarsi et al. (2020), we obtained $\log\varepsilon_{\odot,N}$(NLTE+3D) = 7.88$\pm$0.03 for the Sun. Based on high spectral resolution spectra, the NLTE abundances of nitrogen were derived for 11 unevolved A-F type stars with reliable atmospheric parameters. Non-LTE leads to strengthened N I lines, and the non-LTE effects grow with increasing effective temperature. For each star, non-LTE leads to smaller 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 the LTE case. A $\lambda$ Boo-type star HD 172167 (Vega) also has close-to-solar N abundance. The four Am stars reveal a scatter of the N abundances, from [N/H] = -0.44 to [N/H] = 0.39. The N abundances of the Sun and superficially normal A stars are consistent within 0.09 dex with the nitrogen abundance of the interstellar gas and the early B-type stars.