Phosphorus in cool stars of various metallicities: The non-local thermodynamic equilibrium consideration

TL;DR

This study investigates the non-local thermodynamic equilibrium (NLTE) effects on phosphorus abundance measurements in cool stars across various metallicities, developing an atomic model to refine abundance estimates and assess the robustness of LTE-based observations.

Contribution

The paper introduces a new atomic model for phosphorus in NLTE, providing correction grids that improve the accuracy of phosphorus abundance measurements in stars of different metallicities.

Findings

NLTE corrections are small but significant, e.g., -0.08 dex for the Sun.

NLTE effects do not alter the overall phosphorus abundance distribution.

The characteristic pattern of phosphorus-rich stars remains unchanged.

Abstract

The phosphorus abundance distribution in field stars as a function of metallicity reveals a complex pattern. The LTE data for [P/Fe] in the low-metallicity range are sparse and scattered around [P/Fe]~ 0 dex. Near [Fe/H]~ -2 dex, the relative abundance [P/Fe] increases and reaches a maximum value of around [Fe/H]~ -1 dex. In this domain, P-rich stars and (super)phosphorus-rich stars are observed; the [P/Fe] value can exceed 1 dex. Until now, no attempts have been made to study the NLTE effects on the ultraviolet and infrared phosphorus lines in spectra of cool stars to test the robustness of the observed LTE phosphorus abundance distribution. We developed an atomic model of P I that can be used to analyze phosphorus lines in the spectra of cool dwarfs and giants in the NLTE approximation. The model was tested using the solar flux and intensity spectra, as well as the spectra of Procyon and sigma Boo. Profiles of 14 phosphorus lines in the infrared regions and equivalent widths were analyzed. Our NLTE phosphorus abundance in the Sun is (P/H)=5.35+/-0.04 dex. Using our NLTE model, we selected 12 ultraviolet and infrared phosphorus lines and calculated a grid of NLTE corrections for the following parameter ranges: Teff from 4000 to 6750 K, step 250 K; log g from 1 to 5 dex, step 1 dex; and Vt = 2 km/s, [Fe/H] from -3 to +0.5 dex, step 0.5 dex. The NLTE corrections were calculated for phosphorus abundance ratios of [P/Fe]=-0.4, 0.0, +0.4 dex. For the Sun, the NLTE correction is -0.08 dex. The grid of the NLTE corrections, as well as the direct line profile synthesis, were used to refine the literature data on the phosphorus abundance in metal-poor, intermediate-deficient, and solar-metallicity stars. NLTE corrections do not qualitatively alter the overall phosphorus abundance distribution over a wide metallicity range, and do not change the characteristic pattern of phosphorus-rich stars.