Argon Abundances in the Solar Neighborhood: Non-LTE Analysis of Orion Association B-type Stars

TL;DR

This study determines argon abundances in Orion B-type stars using NLTE models, finding values higher than the solar corona but consistent with nebula measurements, suggesting a revised local argon abundance.

Contribution

It provides a new, independent measurement of argon abundance in the solar neighborhood using stellar spectra and NLTE modeling, challenging previous solar corona-based estimates.

Findings

Argon abundance in Orion B stars is A(Ar) = 6.66 +- 0.06.

The derived argon abundance aligns with Orion nebula measurements.

A revised solar neighborhood argon abundance is proposed as A(Ar) = 6.63 +- 0.10.

Abstract

Argon abundances have been derived for a sample of B main-sequence stars in the Orion association. The abundance calculations are based on NLTE metal line-blanketed model atmospheres calculated with the NLTE code TLUSTY and an updated and complete argon model atom. We derive an average argon abundance for this young population of A(Ar) = 6.66 +- 0.06. While our result is in excellent agreement with a recent analysis of the Orion nebula, it is significantly higher than the currently recommended solar value which is based on abundance measurements in the solar corona. Moreover, the derived argon abundances in the Orion B stars agree very well with a measurement from a solar impulsive flare during which unmodified solar photospheric material was brought to flare conditions. We therefore argue that the argon abundances obtained independently for both the Orion B stars and the Orion nebula are representative of the disk abundance value in the solar neighborhood. The lower coronal abundance may reflect a depletion related to the FIP effect. We propose a new reference value for the abundance of argon in the solar neighborhood, A(Ar) = 6.63 +- 0.10, corresponding to Ar/O = 0.009.