Letter to the Editor

Does the solar oxygen abundance change over the solar cycle?

An investigation into activity-induced variations in the O I infrared triplet

¹ Leibniz-Institut für Astrophysik Potsdam (AIP) An der Sternwarte 16 14482 Potsdam, Germany
² ESO – European Southern Observatory, Alonso de Cordova 3107 Vitacura Santiago, Chile

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 October 2025
Accepted: 22 January 2026

Abstract

The determination of the solar oxygen abundance remains a central problem in astrophysics because its accuracy is limited not only by models, but also by systematics. While many of these factors have been thoroughly characterized, the effect of the solar activity cycle has remained unexplored so far. Because of its relative strength and accessibility, the O I infrared triplet is typically the primary choice for abundance studies. Previous investigations have shown, however, that abundances inferred from this triplet tend to be higher than expected on active stars, but no such overabundance effect is observed for the much weaker forbidden O I 6300 Å line. This raises the question of whether a similar trend can be found for the Sun. To address this question, we analyzed synoptic disk-integrated Sun-as-a-star datasets of two decades from the FEROS, HARPS-N, PEPSI, and NEID spectrographs with a focus on the infrared triplet (7772, 7774, and 7775 Å) and the forbidden O I 6300 Å line. The excellent signal-to-noise ratio of the PEPSI observations allowed us to detect a weak but significant variation in the equivalent widths of the infrared triplet that corresponds to an abundance difference of about 0.01 dex between activity minimum and maximum. This value is significantly lower than the typical uncertainties on the solar oxygen abundance. No comparable trend is found in the other datasets because the scatter is higher. Based on these results, we conclude that within the typical uncertainties presented in other works, we can assume the inferred solar oxygen abundance to be stable throughout the solar cycle, but that this effect might be significant for other more active stars.