Does the solar oxygen abundance change over the solar cycle? An investigation into activity-induced variations of the O I infrared triplet

TL;DR

This study tests whether the solar oxygen abundance $A( ext{O})$ changes with the solar activity cycle by analyzing Sun-as-a-star spectra from four independent long-term datasets (FEROS, HARPS-N, PEPSI, NEID) across significant portions of the solar cycle. Focusing on the O I infrared triplet ($7772$, $7774$, $7775$ Å) and the forbidden O I 6300 Å line, the authors measure equivalent widths and examine correlations with solar activity proxies, converting the EW variations to abundance changes via $3D$ NLTE curves-of-growth. They find a weak but statistically significant EW–activity trend only in the high-S/N PEPSI data, corresponding to about $0.01$ dex in $A( ext{O})$, while the other datasets show no robust trend and the 6300 Å line remains activity-insensitive within precision. The results imply the solar oxygen abundance is stable across the solar cycle within typical uncertainties, though activity-related biases may be more pronounced in younger, more active stars; the work also highlights the crucial role of high-S/N data in detecting subtle activity-induced signatures.

Abstract

The determination of the solar oxygen abundance remains a central problem in astrophysics, as 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 so far remained unexplored. Due to its relative strength and accessibility, the O I infrared triplet is typically the primary choice for abundance studies. However, previous investigations have shown that abundances inferred from this triplet tend to be higher than expected on active stars, whereas such an overabundance effect is not 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 analyze two decades' worth of synoptic disk-integrated Sun-as-a-star datasets from the FEROS, HARPS-N, PEPSI, and NEID spectrographs, focusing on the infrared triplet (7772, 7774, 7775 Å) and the forbidden O I 6300 Å line. The excellent signal-to-noise ratio of the PEPSI observations allows us to detect a weak but significant variation in the equivalent widths of the infrared triplet, corresponding to about 0.01 dex difference in abundance between activity minimum and maximum. This value is significantly smaller than the typical uncertainties on the solar oxygen abundance. Due to higher scatter, no comparable trend is found in the other data sets. 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 across the solar cycle, but that this effect may be significant for other, more active stars.

Figures (5)

• Figure 1: Monthly mean sunspot number (black line)) showing the last six solar cycles. Shaded bands indicate the windows used for observing in past solar oxygen abundance studies. These windows are labeled according to their color.
• Figure 2: Measured EW of the OI 7772 Å line with FEROS (blue), PEPSI (orange), and NEID (green), including corresponding uncertainties. The gray curve shows the sunspot number as an activity reference, while the black horizontal line at 50 distinguishes active from quiet times. Corresponding plots for the three remaining lines are shown in Fig. \ref{['fig:abundanceVSActivity_full']}.
• Figure 3: EWs of the OI 7772 Å line for FEROS (blue), PEPSI (orange), and NEID (green) as a function of spot number. Corresponding plots for the three remaining lines are shown in Fig. \ref{['fig:abundanceVSActivity_2']}.
• Figure 4: Same as Fig. \ref{['fig:abundanceovertime']} but for the remaining IR triplet lines and the forbidden line.
• Figure 5: Same as Fig. \ref{['fig:abundanceVSActivity']} but for the remaining lines.