Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements

TL;DR

This study investigates the systematic negative biases in Ca II infrared triplet activity indices in FGK stars, attributing them mainly to underestimation of line core depths by photospheric templates and exploring mitigation strategies.

Contribution

It identifies the primary cause of negative Ca II IRT residuals as missing chromospheric structure in templates and proposes empirical adjustments to improve measurement accuracy.

Findings

Negative biases are mainly due to underestimated IRT core depths in templates.

Empirical increase in microturbulent velocity partially mitigates the bias.

Different synthesis configurations show systematic offsets but maintain strong linear correlations.

Abstract

Synthetic-template subtraction is widely used to measure chromospheric activity in large spectroscopic surveys. However, many solar-like FGK stars show systematically negative Ca II infrared triplet (IRT) residual indices, implying that the observed line cores are deeper than those predicted by parameter-matched templates. We investigate this effect using solar-like stars from LAMOST DR9, MaStar, and XSL DR3, measuring activity indices (R+) for both the Ca II H&K and IRT lines in a uniform framework. We find that observational effects, including atmospheric-parameter offsets, treatment of the instrumental line-spread function, and propagated measurement uncertainties, contribute to scatter but do not explain the systematic negative bias in R+_IRT. The results instead suggest that the negative bias most likely arises because photospheric templates underestimate the depth of the IRT cores, likely owing to missing chromospheric structure and, to a lesser extent, NLTE effects. An empirical increase in the adopted microturbulent velocity deepens the synthetic IRT cores and partially mitigates the negative offset. In addition, R+ values derived from different synthesis configurations show systematic offsets but generally preserve strong linear correlations, indicating that they can be cross-calibrated. These results clarify the origin of negative Ca II IRT residual indices and help interpret template-dependent systematics in chromospheric activity measurements based on synthetic-template subtraction.