Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements
Researchers investigated systematic negative biases in Ca II infrared triplet residual indices (R+ IRT) measured using synthetic-template subtraction methods for chromospheric activity. They found that observational effects do not explain the bias, suggesting photospheric templates underestimate IRT core depth due to missing chromospheric structure or NLTE effects. An empirical adjustment to microturbulent velocity partially mitigated this offset.