Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements
- 1Chinese Academy of Sciences
- 2University of Chinese Academy of Sciences
- 3University of Cambridge
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) activity indices. 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 within a uniform framework. We find that observational effects contribute to the scatter but do not explain the systematic negative bias in R IRT + . This bias mainly arises because radiative-equilibrium photospheric models lack the chromospheric layers, which exhibit a temperature rise and provide additional Ca II IRT opacity. This additional opacity makes the observed IRT cores deeper than those in the synthetic photospheric profiles. Non–local thermodynamic equilibrium (NLTE) effects may contribute but are unlikely to dominate. The different behavior of the H & K and IRT diagnostics can be related to their different source-function behavior: the stronger chromospheric source-function enhancement of H & K makes their cores more likely to appear in emission. Increasing the adopted microturbulent velocity partially mitigates the negative offset, serving as an empirical compensation for missing atmospheric physics. 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.
Related
- Revisiting the interpretation of HI Zeeman measurements in the diffuse interstellar medium
- Influence of chemical and morphological properties on the mid-infrared refractive indices of Titan aerosol analogs
- Joint measurements of thermal Sunyaev-Zel'dovich and cosmic infrared background cross-correlations with cosmic shear
- The SPHEREx Instrument: Calibration, Testing, and Performance Measurements of the Near-infrared Spectroscopic Surveyor from the Laboratory to In-orbit Commissioning
- MAJIS infrared performances during the JUICE 2024 Earth fly-by: comparisons with IASI measurements and detection of atmospheric trace species
BibTeX
@article{杨2026revisiting,
title = {{Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements}},
author = {Xiaozhen 肖振 Yang 杨 and Xiaoting 晓婷 Fu 符 and Mingjie 明杰 Jian 简 and Jingkun 景昆 Zhao 赵 and Hailong 海龙 Yuan 袁 and Zhongrui 仲瑞 Bai 白 and Mengxin 梦欣 Wang 汪 and Yiqiao 义乔 Dong 董 and Mingkuan 明宽 Yang 杨 and Ziyue 子悦 Jiang 蒋 and Qian 倩 Liu 刘 and Ganyu 甘雨 Li 李 and Haotong 昊彤 Zhang 张},
journal = {The Astrophysical Journal Supplement Series},
year = {2026},
month = nov,
doi = {10.3847/1538-4365/aeb011},
url = {https://iopscience.iop.org/article/10.3847/1538-4365/aeb011},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Revisiting Ca II Activity Indices in FGK Stars: Systematic Biases in Infrared Triplet Measurements
AU - Xiaozhen 肖振 Yang 杨
AU - Xiaoting 晓婷 Fu 符
AU - Mingjie 明杰 Jian 简
AU - Jingkun 景昆 Zhao 赵
AU - Hailong 海龙 Yuan 袁
AU - Zhongrui 仲瑞 Bai 白
AU - Mengxin 梦欣 Wang 汪
AU - Yiqiao 义乔 Dong 董
AU - Mingkuan 明宽 Yang 杨
AU - Ziyue 子悦 Jiang 蒋
AU - Qian 倩 Liu 刘
AU - Ganyu 甘雨 Li 李
AU - Haotong 昊彤 Zhang 张
PY - 2026
DA - 2026/11/06
JO - The Astrophysical Journal Supplement Series
DO - 10.3847/1538-4365/aeb011
UR - https://iopscience.iop.org/article/10.3847/1538-4365/aeb011
AB - 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) activity indices. 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 within a uniform framework. We find that observational effects contribute to the scatter but do not explain the systematic negative bias in R IRT + . This bias mainly arises because radiative-equilibrium photospheric models lack the chromospheric layers, which exhibit a temperature rise and provide additional Ca II IRT opacity. This additional opacity makes the observed IRT cores deeper than those in the synthetic photospheric profiles. Non–local thermodynamic equilibrium (NLTE) effects may contribute but are unlikely to dominate. The different behavior of the H & K and IRT diagnostics can be related to their different source-function behavior: the stronger chromospheric source-function enhancement of H & K makes their cores more likely to appear in emission. Increasing the adopted microturbulent velocity partially mitigates the negative offset, serving as an empirical compensation for missing atmospheric physics. 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.
SN - 0067-0049
SN - 1538-4365
ER -