The Near-infrared Ca ii Triplet as a Stellar Activity Indicator: A Library and Comparative Study

We have established and released a new stellar index library of the Ca ii triplet, which serves as an indicator for characterizing the chromospheric activity of stars. The library is based on data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) Low-Resolution Spectroscopic Survey (LRS) Data Release 9. To better reflect the chromospheric activity of stars, we have defined new indices R and R +. The library includes measurements of R and R + for each Ca ii infrared triplet (IRT) from 699,348 spectra of 562,863 F, G, and K-type solar-like stars with a signal-to-noise ratio higher than 100, as well as the stellar atmospheric parameters and basic information inherited from the LAMOST LRS catalog. We compared the differences between the three individual indices of the Ca ii triplet and also conducted a comparative analysis of Rλ8542+ to the Ca ii H and K S and RHK+ index databases. We observe the fraction of less active stars decreases with T eff and the fraction of more active stars first decreases with decreasing temperature and turns to increase with decreasing temperature at 5800 K. We also find that a significant fraction of stars that show a high activity index in both Ca ii H and K and IRT are binaries with low activity; some of them could be discriminated in the Ca ii H and K S index and Rλ8542+ space. This new stellar library serves as a valuable resource for studying chromospheric activity in stars and can be used to improve our comprehension of stellar magnetic activity and other astrophysical phenomena.


Introduction
Stars with outer convective envelopes tend to exhibit magnetic activity.Starspots and faculae in the photosphere, plages in the chromosphere, and X-rays in the corona are all related to magnetic activity.Studies of stellar activity are essential for improving our understanding of stellar dynamo models and related studies such as stellar age and rotation or activity relation, and stellar flares and the stellar-activity cycle.On the other hand, stellar activity is important for exoplanet studies, since magnetic activity, especially flares, will have an impact on planetary habitability (Shields et al. 2016;Howard et al. 2018;Lillo-Box et al. 2022).Also, jitters in both photometry and radial velocity measurements caused by stellar magnetic activity will hinder the detection of Earthlike exoplanets (Wright 2005).Finding stars with low activity is crucial to the detection of low-mass exoplanets.
The emission core of lines originating from the chromosphere can serve as an indicator to quantify activity.One wellknown measurement of activity is the Ca II H and K S MWO index, proposed by the Mount Wilson Observatory (Wilson 1968).However, the photosphere also contributes to the Ca II H and K line flux, and the contribution varies with effective temperatures, leading to potential misestimation of stellar activity.To overcome this issue, Linsky et al. (1979a) proposed the R HK ¢ index, which subtracts the empirical photospheric flux from the flux.Building on the R HK ¢ index, Mittag et al. (2013Mittag et al. ( , 2019) ) proposed the R HK + index, which subtracts the basal flux in addition to the photospheric flux.The H α line can also serve as an indicator of activity and is more suitable for latetype stars than Ca II H and K (Cincunegui et al. 2007).They defined the S + index for H α , which correlates well with the S MWO index.
The Ca II infrared triplet (IRT) lines represent another set of indices of activity: They serve as indicators of stellar chromospheric activity, as demonstrated by Linsky et al. (1979b).Linsky et al. (1979b) proposed Ca II λ8542 as an activity indicator, while Andretta et al. (2005) defined the R IRT index based on the central depression in the Ca II IRT lines, taking into account rotational broadening.Notsu et al. (2015) employed r 0 (IRT), which is the residual flux normalized by the continuum at the line cores of IRT lines, and H α to study superflares, and suggested that the brightness variation of superflare stars can be explained by the rotation with large starspots.Žerjal et al. (2013) employed the observed spectra of nonactive stars as a template and measured the templatesubtracted equivalent width (EW) of the Ca II IRT lines to represent stellar activity.
It is important to build large databases to statistically understand the physical mechanisms of stellar magnetic activity.As part of this effort, we have previously established large sample databases for solar-like stars' activity utilizing Ca II H and K (Zhang et al. 2022) and H α (He et al. 2023) indices based on LAMOST spectra.In this study, we will build a stellar-activity database of F, G, and K stars based on the measurements of Ca II IRT lines.
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) located in Xinglong, China, offers low-resolution spectra with a resolving power of λ/Δλ = 1800 covering the wavelength range of 3700-9100 Å (Zhao et al. 2012).Additionally, it provides midresolution spectra via the Medium-Resolution Spectroscopic Survey (MRS) with R ∼ 7500 in the 4950-5350 and 6300-6800 Å bands.The observed data is first reduced by the LAMOST 2D pipeline (Bai et al. 2017(Bai et al. , 2021)), and then the LAMOST stellar parameters pipeline (Wu et al. 2011) is applied.The released data, including extracted spectra files as well as the stellar parameters, are available at the LAMOST website at http:// www.lamost.org.
There have been several studies of stellar activity based on LAMOST data.For example, Zhang et al. (2020) employed the R HK + index using LAMOST spectra to investigate the relationship between stellar activity, period, and the amplitude of brightness variation, along with Kepler light-curve data; In this study, our focus is on the Ca II IRT lines of solar-like stars; all the spectra utilized come from the LAMOST LRS Data Release 9 (DR9) database.Due to the low spectral resolution, the core emission of lines is not sensitive to EW and may be compromised by deviations in rotational velocity estimations.Instead, we introduce a new R index that specifically considers the flux near the center of spectral lines.To remove the photospheric flux components, we employed the BT-Settl stellar spectral models (Allard et al. 1997(Allard et al. , 2011(Allard et al. , 2013) ) and calculated the template-subtracted index, R + , to represent pure activity levels.Furthermore, we compared our results with the existing database of Ca II H and K lines and discussed the nature of stars in the distributions of Ca II H and K and IRT activity indices.
This paper is organized into six sections.Section 2 introduces the data selection criteria, while Section 3 defines the indices R and R + and provides a detailed description of the data-processing steps.Section 4 shows the detail of our database.In Section 5 we compare the strengths of the three lines, and discuss the relationship and differences between the indices measured from Ca II H and K. Section 6 is the summary.

Data Preparation
Our analysis is centered on F, G, and K-type solar-like stars, and all stellar parameters are sourced from the LAMOST LRS Stellar Parameter of A, F, G, and K Stars catalog (AFGK Catalog; http://www.lamost.org/dr9/).To ensure comparability with the prior Ca II H and K index study by Zhang et al. (2022), the following parameter restrictions are applied: 1. 100 S/N i , S/N z .This is to ensure the high quality of the Ca II IRT lines located between the i and z band.2. 4800 K T eff 6800 K, This criterion is same as Zhang et al. (2022); the temperature range of solar-like stars covers most F, G, and K samples in the AFGK Catalog.3.For surface gravity, the empirical formula of Zhang et al.
(2022) is adopted to select main sequence stars: After rejecting spectra with issues such as fiber failure in the IRT bandpass, heavy skylight pollution, and wavelength calibration failure, we selected a total of 699,348 spectra from the LAMOST database.Considering multiple observations for the same star, these spectra correspond to 562,863 stars.The number of spectra cross correlated with the previous work of Ca II H and K S and R HK + index databases is listed in Table 1.

Index Definitions
We defined the R, R + index for each line of Ca II IRT as the following equations: where F(λ) is the spectrum, C(λ) is the linear function fitting the local continuum in the IRT bandpass, and subscripts "o" and "m" stand for observation and model, respectively.The normalized spectrum is expressed as F(λ)/C(λ).λ 1 , λ 2 are the starting and ending wavelengths of the sampling range, which is 1 Å around the central wavelength of each Ca II IRT line.
The corresponding central wavelengths and the sampling ranges are listed in Table 2.As the LAMOST spectral data points are in approximately 2 Å intervals, a cubic spline function is applied to interpolate the spectrum to a finer grid.
LAMOST DR9 provides normalized spectra for most spectra, typically generated for the entire spectrum.To achieve a better performance, we renormalized the spectra within the IRT bandpass with a normalization method that utilizes the LinearLSQFitter provided by the Astropy module, which is a linear least-square fitting method (Robitaille et al. 2013;Astropy Collaboration et al. 2018, 2022).Two examples are illustrated in Figure 1 to show the difference between global and local normalization.Both methods perform similarly for the absorption-line spectra, but in the case of emission lines, our method clearly outperforms the LAMOST approach.

Templates
For late-type stars.the dissipation of acoustic energy (Schrijver et al. 1989) and turbulent dynamo activity from nonrotating plasma (Bercik et al. 2005) in the upper photosphere contributes to the core of Ca II H and K and Ca II IRT lines.Therefore, it is better to subtract this "basal" flux from the spectrum to derive the true chromosphere activity.Andretta et al. (2005) investigated the non-local thermodynamic equilibrium (NLTE) effect on Ca II IRT lines, and found that the central depression (CD) index can be affected by an NLTE of more than 20%.Since our R + and R indices are defined on a narrow band of 1 Å, similar to the CD index, NLTE should be considered in the R + index to remove the basal flux.The LTE BT-Settl spectral model and the NLTE model for Ca II lines (Allard et al. 2013) based on the Phoenix code (Husser et al. 2013) were applied to subtract the basal flux in the IRT bandpass.
The grids of BT-Settl templates are listed in Table 3.These templates were interpolated with intervals of ΔT eff = 10 K, g log 0.01 D = , and Δ[Fe/H] = 0.01 to ensure a precise match with our observational spectra.The templates are degraded to R ≈ 1800 and subtracted from the observed spectra, as in Equation (2).

Estimation of Uncertainties
Similar to the LAMOST Ca II H and K index error budget analysis in Zhang et al. (2022), for the Ca II IRT R index, we consider three factors of uncertainty as follows: 1. Uncertainty of spectral flux.LAMOST releases the target spectrum along with the corresponding spectrum of inverse variance (1/δ 2 ), which could be used to estimate the flux uncertainty:   where C(λ) is the continuum, the same as defined in Equation (1). 2. Uncertainty of interpolation.As the wavelength intervals of LAMOST spectra are 2 Å, the spectrum is interpolated.
A different interpolation method lead to the R index uncertainty, as illustrated in Figure 2. The uncertainty of interpolation is derived as To ensure that our choice of a 1 Å window does not impact our conclusions, we compared the R indices of each Ca II IRT line measured in a 1 Å window with those  of the 2 Å window.For the majority of targets, the difference is negligible, as shown in Figure 3. 3. Uncertainty of redshift (or radial velocity).By using z + z err , z, and z − z err provided by LAMOST DR9, we can obtain R + , R, and R − respectively, for each line, so the δR z is represented as the following: Combining Equations (3), ( 4) and (5), the total uncertainty δR is given by For the R + index, the additional uncertainty comes from the templates.Utilizing the stellar parameter errors provided by LAMOST DR9, we calculated a series of R indices for each template around the best template, The maximum and minimum of the template index R T are denoted as R T max and R T min respectively.The uncertainty of the template index R T is then calculated as and the uncertainty of R + is given by illustrates the contribution of different components to δR and δR + .It can be observed that the uncertainty of R + is mainly dominated by interpolation and flux error.

Stellar Activity Database
We calculated the R and R + indices and their corresponding errors for 699,348 F, G, and K-type spectra selected from the LAMOST DR9 database.The result are written in a machinereadable format and uploaded to the China-VO repository at doi:10.12149/101245.The column descriptions of the database can be found in Table 4.Our R and R + index database can be used as an indicator for stellar-activity studies.
Theoretically, the R + index should be close to zero for inactive stars, but there is a significant fraction of stars with R + values below zero (see Figure 6).Similar negative values are also found in Gaia (Lanzafame et al. 2023) and RAVE (Žerjal et al. 2013) Ca II IRT index measurements.We believe that the following reasons may have contributed to this: 1.The parameters of the LAMOST spectra may not have been measured accurately.2. Low or moderate chromospheric activity could produce some extra absorption (Mullan 1979;Lanzafame et al. 2023).

Relationship between IRT Indices and Stellar Parameters
In Figure 5, we plotted the Ca II IRT R + against each other.There are clear linear correlations in all plots.We calculated the Pearson correlation coefficient and marked it at the lower part of each panel.For each pair, the ridge of the density distribution is fitted with a linear function using the Bayesian Ridge Regression algorithm from the sklearn module (Pedregosa et al. 2011).The functions are shown on the top of each panel in Figure 5. From the figure, we can see that R 8542 l + versus R 8662 l + exhibits the strongest linear relationship, with a higher Pearson coefficient than other pairs.The λ8542 line is the most opaque member of the Ca II IRT lines and usually considered as a better diagnostic for chromospheric activities (Linsky et al. 1979b).Based on the linear function slopes, the strength of R 8542 l + is greater than the other two lines, and our results confirm the conclusion of Linsky et al. (1979b) and are also consistent with the findings of Žerjal et al. (2013) and Martin et al. (2017).Henceforth, we limit our discussion to λ8542, although all the other line indices are available in our database for possible use.
The distributions of R 8542 l + and R λ8542 with stellar parameters are presented in Figure 6.As can be noticed, there is a native bias in the R + index plot.The R + index is measured in a narrow band (1 Å) around the line core.As pointed out in Linsky et al. (1979b), for quiet-chromosphere stars, even when the stellar parameters are the same, different turbulence, rotation, or other possible parameters could lead to the uncertainty in the observed line profile.Also, the released LAMOST stellar parameters are measured in the blue part of the spectra and the theoretical stellar template may not fully fit the observed nearinfrared lines, especially in the narrow line core.Furthermore, the stellar activity may also bias the stellar parameter measurement.So we suspect the mismatch between the template and the observed line core may cause the negative bias, but the relative R + index may still reflect the stellar activity.To test the Note.If some of the stellar parameter errors or the index errors are not available in the data release, the corresponding error values in the table are filled with −9999.
reliability of our R + index, we applied our method to the wellstudied nearby northern field stars listed in Baliunas et al. (1995) and Hall et al. (2007Hall et al. ( , 2009)).Those stars are confirmed as active or inactive by both Ca II H and K time and photometric time series.As those stars are too bright for LAMOST surveys, we accessed the high-quality spectra of these stars from the ESO archive,6 and found 18 stars with available data in both Ca II H and K and IRT bands.The spectra were degraded to LAMOST resolution and the R and R + indices of those stars are presented in Figure 6.The S index versus the R + index of these stars are also plotted in Figure 13.The inactive, moderately active, and highly active stars are shown by different colors in Figure 6; their relative position in the plot is consistent with their defined activity.However, the moderately and highly active stars do not show a higher R + index in the plot; this is not uncommon in the active stars defined by Ca II H and K lines, as Ca II IRT lines do not have to show emission cores as strong as H and K lines (Linsky et al. 1979b).As an example, in Figure 7, we plot the spectrum in both the Ca II H and K and IRT bands for one of the highly active stars in the sample, HD 22049, with an average S index S á ñ = 0.46 (Baliunas et al. 1995).The highresolution spectra show a strong emission core in the H and K lines, while it is much less evident in the case of λ8542.The tiny bump will be smeared in the low-LAMOST-resolution spectra.So it reasonable that HD 22049 does not show strong activity in R 8542 l + index.Stars with less activity are also important for low-mass exoplanet studies, since life may be more likely to exist on a planet hosted by a less active star, and exoplanets may be more easily discovered around less active stars compared with more active stars because both the observed light curve and radial velocity curve will be more stable due to fewer spots on the star (e.g., Korhonen et al. 2015;Hojjatpanah et al. 2020).To take a peek at the distribution of the chromospheric active and inactive stars, the star are divided into 20 temperature bins, and the number count in each bin is plotted in the bottom panel of Figure 8.The mean and variance of R 8542 l + are calculated for each bin.Stars with an R + index higher than 2σ are defined as highly active stars and those lower than 2σ are inactive stars.The fractions of active and inactive stars are plotted in the upper two panels of Figure 8.The fraction of inactive stars decreases with temperature.The fraction of active stars increases with the decreasing temperature below 5800 K and increases with temperature above 5800 K.As a large fraction of high R + index stars are actually binaries (see Section 5.2), the increasing fraction of active stars with temperature may reflect the increasing binary fraction with mass rather than the increasing activity.Further work is needed to clarify this.
The distributions of highly active, inactive, and moderately active stars in the stellar parameter space are shown in Figure 9.The inactive stars exhibit high metallicity in Figure 9, indicating that they are a thin-disk population; similarly, the low-metallicity population in the active stars plot may possibly come from the local thick-disk population.As some stars were observed several times by LAMOST, for Figures 6, 8, and 9 only one spectrum was kept for stars with multiple visits to ensure the fraction is not biased by a repeat count.As the stellar activity is a complicated function of mass, age, metallicity, and rotation, which is beyond the scope of the current paper, we will leave the detailed analysis for future work.

Comparison with the S Index
Comparing our database with the Ca II H and K S index of Zhang et al. (2022), there are 0.58 million spectra in common (Table 1).The S index versus R 8542 l + and S index versus R λ8542 are plotted in Figure 10.Both plots show a linear relation between the S index and λ8542 indices, with R + being less scattered than the R index, as the basal photospheric contribution was removed.In Figure 10, we also plotted the S index versus the R + and R indices with T eff in every 200 K bin.It shows that as the temperature decreases, the relationship between the S and R (+) indices becomes increasingly nonlinear.Visually inspecting Figure 10, the high-activity index star seems to be divided into three branches.We label the three branches in Figure 11 and plot their distributions in stellar parameter space in the lower panels of Figure 11.For Branch 1, we did not find any specific tendencies in the distribution of T eff and [Fe/H], but they almost located at g log 4.5 < . Branch 2 has a lower R + index than Branch 1 and extends to a very high S index end.They are distributed at temperatures below 5750 K and exhibit high metallicity.Branch 3 has a high S index but a low R 8542 l + index.The sample size of Branch 3 is small, but they have a broad temperature range.They show high metallicity in the low-temperature end and low metallicity in the hightemperature end.To investigate the properties of the three groups, we checked the spectra by eye, and the typical spectra are show in Figure 12.
1. Most of the spectra in Branch 1 show the characteristic double lines at the IRT bandpass and that H and K lines are broader than the template, which is typical in spectral binaries.For LAMOST LRS, the radial velocity (RV) separation should be more than 150 km s −1 for the two lines to be clearly discerned.Therefore, those are highly possible to be close binaries with a larger RV difference and similar luminosity.To confirm this conclusion, we crossmatched the gaiadr3.vari_eclipsing_binarycatalog (Gaia Collaboration et al. 2016, 2023;Mowlavi et al. 2023), which yielded 1727 common spectra (1507 common sources).About 66% (997/1507) of stars in our defined Branch 1 region (see Figure 11) coincide with the Gaia DR3 eclipsing binaries.As the Gaia samples are selected by light curves and thus are highly dependent on the inclination angle, the remaining  34% of Branch 1 may consist of either spectral binaries with low inclination that show no eclipse in light curves, or possibly some single stars that show very high activity.Therefore, a large fraction of this branch should be close binaries that mimic the chromospheric emission due to the index calculation algorithm; most of them are not active stars, or at least not as high as the S or R indices indicated.Further investigation is necessary to determine their nature.The Gaia eclipsing binary samples extend linearly to the low-active-index end in the S versus R + plot (see Figure 13).We fitted the Gaia samples with the Random Sample Consensus regression algorithm provided by the sklearn package (Pedregosa et al. 2011), and the result is shown in Figure 13.The spectra and the corresponding templates of the Ca II IRT region and the Ca II H and K region are plotted.(Holl et al. 2023), TESS light curves (Ricker et al. 2015;Sullivan et al. 2015), and Kepler light curves (Howell et al. 2014) were collected to help understand the nature of these targets, and this information is listed in the last column of Table A1.From the table, 21 stars are binaries or spatial coincidences, supporting our speculation.Twenty-seven stars are variables that may show high activity, 10 stars have no particular class and the remainder are due to pollution or have the wrong spectral type.Further study is necessary to know their nature.7), though they used EW IRT as an infrared-activity indicator.Binary stars are more likely to appear in the regions of both high H and K and high IRT indices, similar to Žerjal et al. (2013).From Figures 13 and  14, high H and K or a high IRT indices alone are not a good indicator of activity, as there is a large population of binaries with low stellar activity that mimic high-activity stars.Combining the H and K index and the IRT index, especially in the S versus R 8542 l + distribution plot, will be more helpful to discern different populations of stellar activity.

Conclusion
We defined new near-infrared Ca II triplet stellar-activity indices, R and R + , and derived the indices for 699,348 spectra of 562,863 solar-like F, G, and K-type stars.These activity indices, as well as their estimated uncertainties and other basic information, are integrated in a database available at the China-VO repository at doi:10.12149/101245.
Comparing the indices of the λ8498, λ8542, and λ8662 lines shows linear correlation within each pair.R 8542 l + is the strongest among the three lines, and could be used as the indicator to represent the Ca II IRT activity.We presented the distribution of the λ8542 index in stellar parameter space, and selected samples of highly active and less active stars, respectively.The fraction of less active stars decreases with the temperature, while the fraction of highly active stars first decreases with the temperature above 5800 K, then below 5800 K, the fraction increases with decreasing temperature.We further compared our infrared-activity index with the Ca II H and K index and found that the high S index star could be divided into three branches.Branch 1 is mostly spectral binaries with double lines that mimic the emission-line core, Branch 2 comprises RS-CVns showing high activity, and Branch 3 includes stars with a high S index but relatively low R 8542

Figure 2 .
Figure 2. Difference of two interpolation methods.Black dots are the observed spectrum; the blue curve is the cubic spline interpolation of the spectrum; the orange-dashed curve is the linear interpolation; the red dotted-dashed line shows the vacuum wavelength of λ8542.

Figure 3 .
Figure 3.Comparison of R index derived from 1 Å to 2 Å widths, respectively, for each IRT line.Red dashed lines are obtained by least-squares fitting of the data.

Figure 4 .
Figure 4. Distribution of uncertainties for the spectral lines λ8498, λ8542, and λ8662 in three columns from left to right.Each column includes two panels, with the top one showing the distribution of uncertainty for R and its individual component, and the bottom one displaying the distribution of uncertainty for R + , both represented by the red histograms.

Figure 6 .
Figure 6.The distributions of R λ8542 and R 8542 l + with stellar parameters.From left to right are T eff , [Fe/H], and g log .The upper section in each panel is for the R index and the lower is for R + , as indicated in the plots.The red dashed line in the lower left panel is the lower 2σ line to selected inactive stars in Figure 8. Star symbols are bright stars with well-studied activity in Baliunas et al. (1995) and Hall et al. (2007, 2009).Spectra are extracted from the ESO archive.Inactive stars: HD 1461, HD 3795, HD 9562, HD 45067, HD 126053, HD 187691, HD 197076.Moderately active stars: HD 16160, HD 16673, HD 20630, HD 30495, HD 35296, HD 39587, HD 88873, HD 155885, HD 160346.Highly active stars: HD 17925, HD 22049.See text for details.

Figure 5 .
Figure 5. Linear regression is performed for each pair of R + values, with the corresponding residuals between the data and the fitted line shown in the lower panels.The left column displays R 8498 l + -R 8542 l + , the middle column shows R 8498 l + -R 8662 l + , and the right column depicts R 8542 l + -R 8662 l +.The red dashed lines represent the regression equations obtained from fitting the data, while ρ corresponds to the Pearson correlation coefficient.

Figure 7 .
Figure 7. High-resolution spectrum of HD 22049.Top left: Ca II K; top right: Ca II H; bottom: Ca II λ8542.

Figure 8 .
Figure 8.The top and middle panels show the proportion of highly active and inactive stars, respectively.The bottom panel shows the number count of different categories in different temperature bins, as indicated by the different colors.

Figure 9 .
Figure 9. Distribution of inactive, normal, and highly active stars in the T eff and log g space.Different colors represent different [Fe/H], as indicated by the color bar.

Figure 10 .
Figure 10.The top-left panel shows the relationship between the S index and R λ8542 , while the bottom-left panel illustrates the relationship of the S index and R 8542 l + .The colors of each point represent their temperature, as indicated by the color bar.To make a clear view of how the indices' distributions change with the temperature, the small panels on the right-hand side show a similar distribution of indices as the left panels, but with the T eff range limited to 200 K.
Figure 11.(a) Distribution of S−R 8542 l + ; different branches are defined by eye and plotted in different colors, as indicated in the frame.The background stars are shown in gray.(b) Distribution of Branch 1 stars in stellar parameter space.Metallicity is indicated by color, as shown in the bottom color bar.(c) Distribution of Branch 2 stars.(d) Distribution of Branch 3 stars.

Figure 12 .
Figure 12.Typical spectra from the three branch stars are shown in Figure 11.The panels from top to bottom show a star of Branches 1, 2, and 3 respectively.The spectra and the corresponding templates of the Ca II IRT region and the Ca II H and K region are plotted.
different reasons.Combining the Ca II H and K S index and R 8542 l + is particularly useful in selecting true chromospherically active stars.Future work is necessary to exclude contamination from less active binaries and establish a pure sample of highly active stars.

Figure 13
Figure 13.R 8542 l + vs. S index distribution.The background gray points are the same as Figure 10.The overlaid color samples are stars crossmatched with Gaia DR3 eclipsing binaries and RS-CVn variables.As indicated in the plot, these two samples coincide with Branch 1 and Branch 2, as defined in Figure 11.The black-dashed line is the linear regression of the eclipsing binary sample, with the function given.Star symbols are the same as in Figure 6.
The above studies are based on the measurement of Ca II H and K or H α ; the capability of Ca II IRT lines has not been fully explored yet.
Zhang et al. (2019))ured the R H a index using LAMOST MRS;Zhang et al. (2022)established a Ca II H and K S index database based on the LAMOST Low-Resolution Spectroscopic Survey (LRS);Karoff et al. (2016)explored superflares using the S index along with Kepler light-curve data and found that superflare stars are characterized by enhanced activity;Zhang et al. (2019)proposed that stellar chromospheric activity indices can be used to roughly estimate stellar ages for dwarfs.

Table 1
Ca II Index Database Using LAMOST Data

Table 4
Columns of the Catalog Figure 14.The stars in Branch 1 and Branch 2 in the previous section are also plotted.The overall distribution is similar to Žerjal et al. (2013, their Figure