iSTARMOD: A Python Code to Quantify Chromospheric Activity by Using the Spectral Subtraction Technique

TL;DR

The paper tackles the challenge of quantifying chromospheric activity in late-type stars across multiple indicators by employing spectral subtraction, implemented in iSTARMOD, a Python-based successor to STARMOD. It details an order-by-order subtraction pipeline that aligns, broadens, and combines reference spectra to isolate excess chromospheric emission, with automated EW measurements and uncertainty estimates. A comprehensive calibration framework for the χ-factor is presented, enabling conversion of excess EWs to absolute fluxes $L_{ ext{line}}/L_{ ext{bol}}$ across indicators such as Hα, Ca II H&K, Ca II IRT, He I D3, Na I D, and Paschen lines, thereby enabling flux-flux analyses and cross-indicator comparisons. The authors demonstrate the method on single stars and spectroscopic binaries, highlight the tool’s applicability to large surveys (e.g., CARMENES), and emphasize its utility for mitigating activity-induced RV signals in exoplanet searches. Overall, iSTARMOD offers a scalable, modular, and publicly available solution for systematic chromospheric activity studies and RV contamination assessment in stellar spectroscopy.

Abstract

The use of the spectral subtraction technique allows measurements of chromospheric activity in late-type stars across several activity indicators, such as H$α$ and the other Balmer lines in the visible, He I D3 and Na I D1, D2, Ca II H and K, and Ca II infrared triplet, as well as the Paschen series and He I $λ$10830 lines in the near-infrared. iSTARMOD is an updated and extended version of the original STARMOD code and its subsequent modifications. iSTARMOD is presented in this paper as a Python code developed to quantify chromospheric activity by using the spectral subtraction technique. iSTARMOD improves usability, modularity, and integration with modern data analysis workflows and is publicly available, including several examples that help one learn how to use and test the code. The iSTARMOD code is accompanied here with a series of calibrations of $χ$-functions, to transform the excess emission equivalent widths measured through iSTARMOD into absolute surface fluxes. The method provided with this code and the corresponding flux calibrations allows for the automatic characterization of the chromospheric activity of a large number of spectra or a large number of stars and is also very useful for mitigating the effect of activity on radial velocities in the search for exoplanets.

Figures (8)

• Figure 1: Comparison between Gray, Gaussian, and Lorentzian broadening profiles
• Figure 2: Synthetic line profiles with varying rotational velocities ($v\text{ sin}i$). The iSTARMOD code has been applied to an example H$\alpha$ absorption line, taken from a quiescent K2V star. Note that minor and noisy features of the intrinsic profile are erased for $v\,>\,10 km s^{-1}$
• Figure 3: Flowchart of iSTARMOD process, showing the implementation of the algorithm of spectral subtraction technique, the steps followed and the information flow
• Figure 4: Comparison between the polynomial fit provided in reinersbasri (blue) in the range [1200, 4000] and this paper calculations (red), in the range [2000, 7000]. The points obtained from the set of synthetic spectra are the black crosses.
• Figure 5: Spectral Subtraction Example from PW And, a K2V star. The different spectra shows chromospheric indicators in Ca ii H, Ca ii K (top panels), H$\beta$ and Na i$D_2$ (middle panels), and H$\alpha$ and Ca ii IRT-a lines (bottom panels). Using FOCES provided spectra Lower panel: observed target spectrum (blue) and synthetic spectrum (red), obtained from a reference star spectrum. Upper panel: subtracted spectrum (green). In both panels: the vertical blue dashed lines mark the integration limits for the chromosoheric excess emission EW determination.