Forward modeling solar spectra onto Doppler images of λ And

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Institut für Physik und Astronomie, Universität Potsdam, 14476 Potsdam, Germany

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 12 February 2026
Accepted: 27 April 2026

Abstract

Context. Due to their high chromospheric activity and photometric variability, RS CVn-type binaries are ideal laboratories for studying stellar surface structures and the corresponding stellar activity relations. However, the atmospheric nature of their primary evolved components (luminosity classes III-IV) are more complex than those of main-sequence stars. Additionally, detailed models are still lacking for the sub(giant) systems. Therefore, comparative techniques represent the most effective approach for probing the connection between chromospheric emission and surface structures.

Aims. Using the Doppler images of λ And, we investigated whether surface temperature information can be reversed to create its activity parameters by feeding a toy model solar spectra based on surface images. At the same time, we examined whether spot contributions alone are sufficient to explain the observed activity modulation of the RS CVn star λ And while quantifying the differences with the actual observations of this star obtained simultaneously with the Doppler images we used.

Methods. Due to a lack of publicly available starspot models for its stellar type, we adopted the observed solar spectra as the only available approximation of λ And’s spots. These spectra were injected into a sequence of a full-disk temperature map derived from Doppler imaging that represents a full stellar rotation. These disks were then forward modeled into disk-integrated spectra with the Numerical Empirical Sun-as-a-star Integrator (NESSI). This experiment was performed on three photospheric lines (Fe I 6173 Å, Fe I 6301 Å, and K I 7699 Å) and four chromospheric lines (Hα, Ca II H&K, and Ca II 8542 Å). Finally, we used these spectra to calculate the radial velocities and chromospheric emissions diagnostics, which in turn were compared to the original photospheric and chromospheric characteristics of the star.

Results. Despite the very different stellar structures and atmospheric stratification between λ And and the Sun, we show that the chromospheric emissions produced by our toy model largely follow the same trend as the original observations of λ And. This indicates that the modulation of the chromospheric activity is dominated by magnetic activity associated with the active regions with dark spots. In addition, the differences in the emission amplitudes quantify the different chromospheric heating mechanisms for these two very different types of stars.

Conclusions. Using this approach, we show that even with simplified assumptions the spectral behavior of λ And can be qualitatively reproduced. Toy models such as the one presented in this work make an additional dimension available by providing a relation between the surface structures and chromospheric emissions. Such models also help in developing a further understanding of the heating mechanisms of these active giants through comparative techniques, where in this case the spot activity seemingly modulates the chromospheric signal and can explain the bulk of its variations over a rotation.