Sensitivity of Spectral Lines to Granulation: From the Sun to K-type Stars

Abstract Stellar granulation is a source of radial-velocity (RV) jitter of 1 m s −1 on Sun-like stars, limiting the detection of Earth analogs. One way to move beyond this limit is to use line-weighting schemes to mitigate granulation-driven RV variability in a spectral-type-dependent manner. A physics-based granulation-sensitivity diagnostic is essential. Here, we use a line-by-line sensitivity measure from 3D magnetoconvection simulations that quantifies how strongly each spectral line’s Doppler shift and strength respond to convective velocity and thermodynamic fluctuations. Extending our previous solar analysis, which used the spatial variability of line profiles across a single granulation snapshot as a computationally efficient proxy for temporal variability, we test the transferability of this diagnostic to cooler stars and characterize how line sensitivity varies with spectral type. We synthesize high-resolution spectra with the Merged Parallelised Simplified ATLAS code from 3D time-dependent MURaM simulations representative of the Sun and late-G and K dwarfs, focusing on neutral and singly ionized iron lines spanning a wide range of excitation potentials and strengths. As T eff decreases, convective velocities weaken, and the ionization balance shifts, producing a clearer separation between line families: Fe i lines show lower velocity sensitivity and smaller fractional strength variability, whereas Fe ii lines show the opposite. Using cumulative contribution functions, we further relate spectroscopic velocity jitter to characteristic line-formation temperature. The diagnostic continues to separate robust and sensitive lines in late-G and K dwarfs, enabling spectral-type-aware cross-correlation masks and line-by-line weights. Solar-optimized line selections are not generally portable to cooler stars, especially when based on equivalent-width stability rather than velocity sensitivity.