Sensitivity of spectral lines to granulation: from the Sun to K-type stars

TL;DR

This study investigates how spectral line sensitivity to stellar granulation varies across different star types, aiming to improve radial velocity measurements by selecting lines less affected by granulation noise.

Contribution

It extends a line-by-line diagnostic method from solar to cooler stars, demonstrating its effectiveness in identifying granulation-sensitive lines across spectral types.

Findings

FeI lines show lower velocity sensitivity in cooler stars.

FeII lines are more sensitive to granulation effects.

The diagnostic can distinguish stable and sensitive lines in late-G and K dwarfs.

Abstract

Stellar granulation produces radial-velocity (RV) jitter at the 1 m/s level in Sun-like stars, limiting Earth-analog detection. A route beyond this limit is to weight spectral lines according to their granulation sensitivity. We apply a line-by-line diagnostic from 3D magneto-convection simulations that measures how each line's Doppler shift and strength respond to convective velocity and thermodynamic fluctuations. Extending our solar study, which used spatial line-profile variability across one granulation snapshot as an efficient proxy for temporal variability, we test whether this diagnostic transfers to cooler stars and examine how sensitivity changes with spectral type. We synthesize high-resolution spectra with MPS-ATLAS from 3D time-dependent MURaM simulations of the Sun and late-G and K dwarfs, focusing on FeI and FeII lines spanning broad ranges of excitation potential and strength. With decreasing $T_{\mathrm{eff}}$, weaker convective velocities and changing ionization balance produce a clearer separation between line families: FeI lines show lower velocity sensitivity and smaller fractional strength variability, while FeII lines are more sensitive. Cumulative contribution functions link spectroscopic velocity jitter to characteristic line-formation temperature. The diagnostic robustly separates stable and granulation-sensitive lines in late-G and K dwarfs, enabling spectral-type-aware cross-correlation masks and line-by-line RV weights. Solar-optimized line selections are therefore not generally portable to cooler stars, particularly when based on equivalent-width stability rather than velocity sensitivity.