2025AJ....170..209K

We present a comparative spectral analysis to explore the ability of a cooler Sun model to accurately capture the spectral line shape changes caused by sunspots. In the search for small Earth-like planets, the effects of stellar surface activity can overwhelm the ∼10 cm s^–1 planetary radial velocity signal. This necessitates the development of new stellar modeling methods and a greater understanding of the impact of surface activity on stellar spectra. Some attempts to model out noise from sunspot activity, in particular, have used a sum of a stellar model with a model of a cooler, but otherwise identical star. From our analysis, we find that a cooler effective temperature alone cannot capture the numerous spectral line shape variations seen in a sunspot observation. The cooler temperature of a sunspot not only deepens the cores of atomic lines, but it also increases and strengthens molecular lines that are not fully represented in our line list. Furthermore, our LTE models and a comparison cool star also fail at capturing line strengthening, broadening, blending, and splitting induced by the magnetic field in the sunspot.