Star spots mimic the proposed abundance signatures of planet formation

Abstract

Trends with stellar elemental abundances and condensation temperature have been proposed as a possible tracer of rocky planet formation. Here we present an alternative explanation, where we show that stellar activity (star spots) can also cause apparent abundance changes in refractory elements preferentially over volatile elements. This effect arises naturally from atomic physics: refractory (rocky; high-$T_c$) elements preferentially have low first-ionization potentials, with strong transitions that are most sensitive to the cooler, magnetically-perturbed spot conditions, making any spot-induced change in line strength correlated with $T_c$. We re-examine literature on Sun-like stars in the Solar neighborhood and find that the $\log{R'_{HK}}$ stellar activity indicator is correlated with the so-called $T_c$ slope: the gradient of chemical abundances with respect to condensation temperature. Using 1D LTE Korg.jl simulations of the Sun with spots of varying size and contrast, we show that a wide range of spot configurations can manifest as abundance patterns that mimic a signal of abundance trends with condensation temperature. Finally, we conduct a line-by-line differential analysis of an ambient Solar spectrum relative to a Solar sunspot umbra to represent an extreme case, where we find that elements with high condensation temperature can show a factor of three (0.5 dex) increase. We discuss implications that stellar activity poses for precision spectroscopy, and provide some direction on how this effect might be mitigated.