A simple model for spectroscopic analyses of active stars

ABSTRACT Spectroscopic analyses of young late-type stars suffer from systematic inaccuracies, typically underestimating metallicities but overestimating abundances of certain elements including oxygen and barium. Effects are stronger in younger and cooler stars, and recent evidence specifically indicates a connection to the level of chromospheric activity. We present here a two-component spectroscopic model representing a non-magnetic baseline plus a magnetic spot, and analyse the resulting synthetic spectra of young solar analogues using a standard spectroscopic technique. For a moderately active star with solar parameters and chromospheric activity index $\log R_\text{HK}^{\prime }= -4.3$ ($\sim 100$ Myr), we predict that $\left[\textrm {Fe/H}\right]$ is underestimated by 0.06 dex while $v_\textrm {mic}$ is overestimated by 0.2 km s$^{-1}$; for higher activity levels we predict effects as large as 0.2 dex and 0.7 km s$^{-1}$. Predictions are in agreement with literature data on solar twins, and indicate that the model is a plausible explanation to the observed effects. The model is simple enough that it can be included in spectroscopic packages with only changes to the underlying spectrum synthesis modules, if a $\log R_\text{HK}^{\prime }$ value is provided.