Cool and Data-Driven: An Exploration of Optical Cool Dwarf Chemistry with Both Data-Driven and Physical Models

Detailed chemical studies of F/G/K – or Solar-type – stars have long been routine in stellar astrophysics, enabling studies in both Galactic chemodynamics, and exoplanet demographics. However, similar understanding of the chemistry of M and late-K dwarfs – the most common stars in the Galaxy – has been greatly hampered both observationally and theoretically by the complex molecular chemistry of their atmospheres. Here we present a new implementation of the data-driven $\mathit{\text{Cannon}}$ model, modelling $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$, $\log g$, [Fe/H], and [Ti/Fe] trained on low-medium resolution optical spectra ($4000-7000$\SI{}{\angstrom}) from 103 cool dwarf benchmarks. Alongside this, we also investigate the sensitivity of optical wavelengths to various atomic and molecular species using both data-driven and theoretical means via a custom grid of MARCS synthetic spectra, and make recommendations for where MARCS struggles to reproduce cool dwarf fluxes. Under leave-one-out cross-validation, our $\mathit{\text{Cannon}}$ model is capable of recovering $T_{\mathrm{e}\mathrm{f}\mathrm{f}}$, $\log g$, [Fe/H], and [Ti/Fe] with precisions of 1.4%, $\pm 0.04$dex, $\pm 0.10$dex, and $\pm 0.06$dex respectively, with the recovery of [Ti/Fe] pointing to the as-yet mostly untapped potential of exploiting the abundant – but complex – chemical information within optical spectra of cool stars.