Ground-based Optical Transmission Spectroscopy of the Nearby Terrestrial Exoplanet LTT 1445Ab

Nearby M dwarf systems currently offer the most favorable opportunities for spectroscopic investigations of terrestrial exoplanet atmospheres. The LTT~1445 system is a hierarchical triple dwarfs with two known planets orbiting primary star, LTT~1445A. We observe four transits world LTT~1445Ab ($R=1.3$ R$_\oplus$, $M=2.9$ M$_\oplus$) at low resolution Magellan II/LDSS3C. use combined flux LTT~1445BC pair as comparison marking first time that an used to remove telluric variability from time-series observations another dwarf. find H$\alpha$ in emission both LTT~1445B and C, well flare one data sets LTT~1445C. These contaminated are removed analysis. construct broadband transit light curve 620--1020 nm. Binned 3-minute bins we achieve rms 49 ppm curve. transmission spectrum 20 spectrophotometric each spanning nm compare it models clear, 1$\times$ solar composition rule out this atmospheric case surface pressure 10 bars $3.2 \sigma$ confidence, 1 bar $3.1 confidence. Upcoming secondary eclipse JWST will further probe cases high mean molecular weight atmosphere, hazy or cloudy no atmosphere all on world.