Precise Radial Velocities of Cool Low Mass Stars With iSHELL

We present updates to our program of obtaining precise near infrared (NIR) radial velocities (RVs) with the R ~ 80,000 iSHELL spectrograph on the NASA Infrared Telescope Facility (IRTF) using a methane isotopologue gas cell in the calibration unit. Observing cool low mass stars provides a "Habitable Zone" shortcut through their lower mass, effective temperature, and larger reflex velocities from orbiting bodies. It is advantageous to observe these stars at NIR wavelengths where they emit the bulk of their bolometric luminosity and are most quiescent from rotationally modulated stellar activity. Our novel analysis pipeline extracts RVs by minimizing the RMS of the residuals between the observed spectrum and a forward model, and accounts for the gas cell, tellurics, blaze function, multiple sources of quasi-sinusoidal fringing, and the line spread function of the spectrograph (LSF). The stellar template is derived iteratively using the target observations themselves through averaging barycenter-shifted residuals. With iSHELL we are currently monitoring transiting candidates identified with the NASA TESS mission to determine dynamical masses. We have demonstrated < 3 m/s precision over one-year timescales for the M4 dwarf Barnard's Star, sufficient to detect Neptunes on a wide range of orbits, terrestrial planets on close in orbits, and measure the expected wavelength dependence of stellar activity for young and active stars.