Precise Radial Velocities of Cool Low Mass Stars With iSHELL

TL;DR

This paper presents a new methodology for obtaining precise near-infrared radial velocity measurements of cool low-mass stars using the iSHELL spectrograph, achieving high precision suitable for exoplanet detection.

Contribution

The authors develop a novel analysis pipeline and demonstrate high-precision RV measurements in the NIR, enabling better exoplanet searches around faint, active low-mass stars.

Findings

Achieved 5 m/s RV precision over a year for Barnard's Star and 61 Cygni A.

Achieved 3 m/s RV precision over a month for GJ 15 A.

Demonstrated iSHELL's potential for exoplanet mass determination and searches around active stars.

Abstract

The coolest dwarf stars are intrinsically faint at visible wavelengths and exhibit rotationally modulated stellar activity from spots and plages. It is advantageous to observe these stars at near infrared (NIR) wavelengths (1-2.5 microns) where they emit the bulk of their bolometric luminosity and are most quiescent. In this work we describe our methodology and results in obtaining precise radial velocity (RV) measurements of low mass stars using K-band spectra taken with the R~80,000 iSHELL spectrograph and the NASA Infrared Telescope Facility (IRTF) using a methane isotopologue gas cell in the calibration unit. Our novel analysis pipeline extracts RVs by minimizing the RMS of the residuals between the observed spectrum and a forward model. The model accounts for the gas cell, tellurics, blaze function, multiple sources of quasi-sinusoidal fringing, and line spread function of the spectrograph (LSF). The stellar template is derived iteratively using the target observations themselves through averaging barycenter-shifted residuals. We have demonstrated 5 ms^-1 precision over one year timescales for the M4 dwarf Barnard's Star and K dwarf 61 Cygni A, and 3 ms^-1 over a month for the M2 dwarf GJ 15 A. This work demonstrates the potential for iSHELL to determine dynamical masses for candidate exoplanets discovered with the NASA TESS mission, and to search for exoplanets orbiting moderately active and/or young K and M dwarfs.