Multiepoch radial velocity observations of L dwarfs

We report on the development of a technique for precise radial velocity measurements of cool stars and brown dwarfs in the near infrared. Our technique is analogous to the iodine (I₂) absorption cell method that has proven so successful in the optical regime. We rely on telluric CH₄ absorption features to serve as a wavelength reference, relative to which we measure Doppler shifts of the CO and H₂O features in the spectra of our targets. We apply this technique to high-resolution (R ≈ 50,000) spectra near 2.3 μm of nine L dwarfs taken with the Phoenix instrument on Gemini South and demonstrate a typical precision of 300 m s^-1. We conduct simulations to estimate our expected precision and show that our performance is currently limited by the signal-to-noise ratio (S/N) of our data. We present estimates of the rotational velocities and systemic velocities of our targets. With our current data we are sensitive to companions with M sin i > 2 M _J in orbits with periods less than 3 days. We identify no companions in our current data set. Future observations with improved S/N should result in radial velocity precision of 100 m s^-1 for L dwarfs.