High precision radial velocity measurements in the infrared A first assessment of the RV stability of CRIRES

High precision radial velocity (RV) measurements in the near infrared are in high demand, especially in the context of exoplanet search campaigns shifting their interest to late type stars in order to detect planets with ever lower mass or targeting embedded pre-main-sequence objects. ESO commissioned a new spectrograph at the VLT - CRIRES - designed for high resolution near-infrared spectroscopy with a comparably broad wavelength coverage and the possibility of using gascells to provide a stable RV zero-point. Here, we investigate the intrinsic short-term RV stability of CRIRES, both with gascell calibration data and on-sky measurements using the absorption lines of the Earth's atmosphere imprinted in the source spectrum as a local RV rest frame. Moreover, we also investigate for the first time the intrinsic stability of telluric lines at 4100 nm for features originating in the lower troposphere. Our analysis of nearly 5 h of consecutive observations of MS Vel, a M2II bright giant centred on two SiO first overtone band-heads at 4100 nm, demonstrates that the intrinsic short- term stability of CRIRES is very high, showing only a slow and fully compensateable drift of up to 60m/s after 4.5 h. The radial velocity of the telluric lines is constant down to a level of approx. +/- 10 m/s (or 7/1000 of one pixel). Utilising the same telluric lines as a rest frame for our radial velocity measurements of the science target, we obtain a constant RV with a precision of approx. +/- 20m/s for MS Vel, as expected for a M-giant.