Evaluating the stability of atmospheric lines with HARPS

Context. When searching for extrasolar systems using the radial velocity technique, the need for high-precision measurements implies that a precise wavelength calibration is required. The choice of the calibrator is a particularly important open question in the infra-red domain, where precision and achievements remain inferior to those in the optical. Aims. We investigate the long-term stability of atmospheric lines as a precise wavelength reference and analyze their sensitivity to different atmospheric and observing conditions. Methods. We use HARPS archival data for three bright stars, TauCeti, mu Arae, and e Eri, which span 6 years and include high-cadence measurements performed over several nights. We cross-correlate this data with an O(2) mask and evaluate both radial velocity and bisector variations to a photon noise level of 1 m/s. Results. We find that the telluric lines in the three data-sets are stable down to 10 m/s (rms) over the 6 years. We also show that the radial velocity variations can be modeled by simple atmospheric models, yielding a final precision of 1-2 m/s. Conclusions. The long-term stability of atmospheric lines was 10 m/s over six years, in spite of atmospheric phenomena. Atmospheric lines can be used as a wavelength reference for short timescale programs, yielding a precision of 5 m/s without any correction. A higher precision, of 2 m/s, can be reached if the atmospheric phenomena are corrected for using the simple atmospheric model described, making it a very competitive method even on long timescales.