Comparing radial velocities of atmospheric lines with radiosonde measurements

The precision of radial velocity (RV) measurements depends on the precision attained on the wavelength calibration. One of the available options is to use atmospheric lines as a natural, freely available wavelength reference. Figueira et al. measured the RV of O2 lines using High Accuracy Radial velocity Planet Searcher (HARPS) and showed that the scatter was only of ∼10 m s−1 over a time-scale of 6 yr. Using a simple but physically motivated empirical model, they demonstrated a precision of 2 m s−1, roughly twice the average photon noise contribution. In this paper, we take advantage of a unique opportunity to confirm the sensitivity of the telluric absorption lines’ RV to different atmospheric and observing conditions by means of contemporaneous in situ wind measurements. This opportunity is a result of the work done during site testing and characterization for the European Extremely Large Telescope (E-ELT). The HARPS spectrograph was used to monitor telluric standards while contemporaneous atmospheric data were collected using radiosondes. We quantitatively compare the information recovered by the two independent approaches. The RV model fitting yielded results similar to that of Figueira et al., with lower wind magnitude values and varied wind direction. The probes confirmed the average low wind magnitude and suggested that the average wind direction is a function of time as well. However, these results are affected by large uncertainty bars that probably result from a complex wind structure as a function of height. The two approaches deliver the same results in what concerns wind magnitude and agree on wind direction when fitting is done in segments of a couple of hours. Statistical tests show that the model provides a good description of the data on all time-scales, being always preferable to not fitting any atmospheric variation. The smaller the time-scale on which the fitting can be performed (down to a couple of hours), the better the description of the real physical parameters is. We then conclude that the two methods deliver compatible results, down to better than 5 m s−1 and less than twice the estimated photon noise contribution on O2 lines’ RV measurement. However, we cannot rule out that parameters α and γ (dependence on airmass and zero-point, respectively) have a dependence on time or exhibit some cross-talk with other parameters, an issue suggested by some of the results.