2019MNRAS.483.5026L

We study the accuracy of spectroscopic radial velocities (RVs) by comparing spectroscopic and astrometric RVs for stars of the Hyades open cluster. We obtained High-Accuracy Radial velocity Planet Searcher (HARPS) spectra for a large sample of Hyades stars and combined them with accurate astrometric RVs based on Hipparcos and Gaia data. After cleaning the sample from binaries, RV variables, and outliers, 71 stars remained. The distribution of the observed RV difference (between spectroscopic and astrometric) is skewed and depends on the star right ascension. This is consistent with the Hyades cluster rotating at 42.3 m s^–1pc^–1. The two Hyades giants in the sample show, as predicted by gravitational redshift (GR), a spectroscopic RV that is blue-shifted with respect to the dwarfs, and the empirical GR slope is of 626 ± 131 m s^–1, in agreement with the theoretical prediction. The difference between spectroscopic and astrometric RVs is close to zero when considering the Gaia-based RVs corrected for cluster rotation. The mean difference is of -33 m s^–1 and the median is of -16 m s^–1, with a standard deviation of 347 m s^–1 being close to the expected cluster velocity dispersion. We also determine a new value of the cluster centroid spectroscopic RV: 39.36 ± 0.26 km s^–1. We finally discuss other phenomena that can influence the RV difference, such as cluster expansion, stellar rotation, stellar activity, general relativity, and Galactic potential. Clusters within the reach of current telescopes are expected to show differences of several hundreds m s^–1, depending on their position in the Galaxy.