When the processing was finished, the CU6 carried out a validation campaign off-line on all pipeline results in order to assess the properties of the entire dataset and to identify the bad quality data to exclude from the Gaia DR2 publications (Katz et al. 2018).
The spectroscopic pipeline produced the median radial velocity of 9 816 603 stars from which 2 344 881 have been excluded due to bad quality identified after the internal validation. The radial velocities of the stars with the following properties have been excluded. The approximate percentage of the sources removed by each criterion is also indicated. Some of the sources satisfy more than one criterion, and the percentage depends on the order the filtering criteria are applied:
Some other data have been excluded by the validation campaign carried out by the CU9 (Section 1.2.2). In particular, some stars have been excluded because they have been found to be the duplication of other stars. In total, in Gaia DR2 there are radial velocities for million stars. They have radial_velocity_error 20 km s(the median value is km s). Their associated template is in the range: rv_template_teff K.
The accuracy of the measurements has been estimated by comparison with the external catalogues (Table 6.3) even if also the external catalogues can be affected by systematic zero-point shifts. Figure 6.8 shows the median radial velocity residuals () as a function of the external . The RVS of the bright stars do not exhibit a significant offset with the APOGEE and the CU6GB , but they show an offset with SIM and RAVE. For stars fainter than , the RVS exhibit an offset increasing with magnitude with the other catalogues, and reaching approximately 500 m s at the faint end. The reason for this is not yet understood but may be attributed, at least in part, to Charge Transfer Inefficiency due to radiation damage. The pre-launch end-of-mission requirement on the systematic uncertainty was m s (Cropper et al. 2018), and the 500 m s shift for the stars at the faint end is acceptable for this first data release.
Figure 6.9 presents the Gaia DR2 precision as a function of the external magnitude and
of effective temperature of the templates associated to the stars, rv_template_teff (the different curves). In general the radial velocity precision improves as the effective temperature decreases. This is the direct consequence of the morphology of the spectra depending on the temperature (see Figure 6.10). However some nearby curves show the opposite behaviour: the stars processed with = 6750 K templates (red curve) have better precision than those processed with = 6500 K templates (orange curve). The reason is that temperatures of 6500 K are associated to the star by the module DetermineAP (Section 6.4.4) and are more affected by template mismatch than the stars with = 6750 K which comes from the ground-based parameters (Section 6.2.3). The pre-launch end-of-mission requirement on the precision was km sfor the G2V stars with (Cropper et al. 2018). The radial velocities obtained for in this first data release are already exceeding the end-of-mission requirement.