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gaia early data release 3 documentation

6.5 Quality assessment and validation

6.5.2 Validation

When the Gaia DR2 processing was finished, the CU6 carried out a validation campaign off-line on all pipeline Vrad 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. 2019). 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:

  • Vrad outside the limit STA range ]-1000; 1000[  km s-1(only 70 stars satisfy this criterion);

  • coordinate uncertainties larger than 100 mas: i.e. σα2+σδ2>100 mas ( 0.3 %);

  • Vradt flagged as ambiguous in all transits by STA ( 20.5 %);

  • large radial velocity uncertainty (radial_velocity_error) σVrad20  km s-1( 49.4 %);

  • ‘hot’ stars having the associated template rv_template_teff Teff  7000 K ( 19 %);

  • ‘cool’ stars with rv_template_teff Teff  3500 K ( 5.5 %);

  • detected emission lines (templates for emission-line stars are not available and the Vradt estimation cannot be done properly) ( 0.2 %);

  • too faint flux measured in the window: internal GRVS >14 ( 0.08 %);

  • suspected binaries detected as double-lined in > 10 % of the transits ( 2.3 %);

  • 397 stars with |Vrad| 500  km s-1(the spectra of the 627 stars found to have |Vrad| 500  km s-1, after the above filters had been applied, have been visually inspected and the radial velocities obtained with damaged spectra have been excluded).

Some other data have been excluded by the Gaia DR2 validation campaign carried out by the CU9 (Arenou et al. 2018). In particular, some stars have been excluded because they have been found to be the duplication of other stars. In Gaia EDR3 additional 14 800 radial velocities are excluded (Section 6.1.1) and in total there are 7 209 832 stars with a radial velocity. The radial velocity uncertainties (dr2_radial_velocity_error) are < 20  km s-1(the median value is 1  km s-1). The Teff of the associated templates is in the range: 3500< rv_template_teff <7000 K.

The accuracy of the Vrad 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 (ΔVres=Vrad-Vradcat) as a function of the external GRVS. The RVS Vrad of the bright stars do not exhibit a significant offset with the APOGEE and the CU6GB Vradcat, but they show an offset with SIM and RAVE. For stars fainter than GRVS 9, the RVS Vrad exhibit an offset increasing with magnitude with the other catalogues, and reaching approximately 500  m s-1 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 300  m s-1 (Cropper et al. 2018), and the 500  m s-1 shift for the stars at the faint end is acceptable for this first data release.

Figure 6.8: Median of the radial velocity residuals as a function of the external GRVS magnitude. A minimum of 20 stars per bin is required to calculate the median of the radial velocity residuals. The shaded area represent the robust uncertainty and are limited by the 15.85th and 84.15th percentiles. Figure from Katz et al. (2019).

Figure 6.9 presents the Gaia DR2 precision as a function of the external GRVS 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 Teff = 6750 K templates (red curve) have better precision than those processed with Teff = 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 Teff = 6750 K which comes from the ground-based parameters (Section 6.2.3). The pre-launch end-of-mission requirement on the precision was 1  km s-1for the G2V stars with GRVS 12 (Cropper et al. 2018). The radial velocities obtained for GRVS <11.5 in this first data release are already exceeding the end-of-mission requirement.

Figure 6.9: Precision: median of the radial velocity uncertainties (ϵVrad) as a function of the external GRVS magnitude. Curves: Teff templates (rv_template_teff). Figure from Katz et al. (2019).
Figure 6.10: RVS spectra of six Hipparcos stars with different spectral types. The coolest star (M6 type) spectrum is dominated by the molecular TiO band. These stars, with Teff < 3500 K, are excluded from Gaia DR2 . The spectral types K3 and G0 are included in Gaia DR2 , the spectra are dominated by the CaII lines, and as the Teff increases the weak neutral lines such as Fe, Si, Ti are less numerous and their contrast with respect to the continuum decreases, this results in less information to derive the radial velocity, and explains why the precision is better for the cooler stars. In the F0 star the H lines start to appear and template mismatch on logg results in imprecise Vrad. Stars with Teff 7000 K are excluded from Gaia DR2 . Figure by Olivier Marchal, image of the week