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gaia early data release 3 documentation

6.1 Introduction

6.1.2 Data products in Gaia DR2

The data products for each source processed by the spectroscopic pipeline are: radial velocity and uncertainty, the number of transits of the source used to estimate the radial velocity, and the parameters Teff (effective temperature), logg (surface gravity) and [Fe/H] (metallicity) of the template synthetic spectrum used to measure the radial velocity. In Table 6.1 the products of the spectroscopic processing are listed, together with the notation used in this chapter and the name in the Gaia Archive data base. To highlight that the Gaia EDR3 spectroscopic data are not new data but copied from Gaia DR2, the archive name has been changed to contain the prefix ‘dr2’ (Table 6.1).

Table 6.1: Spectroscopic content of Gaia EDR3
Data product Units Notation Gaia Archive name
Median radial velocity  km s-1 Vrad dr2_radial_velocity
Radial velocity uncertainty  km s-1 ϵVrad dr2_radial_velocity_error
Number of transits N dr2_ rv_nb_transits
Template temperature K Teff dr2_ rv_template_teff
Template surface gravity dex logg dr2_rv_template_logg
Template metallicity dex [Fe/H] dr2_rv_template_fe_h

The main data products of the RVS data processing are the radial velocities (Vrad). For each transit of each source that enters the pipeline (only a small number of the sources observed by the RVS are processed in Gaia DR2, see Section 6.2.1) the radial velocity is computed. The median value of these single-transit measurements, corrected by the Gaia velocity with respect to the solar-system barycentre (Vradt), is the dr2_radial_velocity associated to the source:

Vrad=median(Vradt) (6.1)

The dr2_radial_velocity_error (ϵVrad) is the uncertainty on the median, to which a constant shift of 0.11  km s-1 has been added to take into account of a calibration floor contribution:

ϵVrad=σVrad2+0.112 (6.2)

where σVrad is the uncertainty on the median:

σVrad=π2.σ(Vradt)N (6.3)

σ(Vradt) is the standard deviation of the single-transit radial velocities and N the number of transits for which a Vradt has been obtained.

The number of transits, N = dr2_rv_nb_transits, that have been combined to produce the radial velocity Vrad is also provided. It permits to take back the standard deviation and have the information on the dispersion of the single-transit measurements.

The radial velocities Vradt are measured by a fit of synthetic spectra templates (Section 6.4.7). The information on the synthetic spectrum used to determine the radial velocity of the star is provided via its atmospheric parameters Teff, logg and [Fe/H], named respectively dr2_rv_template_teff, dr2_rv_template_logg and dr2_rv_template_fe_h.

In Gaia EDR3 7.2 million stars have a dr2_radial_velocity. The median dr2_radial_velocity_error is 1.05  km s-1.