6.1 Introduction 6.1 Introduction 6.1.2 Overview of the spectroscopic processing

6.1.1 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 $T_{\rm eff}$ (effective temperature), $\log g$ (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.

Table 6.1: Spectroscopic content of Gaia DR2

Data product	Units	Notation	Gaia Archive name
Median radial velocity	km s ${}^{-1}$	$V_{\rm rad}$	radial_velocity
Radial velocity uncertainty	km s ${}^{-1}$	$\epsilon_{V_{\rm rad}}$	radial_velocity_error
Number of transits		$N$	rv_nb_transits
Template temperature	K	$T_{\rm eff}$	rv_template_teff
Template surface gravity	dex	$\log g$	rv_template_logg
Template metallicity	dex	[Fe/H]	rv_template_fe_h

The main data products of the RVS data processing are the radial velocities ( $V_{\rm rad}$ ). 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 ( $V_{\rm rad}^{\rm t}$ ), is the radial_velocity associated to the source:

V_{\rm rad}=median(V_{\rm rad}^{\rm t})

(6.1)

The radial_velocity_error ( $\epsilon_{V_{\rm rad}}$ ) 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:

\epsilon_{V_{\rm rad}}=\sqrt{\sigma^{2}_{V_{\rm rad}}+0.11^{2}}

(6.2)

where $\sigma_{V_{\rm rad}}$ is the uncertainty on the median:

\sigma_{V_{\rm rad}}=\sqrt{\frac{\pi}{2}}.\frac{\sigma(V_{\rm rad}^{\rm t})}{% \sqrt{N}}

(6.3)

$\sigma(V_{\rm rad}^{\rm t})$ is the standard deviation of the single-transit radial velocities and $N$ the number of transits for which a $V_{\rm rad}^{\rm t}$ has been obtained.

The number of transits, $N$ = rv_nb_transits, that have been combined to produce the radial velocity $V_{\rm rad}$ 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 $V_{\rm rad}^{\rm t}$ 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 $T_{\rm eff}$ , $\log g$ and [Fe/H], named respectively rv_template_teff, rv_template_logg and rv_template_fe_h.

In Gaia DR2 $\sim$ 7.2 million stars have a radial_velocity and the median radial_velocity_error is $\sim$ 1.05 km s ${}^{-1}$ .