Radial Velocity with Gaia
The radial velocity of a star, denoted vr, is its velocity along the line of sight of an observer. It is calculated from the Doppler shift in the lines of stellar spectra, taking into account the nature of the star. By comparing the observed spectra of a star with reference spectra we can measure the wavelength shift of the reference lines and thus determine vr. A star receding from us has a redshifted spectrum and a positive radial velocity, while a star coming towards us has a blueshifted spectrum and a negative radial velocity.
Stellar radial velocities are needed if we wish to know the space velocity of stars with respect to the Sun. They are thus essential to our understanding of the kinematics of our Galaxy.
There are three reasons for considering the parallel acquisition of radial velocities with Gaia (as opposed to a ground-based campaign):
- the astrometric measurements supply only two components of the space motion of the target stars (vr is needed for proper kinematical or dynamical studies); the acquisition of such vast numbers of radial velocities from the ground is prohibitive;
- measurements of vr at a number of widely-separated epochs is a powerful method of detecting and characterizing binary systems;
- an independent knowledge of the radial velocity will allow us to disentangle true orbital acceleration (due to multiplicity) and the effect of perspective acceleration.
Onboard acquisition of radial velocities with Gaia is both feasible and relatively simple. In principle one would like to obtain as accurate as possible radial velocity for all Gaia targets. For practical purposes two regimes can be identified:
- bright stars (V < 15 mag), which will be of individual interest, and for which vr will also be useful as an indicator of multiplicity and for the determination of perspective acceleration. For such stars an accuracy of 1-2 km per second in vr may be obtained;
- faint stars (fainter than 15 mag), which will be mostly distant stars and of interest as tracers of Galactic dynamics. For these, an accuracy of 5-10 km per second in vr will be sufficient for statistical purposes.
Most Gaia stars will be intrinsically red, and made even redder by interstellar absorption, so a red spectral region is to be preferred for the Gaia spectrograph. To maximize the radial velocity signal even for metal-poor stars, strong saturated lines are desirable. Broad lines also allow the radial velocity to be accurately derived from even moderate resolution spectra. The Ca II triplet around 860nm will be used.