Impact of stellar companions on precise radial velocities

ASD, IMCCE-CNRS UMR8028, Observatoire de Paris, UPMC, 77 avenue Denfert-Rochereau, 75014 Paris, FranceReceived January 9, 2014; accepted November 29, 2012ABSTRACTContext.With the announced arrival of instruments such as ESPRESSO one can expect that several systematic noise sources on themeasurement of precise radial velocity will become the limiting factor instead of photon noise. A stellar companion within the fiberis such a possible noise source.Aims.With this work we aim at characterizing the impact of a stellar companion within the fiber to radial velocity measurementsmade by fiber-fed spectrographs. We consider the contaminant star either to be part of a binary system whose primary star is the targetstar, or as a background/foreground star.Methods. To carry out our study, we used HARPS spectra, co-added the target with contaminant spectra, and then compared theresulting radial velocity with that obtained from the original target spectrum. We repeated this procedure and used different tunableknobs to reproduce the previously mentioned scenarios.Results.We find that the impact on the radial velocity calculation is a function of the difference between individual radial velocities,of the difference between target and contaminant magnitude, and also of their spectral types. For the worst-case scenario in whichboth target and contaminant star are well centered on the fibe r, the maximum contamination for a G or K star may be higher than 10cm/s, on average, if the difference between target and contaminant magnitude is ∆m < 10, and higher than 1 m/s if ∆m < 8. If thetarget star is of spectral type M, ∆m < 8 produces the same contamination of 10 cm/s, and a contamination may be higher than 1 m/sif ∆m < 6.Key words. Planets and satellites: detection - Techniques: radial velocities