An extensive radial velocity survey toward NGC 6253

The old and metal rich open cluster NGC 6253 was observed with the FLAMES multi-object spectrograph during an extensive radial velocity campaign monitoring 317 stars with a median of 15 epochs per object. All the targeted stars are located along the upper main sequence of the cluster between 14.8 $<$ V $<$ 16.5. Fifty nine stars are confirmed cluster members both by radial velocities and proper motions and do not show evidence of variability. We detected 45 variable stars among which 25 belong to NGC 6253. We were able to derive an orbital solution for 4 cluster members (and for 2 field stars) yielding minimum masses in between $sim$90 M$rm_J$ and $sim$460 M$rm_J$ and periods between 3 and 220 days. Simulations demonstrated that this survey was sensitive to objects down to 30 M$rm_J$ at 10 days orbital periods with a detection efficiency equal to 50%. On the basis of these results we concluded that the observed frequency of binaries down to the hydrogen burning limit and up to 20 days orbital period is around (1.5$pm$1.3)% in NGC 6253. The overall observed frequency of binaries around the sample of cluster stars is (13$pm$3)%. The median radial velocity precision achieved by the GIRAFFE spectrograph in this magnitude range was around $sim$240m$rm,s^{-1}$ ($sim$180 m$rm,s^{-1}$ for UVES). Based on a limited follow-up analysis of 7 stars in our sample with the HARPS spectrograph we determined that a precision of 35 m $rm s^{-1}$ can be reached in this magnitude range, offering the possibility to further extend the variability analysis into the substellar domain. Prospects are even more favourable once considering the upcoming ESPRESSO spectrograph at VLT.