Precise reddening and metallicity of NGC 6752 from FLAMES spectra

Accurate reddenings for Globular Clusters could be obtained by comparing the colour-temperature relation obtained using temperatures from a reddening-free indicator (Hα) with that given by standard colour-temperature calibrations. The main difficulty in such derivations is the large errors in temperatures for individual stars due to uncertainties on the removal of instrumental signature for each individual star. The large multiplexing opportunity offered by FLAMES at VLT2 allowed us to obtain spectra centred on Hα at a resolution of R = 6000 and 5<S/N<50 for 120 stars near the turn-off of NGC 6752 with GIRAFFE from a single 1300 seconds exposure. This set of spectra was used to derive effective temperatures from fittings of Hα profiles with typical errors of about ±200 K and reddening estimates with individual errors of 0.05 mag. Averaging all individual reddenings, a high precision reddening estimate was obtained for the cluster: E(B-V)=0.046 ± 0.005. The same exposure provided UVES spectra of seven stars near the red giant branch bump at a resolution of 40 000 and 20<S/N<40. These spectra, combined with temperatures from colours (corrected for our high precision reddening value), provided Fe abundances with internal errors of 0.026 dex and with average metallicity [Fe/H] = -1.48 ± 0.01 ± 0.06 dex (random + systematic). Abundances were obtained for several other elements, allowing e.g. an accurate estimate of the ratio between the α-elements and Fe ([ α/Fe] = +0.27 ± 0.01). The O-Na anticorrelation is evident from our UVES data, in agreement with past results. This analysis shows the power of FLAMES for analysis of globular clusters: the accurate reddenings and metal abundances obtained by a procedure like that described here, combined with distance determinations from cluster dynamics or main sequence fitting, and high quality colour-magnitude diagrams, could allow derivation of ages with errors below 1 Gyr for individual globular clusters.