A grid of synthetic spectra and indices Fe5270, Fe5335, Mgb and Mg2 as a function of stellar parameters and [α/Fe]

We have computed a grid of synthetic spectra in the wavelength range lambdalambda 4600 - 5600 Angstrom using revised model atmospheres, for a range of atmospheric parameters and values of [alpha- elements / Fe] = 0.0 and + 0.4. The Lick indices Fe5270, Fe5335, Mgb and Mg-2 are measured on the grid spectra for FWHM = 2 to 8.3 Angstrom. Relations between the indices Fe5270, Fe5335 and Mg-2 and the stellar parameters effective temperature T-eff, log g, [ Fe / H] and [alpha / Fe], valid in the range 4000 less than or equal to T-eff less than or equal to 7000 K, are presented. These fitting functions are given for FWHM = 3.5 and 8.3 Angstrom. The indices were also measured for a list of 97 reference stars with well- known stellar parameters observed at ESO and OHP, and these are compared to the computed indices. Finally, a comparison of the indices measured on the observed spectra and those derived from the fitting functions based on synthetic spectra is presented.