Empirical calibration of the lambda 4000 angstrom break

Abstract

Empirical fitting functions, describing the behaviour of the lambda 4000 Angstrom break, D-4000, in terms of effective temperature, metallicity and surface gravity, are presented. For this purpose, the break has been measured in 392 stars from the Lick/IDS Library. We have followed a very detailed error treatment in the reduction and fitting procedures, allowing for a reliable estimation of the break uncertainties. This calibration can be easily incorporated into stellar population models to provide accurate predictions of the break amplitude for, relatively old, composite systems.