Empirical calibration of the λ4000 Å  break^*

¹ Departamento de Astrofísica, Facultad de Ciencias Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
² Instituto de Astronomía, U.N.A.M., Apdo. Postal 70-264, 04510 Mexico D.F., Mexico

Send offprint request to: J. Gorgas

Received: 18 March 1999
Accepted: 11 May 1999

Abstract

Empirical fitting functions, describing the behaviour of the λ4000 Åbreak, D₄₀₀₀, 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.