Empirical calibration of the 4000 A break

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. Table 1 lists the star sample, together with spectral types, adopted atmospheric parameters, break measurements, including errors, and residuals from the fitting functions.