H{gamma} & H{delta} absorption features

For each Balmer feature, two definitions, involving a narrow (~20{AA}) and a wide (~40{AA}) central bandpass, were measured. These four new Balmer indices augment 21 indices previously determined by Worthey et al., and polynomial fitting functions that give index strengths as a function of stellar temperature, gravity, and [Fe/H] are provided.

The new indices are folded into models for the integrated light of stellar populations, and predictions are given for single-burst stellar populations of a variety of ages and metallicities. Contrary to our initial hopes, the indices cannot break a degeneracy between burst age and burst strength in poststarbust objects, but they are successful mean-age indicators when used with sensitive metallicity indicators. An appendix gives data, advice, and examples of how to transform new spectra to the 25-index Lick IDS system.