Hγ and Hδ Absorption Features in Stars and Stellar Populations

The Hγ and Hδ absorption features are measured in a sample of 455 (out of an original 460) Lick/IDS stars with pseudo-equivalent width indices. For each Balmer feature, two definitions, involving a narrow (~20 A) and a wide (~40 A) central bandpass, are 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 post-starburst 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.