THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 111:377, 1997 August

    Hgamma AND Hdelta ABSORPTION FEATURES IN STARS AND STELLAR POPULATIONS

                        GUY WORTHEY AND D. L. OTTAVIANI
   Department of Astronomy, University of Michigan, Ann Arbor, MI 48109-1090


                                   ABSTRACT

        The Hgamma and Hdelta 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, 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.

     Subject headings: galaxies: evolution -- galaxies: stellar content --
                       line: profiles -- stars: abundances