All 25 Lick/IDS spectral indices have been computed for the integrated light of simple stellar populations over broad ranges of age and metallicity and with effects from horizontal-branch stars fully implemented .
Our models employ \alpha -enhanced isochrones at the sub-solar metallicity regime , but solar-scaled ones at solar and super-solar metallicity .
We have also employed the updated response functions of Houdashelt et al .
at the solar and super-solar metallicity regime , so that we could assess the light-element enhancement phenomena seen from metal-rich early-type galaxies .
For Balmer indices a significant response was noted for H \gamma and H \delta when \alpha -elements are enhanced , but H \beta is rather \alpha - \it { in } sensitive .
We also find that our 5 and 12 Gyr models of H \gamma and H \delta overlap in the metal-poor regime because of changing populations of blue horizontal-branch stars .
Furthermore , for populations younger than 1 Gyr , Balmer lines become weaker in the metal-poor regime because the main-sequence turnoff is hotter than 10,000 K. We present models at fixed [ Fe/H ] ( rather than fixed heavy element mass fraction Z ) and compare to Milky Way globular clusters that have independently estimated mean [ Fe/H ] and [ \alpha /Fe ] .
Comparison of our models with observations of Milky Way and M31 globular clusters in index-index space are favorable , tracing the observations at a model age of 12 Gyr without any zero-point shifts that are needed by some other models .
The metallicity range of M31 globular clusters is similar to that of their Galactic counterparts .
We also verify Beasley et al. ’ s recent hypothesis of the existence of young and intermediate-age star clusters in M31 .
Contrary to the literature values , the Milky Way globular cluster NGC 6553 appears more metal-rich than NGC 6528 from metal indices .
We present H \delta and H \gamma Lick/IDS indices for the Lick/IDS sample of galaxies .
We confirm the well-known enhancement of Mg and Na relative to Fe and Ca among early-type galaxies , and its increase with increasing velocity dispersion .
There are distinct differences between globular clusters and galaxies in diagrams involving CN _ { 1 } and CN _ { 2 } , hinting that the globular cluster environment may be a special one in terms of the amount of N incorporated into stars .