Yonsei Evolutionary Population Synthesis (YEPS) Model. I. Spectroscopic
  Evolution of Simple Stellar Populations
  We present a series of papers on the year-2012 version of Yonsei Evolutionary
Population Synthesis (YEPS) model which is constructed on over 20 years of
heritage. This first paper delineates the spectroscopic aspect of integrated
light from stellar populations older than 1 Gyr. The standard YEPS is based on
the most up-to-date Yonsei-Yale stellar evolutionary tracks and BaSel 3.1 flux
libraries, and provides absorption line indices of the Lick/IDS system and
high-order Balmer lines for simple stellar populations as functions of stellar
parameters, such as metallicity, age and {\alpha}-element mixture. Special care
has been taken to incorporate systematic contribution from horizontal-branch
stars which alters the temperature-sensitive Balmer lines significantly,
resulting in up to 5 Gyr difference in age estimation of old, metal-poor
stellar populations. We also find that the horizontal branches exert an
appreciable effect not only on the Balmer lines but also on the
metallicity-sensitive lines including the magnesium index. This is critical to
explain the intriguing bimodality found in index distributions of globular
clusters in massive galaxies and to derive spectroscopic metallicities
accurately from various indices. A full set of the spectroscopic and
photometric YEPS model data of the entire parameter space is currently
downloadable at http://web.yonsei.ac.kr/cosmic/data/YEPS.htm
