Yonsei evolutionary population synthesis (YEPS). II. Spectro-photometric
  evolution of helium-enhanced stellar populations
  The discovery of multiple stellar populations in Milky Way globular clusters
(GCs) has stimulated various follow-up studies on helium-enhanced stellar
populations. Here we present the evolutionary population synthesis models for
the spectro-photometric evolution of simple stellar populations (SSPs) with
varying initial helium abundance ($Y_{\rm ini}$). We show that $Y_{\rm ini}$
brings about {dramatic} changes in spectro-photometric properties of SSPs. Like
the normal-helium SSPs, the integrated spectro-photometric evolution of
helium-enhanced SSPs is also dependent on metallicity and age for a given
$Y_{\rm ini}$. {We discuss the implications and prospects for the
helium-enhanced populations in relation to the second-generation populations
found in the Milky Way GCs.} All of the models are available at
\url{http://web.yonsei.ac.kr/cosmic/data/YEPS.htm}.
