  Irradiation of a stellar atmosphere by an external source (e.g. an AGN)
changes its structure and therefore its spectrum. Using a state-of-the-art
stellar atmosphere code, we calculate the infrared spectra of such irradiated
and transformed stars. We show that the original spectrum of the star, which is
dominated by molecular bands, changes dramatically when irradiated even by a
low-luminosity AGN ($L_{\rm X} = 10^{33}$ erg s$^{-1}$), becoming dominated by
atomic lines in absorption. We study the changes in the spectrum of low-mass
carbon- and oxygen-rich giant stars as they are irradiated by a modest AGN,
similar to the one at the Galactic center (GC). The resulting spectra are
similar to those of the faintest S-cluster stars observed in the GC. The
spectrum of a star irradiated by a much brighter AGN, like that powered by a
tidally disrupted star, is very different from that of any star currently
observed near the GC. For the first time we have discovered that the structure
of the atmosphere of an irradiated giant changes dramatically and induces a
double inversion layer. We show that irradiation at the current level can
explain the observed trend of CO band intensities decreasing as a function of
increasing proximity to Sg $A^{*}$. This may indicate that (contrary to
previous claims) there is no paucity of old giants in the GC, which coexist
simultaneously with young massive stars.
