Model Atmospheres for Irradiated Giant Stars: Implications for the Galactic Center

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-artstellar atmosphere code, we calculate the infrared spectra of such irradiatedand transformed stars. We show that the original spectrum of the star, which isdominated by molecular bands, changes dramatically when irradiated even by alow-luminosity AGN ($L_{\rm X} = 10^{33}$ erg s$^{-1}$), becoming dominated byatomic lines in absorption. We study the changes in the spectrum of low-masscarbon- 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 aresimilar to those of the faintest S-cluster stars observed in the GC. Thespectrum of a star irradiated by a much brighter AGN, like that powered by atidally disrupted star, is very different from that of any star currentlyobserved near the GC. For the first time we have discovered that the structureof the atmosphere of an irradiated giant changes dramatically and induces adouble inversion layer. We show that irradiation at the current level canexplain the observed trend of CO band intensities decreasing as a function ofincreasing proximity to Sg $A^{*}$. This may indicate that (contrary toprevious claims) there is no paucity of old giants in the GC, which coexistsimultaneously with young massive stars.Comment: Submitted to ApJ; typo in name correcte