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 (LX = 1033 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.

Publication Date



This is the pre-print of an article published by the American Astronomical Society. The final, published version is available here: https://doi.org/10.1086/513062

© 2007 The American Astronomical Society.

Also archived in: arXiv:astro-ph/0601527 v2 Jan 27 2006

Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

Document Type


Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus