Discovery of Two Infrared Objects with Strong Ice Absorption in the Akari Slitless Spectroscopic Survey of the Galactic Plane

Abstract

We discover two infrared objects that show deep absorption features of H 2 O, CO 2 , and CO ices in the AKARI/Infrared Camera slitless spectroscopic survey of the Galactic plane in 2.5–13 μ m. Both objects are neither located in known star-forming regions nor in known dense clouds. For one of the objects, Object 1, we successfully extract spectrum from 2.5–13 μ m, which also shows several absorption features in 5–13 μ m, including deep silicate absorption at 10 μ m. For the other object, Object 2, only spectrum from 3.1–5 μ m is reliably extracted due to the presence of nearby overlapping objects and faint nebulosity. Both objects show warm (>100 K) CO gas absorption in addition to the ice absorption features, suggesting that they are embedded young stellar objects (YSOs). On the other hand, both objects have spectral energy distributions (SEDs) that peak at around 5 μ m and decrease toward longer wavelengths. These characteristics of the SEDs and the presence of deep absorption features cannot easily be accounted for by standard YSO models. They may be explained as background stars behind dense clouds. We discuss possible nature of the objects and implications of the present discovery.