Discovery of two infrared objects with strong ice absorption in the AKARI slit-less spectroscopic survey of the Galactic Plane

TL;DR

This paper reports the discovery of two infrared objects with deep ice absorption features in the Galactic plane, suggesting they are embedded young stellar objects or background stars behind dense clouds, challenging standard YSO models.

Contribution

First detection of such ice-rich objects outside known star-forming regions using AKARI's slit-less spectroscopic survey.

Findings

Both objects show deep H2O, CO2, and CO ice absorption features.

Objects exhibit warm CO gas absorption indicating embedded YSO characteristics.

Spectral energy distributions peak at 5 microns, inconsistent with standard YSO models.

Abstract

We discover two infrared objects that show deep absorption features of H2O, CO2, and CO ices in the AKARI/Infrared Camera (IRC) slit-less spectroscopic survey of the Galactic plane in 2.5--13 micron. Both objects are located neither in known star-forming regions nor in known dense clouds. For one of the objects, Object 1, we successfully extract a spectrum from 2.5 to 13 micron, which also shows several absorption features in 5--13 micron, including deep silicate absorption at 10 micron. For the other object, Object 2, only a spectrum from 3.1 to 5 micron 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 micron and decrease towards 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.