2011ApJ...728...78S

The first core is a hydrostatic object which is formed just before the stellar core formation and is one of the most important objects needed to understand the star formation process. However, it has not been observed yet. In this paper, we studied observational characteristics of the first core through radiative transfer calculation. We adopted the rotating first core model from the results of three-dimensional hydrodynamical simulations with a barotropic equation of state. The rotating first core has a radius of ∼10 AU, a thickness of ∼1 AU, and a mass of ∼0.1-0.01M_☉. The first core is covered with a thin (∼0.001 AU) accretion shock layer and radiates mainly in the mid-infrared (mid-IR; a few µm <λ < 50 µm). However, most mid-IR radiations are absorbed by the infalling envelope and this heating forms a 10 AU scale warm region (20-50 K) above the first core. As a result, the rotating first core is observed as an object that has cold spectral energy distribution (SED; the radiation of 12 µm or 24 µm is much darker than that of 70 µm) and low luminosity (∼0.1-0.01 L_☉). The peak of SED corresponds to the blackbody radiation of 20-30 K. The first core increases the luminosity and temperature gradually. We show that it is possible to identify the rotating first core with the ALMA in full operation phase. In the face-on view, the dust thermal emissions from the first core are characterized by a central bright core with a radius of a few AU and a low surface brightness disk of 10-20 AU scale, in which a weak spiral structure is seen with a 0".01 resolution if the object is in a nearby star-forming region with a distance of d = 150 pc. Another way to identify this object is to detect submillimeter OH lines from the shocked layer.