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Observational Characteristics of the First Protostellar Cores

First protostellar cores are young stellar objects in the earliest evolutionary stage. They are hydrostatic objects formed soon after the central portions of star-forming cores become optically thick to dust emission. We consider their characteristics in the emitted radiation, and discuss their evolution with increasing mass of the cores. Particular attention is paid to detailed radiative and chemical processes in the postshock relaxation layer located at the surface of the core, where the majority of radiation is emitted. Most of the radiation is originally emitted in the dust continuum in mid-infrared wavelength (~10-30 micron), which reprocessed to far-infrared with ~100-200 micron. Although some fraction (~0.1) of the radiation energy is emitted in the H2O lines at the accretion shock, most is absorbed and reemitted in the dust continuum in the envelope. The H2O lines account for at most ~1/100 of the observed luminosity. If a cavity is present in the envelope due to outflow or rotation, the dust and H2O line emission in the mid-infrared wavelength from the shock can be observed directly, or as a reflection nebula. Among forthcoming observational facillities, SPICA is the most suitable for detecting either direct or processed radiation from first-core objects.