Cartoon illustrating the inner few 100 AU of a low-mass protostar (not to scale). Where the wind and UV photons interact directly with the envelope, an irradiated shock occurs which compresses the envelope by several orders of magnitude over short distances. On the side of the cavity walls facing the protostar, ions (C⁺, CH⁺, OH⁺) dominate emission and cooling. Neutral species (OH, H₂O, CO) dominate further into the envelope and on larger size scales. The disk hides the innermost parts of the red-shifted outflow lobe. Typical densities and temperatures are provided where relevant.