c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase emission lines

Context. A survey of mid-infrared gas-phase emission lines of H-2, H2O and various atoms toward a sample of 43 embedded low-mass young stars in nearby star-forming regions is presented. The sources are selected from the Spitzer Cores to Disks (c2d) legacy program. Aims. The environment of embedded protostars is complex both in its physical structure (envelopes, outflows, jets, protostellar disks) and the physical processes (accretion, irradiation by UV and/or X-rays, excitation through slow and fast shocks) which take place. The mid-IR spectral range hosts a suite of diagnostic lines which can distinguish them. A key point is to spatially resolve the emission in the Spitzer-IRS spectra to separate extended PDR and shock emission from compact source emission associated with the circumstellar disk and jets. Methods. An optimal extraction method is used to separate both spatially unresolved (compact, up to a few hundred AU) and spatially resolved (extended, thousand AU or more) emission from the IRS spectra. The results are compared with the c2d disk sample and literature PDR and shock models to address the physical nature of the sources. Results. Both compact and extended emission features are observed. Warm (T-ex few hundred K) H-2, observed through the pure rotational H-2 S(0), S(1) and S(2) lines, and [S I] 25 mu m emission is observed primarily in the extended component. [S I] is observed uniquely toward truly embedded sources and not toward disks. On the other hand hot (T-ex greater than or similar to 700 K) H-2, observed primarily through the S(4) line, and [Ne II] emission is seen mostly in the spatially unresolved component. [Fe II] and [Si II] lines are observed in both spatial components. Hot H2O emission is found in the spatially unresolved component of some sources. Conclusions. The observed emission on >= 1000 AU scales is characteristic of PDR emission and likely originates in the outflow cavities in the remnant envelope created by the stellar wind and jets from the embedded young stars. Weak shocks along the outflow wall can also contribute. The compact emission is likely of mixed origin, comprised of optically thick circumstellar disk and/or jet/outflow emission from the protostellar object.