c2d Spitzer IRS spectra of embedded low-mass young stars: gas-phase
  emission lines
  A survey of mid-IR gas-phase emission lines of H2, 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. 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. A key point is to
spatially resolve the emission in the Spitzer-IRS spectra. An optimal
extraction method is used to separate both spatially unresolved (compact, up to
a few 100 AU) and spatially resolved (extended, 1000 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.
Both compact and extended emission features are observed. Warm (Tex few 100 K)
H2, observed through the pure rotational H2 S(0), S(1) and S(2) lines, and [S
I] 25 mu 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 (Tex>700 K) H2, observed primarily through the S(4) line, and
[Ne II] emission is seen mostly in the compact component. [Fe II] and [Si II]
lines are observed in both spatial components. Hot H2O emission is found in the
compact component of some sources. 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.
