A Multiwavelength Study of Young Massive Star-Forming Regions. III.
  Mid-Infrared Emission
  We present mid-infrared (MIR) observations, made with the TIMMI2 camera on
the ESO 3.6 m telescope, toward 14 young massive star-forming regions. All
regions were imaged in the N band, and nine in the Q band, with an angular
resolution of ~ 1 arcsec. Typically, the regions exhibit a single or two
compact sources (with sizes in the range 0.008-0.18 pc) plus extended diffuse
emission. The Spitzer-Galactic Legacy Infrared Mid-Plane Survey Extraordinaire
images of these regions show much more extended emission than that seen by
TIMMI2, and this is attributed to polycyclic aromatic hydrocarbon (PAH) bands.
For the MIR sources associated with radio continuum radiation (Paper I) there
is a close morphological correspondence between the two emissions, suggesting
that the ionized gas (radio source) and hot dust (MIR source) coexist inside
the H II region. We found five MIR compact sources which are not associated
with radio continuum emission, and are thus prime candidates for hosting young
massive protostars. In particular, objects IRAS 14593-5852 II (only detected at
17.7 microns) and 17008-4040 I are likely to be genuine O-type protostellar
objects. We also present TIMMI2 N-band spectra of eight sources, all of which
are dominated by a prominent silicate absorption feature (~ 9.7 microns). From
these data we estimate column densities in the range (7-17)x10^22 cm^-2, in
good agreement with those derived from the 1.2 mm data (Paper II). Seven
sources show bright [Ne II] line emission, as expected from ionized gas
regions. Only IRAS 123830-6128 shows detectable PAH emission at 8.6 and 11.3
microns.
