CHAMP+ observations of warm gas in M17 SW
  Since the main cooling lines of the gas phase are important tracers of the
interstellar medium in Galactic and extragalactic sources, proper and detailed
understanding of their emission, and the ambient conditions of the emitting
gas, is necessary for a robust interpretation of the observations. With high
resolution (7"-9") maps (~3x3 pc^2) of mid-J molecular lines we aim to probe
the physical conditions and spatial distribution of the warm (50 to few hundred
K) and dense gas (n(H_2)>10^5 cm^-3) across the interface region of M17 SW
nebula. We have used the dual color multiple pixel receiver CHAMP+ on APEX
telescope to obtain a 5'.3x4'.7 map of the J=6-5 and J=7-6 transitions of 12CO,
the 13CO J=6-5 line, and the {^3P_2}-{^3P_1} 370 um fine-structure transition
of [C I] in M17 SW. LTE and non-LTE radiative transfer models are used to
constrain the ambient conditions. The warm gas extends up to a distance of ~2.2
pc from the M17 SW ridge. The 13CO J=6-5 and [C I] 370 um lines have a narrower
spatial extent of about 1.3 pc along a strip line at P.A=63 deg. The structure
and distribution of the [C I] {^3P_2}-{^3P_1} 370 um map indicate that its
emission arises from the interclump medium with densities of the order of 10^3
cm^-3. The warmest gas is located along the ridge of the cloud, close to the
ionization front. An LTE approximation indicates that the excitation
temperature of the embedded clumps goes up to ~120 K. The non-LTE model
suggests that the kinetic temperature at four selected positions cannot exceed
230 K in clumps of density n(H_2)~5x10^5 cm^-3, and that the warm T_k>100 K and
dense (n(H_2)>10^4 cm^-3) gas traced by the mid-J 12CO lines represent just
about 2% of the bulk of the molecular gas. The clump volume filling factor
ranges between 0.04 and 0.11 at these positions.
