Disentangling the excitation conditions of the dense gas in M17 SW
  We probe the chemical and energetic conditions in dense gas created by
radiative feedback through observations of multiple CO, HCN and HCO$^+$
transitions toward the dense core of M17 SW. We used the dual band receiver
GREAT on board the SOFIA airborne telescope to obtain maps of the $J=16-15$,
$J=12-11$, and $J=11-10$ transitions of $^{12}$CO. We compare these maps with
corresponding APEX and IRAM 30m telescope data for low- and mid-$J$ CO, HCN and
HCO$^+$ emission lines, including maps of the HCN $J=8-7$ and HCO$^+$ $J=9-8$
transitions. The excitation conditions of $^{12}$CO, HCO$^+$ and HCN are
estimated with a two-phase non-LTE radiative transfer model of the line
spectral energy distributions (LSEDs) at four selected positions. The energy
balance at these positions is also studied. We obtained extensive LSEDs for the
CO, HCN and HCO$^+$ molecules toward M17 SW. The LSED shape, particularly the
high-$J$ tail of the CO lines observed with SOFIA/GREAT, is distinctive for the
underlying excitation conditions. The critical magnetic field criterion implies
that the cold cloudlets at two positions are partially controlled by processes
that create and dissipate internal motions. Supersonic but sub-Alfv\'enic
velocities in the cold component at most selected positions indicates that
internal motions are likely MHD waves. Magnetic pressure dominates thermal
pressure in both gas components at all selected positions, assuming random
orientation of the magnetic field. The magnetic pressure of a constant magnetic
field throughout all the gas phases can support the total internal pressure of
the cold components, but it cannot support the internal pressure of the warm
components. If the magnetic field scales as $B \propto n^{2/3}$, then the
evolution of the cold cloudlets at two selected positions, and the warm
cloudlets at all selected positions, will be determined by ambipolar diffusion.
