VLT near- to mid-IR imaging and spectroscopy of the M17 UC1-IRS5 region
  We investigate the surroundings of the hypercompact HII region M17 UC1 to
probe the physical properties of the associated young stellar objects and the
environment of massive star formation. Five of the seven point sources in this
region show $L$-band excess emission. Geometric match is found between the H_2
emission and near-IR polarized light in the vicinity of IRS5A, and between the
diffuse mid-IR emission and near-IR polarization north of UC1. The H_2 emission
is typical for dense PDRs, which are FUV pumped initially and repopulated by
collisional de-excitation. The spectral types of IRS5A and B273A are B3-B7
V/III and G4-G5 III, respectively. The observed infrared luminosity L_IR in the
range 1-20 micron is derived for three objects; we obtain 2.0x10^3 L_\sun for
IRS5A, 13 L_\sun for IRS5C, and 10 L_\sun for B273A. IRS5 might be a young
quadruple system. Its primary star IRS5A is confirmed to be a high-mass
protostellar object (~ 9 M_\sun, ~1x10^5 yrs); it might have terminated
accretion due to the feedback from the stellar activities (radiation pressure,
outflow) and the expanding HII region of M17. UC1 might also have terminated
accretion because of the expanding hypercompact HII region ionized by itself.
The disk clearing process of the low-mass YSOs in this region might be
accelerated by the expanding HII region. The outflows driven by UC1 are running
in south-north with its northeastern side suppressed by the expanding
ionization front of M17; the blue-shifted outflow lobe of IRS5A is seen in two
types of tracers along the same line of sight in the form of H_2 emission
filament and mid-emission. The H_2 line ratios probe the properties of M17 SW
PDR, which is confirmed to have a clumpy structure with two temperature
distributions: warm, dense molecular clumps with n_H>10^5 cm^-3 and T~575 K and
cooler atomic gas with n_H~3.7x10^3-1.5x10^4 cm-3 and T~50-200 K.
