2015A&A...578A..82C

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. We use diffraction-limited near-IR (VLT/NACO) and mid-IR (VLT/VISIR) images to reveal the different morphologies at various wavelengths. Likewise, we investigate the stellar and nebular content of the region with VLT/SINFONI integral field spectroscopy with a resolution R∼1500 at H+K bands. Five of the seven point sources in this region show L-band excess emission. A geometric match is found between the H₂ 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 H₂ emission is typical for dense photodissociation regions (PDRs), which are initially far-ultraviolet pumped 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 1-20µm is derived for three objects; we obtain 2.0x10³L_☉ for IRS5A, 13L_☉ for IRS5C, and 10L_☉ for B273A. IRS5 might be a young quadruple system. Its primary star IRS5A is confirmed to be a high-mass protostellar object (∼9M_☉, ∼1x10⁵yrs); it might have terminated accretion due to the feedback from stellar activities (radiation pressure, outflow) and the expanding HII region of M17. The object UC1 might also have terminated accretion because of the expanding hypercompact HII region, which it ionizes. The disk clearing process of the low-mass young stellar objects in this region might be accelerated by the expanding HII region. The outflows driven by UC1 are running 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 H₂ emission filament and mid-emission. The H₂ 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>10⁵cm^–3 and T≃575K and cooler atomic gas with n_H∼3.7x10³-1.5x10⁴cm^–3 and T∼50-200K.