VLT near- to mid-IR imaging and spectroscopy of the M 17 UC1 – IRS5 region⋆
1 Purple Mountain Observatory &
Key Laboratory for Radio Astronomy, Chinese Academy of
Sciences, 2 West Beijing Road,
210008 Nanjing, PR China
2 Astronomisches Institut, Ruhr – Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany
3 University of Chinese Academy of Sciences, 100039 Beijing, PR China
4 Instituto de Astronomía, Universidad Católica del Norte, Avenida Angamos 0610, Casilla 1280 Antofagasta, Chile
Accepted: 16 February 2015
Aims. We investigate the surroundings of the hypercompact H ii region M 17 UC1 to probe the physical properties of the associated young stellar objects and the environment of massive star formation.
Methods. 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.
Results. 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 LIR in the range 1−20 μm is derived for three objects; we obtain 2.0 × 103 L⊙ for IRS5A, 13 L⊙ for IRS5C, and 10 L⊙ for B273A.
Conclusions. IRS5 might be a young quadruple system. Its primary star IRS5A is confirmed to be a high-mass protostellar object (∼9 M⊙, ∼1 × 105 yrs); it might have terminated accretion due to the feedback from stellar activities (radiation pressure, outflow) and the expanding H ii region of M 17. The object UC1 might also have terminated accretion because of the expanding hypercompact H ii region, which it ionizes. The disk clearing process of the low-mass young stellar objects in this region might be accelerated by the expanding H ii region. The outflows driven by UC1 are running south-north with its northeastern side suppressed by the expanding ionization front of M 17; 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 M 17 SW PDR, which is confirmed to have a clumpy structure with two temperature distributions: warm, dense molecular clumps with nH> 105 cm-3 and T ≈ 575 K and cooler atomic gas with nH ∼ 3.7 × 103−1.5 × 104 cm-3 and T ∼ 50 − 200 K.
Key words: stars: early-type / stars: individual: M17 UC1 / ISM: individual objects: M17 SW / photon-dominated region (PDR) / stars: formation
© ESO, 2015