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
e-mail: zwchen@pmo.ac.cn
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

Received: 1 September 2014
Accepted: 16 February 2015

Abstract

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 L_IR 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 n_H> 10⁵ cm^-3 and T ≈ 575 K and cooler atomic gas with n_H ∼ 3.7 × 10³−1.5 × 10⁴ cm^-3 and T ∼ 50 − 200 K.