Diverse stages of star formation in the IRAS 18162-2048 region. Emergence of UV Feedback

Abstract

Methods: We obtained adaptive optics-assisted integral field spectroscopy in the near-infrared (IR) $K$ band ($1.93-2.47\mu \mathrm{m}$) with VLT/SINFONI, complemented by VLA X and C bands (3$-$6 cm) and ALMA band 3 ($\sim$3.3 mm) observations. Results: The near-IR continuum reveals two infrared sources, IRS 2 and IRS 7, while the main protostellar core IRAS 18162-2048 remains undetected up to $2.47\mu \mathrm{m}$. IRS 7 shows a peculiar Hydrogen Recombination Line Br$\gamma$ profile with a narrow emission component superimposed on a broad absorption feature, consistent with a B2/B3 zero-age main-sequence star. Extended H${}_2$ emission exhibits a `sawtooth' pattern in the excitation diagram, characteristic of UV radiation in a PDR rather than shock excitation. The radiative transfer model Cloudy reproduces the H${}_2$ ro-vibrational populations for $T_{\mathrm{gas}}=600$ K and $n_{\mathrm{H}}=7.9\times 10^3\mathrm{c}{\mathrm{m}}^{-3}$. VLA X and C bands observations reveal a compact radio source previously reported as a stationary condensation (SC) and coincident with IRS 7. For the first time, we detect IRS 7/SC in mm wavelengths. The spectral index in the 3$-$6 cm and 3.3 mm regime is consistent with optically thin free-free emission. Conclusions: Our near-IR and radio observations reveal that IRS 7/SC is a B2/B3 ZAMS star that has begun to photoionise its environment, giving rise to an extended PDR and a compact \ion{H}{ii} region. The coexistence of this source with the deeply embedded protostar IRAS 18162-2048 and other bubble-like structures in the field, suggests a multigenerational star-forming environment. Future \textit{James Webb Space Telescope} observations targeting the H${}_2$ pure rotational lines ($3-28\mu \mathrm{m}$) and other HRLs less affected by extinction will be essential to characterise the cooler molecular and ionised gas to fully disclose the formation history of the region.

Accepted in A&A. This version is after language editor corrections

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