Diverse stages of star formation in the IRAS 18162-2048 region

Emergence of UV feedback

¹ Instituto de Astrofísica de Andalucía, CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
² INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
³ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), c/ Martí i Franquès 1, 08028 Barcelona, Spain
⁴ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), c/ Martí i Franquès 1, 08028 Barcelona, Spain
⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Instituto de Astronomía Teórica y Experimental (IATE), Universidad Nacional de Córdoba (UNC), Córdoba, Argentina
⁷ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Michoacán 58089, Mexico
⁸ INAF, Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius, Italy
⁹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹⁰ Dublin Institute for Advanced Studies, School of Cosmic Physics, 31 Fitzwilliam Place, Dublin 2, Ireland
¹¹ University College Dublin, School of Physics, Belfield, Dublin 4, Ireland
¹² Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str 77, 50937 Köln, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 8 December 2025
Accepted: 9 February 2026

Abstract

Context. High-mass star formation remains a major open problem in astrophysics, particularly regarding the transition between deeply embedded protostars and the onset of ionising radiation capable of producing photodissociation regions (PDRs) and compact H II regions.

Aims. We aim to characterise the excitation and ionisation conditions of the high-mass star-forming region IRAS 18162−2048, which is where the parsec-scale jet HH80−81 lies.

Methods. We obtained adaptive optics–assisted integral field spectroscopy in the near-IR K band (1.93–2.47 µm) with VLT/SINFONI, complemented by VLA X and C bands (3−6 cm) and ALMA band 3 (∼3.3 mm) observations. We analysed the continuum and line emission to derive visual extinction and excitation conditions and the kinematics of the gas of the region.

Results. The near-IR continuum reveals two IR sources, IRS 2 and IRS 7, while the main protostellar core, IRAS 18162-2048, remains undetected up to 2.47 µm. IRS 7 shows a peculiar hydrogen recombination line (HRL) Brγ profile with a narrow emission component superimposed on a broad absorption feature, consistent with a B2/B3 zero-age main-sequence (ZAMS) star. Extended H₂ 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 H₂ ro-vibrational populations for T_gas = 600 K and n_H = 7.9 × 10³ cm⁻³. The 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 millimetre 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 photo-ionise its environment, giving rise to an extended PDR and a compact 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 multi-generational star-forming environment. Future James Webb Space Telescope observations targeting the H₂ pure rotational lines (3–28 µm) and other HRLs less affected by extinction will be essential to characterising the cooler molecular and ionised gas to fully disclose the formation history of the region.