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

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 Tgas = 600 K and nH = 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.