The GUAPOS project

VII. Physical structure and molecular environment of the G31.41+0.31 HII region^★

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Centro de Astrobiología (CAB), CSIC-INTA, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
³ Institut de Ciències de l’Espai (ICE), CSIC, Campus UAB, Carrer de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), E-08860, Castelldefels, Barcelona, Spain
⁵ INAF, Istituto di Astrofisica e Planetologia Spaziale, Via Fosso del Cavaliere 100, 00133 Roma, Italy

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

Received: 27 November 2025
Accepted: 18 February 2026

Abstract

Context. Ionised regions around OB-type stars, formed at an early stage of their evolution, are important in the investigation of the formation processes of these objects. Thus far, however, only a few observations of their physical structure and interaction with the parental molecular cloud have been carried out. The high resolution and high sensitivity of new instruments such as ALMA and the upgraded VLA allow us to fill in this knowledge gap.

Aims. We investigate the well-known core-halo ultracompact HII region G31.41+0.31 and the surrounding molecular clump to determine the density and temperature of both the ionised and neutral gas, and to possibly obtain a 3D picture of their spacial distribution.

Methods. We took advantage of the full-band frequency coverage at 3 mm obtained with ALMA for the GUAPOS project to image the emission of a plethora of hydrogen recombination lines towards the G31.41+0.31 HII region, as well as several molecular transitions that serve as tracers of medium-density (~10⁴−10⁶ cm⁻³) gas. The line data are complemented by continuum measurements obtained with the VLA at 1 cm and 7 mm. By fitting these lines with a model that takes into account non-local thermal equilibrium (NLTE) effects, we were able to investigate the density and temperature structure and the velocity field of the region.

Results. Our findings, based on a model fit accounting for NLTE effects, indicate that the electron temperature of the HII region mostly spans a range between 5000 and 6000 K, while the density varies between 2500 and 7500 cm⁻³. All in all, the distribution of these parameters, along with the corresponding velocity field hint at a cometary shaped HII region expanding away from the observer to the NW. The molecular gas appears to be still infalling towards the peak of the UC HII region, while its density and temperature are consistent with pressure confinement of the ionised gas to the SE.