The GUAPOS project

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 H II 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 H II region, as well as several molecular transitions that serve as tracers of medium-density (~10 4 −10 6 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 H II 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 H II region expanding away from the observer to the NW. The molecular gas appears to be still infalling towards the peak of the UC H II region, while its density and temperature are consistent with pressure confinement of the ionised gas to the SE.