[Paper Review] The GUAPOS project -- VII: Physical structure and molecular environment of the G31.41+0.31 HII region

R. Cesaroni, M. T. Beltrán+7 more|arXiv (Cornell University)|Feb 20, 2026

Astrophysics and Star Formation Studies0 citations

TL;DR

This study uses ALMA and VLA data from the GUAPOS project to map the ionised and surrounding molecular gas of the G31.41+0.31 UC HII region, deriving electron temperature and density, and characterising gas kinematics and interactions with the parental clump.

ABSTRACT

Ionised regions around OB-type stars are formed at an early stage of their evolution and are important to investigate the formation process of these objects. However, so far only few observations of their physical structure and interaction with the parental molecular cloud have been made. The high resolution and sensitivity of new instruments such as ALMA and the upgraded VLA allow us to fill this gap in our knowledge. We investigate the well known core-halo ultracompact HII region G31.41+0.31 and the surrounding molecular clump with the aim to determine the density and temperature of both the ionised and neutral gas, and possibly obtain a 3D picture of their spacial distribution. We take 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 which are tracers of medium-density ($\sim$$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 also using a model that takes into account non-LTE effects we can investigate the density and temperature structure and the velocity field of the region. Our findings, based on a model fit accounting for non-LTE effects, indicate that the electron temperature of the HII region is mostly spanning 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 as well as 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, and its density and temperature are consistent with pressure confinement of the ionised gas to the SE.

Motivation & Objective

• Investigate the density and temperature structure of the G31.41+0.31 HII region and its surrounding molecular clump.
• Characterise the velocity field and 3D spatial distribution of ionised and neutral gas.
• Assess interaction between the UC HII region and the parental molecular environment.
• Constrain the ionising source and its relation to the region's bolometric luminosity and dust/gas confinement.

Proposed method

• Observe the G31.41+0.31 region with ALMA Band 3 to image hydrogen recombination lines and multiple molecular tracers across the 3 mm band.
• Complement ALMA line data with VLA continuum measurements at 1 cm and 7 mm to trace free-free emission.
• Fit recombination line spectra with a non-LTE expanding-shell HII region model to derive electron density ne and electron temperature Te.
• Derive Te and ne maps by comparing line+continuum spectra with the model, accounting for beam convolution and turbulence.
• Extract physical parameters of the surrounding molecular gas using selected lines (e.g., CN, H13CO+, CS, CH3CCH) and perform Gaussian fits to derive opacities, velocities, and line widths.
• Compare ionised gas properties with continuum morphology to infer the pressure confinement and potential champagne-flow dynamics.

Figure 1: a. Maps of the 1 cm (colour image and white contours) and 7 mm (black contours) continuum emission imaged with the VLA. The contour levels of the 1 cm map are drawn in the colour scale to the right, while those of the 7 mm map range from 0.08 to 1.88 in steps of 0.3 mJy/beam. The black dot

Experimental results

Research questions

• RQ1 What are the electron temperature and density distributions in the G31.41+0.31 HII region?
• RQ2 How does the ionised gas interact with the surrounding molecular clump, and what does this imply about confinement and flow (e.g., champagne flow)?
• RQ3 What is the velocity field of the ionised and neutral gas, and is there evidence of infall, expansion, or outflow at the HII boundary?
• RQ4 Can non-LTE recombination line modeling reproduce observed line spectra and constrain physical conditions across the region?
• RQ5 What is the relationship between the ionising source(s) and the region's bolometric luminosity in light of potential Lyman-excess or dust leakage?

Key findings

• Electron temperature across the HII region is mostly 5000–6000 K, with an inferred mean around 6000 K from model fits; higher values up to ~9000 K may occur near the SE border (potentially due to interface turbulence).
• Electron density varies from ~2500 to ~7500 cm^-3, with a main peak toward the UC HII region and a secondary peak between the HMC and the southern molecular gas.
• The ionised gas shows a cometary morphology expanding away from the observer toward the NW, while the surrounding molecular gas appears infalling toward the HII region, consistent with pressure confinement to the SE.
• Line modeling of H39α, H40α, and H41α (and other lines) yields best-fit ne and Te with uncertainties typically <15% for Te and <5% for ne at well-detected pixels.
• Hydrogen recombination lines trace the whole ionised halo (α lines) whereas γ lines are mainly detected toward the UC HII region, indicating sensitivity differences in tracers.
• Molecular tracers (e.g., C33S, CH3CCH, CN) reveal absorption features against the bright continuum and multiple components in emission/absorption, informing on the neutral gas kinematics and its relation to the HII region.

Figure 2: a. Overlay of the 1 cm map of Fig. 1 (contours) and the 8 $\mu$ m image from the Spitzer/GLIMPSE database (colour image). Contour levels range from 1 to 28 in steps of 9 mJy/beam. The angular resolution of the latter image is shown in the bottom right corner. The dotted rectangle outlines