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

TL;DR

GUAPOS employs full-band 3 mm ALMA observations of hydrogen recombination lines alongside molecular tracers, plus VLA centimeter-continuum data, to map the physical structure and kinematics of the G31.41+0.31 ultracompact H II region and its surrounding molecular clump. Non-LTE recombination-line modelling yields spatially resolved $T_e\sim$5–6×10^3 K and $n_e\sim$a few×10^3 cm$^{-3}$ in the ionised gas, while CH$_3$CCH rotation diagrams and CN absorption reveal the temperature, density, and velocity structure of the adjacent molecular gas, including a NE–SW rotation and ongoing infall toward the HMC. The data support a 3D scenario in which the H II region expands NW within a molecular clump that confines it to the SE, with the HMC lying in front of the H II region and exhibiting rotation and residual infall; this work demonstrates the power of simultaneous ionised and neutral gas diagnostics for characterising feedback and accretion in massive star-forming environments, and suggests potential leakage of Lyman-continuum photons from the clump. Overall, the study provides a cohesive picture of the interplay between ionising feedback and the parental cloud in G31.41+0.31 and establishes a framework for similar analyses with next-generation interferometers.

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.

Figures (27)

• 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 dotted pattern outlines the approximate border of the HMC (see text). The dashed rectangle frames the region shown in panel b. The synthesised beams (1$\overset{\prime\prime}{.}$2 at 1 cm and 0$\overset{\prime\prime}{.}$21$\times$0$\overset{\prime\prime}{.}$17 with PA --0$\overset{\circ}{.}$5 at 7 mm) are shown in the bottom left and right corners. b. Enlargement of the region that contains the UC H ii region and the HMC, corresponding to the dashed rectangle in panel a. The symbols have the same meaning as in panel a, with the exception of the colour image that shows the 7 mm continuum emission. We note that the colour scale is saturated (the peak emission at 7 mm is 1.96 mJy/beam) to emphasise the structure of the UC H ii region. c. Contour map of the 3 mm continuum emission obtained from the ALMA data overlaid on the same 1 cm continuum image as in panel a. Contour levels range from 0.5 to 180 mJy/beam in 10 logarithmic steps. The synthesised beam (1$\overset{\prime\prime}{.}$2) is the same for both maps and is shown in the bottom right corner.
• Figure 2: a. Overlay of the 1 cm map of Fig. \ref{['fvmaps']} (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 the region shown in Figs. \ref{['fvmaps']}a and \ref{['fvmaps']}c. b. Same as panel a, for the 70 $\mu$m image obtained from the Herschel/Hi-GAL survey.
• Figure 3: Maps of the integrated emission over the hydrogen recombination lines indicated in the top right corner of each panel. The emission from the HMC is not plotted because the recombination lines are heavily blended with stronger molecular transitions. The circle in the bottom right corner is the synthesised beam. The offsets are relative to the phase center of the ALMA observations. The emission of the H54$\gamma$ line is contaminated by two hyperfine components of the NS $J=5/2\rightarrow3/2$ transition.
• Figure 4: Same as Fig. \ref{['frint']}, for the peak velocity of the recombination lines.
• Figure 5: Same as Fig. \ref{['frint']}, for the full width at half maximum of the recombination lines.