Exploring the interplay between molecular and ionized gas in HII regions

Massive stars strongly impact their natal environments and influence subsequent star formation through feedback mechanisms such as shocks, outflows, and radiation. regions are key laboratories for studying this impact. To understand such feedback, it is crucial to characterize the physical conditions of the dense molecular gas in which these regions are embedded. We aim to constrain the kinetic temperature and ̊m H_2 volume density of massive star-forming clumps associated with regions using multiple transitions. In addition, we investigate the interplay between ionized gas, molecular gas, and dust to probe how massive stars influence their parental clumps. We observed the J_ K_ ̊m a K_ ̊m c transitions of (within its J = 3-2 and 4-3 states) with the Atacama Pathfinder EXperiment (APEX) 12,m submillimeter telescope, using the nFLASH230 and SEPIA345 receivers toward a sample of 61 regions. We derived spectral line parameters via multicomponent Gaussian fitting, which was then used to constrain the physical conditions determined using PyRADEX, a non–local thermodynamic equilibrium (LTE) radiative transfer code in combination with Markov chain Monte Carlo analysis. The non-LTE analysis yielded kinetic temperatures ( ), show stronger and more systematic correlations. These findings emphasize the role of the central star in governing the interplay between the molecular and ionized gas. In our sample of ranging from 33.7,K to 265,K and ̊m H_2 densities ( and ̊m 1.05 ,cm^ , providing a detailed characterization of the dense molecular gas contained in these clumps. In addition to the ) and Lyman continuum photon rate (N̊m _ between ̊m 0.8 10^ 4 10^ 7 -3 emission arising from the targeted clump, a large fraction (57%) of the sources exhibited multiple components, with the secondary components being characterized by a higher and broader line widths. Investigation of the nature of the secondary component revealed its association with supersonic nonthermal motions and turbulent gas. When comparing the physical properties of the molecular gas and dust components with those of the ionized gas, we found that parameters directly linked to the central high-mass star, such as bolometric luminosity (L̊m _ bol Lyc regions, the pressure of the neutral gas systematically exceeds that of the ionized gas. This suggests that the surrounding neutral molecular medium can hinder or slow down the expansion of regions due to its higher pressure. However, given the limited spatial resolution, a definitive conclusion on the role of molecular gas in confining regions cannot be made until high resolution observations are obtained.