Exploring the interplay between molecular and ionized gas in HII regions

TL;DR

This study uses para-H2CO transitions observed with APEX to characterize the physical conditions of dense molecular gas in HII regions and explores how massive stars influence the surrounding ionized and molecular gas components.

Contribution

It provides detailed non-LTE analysis of molecular gas properties in HII regions and investigates the interaction between ionized, molecular, and dust components influenced by massive stars.

Findings

Kinetic temperatures range from 33.7 K to 265 K.

H2 densities range from 0.8×10^4 to 1.05×10^7 cm^-3.

Secondary components show higher temperatures and turbulence.

Abstract

We aim to constrain the kinetic temperature and H$_2$ volume density of massive star-forming clumps associated with HII regions using multiple para-H$_2$CO 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 $J_{K_aK_c}$ transitions of para-H2CO (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 towards a sample of 61 HII regions. Spectral line parameters are derived via multi-component 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 (MCMC) analysis. The non-LTE analysis yielded kinetic temperatures ($T_{kin}$) ranging from 33.7 K to 265 K and H2 densities (n(H$_2$)) between 0.8 X $10^4$ to 1.05 X $10^7$ cm$^{-3}$, providing a detailed characterization of the dense molecular gas contained in these clumps. In addition to the para-H$_2$CO emission arising from the targeted clump a large fraction (57%) of the sources exhibit multiple para-H$_2$CO components, with the secondary components being characterized by higher $T_{kin}$ and broader linewidths. Investigating the nature of the secondary component revealed its association with supersonic non-thermal motions and turbulent gas. When comparing the physical properties of the molecular gas and dust components with those of the ionizing gas, we find that parameters directly linked to the central high-mass star such as bolometric luminosity and Lyman continuum photon rate, show stronger and more systematic correlations. Emphasizing the role of the central star in governing the interplay between the molecular and ionized gas.(Abridged)