Exploring the interplay between molecular and ionized gas in H II regions

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Deutsches Zentrum für Astrophysik, Postplatz 1, 02826 Görlitz, Germany
⁴ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
⁵ Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK
⁶ Department of Earth & Space Sciences, Indian Institute of Space Science and Technology, Trivandrum 695547, India
⁷ Purple Mountain Observatory, and Key Laboratory of Radio Astronomy, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, PR China

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 11 November 2025
Accepted: 18 December 2025

Abstract

Context. Massive stars strongly impact their natal environments and influence subsequent star formation through feedback mechanisms such as shocks, outflows, and radiation. H II 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.

Aims. We aim to constrain the kinetic temperature and H₂ volume density of massive star-forming clumps associated with H II regions using multiple p–H₂CO transitions. In addition, we investigate the interplay between ionized gas, molecular gas, and dust to probe how massive stars influence their parental clumps.

Methods. We observed the J_KaKc transitions of p–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 toward a sample of 61 H II 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.

Results. The non-LTE analysis yielded kinetic temperatures (T_kin) ranging from 33.7 K to 265 K and H₂ densities (n(H₂)) between 0.8 × 10⁴ and 1.05 × 10⁷ cm⁻³, providing a detailed characterization of the dense molecular gas contained in these clumps. In addition to the p–H₂CO emission arising from the targeted clump, a large fraction (57%) of the sources exhibited multiple p–H₂CO components, with the secondary components being characterized by a higher T_kin 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 (L_bol) and Lyman continuum photon rate (N_Lyc), 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 H II 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 H II regions due to its higher pressure. However, given the limited spatial resolution, a definitive conclusion on the role of molecular gas in confining H II regions cannot be made until high resolution observations are obtained.