Gravitational trapping and ram pressure trapping of ultracompact and hypercompact H II regions

¹ Faculty of Physics, University of Duisburg-Essen, Lotharstraße 1, 47057 Duisburg, Germany
² Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Space Research Center (CINESPA), School of Physics, University of Costa Rica, 11501 San José, Costa Rica
⁴ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

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

Received: 11 October 2024
Accepted: 23 November 2025

Abstract

Context. Early H II regions are observationally classified by size into ultracompact and hypercompact configurations. It remains unclear whether these phases are long-lived or transient. Understanding the physical processes that stall H II region growth may help to solve the “lifetime problem”: the observation of more compact H II regions than expected from theory.

Aims. Utilizing two-dimensional, axially symmetric radiation hydrodynamic simulations of young expanding H II regions, including early star and disk formation, we seek to better understand the trapping of H II regions. Trapping forces include gravity and ram pressure, which oppose forces such as thermal pressure expansion, radiation pressure, and centrifugal force.

Methods. We used the open-source magnetohydrodynamical code Pluto, the radiation transport module Makemake, the photoionization module Sedna, and the self-gravity module Haumea. This approach resembles the setup of previous works, but without the manual injection of protostellar outflows.

Results. Without radiation pressure, the H II region remains gravitationally trapped in the ultracompact phase indefinitely. With radiation pressure, the H II region escapes gravitational trapping but experiences ram pressure trapping on larger scales. For initial mass reservoirs with a high central density, no trapping occurs, while a less steep density gradient yields clear trapped phases. Hypercompact trapped phases exhibit a “flickering” variation in H II region radius, in agreement with observations of stalling and even contraction over small timescales. With radiation pressure, low-mass or low-density reservoirs experience both gravitational and ram pressure trapping, while high-mass reservoirs undergo only the latter.

Conclusions. During the early evolution of H II regions, gravitational and ram pressure trapping may occur at different times and distances from the (proto)star, depending on environmental conditions. The mean expansion velocity is influenced by thermal pressure, gravity, centrifugal forces, and radiation pressure. For the cases studied herein, the inclusion of radiation pressure is crucial to obtaining physically meaningful results.