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

TL;DR

This study addresses how HC and UC H II regions are trapped and how their lifetimes are prolonged by physical feedback. It uses 2D axisymmetric radiation-hydrodynamics with self-gravity and disk formation to model how gravity, ram pressure, radiation pressure, and centrifugal forces interact during early massive-star formation. Key findings show that radiation pressure is essential to overcome gravitational trapping, while ram pressure from infalling gas can induce later trapping and flickering in H II region size, aligning with observed variability; density structure determines whether trapping occurs at all, with centrally concentrated profiles reducing trapping. The results offer a plausible mechanism for the observed long-lived compact H II phases and highlight the role of environmental conditions in shaping early massive-star feedback.

Abstract

Observationally, early H II regions are 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 so-called lifetime problem: the observation of more compact H II regions than expected from theory. Utilizing two-dimensional, axially symmetric radiation hydrodynamic simulations of young expanding H II regions, including the phase of 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. 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 high central density, no trapping occurs, while a less steep density gradient yields clear trapped phases. Hypercompact trapped phases exhibit a so-called flickering variation in H II region radius, in agreement with observations of stalling and even contraction over small time scales. With radiation pressure, low-density reservoirs experience both gravitational and ram pressure trapping, while high-mass reservoirs undergo only the latter.

Figures (10)

• Figure 1: Example snapshot of typical H II region "butterfly wing" morphology taken from run #0 in this work, as a density color-map. The boundary depicted with the white outline is the region where more than 50% of the hydrogen was ionized.
• Figure 2: The cloud at the start of the simulation before collapse, displayed in 3D. Rotation and the midplane are indicated.
• Figure 3: Accretion rate ($\dot{\mathrm{M}}_{*}$) onto the protostar as a function of stellar mass for the representative $\,500\,\mathrm{M}_\odot$ simulation (run #2).
• Figure 4: The ionizing photon rate ($S_*$) as a function of stellar mass for the representative $\,500\,\mathrm{M}_\odot$ simulation (run #2).
• Figure 5: From left to right: H II region extent with (magenta) and without (blue) radiation pressure switched on, in the $M_{\text{core}} = \,250\,\mathrm{M}_\odot$ (runs #0, #5), $\,500\,\mathrm{M}_\odot$ (runs #2, #6) and $\,1000\,\mathrm{M}_\odot$ (runs #4, #7) cases.