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

TL;DR

This study uses radiation hydrodynamic simulations to explore how gravity and ram pressure can trap ultracompact and hypercompact H II regions, shedding light on their observed long lifetimes and flickering behaviors.

Contribution

It introduces detailed 2D simulations that reveal the roles of gravity and ram pressure in trapping H II regions, providing new insights into their physical evolution and observed variability.

Findings

Gravitational trapping can indefinitely hold ultracompact H II regions without radiation pressure.

Radiation pressure allows H II regions to escape gravitational trapping but can cause ram pressure trapping.

Hypercompact regions exhibit flickering, consistent with observed stalling and contraction behaviors.

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.