Contributed Talk - Splinter Stars

Wednesday, 15 September 2021, 09:20   (virtual Stars)

On gravitational trapping and ram pressure trapping of ultracompact and hypercompact HII regions

L. Martini, G. A. Oliva, R. Kuiper
Institute of Astronomy and Astrophysics, University of Tübingen

HII regions are key observational signposts for massive star formation. During late evolution, they influence their environments on scales from their natal star forming regions to entire galaxies. During early evolution, they commonly exhibit hypercompact and ultracompact phases in which their expansion is limited. The physical processes driving and hindering expansion during these phases are not fully understood but are essential to complete our understanding of high mass star formation. As a follow-up to Kuiper & Hosokawa (2018), we perform state-of-the-art, high-performance simulations of collapsing massive pre-stellar cores of gas and dust, the formation of high-mass stars, their accretion disks, and their expanding HII regions. This high-resolution setup allows us to follow the evolution of early HII regions from near-star scales of 3 au up to cloud scales of 1.0 pc, while monitoring the quantitative impact of photoionization feedback and thermal pressure, radiation pressure, centrifugal forces, gravitational trapping due to stellar gravity and self-gravity of the gas, as well as trapping due to ram pressure from the infalling envelope. To reinforce the robustness of our findings, we perform a variety of simulations with and without radiation pressure for different initial mass reservoirs and density profiles to determine the dependence of the HII region growth on environmental parameters. We find that HII regions remain gravitationally trapped in a hypercompact configuration when radiation pressure is neglected. Centrifugal forces shape the HII region such that its maximum extent does not align with the rotation axis, and gravitational trapping continues for several hundred thousand years. Photoionization feedback together with radiation pressure produce outward radiative forces that outrace gravitational trapping, resulting in sudden HII region growth beyond the gravitational binding radius. Then, at scales on the order of a few 1000 au, ram pressure from the infalling large-scale environment intermittently stalls expansion again. The HII regions remain in an ultracompact configuration for 40-90 kyr, varying with initial mass. We conclude that expanding HII regions experience a series of trapped phases due to the balance of internal and external forces. The total duration of the trapped phases can explain the so-called “lifetime” problem of early HII regions.