Fragmentation and evolution for the molecular shells of the compact H ii regions are less explored compared to their evolved counterparts. We map nine compact H ii regions with a typical diameter of 0.4 pc that are surrounded by molecular shells traced by CCH. Several to a dozen dense gas fragments probed by H$^{13}$CO$^+$ are embedded in these molecular shells. These gas fragments, strongly affected by the H ii region, have a higher surface density, mass, and turbulence than those outside the shells but within the same pc-scale natal clump. These features suggest that the shells swept up by the early H ii regions can enhance the formation of massive dense structures that may host the birth of higher mass stars. We examine the formation of fragments and find that fragmentation of the swept-up shell is unlikely to occur in these early H ii regions, by comparing the expected time scale of shell fragmentation with the age of H ii region. We propose that the appearance of gas fragments in these shells is probably the result of sweeping up pre-existing fragments into the molecular shell that has not yet fragmented. Taken together, this work provides a basis for understanding the interplay of star-forming sites with an intricate environment containing ionization feedback such as those observed in starburst regions.
Modelling galaxy formation in hydrodynamic simulations has increasingly adopted various radiative transfer methods to account for photoionization feedback from young massive stars. However, the evolution of H ii regions around stars begins in dense star-forming clouds and spans large dynamical ranges in both space and time, posing severe challenges for numerical simulations in terms of both spatial and temporal resolution that depends strongly on gas density (∝n−1). In this work, we perform a series of idealized H ii region simulations using the moving-mesh radiation-hydrodynamic code arepo-rt to study the effects of numerical resolution. The simulated results match the analytical solutions and the ionization feedback converges only if the Strömgren sphere is resolved by at least 10–100 resolution elements and the size of each time integration step is smaller than 0.1 times the recombination time-scale. Insufficient spatial resolution leads to reduced ionization fraction but enhanced ionized gas mass and momentum feedback from the H ii regions, as well as degrading the multiphase interstellar medium into a diffuse, partially ionized, warm (∼8000 K) gas. On the other hand, insufficient temporal resolution strongly suppresses the effects of ionizing feedback. This is because longer time-steps are not able to resolve the rapid variation of the thermochemistry properties of the gas cells around massive stars, especially when the photon injection and thermochemistry are performed with different cadences. Finally, we provide novel numerical implementations to overcome the above issues when strict resolution requirements are not achievable in practice.
more » « less- PAR ID:
- 10472848
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 527
- Issue:
- 1
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 478-500
- Size(s):
- p. 478-500
- Sponsoring Org:
- National Science Foundation
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