ATOMS: ALMA three-millimeter observations of massive star-forming regions -- XVIII. On the origin and evolution of dense gas fragments in molecular shells of compact HII regions

TL;DR

This study investigates how molecular shells around early-stage compact HII regions influence dense gas fragment formation, suggesting shells enhance massive star formation by sweeping up pre-existing fragments rather than fragmenting themselves.

Contribution

It provides new insights into the formation and evolution of dense gas fragments in molecular shells of compact HII regions, emphasizing the role of pre-existing structures and feedback effects.

Findings

Dense gas fragments are more massive and turbulent within shells.

Shell fragmentation is unlikely in early HII regions due to timescale constraints.

Fragments likely result from sweeping up pre-existing structures, not shell fragmentation.

Abstract

Fragmentation and evolution for the molecular shells of the compact HII regions are less explored compared to their evolved counterparts. We map nine compact HII 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 H13CO+ are embedded in these molecular shells. These gas fragments, strongly affected by the HII 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 HII 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 HII regions, by comparing the expected time scale of shell fragmentation with the age of HII 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.