Direct observational evidence of multi-epoch massive star formation in G24.47+0.49

TL;DR

This study provides direct observational evidence of multi-epoch massive star formation in G24.47+0.49, revealing hierarchical triggering, expanding molecular rings, and multiple evolutionary phases of star-forming cores.

Contribution

It presents the first direct detection of hierarchical, multi-epoch massive star formation with detailed molecular and ionized gas observations.

Findings

Detection of an expanding molecular gas ring at 9 km/s.

Identification of twelve dense cores with high surface mass density.

Evidence of multiple star formation epochs within the region.

Abstract

Using new continuum and molecular line data from the ALMA Three-millimeter Observations of Massive Star-forming Regions (ATOMS) survey and archival VLA, 4.86 GHz data, we present direct observational evidence of hierarchical triggering relating three epochs of massive star formation in a ring-like H II region, G24.47+0.49. We find from radio flux analysis that it is excited by a massive star(s) of spectral type O8.5V-O8V from the first epoch of star formation. The swept-up ionized ring structure shows evidence of secondary collapse, and within this ring a burst of massive star formation is observed in different evolutionary phases, which constitutes the second epoch. ATOMS spectral line (e.g., HCO$^+$(1-0)) observations reveal an outer concentric molecular gas ring expanding at a velocity of $\sim$ 9 $\rm km\,s^{-1}$, constituting the direct and unambiguous detection of an expanding molecular ring. It harbors twelve dense molecular cores with surface mass density greater than 0.05 $\rm g\,cm^{-2}$, a threshold typical of massive star formation. Half of them are found to be subvirial, and thus in gravitational collapse, making them third epoch of potential massive star-forming sites.