Direct Observational Evidence of Multi-epoch Massive Star Formation in G24.47+0.49
Sanhueza, Patricio; Tej, Anandmayee; Hwang, Jihye; Lee, Chang Won; Liu, Hong-Li; Liu, Tie; Li, Shanghuo; Garay, Guido; Saha, Anindya; Tóth, L. Viktor; Juvela, Mika; Goldsmith, Paul F.; Xu, Feng-Wei; Bronfman, Leonardo; He, Jinhua; Issac, Namitha; Baug, Tapas; Vázquez-Semadeni, Enrique; Chibueze, James O.; Bhadari, N. K.; Dewangan, Lokesh K.; Das, Swagat Ranjan
India, China, Chile, United States, South Korea, Finland, México, Japan, Germany, South Africa, Nigeria, Hungary
Abstract
Using new continuum and molecular line data from the Atacama Large Millimeter/submillimeter Array Three-millimeter Observations of Massive Star-forming Regions (ATOMS) survey and archival Very Large Array, 4.86 GHz data, we present direct observational evidence of hierarchical triggering relating three epochs of massive star formation in a ringlike 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 ∼9 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 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 the third epoch of potential massive star-forming sites.