Warm Inner Envelopes of Massive Star-Forming Regions

Abstract

High-mass stars play crucial roles in the evolution of galaxies, but their formation process is still not fully understood. Our work investigates this topic by studying the warm inner gas envelopes around high-mass star-forming regions in the Top100 sample, selected from the brightest clumps in the unbiased ATLASGAL survey of cold dust in the Milky Way. Using mid- and high-<em>J</em> transitions of CO and its rare isotopologues ¹³CO and C¹⁸O, the work probes the morphology and kinematic properties of these envelopes across different evolutionary stages. Observing such lines is traditionally difficult, but it becomes more feasible in this work thanks to the advancement of technology at the APEX/CHAMP⁺ and SOFIA/GREAT instruments. The ¹³CO(6-5) data obtained with APEX/CHAMP⁺ reveal correlations between emission and clump properties, indicating that the excitation of this line, which traces the warm envelopes, increases with the evolution of star formation. Envelope morphology, as revealed by ¹³CO(6-5) integrated intensity maps, appears mostly as a single core in all evolutionary stages, indicating either that the shapes of the envelopes do not evolve with star formation, or that higher angular resolutions observations are needed to resolve such transformations. Radial intensity gradients of the ¹³CO(6-5) emission are well described by power-law functions, with steeper slopes at evolved stages suggesting rising gas temperature and/or density at the source centre. The envelope kinematics, however, are complex and could be driven by multiple processes such as outflow entrainment, envelope rotation, or inherited motions from larger-scale envelopes. Spectra of high-<em>J</em> CO(11-10) and CO(16-15) lines obtained with SOFIA/GREAT reveal broad wing emission, which we were able to extract. The wing emission appears already in young sources, suggesting the early existence of outflows in high-mass star formation. Radiative transfer modelling with RADEX suggests that shocks are responsible for the excitation of the gas producing the wing emission. Finally, we extend our work to detailed emission modelling for ten mid- and high-<em>J</em> transitions of CO, ¹³CO, and C¹⁸O towards six Top100 clumps, which helps constrain the physical properties of molecular gas in the warm envelopes. Additionally, we intend to test whether one gas component can reproduce all the observed lines. However, we find that multiple gas components are required to account for all the observed lines, aligning with previous suggestions that multiple mechanical and radiative processes could be responsible for the excitation of warm gas in the envelopes.