2025A&A...695A..24H

Context. High-mass stellar embryos are embedded in warm envelopes that provide mass reservoirs for the accretion process onto final stars. Feedback from star formation activities in return impacts the properties of the envelopes, offering us a unique opportunity to investigate star formation processes.
Aims. Our goals are to characterise the properties of warm envelopes of proto- or young stellar objects at different evolutionary stages based on the morphology and kinematics of submillimetre emission from the ¹³CO (6-5) line and to examine their relations with star formation processes.
Methods. Using the Atacama Pathfinder EXperiment (APEX) telescope, we obtained maps of ¹³CO (6-5) emission with an angular size of 80″ × 80″ (ranging from 0.3 pc × 0.3 pc to 4.9 pc × 4.9 pc in physical size) of 99 massive clumps from the ATLASGAL survey of submillimetre dust continuum emission. Our maps are classified based on morphological complexities, and the radial structure of ¹³CO (6-5) emission is characterised for simple single-core sources. In addition, the velocity centroids of ¹³CO (6-5) emission are compared to small- and large-scale gas kinematics (traced by ¹²CO (6-5) and ¹³CO (2-1) emission, respectively), aiming to shed light on the origin of envelope kinematics.
Results. ¹³CO (6-5) emission is detected towards sources in all stages of high-mass star formation, with a detection rate of 83% for the whole sample. The detection rate, line width, and peak brightness temperature increase with evolutionary stage, and the line luminosity is strongly correlated with the bolometric luminosity and the clump mass. These results indicate that the excitation of ¹³CO (6-5) emission is closely related to star formation processes. In addition, the radial distributions of ¹³CO (6-5) emission for single-core sources can be well fitted by power-law functions, suggesting a relatively simple envelope structure for the majority of our sources (52 out of 99). The slopes of the radial distributions are systematically steeper for the most evolved group of sources (that host HII regions), which likely results from enhancements in density and/or temperature at the central parts of the warm envelopes. As for the ¹³CO (6-5) kinematics, linear velocity gradients are common among the single-core sources (44 out of 52), and the measured mean velocity gradients are on average 3 km s^–1 pc^–1. Our comparison of the ¹³CO (6-5), ¹²CO (6-5), and ¹³CO(2-1) kinematics suggests that the origin of the linear velocity gradients in the warm envelopes is complex and unclear for many sources.
Conclusions. ¹³CO (6-5) emission is ubiquitous in a wide variety of massive clumps, ranging from young sources where protostars have not yet been formed to evolved sources with fully developed HII regions. The excitation of ¹³CO (6-5) emission in warm envelopes is likely impacted by different processes at different epochs of high-mass star formation, while the origin of the ¹³CO (6-5) kinematics remains elusive and needs further investigation.