A research team led by Prof. ZHOU Jianjun, Prof. Jarken Esimbek and PhD student ZHANG Wenjun from the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences, together with their collaborators, selected 27 relatively isolated giant molecular clouds based on the CO data from the SEDIGISM survey (Schuller et al. 2017) and the COHRS survey (Dempsey et al. 2013) of the Milky Way. They used the dense gas mass fraction (DGMF) of giant molecular clouds as a tracer of their evolution, and studied the coevolution of giant molecular cloud, filaments and clumps in relation to DGMF. 

Recent observations show that filaments play a key role in star formation process. The new paradigm of star formation can be summarized as follows: molecular clouds → filaments → clumps/cores → protostars → young stellar objects. However, the mechanisms behind the formation and evolution of filaments within giant molecular clouds, the development of clumps within these filaments, and the relationships among these three structures across different scales remain poorly understood. Therefore, further research is necessary to elucidate how dense clumps and cores form from diffuse gas, as this is the initial step in the formation of new stars.

The results suggest that the free fall time, virial parameter, surface density, star formation rate, surface star formation rate, and star formation efficiency of dense gas all show regular variations with DGMF. The column density and line mass of filaments increase with DGMF. The densities of clumps increase with increasing DGMF, while their masses and radii decrease. 

Clumps capable of forming massive stars are spatially associated with filaments, they usually have much larger masses, radius, densities and velocity dispersion. Consequently, the filaments with clumps capable of forming massive stars have much larger masses, lengths and line mass. Therefore, the evolution of giant molecular clouds is a multiscale coevolution process, DGMF seems to be a good tracer for understanding the evolution trajectory of GMCs.

The research work was supported by the National Key R&D Programs of China and the Natural Science Foundation of China.

Fig.1. Variations of star formation rate and star formation efficiency as a function of dense gas mass fraction (DGMF). 

Fig.2. Clumps in giant molecular clouds with higher DGMF have higher fraction forming massive stars.