Recently, an international research team led by the Shanghai Astronomical Observatory (CAS), in collaboration with institutions from China, South Korea, and Europe, conducted a comprehensive observational study of massive star-forming regions in the Milky Way using the Shanghai Tianma Radio Telescope (TMRT) and the Spanish Yebes 40m radio telescope. The team confirmed that the traditional method of measuring molecular cloud temperatures using ammonia (NH₃) may underestimate the true values under high-temperature conditions (T > 20 K). In contrast, methyl acetylene (CH₃CCH) proved to be a more accurate tracer for the temperature variations of warm molecular gas (T = 20–100 K)，these findings provide critical observational evidence for characterizing the physical conditions within star-forming regions.

Gas temperature is a fundamental physical parameter in molecular clouds, playing a decisive role in understanding star formation processes and chemical evolution. It governs the delicate balance between thermal pressure and gravity, determining whether a cloud will collapse to form stars, while also regulating chemical pathways and molecular evolution. Consequently, accurate temperature measurements are essential for understanding star formation efficiency, evolutionary stages, and their physical environments.

Due to the immense distances to molecular clouds, astronomers must rely on molecular spectral lines as indirect "thermometers". However, different molecules exhibit varying sensitivities to physical conditions; some commonly used probes lose their sensitivity as temperatures rise. Therefore, evaluating the applicability of different molecular probes across diverse environments is essential for establishing reliable diagnostic methods.

In this study, the TMRT 65m telescope was responsible for observing NH₃(1,1) and (2,2) spectral lines in the 23 GHz band. The team conducted high-sensitivity, high-spectral-resolution surveys of 37 massive star-forming regions, obtaining high-quality data. The TMRT backend system’s exceptional velocity resolution at 23 GHz allowed researchers to accurately fit the hyperfine structures of NH₃ and reliably derive its rotation temperature.

CH₃CCH, observed in the millimeter waveband, is considered an ideal temperature probe for warm molecular gas. Statistical equilibrium analysis (Figure 1) indicates that under typical warm gas densities (n > 10⁴ cm⁻³), the rotation temperature of CH₃CCH effectively reflects the gas's kinetic temperature. Furthermore, millimeter-wave observations allow for the simultaneous acquisition of multiple CH₃CCH lines, reducing systematic errors from instrument calibration or changing conditions. Leveraging high-quality NH₃ reference data from TMRT, the team systematically analyzed the reliability of different probes in warm environments.

Figure 1: Statistical equilibrium calculation results for CH₃CCH. At densities above 10⁴ cm⁻³, the rotation temperature derived from CH₃CCH is essentially consistent with the gas temperature.

The results show that rotation temperatures derived from NH₃(1,1) and (2,2) lines tend to saturate above approximately 20 K, exhibiting a much narrower dynamic range than those provided by CH₃CCH (Figure 2). In contrast, CH₃CCH responds more sensitively to environmental changes, allowing for clearer differentiation of physical conditions among star-forming regions. This discrepancy stems not from uncertainty but from the distinct response mechanisms of the two molecules. Therefore, while NH₃(1,1) and (2,2) may underestimate temperatures in warm environments, CH₃CCH is more suitable for such measurements.

Figure 2: Comparison of rotation temperatures derived from CH₃CCH and NH₃. The rotational temperature obtained from NH₃ is significantly lower than that from CH₃CCH.

The TMRT played a vital role in this study, demonstrating the significant scientific value of China’s large radio telescopes in molecular cloud physics. In the future, with expanded millimeter-wave capabilities and the commissioning of the KQW triple-frequency receiver, multi-molecular collaborative research will further clarify the mechanisms of star formation.

This research was a joint effort by the Shanghai Astronomical Observatory, Guangxi University, Korea Astronomy and Space Science Institute, and the Spanish National Astronomical Observatory.The study was supported by the National Key Research and Development Program of China, National SKA Program of China, the Oriental Talent Plan, and the State Key Laboratory of Radio Astronomy and Technology.