2026ApJ...998..182L

Kinetic temperature is a fundamental parameter in molecular clouds. Symmetric top molecules, such as NH₃ and CH₃CCH, are often used as thermometers. However, at high temperatures, NH₃(2,2) can be collisionally excited to NH₃(2,1) and rapidly decay to NH₃(1,1), which can lead to an underestimation of the kinetic temperature when using rotation temperatures derived from NH₃(1,1) and NH₃(2,2). In contrast, CH₃CCH is a symmetric top molecule with lower critical densities of its rotational levels than those of NH₃, which can be thermalized close to the kinetic temperature at relatively low densities of about 10⁴ cm^–3. To compare the rotation temperatures derived from NH₃(1,1) and (2,2) and CH₃CCH rotational levels in warm molecular gas, we used observational data toward 55 massive star-forming regions obtained with Yebes 40 m and TMRT 65 m. Our results show that rotation temperatures derived from NH₃(1,1) and (2,2) are systematically lower than those from CH₃CCH 5-4. This suggests that CH₃CCH rotational lines with the same J+1 → J quantum number may be a more reliable thermometer than NH₃(1,1) and (2,2) in warm molecular gas located in the surroundings of massive young stellar objects or, more generally, in massive star-forming regions.