CH ₃ CCH as a Thermometer in Warm Molecular Gas

Abstract Kinetic temperature is a fundamental parameter in molecular clouds. Symmetric top molecules, such as NH 3 and CH 3 CCH, are often used as thermometers. However, at high temperatures, NH 3 (2,2) can be collisionally excited to NH 3 (2,1) and rapidly decay to NH 3 (1,1), which can lead to an underestimation of the kinetic temperature when using rotation temperatures derived from NH 3 (1,1) and NH 3 (2,2). In contrast, CH 3 CCH is a symmetric top molecule with lower critical densities of its rotational levels than those of NH 3 , which can be thermalized close to the kinetic temperature at relatively low densities of about 10 4 cm −3 . To compare the rotation temperatures derived from NH 3 (1,1) and (2,2) and CH 3 CCH 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 NH 3 (1,1) and (2,2) are systematically lower than those from CH 3 CCH 5–4. This suggests that CH 3 CCH rotational lines with the same J +1 → J quantum number may be a more reliable thermometer than NH 3 (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.