CH3CCH 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)$\&$(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 40m and TMRT 65m. Our results show that rotation temperatures derived from NH${}_3$(1,1)$\&$(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$\to$$J$ quantum number may be a more reliable thermometer than NH${}_3$(1,1)$\&$(2,2) in warm molecular gas located in the surroundings of massive young stellar objects or, more generally, in massive star-forming regions.

Accepted for publication in ApJ, 27 pages, 15 figures