CH3CCH as a thermometer in warm molecular gas
Yuqiang Li, Junzhi Wang, Juan Li, Xing Lu, Siqi Zheng, Chao Ou, Qian Huang, Miguel Santander-García, José Jairo Díaz Luis, Seokho Lee, Tie Liu, Zhiqiang Shen
TL;DR
This study directly compares CH3CCH and NH3 as thermometers in warm molecular gas around massive young stars. Using CH3CCH J=5-4 observations (55 detections) and NH3 (1,1)–(2,2) data (33 sources) across 55 regions, it shows that CH3CCH rotation temperatures better reflect the kinetic temperature than NH3 in the 20–100 K range. Statistical-equilibrium modeling confirms CH3CCH tracks $T_{kin}$ for densities above ~1×10^4 cm^-3, while NH3 becomes insensitive at higher temperatures, leading to systematic underestimation. The results advocate CH3CCH as the preferred thermometer for warm gas in massive star-forming regions and highlight the limitations of NH3-based thermometry in this regime.
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$\rightarrow$$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.