CH3CCH as a thermometer in warm molecular gas

Technical Summary: CH3CCH as a Thermometer in Warm Molecular Gas

Problem Statement
Kinetic temperature ($T_{kin}$) is a fundamental parameter governing the chemical evolution and star formation processes within molecular clouds. While symmetric top molecules like ammonia (NH$_3$) and methyl acetylene (CH$_3$CCH) are standard thermometers, the reliability of NH$_3$(1,1) and NH$_3$(2,2) transitions diminishes in warm environments ($T \gtrsim 20-25$ K). At elevated temperatures, NH$_3$(2,2) can be collisionally excited to non-metastable levels (e.g., NH$_3$(2,1)), which rapidly decay to NH$_3$(1,1). This process causes the populations of the (1,1) and (2,2) levels to no longer accurately reflect the kinetic temperature, leading to systematic underestimations. Conversely, CH$_3$CCH possesses lower critical densities for its rotational levels, allowing it to thermalize with the kinetic temperature at densities as low as $\sim 10^4$ cm$^{-3}$. However, systematic comparisons between rotation temperatures derived from NH$_3$(1,1)&(2,2) and CH$_3$CCH in warm, massive star-forming regions have been limited.

Methodology
The authors analyzed observational data from 55 massive star-forming regions associated with 6.7 GHz CH$_3$OH masers, indicative of warm gas ($T_{kin} \approx 20-100$ K).

• Observations: CH$_3$CCH $J=5-4$ transitions were observed using the Yebes 40m telescope (85.4 GHz) toward 101 targets, with 55 showing detections above the $3\sigma$ level. NH$_3$(1,1) and NH$_3$(2,2) data were obtained from 18 sources previously observed with the Effelsberg 100m telescope and 37 new observations using the Shanghai Tianma 65m Radio Telescope (TMRT) at 23.7 GHz.
• Data Reduction: Spectra were reduced using CLASS/GILDAS. Velocity resolution was smoothed to $\sim 0.6$ km s$^{-1}$. Main beam efficiency corrections were not applied as the study focused on relative line ratios for temperature derivation rather than absolute column densities.
• Analysis:
  • CH$_3$CCH: Rotation temperatures ($T_{rot}$) were derived by fitting observed spectra with a multi-Gaussian model under Local Thermodynamic Equilibrium (LTE) assumptions, treating $T_{rot}$, velocity, FWHM, and column density as global free parameters shared across $K$-components.
  • NH$_3$: Rotation temperatures ($T_{rot}(2,2;1,1)$) were derived using a Python code based on the frameworks of Ho & Townes (1983) and Mangum et al. (1992), fitting NH$_3$(1,1) and (2,2) hyperfine structures simultaneously.
  • Statistical Equilibrium: The authors performed statistical equilibrium calculations for CH$_3$CCH using collisional data from Ben Khalifa et al. (2024) to model the relationship between $T_{rot}$, gas density, and $T_{kin}$.

Key Results

• Temperature Discrepancy: The derived rotation temperatures for CH$_3$CCH ($T_{rot}(\text{CH}_3\text{CCH})$) range from 23.0 K to 88.8 K (median 33.1 K), whereas NH$_3$ temperatures ($T_{rot}(2,2;1,1)$) range from 15.7 K to 29.5 K (median 20.7 K).
• Systematic Bias: For the majority of sources, particularly those with $T_{rot}(\text{CH}_3\text{CCH}) > 30$ K, the NH$_3$-derived temperatures are significantly lower than the CH$_3$CCH temperatures. As $T_{rot}(\text{CH}_3\text{CCH})$ increases, $T_{rot}(2,2;1,1)$ shows negligible increase, indicating a loss of sensitivity in the NH$_3$ tracer at higher temperatures.
• Thermalization: Statistical equilibrium calculations confirm that CH$_3$CCH levels are nearly thermalized ($T_{rot} \approx T_{kin}$) at gas densities $n \gtrsim 10^4$ cm$^{-3}$. Given that the sample sources also show H$^{13}$CN 1-0 emission (critical density $\sim 9 \times 10^5$ cm$^{-3}$), the assumption that $T_{rot}(\text{CH}_3\text{CCH})$ traces $T_{kin}$ is robust.

Significance and Claims
The paper concludes that CH$_3$CCH rotational lines (specifically the $J=5-4$ transition) serve as a more reliable thermometer than NH$_3$(1,1)&(2,2) in warm molecular gas surrounding massive young stellar objects. The systematic underestimation of temperature by NH$_3$ is attributed to the collisional excitation of NH$_3$(2,2) into non-metastable levels at high temperatures, followed by rapid decay to the (1,1) state.

The authors note that while CH$_3$CN is also a symmetric top molecule, it often suffers from optical thickness in low-$K$ lines and traces denser, more compact cores, whereas CH$_3$CCH traces the warmer, more extended envelope gas with lower optical depth. The study suggests that caution is required when using NH$_3$(1,1)&(2,2) to derive kinetic temperatures in environments where $T_{kin} > 20$ K, as empirical corrections may no longer be accurate. The discrepancy between the two tracers may also be influenced by spatial resolution differences (beam sizes of 20.5$''$ for CH$_3$CCH vs. 43$''$–50$''$ for NH$_3$) and temperature gradients within the sources, but the primary driver identified is the physical limitation of the NH$_3$ thermometer in warm gas.