Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde VI. The photodissociation region M17SW

Technical Summary: Kinetic Temperature of Massive Star-Forming Molecular Clumps Measured with Formaldehyde VI: The Photodissociation Region M17SW

Problem Statement
The physical properties of the interstellar medium (ISM), particularly the kinetic temperature of dense gas, fundamentally influence star formation rates, spatial distributions, and the initial mass function. In massive star-forming regions like the M17 complex, feedback mechanisms such as ultraviolet (UV) radiation, stellar winds, and expanding H II regions significantly alter the thermal balance of molecular clouds. While the M17SW region is a well-studied photodissociation region (PDR) adjacent to the OB cluster NGC 6618, previous temperature studies using tracers like NH3 and FIR dust continuum have been limited by angular resolution (typically >0.3 pc). These limitations introduce uncertainty regarding small-scale physics, potentially masking discrete temperature fluctuations or unresolved gradients. There is a need for high-resolution mapping of dense gas temperatures to distinguish between large-scale external radiative heating and small-scale internal heating or turbulent dissipation.

Methodology
The authors utilized the IRAM 30 m telescope to map the kinetic temperature structure of the M17SW region at a spatial scale of approximately 0.2 pc (18 arcseconds). The observations focused on the para-H2CO triplet transitions near 218 GHz: $3_{03}-2_{02}$, $3_{22}-2_{21}$, and $3_{21}-2_{20}$.

• Data Acquisition: Observations were conducted in May 2017 and October 2018 using the HERA receiver. The data were reduced using GILDAS, smoothed to a velocity resolution of ~0.8 km s⁻¹ and a spatial resolution of 18 arcseconds.
• Temperature Derivation: Kinetic temperatures ($T_{kin}$) were derived by modeling the average line ratios of the H2CO triplet ($0.5 \times [(3_{22}-2_{21} + 3_{21}-2_{20})/3_{03}-2_{02}]$) using the RADEX non-local thermodynamic equilibrium (non-LTE) radiative transfer code.
• Assumptions: The modeling assumed a background temperature of 2.73 K, a gas volume density of $n(H_2) = 5.5 \times 10^5$ cm⁻³, and a para-H2CO column density of $N(\text{para-H}_2\text{CO}) = 6.5 \times 10^{13}$ cm⁻². An ortho-to-para ratio of 3 was adopted based on previous measurements.
• Analysis: The study compared H2CO-derived temperatures with those from NH3 transitions and FIR dust continuum. Additionally, non-thermal line widths ($\sigma_{NT}$) and Mach numbers were calculated to investigate the relationship between turbulence and gas heating.

Key Contributions and Results

1. Temperature Mapping: The study produced the first high-resolution kinetic temperature map of M17SW using H2CO. The derived temperatures range from 28 K to 181 K, with an average of $54.2 \pm 0.3$ K.
2. Spatial Distribution:
   • High Temperatures ($>100$ K): Found associated with the ultra-compact H II region UC1 and the eastern edge of M17SW.
   • Elevated Temperatures ($>70$ K): Detected at the eastern edge of the primary structure, near specific dense clumps (e.g., G15.03-0.67, G14.99-0.70), and in proximity to H2O and CH3OH masers.
   • Moderate/Low Temperatures: Temperatures decrease toward the western edge of the region, dropping below 50 K.
3. Comparison with Other Tracers: H2CO-derived temperatures are generally consistent with those measured by NH3 but slightly higher than values derived from FIR dust continuum. The authors attribute this to the higher excitation energy of the H2CO lines ($E_u \approx 68$ K) compared to NH3, making H2CO a more sensitive tracer of warmer gas components.
4. Heating Mechanisms:
   • External Radiative Heating: A significant temperature gradient was observed, decreasing from the northeast (near the NGC 6618 cluster) to the southwest. Power-law fits of temperature versus distance from the radiation source ($T_{kin} \propto R^{-0.85}$ for H2CO) align with the modified Stefan-Boltzmann blackbody radiation law, confirming that the dense gas is predominantly heated by external UV radiation from the OB cluster.
   • Internal Heating: On smaller scales (~0.1–0.4 pc), dense clumps associated with masers and star formation activity show elevated temperatures that cannot be explained solely by external radiation. This suggests significant heating from internal protostars and/or young stellar objects (YSOs).
   • Turbulent Heating: A positive correlation was found between non-thermal line widths and kinetic temperature ($T_{kin} \propto \sigma_{NT}^{0.47}$), indicating that turbulence on the ~0.2 pc scale contributes to gas heating, consistent with previous findings in other massive star-forming regions.
5. Implications for Star Formation: The high kinetic temperatures in M17SW ($T_{kin} \sim 54$ K) result in a Jeans mass of approximately $7 M_\odot$, significantly higher than in typical solar neighborhood clouds. This suggests that the star-forming clumps in M17SW are massive, potentially facilitating the formation of high-mass stars and leading to a top-heavy initial mass function (IMF).

Significance
The paper claims that the complex temperature structure of M17SW is revealed by combining H2CO and NH3 observations, attributing the thermal state of the gas to a combination of large-scale external radiative heating from the NGC 6618 cluster and small-scale internal radiative and turbulent heating from embedded protostars. The study provides direct evidence for gas heating mechanisms in a PDR environment at sub-parsec scales. By demonstrating that H2CO is an effective tracer for warm, dense gas in PDRs, the authors reinforce its utility for analyzing the thermal physics of star-forming regions. The findings underscore the necessity of high-resolution observations to disentangle the various heating sources that shape the conditions for massive star formation.