2018A&A...609A..16T

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H₂CO (J_Ka_Kc=3₀₃-2₀₂, 3₂₂-2₂₁, and 3₂₁-2₂₀) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH₃ (2,2)/(1,1) inversion lines and the dust emission. Using the RADEX non-LTE model, we derive the gas kinetic temperature modeling the measured averaged line ratios of para-H₂CO 3₂₂-2₂₁/3₀₃-2₀₂ and 3₂₁-2₂₀/3₀₃-2₀₂. The gas kinetic temperatures derived from the para-H₂CO line ratios are warm, ranging from 30 to >200K with an average of 62±2K at a spatial density of 10⁵cm^–3. These temperatures are higher than those obtained from NH₃ (2,2)/(1,1) and CH₃CCH (6-5) in the OMC-1 region. The gas kinetic temperatures derived from para-H₂CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H₂CO (3-2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH₃ (2,2)/(1,1) and the CH₃CCH (6-5) line series. With dust at MIR wavelengths and para-H₂CO (3-2) on one side, and dust at FIR wavelengths, NH₃ (2,2)/(1,1), and CH₃CCH (6-5) on the other, dust and gas temperatures appear to be equivalent in the dense gas (n(H₂)≥10⁴cm^–3) of the OMC-1 region, but provide a bimodal distribution, one more directly related to star formation than the other. The non-thermal velocity dispersions of para-H₂CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number ≥2.5) of the OMC-1, implying that the higher temperature traced by para-H₂CO is related to turbulence on a ∼0.06pc scale. Combining the temperature measurements with para-H₂CO and NH₃ (2,2)/(1,1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10km/s filament heated by radiation from the central Orion nebula.