Kinetic temperature of massive star-forming molecular clumps measured with formaldehyde - III. The Orion molecular cloud 1

We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H 2 CO ( J K a K c  = 3 03 –2 02 , 3 22 –2 21 , and 3 21 –2 20 ) using the APEX 12 m telescope. This is compared with the temperatures derived from the ratio of the NH 3  (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-H 2 CO 3 22 –2 21 /3 03 –2 02 and 3 21 –2 20 /3 03 –2 02 . The gas kinetic temperatures derived from the para-H 2 CO line ratios are warm, ranging from 30 to >200 K with an average of 62 ± 2 K at a spatial density of 10 5  cm -3 . These temperatures are higher than those obtained from NH 3  (2, 2)/(1, 1) and CH 3 CCH (6–5) in the OMC-1 region. The gas kinetic temperatures derived from para-H 2 CO agree with those obtained from warm dust components measured in the mid infrared (MIR), which indicates that the para-H 2 CO (3–2) ratios trace dense and warm gas. The cold dust components measured in the far infrared (FIR) are consistent with those measured with NH 3  (2, 2)/(1, 1) and the CH 3 CCH (6–5) line series. With dust at MIR wavelengths and para-H 2 CO (3–2) on one side, and dust at FIR wavelengths, NH 3  (2, 2)/(1, 1), and CH 3 CCH (6–5) on the other, dust and gas temperatures appear to be equivalent in the dense gas ( n (H 2 ) ≳ 10 4  cm -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-H 2 CO 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-H 2 CO is related to turbulence on a ~0.06 pc scale. Combining the temperature measurements with para-H 2 CO and NH 3  (2, 2)/(1, 1) line ratios, we find direct evidence for the dense gas along the northern part of the OMC-1 10 km s -1 filament heated by radiation from the central Orion nebula.