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-H2CO(J(KaKc) = 3(03)- 2(02), 3(22)- 2(21), and 3(21)-2(20)) using the APEX 12m 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-H(2)CO3(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)COline ratios are warm, ranging from 30 to > 200K with an average of 62 +/- 2K at a spatial density of 105 cm(-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(H-2) greater than or similar to 104 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-H2CO are positively correlated with the gas kinetic temperatures in regions of strong non-thermal motion (Mach number > greater than or similar to 2 : 5) of the OMC-1, implying that the higher temperature traced by para-H2CO is related to turbulence on a similar to 0.06 pc 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 10 km s(-1) filament heated by radiation from the central Orion nebula.