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Description
We mapped the kinetic temperature structure of the Orion molecular cloud 1 (OMC-1) with para-H_2_CO(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 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 >200K with an average of 62+/-2K 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-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-H_2_CO 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.
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