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
with para-H2CO(303-202, 322-221, and 321-220) 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-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 >200 K with an average of 62 K at a spatial density of 10$^5$ 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 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 0.06 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.
