Fig. 4

RADEX non-LTE modeling of the relation between the kinetic temperature and the average ratio of para-H2CO 322–221/303–202 and 321–220/303–202 with an assumed density of n(H2) = 105 cm-3, an averaged linewidth of 2.0 km s-1, and column density of N(para-H2CO) = 1 × 1013 (red line), 5 × 1013 (green line), and 1 × 1014 cm-2 (blue line). The black points are derived from our observed H2CO line ratios for a column density N(para-H2CO) = 5 × 1013 cm-2, the average value for the OMC-1 (see Sect. 3.2). The temperature uncertainties are obtained from observed para-H2CO line ratio errors.
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