N(HCO)/N(H¹³CO⁺) vs. N(H₂) (top panel) and dust temperature (middle panel). The bottom panel shows the correlation of N(HCO)/N(H¹³CO⁺) with R_D(DCO⁺). The purple points represent pixels in the DR21 filament above 4σ and the gray point shows the ATLASGAL sources from Kim et al. (2020). The black vertical line represents the indicator of ongoing FUV chemistry ≥1 (Gerin et al. 2009; Kim et al. 2020). The blue star mark the surroundings of DR21(OH), where HCO, H¹³CO⁺ and DCO⁺ are detected.