Abstract We present observations of HCN and HCO+ J = 3 → 2 in the central 424″ × 424″ region of the W40 massive star forming region. The observations were taken as part of a pilot project for the MAJORS large program at the JCMT telescope. By incorporating prior knowledge of N( H2) and TK, assuming a constant density, and using the radex radiative transfer code we found that the HCN and HCO+ abundances range from X(HCN) = 0.4 − 7.0 × 10−8 and X( HCO+) = 0.4 − 7.3 × 10−9. Additional modelling using the nautilus chemical evolution code, that takes H2 density variations into account, however, suggests the HCN and HCO+ abundances may be fairly constant. Careful modelling of three different positions finds X(HCN) = 1.3 − 1.7 × 10−8, X( HCO+) = 1.3 − 3.1 × 10−9. Cross-comparison of the two models also provides a crude estimate of the gas density producing the HCN and HCO+ emission, with H2 densities in the range 5 × 104–5 × 105 cm−3– suggesting that the HCN and HCO+ emission does indeed arise from dense gas. High UV intensity (e.g. Go a few thousand) has no effect on the abundances in regions where the visual extinction is large enough to effectively shield the gas from the UV field. In regions where AV 6, however, the abundance of both species is lowered due to destructive reactions with species that are directly affected by the radiation field.
Plume et al. (Tue,) studied this question.