MAJORS II: HCO+ and HCN abundances in W40

ABSTRACT We present observations of HCN and HCO$^{+}$ J = $3 \rightarrow 2$ in the central $424\ \mathrm{ arcsec} \times 424\ \mathrm{ arcsec}$ 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(H$_{2}$) and $T_K$, assuming a constant density, and using the radex radiative transfer code we found that the HCN and HCO$^{+}$ abundances range from X(HCN) = $0.4\!-\!7.0 \times 10^{-8}$ and X(HCO$^{+}$) = $0.4\!-\!7.3 \times 10^{-9}$. Additional modelling using the nautilus chemical evolution code, which takes H$_{2}$ density variations into account, however, suggests that the HCN and HCO$^{+}$ abundances may be fairly constant. Careful modelling of three different positions finds X(HCN) = $1.3\!-\!1.7 \times 10^{-8}$ and X(HCO$^{+}$) = $1.3\!-\!3.1 \times 10^{-9}$. Cross-comparison of the two models also provides a crude estimate of the gas density producing the HCN and HCO$^{+}$ emission, with H$_{2}$ densities in the range $5\times 10^4$–$5\times 10^5$ cm$^{-3}$, suggesting that the HCN and HCO$^{+}$ emission does indeed arise from dense gas. High UV intensity (e.g. $G_o >$ 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 $A_\text{V} \lt 6$, however, the abundance of both species is lowered due to destructive reactions with species that are directly affected by the radiation field.