MAJORS II: HCO+& HCN Abundances in W40

We present observations of HCN and HCO$^+$ J = $3 - 2$ in the central $424’’ \times 424’’$ 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(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, that takes H$_2$ density variations into account, however, suggests the HCN and HCO$^+$ abundances may be fairly constant. Careful modelling of three different positions finds $X$(HCN) = $1.3-1.7 \times 10^{-8}$, $X$(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 $A_V < 6$, however, the abundance of both species is lowered due to destructive reactions with species that are directly affected by the radiation field.

💡 Research Summary

This paper presents a detailed study of the dense‑gas tracers HCN and HCO⁺ in the massive star‑forming region W40, using observations from the James Clerk Maxwell Telescope (JCMT) as part of the pilot phase of the MAJORS (Massive, Active, JCMT‑Observed Regions of Star formation) large program. The authors mapped the J = 3 → 2 transitions of HCN (265.886 GHz) and HCO⁺ (267.558 GHz) over a 424″ × 424″ area centered on the W40 complex. The observations were carried out on 21 August 2020 under moderate weather conditions (225 GHz opacity 0.225–0.315) with a beam size of ≈18″, corresponding to a spatial resolution of ≈0.04 pc at the Gaia‑derived distance of 502 pc.

The data reduction produced integrated intensity maps and spectra for each line. The authors first derived the H₂ column density and kinetic temperature distributions from Herschel Gould Belt Survey data (N(H₂) ≈ 10²²–10²³ cm⁻², T_K ≈ 15–30 K). They then performed two complementary modelling approaches:

1. RADEX non‑LTE radiative‑transfer analysis – Assuming a uniform gas density, the authors input the observed line intensities, N(H₂), and T_K into the 1‑D RADEX code. By exploring a grid of H₂ densities (10⁴–10⁶ cm⁻³) they identified the best‑fit density range of 5 × 10⁴–5 × 10⁵ cm⁻³. Within this density regime the derived fractional abundances are X(HCN) = 0.4–7.0 × 10⁻⁸ and X(HCO⁺) = 0.4–7.3 × 10⁻⁹. These values are broadly consistent with earlier studies of similar regions but display a larger spread, reflecting uncertainties in the assumed uniform density.

2. NAUTILUS gas‑grain chemical evolution modelling – To account for spatial variations in density and time‑dependent chemistry, the authors employed the NAUTILUS code. They fixed the observed N(H₂) and T_K, and varied parameters such as the cosmic‑ray ionisation rate (ζ ≈ 1.3 × 10⁻¹⁷ s⁻¹), the external UV field (G₀ ≈ 200–8000 in Habing units), and the visual extinction (A_V). Simulations were run up to 10⁶ yr. The chemical model predicts that, for regions with A_V > 6 (i.e., well‑shielded dense gas), the abundances remain essentially constant: X(HCN) ≈ 1.3–1.7 × 10⁻⁸ and X(HCO⁺) ≈ 1.3–3.1 × 10⁻⁹. In contrast, for A_V < 6 the abundances drop by 30–50 % because UV‑driven photodissociation and ion‑neutral reactions become efficient.

To test the robustness of these results, the authors selected three representative positions: (i) the central cluster core, (ii) a bright filament/pillar region, and (iii) a low‑density peripheral zone. Both RADEX and NAUTILUS converge on similar abundances and densities for these locations, reinforcing the conclusion that the J = 3 → 2 lines of HCN and HCO⁺ trace gas with densities ≳10⁴ cm⁻³.

The paper also discusses the broader astrophysical implications. The near‑constant abundances in UV‑shielded dense gas support the widely used L_IR–L_HCN correlation, suggesting that the dense‑gas mass traced by HCN (and HCO⁺) is a reliable proxy for the star‑formation rate, independent of local UV field strength. The sensitivity of the abundances to visual extinction highlights the importance of dust shielding in regulating dense‑gas chemistry, a factor that must be considered when comparing Galactic and extragalactic samples. Moreover, the derived density range (5 × 10⁴–5 × 10⁵ cm⁻³) aligns with theoretical expectations for the onset of efficient star formation (Lada et al. 2012), providing an observational link between chemical tracers and the physical threshold for collapse.

In summary, this pilot study validates a consistent methodology—combining high‑resolution JCMT spectroscopy, RADEX radiative‑transfer analysis, and NAUTILUS chemical modelling—to derive HCN and HCO⁺ abundances and associated gas densities across a complex star‑forming region. The findings demonstrate that HCN (3‑2) and HCO⁺ (3‑2) are indeed reliable dense‑gas tracers in well‑shielded environments, while their abundances can be suppressed in low‑extinction, high‑UV zones. The approach will be applied to the full MAJORS sample, enabling a systematic investigation of how Galactic environment influences dense‑gas chemistry and star‑formation efficiency.