2025A&A...694A..30K

Context. The Cygnus-X complex is a massive (a few 10⁶ M_⊙ molecular gas mass), nearby (1.4 kpc) star-forming region with several OB associations. Of these, Cyg OB2 is the largest, with at least 169 OB stars. DR18 is the largest globule near the OB2 association, making it a perfect target for investigating the influence of ultraviolet radiation on molecular clouds.
Aims. By analyzing emission from different molecular species, we aim to study the molecular gas structures toward DR18 using high angular-resolution molecular line observations.
Methods. As part of the Cygnus Allscale Survey of Chemistry and Dynamical Environments (CASCADE) program, we carried out 3.6 millimeter (mm) continuum and spectral line high-resolution observations (∼3-4″) toward DR18, covering several molecular species (e.g., HCN, HNC, H₂CO, N₂H⁺, SiO, C₂H, deuterated species, etc.) with the Northern Extended Millimeter Array (NOEMA) and the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. In addition, multi-wavelength archival datasets from midinfrared (MIR) to centimeter (cm) wavelengths were used to provide a comprehensive analysis of the region.
Results. The spectral index analysis shows significant contamination of the 3.6 mm continuum by free-free emission from ionized gas. A comparison of the 3.6 mm and 6cm continuum emission confirms that a B2 star (DR18-05) shapes the cometary HII region in the DR18 cavity, with ionized gas escaping toward the OB2 association. On the other hand, the extended 3.6 mm and 6 cm continuum emission are likely to trace photoevaporating ionized gas from ultraviolet radiation from the Cyg OB2 association - not from DR18- 05. To study the feedback of the B2 star and the OB2 association on surrounding molecular regions, we analyzed the HCO⁺, HCN, HNC, N₂H⁺, and SiO emission lines. The shell structure around DR18-05 indicates photodissociation regions (PDRs) formed by the expanding HII region and photo-erosion from DR18-05 and OB2 stars. We also identified 18 compact cores with N₂H⁺ emission, half of which are gravitationally bound (virial parameter, α_vir, ≲ 2.0), and mostly located in colder regions (T_HCN/HNC < 30 K) behind the PDRs. The SiO emission is found only in PDRs, with narrow-line widths (∼0.8-2.0 km s^–1) and lower abundances (X(SiO) ∼5 × 10^–11-1 × 10^–10). Comparing with the UV irradiated shock models, we suggest that the SiO emission partially encompassing the HII region arises from the molecular gas region, marginally compressed by low-velocity shocks with ∼5 km s^–1, irradiated by external UV radiation (G₀ ∼ 10²-10³), as they traverse through a medium with n_H ∼ 10⁴ to 10⁵ cm^–3. These shocks can be generated by the initial expansion of the HII region and potentially by stellar winds.