The Cygnus Allscale Survey of Chemistry and Dynamical Environments: CASCADE

Context . The Cygnus-X complex is a massive (a few 10 6 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, H 2 CO, N 2 H + , SiO, C 2 H, 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 H II 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, N 2 H + , and SiO emission lines. The shell structure around DR18-05 indicates photodissociation regions (PDRs) formed by the expanding H II region and photo-erosion from DR18-05 and OB2 stars. We also identified 18 compact cores with N 2 H + 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 ( G 0 ~ 10 2 –10 3 ), as they traverse through a medium with n H ~ 10 4 to 10 5 cm −3 . These shocks can be generated by the initial expansion of the H II region and potentially by stellar winds.