A near infrared study of the HII / photodissociation region DR 18 in Cygnus ?

Near infrared observations of DR 18, a HII region in the Cygnus X molecular complex, are presented in this paper. These observations reveal DR 18 as an arc-shaped nebula in the 2.2 μm region, with a central star of V = 15.6 obscured by AV ' 8 magnitudes. Visible and near-infrared spectroscopy and photometry indicate a spectral type around B0.5V for this star, while a near-infrared color-color diagram of the stars in the area shows that the central star is the most luminous one of a loose aggregate. Analysis of the narrow band imaging in the K band suggests that the arc nebulosity is principally due to emission by small grains, heated by the central star, in a photodissociation region. We interpret the arc nebula as the interface between a molecular cloud that is being eroded by the central star and the resulting HII region. Using published models of photodissociation regions, we estimate the density in the arc nebula to be a few times 103 cm−3. We briefly discuss the possible relation of the structures observed in the near infrared with the source IRAS 20333+4102, which has been included in several far infrared and radio studies of the area. We conclude that IRAS 20333+4102 is not directly related to any of the structures that we describe here, and could be an intermediate mass protostar embedded deeper in the molecular cloud. The emission associated to ionized gas in DR 18 has a morphology fairly different from that of the arc nebula, being brighter near the position of the central star. A crescent-shaped peak is observed beside the central star and facing the arc nebula, suggesting an interaction between a stream of ionized gas from the nebula and the wind from the central star. We present two dimensional gas dynamical simulations which successfully reproduce such gas stream, the bow shock ahead of the central star, and the overall appearance of the nebula. An essential component of our model is the existence of an outward-decreasing density stratification in the cloud being eroded, as is commonly observed in dense molecular clumps. The simple geometry of the nebula and the observability of the central star at short wavelengths make the derivation of the physical conditions of the region and the modeling of its Send offprint requests to: F. Comerón ? Based on observations collected at the German-Spanish Astronomical Center (Calar Alto, Spain), and at the European Southern Observatory (La Silla, Chile) dynamical evolution comparatively easier than in other, similar regions. DR 18 thus provides a good case study of several features associated to the interaction of an early B star with a molecular cloud.