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 a n are-shaped nebula in the 2.2 mu m region, with a central star of V = 15.6 obscured by A(V) similar or equal to 8 magnitudes. Visible and near-infrar ed spectroscopy and photometry indicate a spectral type around B0.5V for th is 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 are ne bulosity is principally due to emission by small grains, heated by the cent ral star, in a photodissociation region. We interpret the are nebula as the interface between a molecular cloud that is being eroded by the central st ar and the resulting HII region. Using published models of photodissociatio n regions, we estimate the density in the are nebula to be a few times 10(3 ) cm(-3). We briefly discuss the possible relation of the structures observ ed in the near infrared with the source IRAS 20333+4102, which has been inc luded in several far infrared and radio studies of the area. We conclude th at 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 dif ferent from that of the are nebula, being brighter near the position of the central star. A crescent-shaped peak is observed beside the central star a nd facing the are nebula, suggesting an interaction between a stream of ion ized gas from the nebula and the wind from the central star. We present two dimensional gas dynamical simulations which successfully reproduce such ga s stream, the bow shock ahead of the central star, and the overall appearan ce of the nebula. An essential component of our model is the existence of a n outward-decreasing density stratification in the cloud being eroded, as i s 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 dynamical evolution comparatively easier th an in other, similar regions. DR 18 thus provides a good case study of seve ral features associated to the interaction of an early B star with a molecu lar cloud.