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Astron. Astrophys. 349, 605-618 (1999)

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1. Introduction

The Milky Way in Cygnus offers one of the finest displays of large scale structures related to star forming activity in our Galaxy, mostly due to the fact that the line of sight runs approximately along the local spiral arm in this direction (Bochkarev & Sitnik 1985, Odenwald 1989, Odenwald & Schwarz 1993). One of the dominant features of this area is Cygnus X, a giant molecular complex containing numerous thermal and non-thermal arcminute-sized sources (see Wendker et al. 1991, and references therein) which reveal the existence of both embedded and emerged HII regions in different evolutionary stages, as well as supernova remnants, thus indicating a vigorous present and past massive star forming activity.

Among the HII regions of Cygnus X is DR 18 (Downes & Rinehart 1966), a rather inconspicuous object at visible wavelengths which so far has received relatively little attention. In visible light pictures of the area, DR 18 appears as a small ([FORMULA]), amorphous patch of nebulosity surrounding a [FORMULA] star. However, the morphology becomes more interesting at infrared wavelengths, as revealed by the new observations that we present here. DR 18 appears as a distinctly arc-shaped nebula in the H and K bands, while narrow band infrared imaging suggests that most of the emission in that wavelength arises in a photodissociation region (PDR) outlining the interface between the HII region and the adjacent molecular cloud being eroded. The simple geometry of the PDR, and the fact that most of its illumination is produced by a single star which can be directly observed at visible and near-infrared wavelengths, makes DR 18 a promising target for further studies of the structures produced by the interaction of stellar ultraviolet radiation with a molecular cloud, as well as the chemical processes inside them.

In this paper we present and discuss our broad and narrow band observations of the DR 18 nebula in the J, H, and K windows, and their implications on the nature of the region. We also present JHK photometry of stars in the area, which reveals a loose cluster of young stars to which the one which ionizes the region probably belongs, and spectra of the ionizing star in the visible and in the 2 µm region. Finally, a comprehensive gas dynamical simulation accounting for the observed features is proposed.

In Sect. 2 we describe our observations. The results are presented in Sect. 3, and discussed in Sect. 4. A summary is given in Sect. 5.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999