In this paper we have presented near infrared observations of DR 18, a faint HII region in the Cygnus X molecular complex. The ionizing star is found to have an early B spectral type from both photometry and spectroscopy. Many lightly obscured stars in the imaged area occupy the area of the , diagram that indicates the presence of considerable amounts of circumstellar material, suggesting that the ionizing star is the brightest member of a loose aggregate of young stars.
The near-infrared morphology of the nebula in the K band is markedly different from that at visible wavelengths, being dominated by a bright, thick arc. Near the position of the central star, a crescent-shaped peak in the emission is seen, most prominently in the bands containing recombination emission lines from hydrogen. This peak is also observed in aperture synthesis observations in the centimeter radio continuum. By considering the emission as seen in different narrow bands, we conclude that the arc nebula actually traces a photodissociation region, with most of the emission being produced by small grains transiently heated to high temperatures by the central star. The photodissociation region is the interface between a faint HII region ionized by the central star, and a nearby molecular cloud, in which the central star was probably born. The HII region is produced by the Lyman continuum flux of the central star as it dissociates and ionizes the molecular gas. The cloud contains an embedded source, IRAS 20333+4102, inconspicuous or invisible in the near infrared. This source does not appear to be playing so far any significant role in the structure and dynamics of the region as observed in the near infrared, and it is probably embedded in the cloud at a depth still unreached by the ionization front.
The ionized gas reaches its peak brightness in the proximity of the central star, on the side of it facing the nebula. The shape of this peak suggests the interaction of a flow of ionized gas away from the molecular cloud with the stellar wind from the star, in a version of the champagne phase of the evolution of a HII region. We have simulated this interaction by means of numerical gasdynamic simulations, and conclude that a density gradient must exist in the molecular cloud in order to produce supersonic velocities inside the cavity digged by the ionizing radiation of the central star. The overall shape of the nebula, and the shape and position of the peak in the emission of ionized gas, are successfully accounted for by this model assuming a density profile of the cloud decreasing with the square of the distance to the center, as is commonly observed in molecular cores. We speculate that IRAS 20333+4102 may be a protostar being born at the center of this core.
DR 18 is an object with a simple geometry, in which the main components of the structure resulting from the erosion of a molecular cloud by a star with moderate ionizing flux can be readily discerned. The ionizing star itself can be easily studied spectroscopically, both in the visible and in the near-infrared. DR 18 thus provides a very useful example for the study of the chemical and dynamical processes taking place when molecular clouds are destroyed by stars at the faint end of the OB class, and it is an excellent target for follow-up multiwavelength observations probing in detail the different components that we have described in this work.
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999