We have described the results of our JHK imaging survey of compact HII regions in the direction of the Cygnus X molecular complex. Observations at these wavelengths are largely unimpeded by dust extinction and are optimally suited for simultaneously studying both the nebulosity and the stars ionizing it. Our images illustrate indeed a large variety of morphologies among the former, and reveal aggregates of varying richness among the latter. A very brief summary of the main characteristics of each region is given in Table 1 to illustrate this diversity.
We have provided rough non-kinematical estimates of distances to our targets, based on a comparison of the stellar contents to the radio continuum flux, that are also listed in Table 1. In practice, the validity of the method is limited by a number of assumptions concerning membership, dereddening, and completeness of the stellar component. Actual cases in which these assumptions appear to be invalid or in which unrealistic distances are obtained have been discussed.
Our descriptions of individual regions in the light of previously
published material in other domains stress the importance of
multiwavelength observations in disclosing their structures and
the relationship between their stellar and gaseous components. A
variety of morphological features (bubble-like, ring-shaped, arc-shaped,
fan-shaped) are revealed by our images that are not seen at other
wavelengths and resolutions. Such features suggest these regions as test
cases for models of the dynamical interaction between the energetic
output of massive stars and their surrounding gas. On the other hand,
the observations presented here provide likely near-infrared counterparts
of a number of known ultracompact HII regions, which appear unresolved
or only barely resolved at an arcsecond scale, implying linear sizes of
order 
0.01 pc or smaller. The new detection at K of these
objects, so far observed only in the radio domain, suggests that
spectroscopic studies of the embedded star or its immediate surroundings
should be possible, thus providing a valuable complement to the existing
radio observations.
We conclude by pointing out that the broad band, survey nature of our near-infrared observations leaves much room for follow-up detailed studies in the same spectral region. Narrow-band imaging through selected filters would be very helpful in exploring the nature of the extended emission, and particularly in separating the respective contributions of recombination emission, H2deexcitation, and continuum emission of small heated grains, leading to a much better characterization of the physical and dynamical conditions in the nebulae. On the stellar side, higher resolution imaging is needed for improved photometry, given the crowdedness of the fields. At the same time, deeper observations would be helpful to sample not only the lower-mass component of the associated aggregates, but also more deeply embedded massive objects that may not have been detected in our observations. Finally, the regions studied here offer a fairly large sample of young O and B-type stars suitable for spectral classification in the infrared, which will allow a much better characterization of the intrinsic properties of the stellar component of each HII region.
Acknowledgements
It is a pleasure to thank the staff of the Observatorio del Teide in Tenerife, and very especially to Dr. María Rosa Zapatero Osorio, for excellent support during the observations. The help of Ms. Petia Andreeva during the observing run and in the preparation of the figures presented in this paper is warmly acknowledged. Useful comments from the referee, Dr. S. Molinari, are also appreciated. This paper made extensive use of the SIMBAD Database maintained at the Centre de Données Stellaires in Strasbourg, France.
Copyright ESO 2001