The Cygnus X complex of molecular clouds (Bolton & Westford 1950; Piddington & Minnett 1952) stands out as one of the richest aggregates of star forming regions in our galactic neighborhood. It appears projected in an area of the sky containing many other structures related to massive star formation and violent processes in the interstellar medium (Bochkarev & Sitnik 1985). The inventory includes the OB associations of the Cygnus arm (including the massive association Cygnus OB2; e.g. Knödlseder 2000 and references therein), the X-ray emitting Cygnus Superbubble, and several other star forming regions revealed by IRAS (Odenwald & Schwartz 1993). A vast amount of observations of Cygnus X, mostly at radio, far, and mid-infrared wavelengths (e.g. Dobashi et al. 1996), have produced comprehensive pictures at various resolution scales of the different individual components of the complex, as well as of its large scale structure. Numerous individual HII regions, first noted by Downes & Rinehart (1968), as well as young supernova remnants, have been revealed in these studies.
The many individual compact HII regions of Cygnus X have important potential for the study of the three-dimensional structure of the complex and its present star-forming activity. They also make up an appropriate sample showing the variety of morphologies that can arise from the interaction of small- and medium-sized aggregates of massive stars with the interstellar medium, while the fact that many of them may be physically related to a single, large complex allows one to study them at a similar spatial resolution using a single instrumental setup. Besides providing flux-limited complete samples of compact HII regions in Cygnus X, observations published to date at radio wavelengths have yielded insights on the morphological and kinematical properties of the ionized gas, while IRAS observations in the mid-infrared (Odenwald 1989; Odenwald & Schwartz 1993) have allowed the determination of some bulk properties of their embedded populations. Unfortunately, detailed studies of their stellar components have been hampered so far by the heavy obscuration towards most of these regions, rendering both the ionized gas and the stars ionizing it unobservable at visible wavelengths. Observations in the near infrared, being much less affected by intervening extinction, can however overcome this limitation, providing detailed information on the properties of the stellar population and the distribution and dynamics of the interstellar medium.
As a first step in the study of their stellar components and the detailed morphology of their associated gas, we present in this paper a broad-band JHK imaging survey of compact HII regions in Cygnus X. The power of near-infrared imaging in a particular example, also located in Cygnus X, is demonstrated by the study of Comerón & Torra (1999) of the DR 18 HII region. The more limited observational material available in the present case, in particular the unavailability of narrow-band imaging centered on selected lines in the K band, does not allow us to go to the level of detail reached in our previous study of DR 18. However, it is still useful in revealing the morphology of the different HII regions and the diverse degrees of richness, concentration, and obscuration of the stellar aggregates that power them.
Our sample selection criteria, observations, and data reduction are described in Sect. 2. Section 3 presents the results and introduces our method for distance estimates. We also supply a detailed object-by-object discussion, placing each one in the context defined by previous observational work. Our conclusions are summarized in Sect. 4.
Copyright ESO 2001