SEQUENTIAL STAR-FORMATION IN THE CEPHEUS MOLECULAR CLOUD - THE S-155 CEPHEUS-B INTERFACE/

In this paper we present new radio and near-infrared observations of a bright H alpha knot located south-east of the Cepheus OB3 association , near the ionization front that divides the S 155 diffuse H II region from the Cepheus B molecular cloud. The arcsec resolution VLA radio c ontinuum images reveal that the previously unresolved radio source # 9 , coincident with the H alpha knot, is composed of four distinct sourc es: an extended RIDGE and three smaller diameter components. From the radio continuum spectrum and the general morphology it is possible to deduce that the RIDGE represents an ionization front in the northern p art of the H alpha knot. The larger of the small-diameter components i s a blister-type compact H II region, still partly bounded by high den sity molecular gas. Another one of these components, which is very wea k, is found to be variable. The third source is definitely unresolved even at 0.4 arcsec resolution and has a non-thermal spectral index. In the latter two sources, the proximity to sources with strong NIR exce ss in the field suggests that they are not background objects but, mos t probably, the non-thermal emission of PMS stars. Near-infrared J, H, and K observations of the same region with similar resolution have de tected several objects, either inside the H alpha knot or deeply embed ded in the dust cloud/molecular clump to the south of it. Three red so urces (previously undetected even in I-band) are found to be close to the positions of the three small-diameter radio continuum sources. One of these is located at the center of the blister type H II region and clearly is produced by its highly obscured exciting star. This miniat ure near-infrared stellar cluster (in terms of total extension, if com pared to the size of the neighboring OB association or of the Cepheus B molecular cloud) may be the product of an outburst of star formation triggered by the compression of the Cepheus B molecular cloud by the ionization front produced by the brightest stars of the previous-gener ation OB association.