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 Hα knot located south-east of the Cepheus OB3 association, near the ionization front that divides the S 155 diffuse HII region from the Cepheus B molecular cloud. The arcsec resolution VLA radio continuum images reveal that the previously unresolved radio source # 9, coincident with the Hα knot, is composed of four distinct sources: 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 part of the Hα knot. The larger of the small-diameter components is a blister-type compact HII region, still partly bounded by high density molecular gas. Another one of these components, which is very weak, 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 excess in the field suggests that they are not background objects but, most probably, the non-thermal emission of PMS stars. Near-infrared J, H, and K observations of the same region with similar resolution have detected several objects, either inside the Hα knot or deeply embedded in the dust cloud/molecular clump to the south of it. Three red sources (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 HII region and clearly is produced by its highly obscured exciting star. This miniature near-infrared stellar cluster (in terms of total extension, if compared 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-generation OB association. While the large-scale general morphology and the ionization balance of the S 155/Cepheus B molecular cloud is dictated by the brightest members of the OB association, the energy for the far-IR emission from the region of the Hα knot and the heating of the adjacent molecular hotspot may come entirely from the stars of the newly-formed cluster, without the need of external input from the luminous stars of the OB association.