S 106

S106 is a bipolar nebula illuminated and excited by a central star called S106 IR. Depending on its distance (literature values range from 500 pc to 5.7 kpc) this star is of late O to early B spectral type. There is evidence that S106 is physically associated with the molecular cloud complex of Cygnus X. The bipolar nebula is the dominant feature in an embedded cluster of young stars forming in a molecular cloud. The bipolar morphology is visible from optical wavelengths into radio wavelengths, implying that it is not caused by foreground extinction. Instead, it is caused by the shadowing effect of a small disk surrounding the exciting star that projects into the surrounding molecular cloud and confines ionizing radiation to the lobes. With the discovery of new star formation sites in S106, i.e. YSOs in the S106 cluster, additional sub-clusters, the Class 0 source S106 FIR, and dense cold cores as reservoirs for new stars, S106 can nowadays be seen as a whole region of active star formation. This view revises earlier interpretations of radio and molecular line observations that saw S106 IR as a single massive protostar with a large accretion disk. 1. Discovery and Early Work The Galactic HII region S106 (also referred to as Sharpless 106, Sh 2-106, G 76.40.6, CRL 2584, and M 1-99) in Cygnus was first noted by Minkowski (1946) in an objective prism survey originally intended to find planetary nebulae. Since S106 does not have the spectral characteristics of a planetary nebula, he listed it among the peculiar objects. Sharpless (1959) independently found this object on the Palomar Sky Survey plates in the course of a search for Galactic nebulae. He listed it as number 106 in his table, giving it its most commonly used name, and noted its extent of about 3, and its irregular, somewhat filamentary structure. On the Palomar Sky Survey red plate, S106 appears as a small, but quite prominent emission nebula of bipolar (or bi-conical) (Morris 1981) appearance, with the southern lobe being much brighter than the northern one. On the Palomar plates, there was no star visible that was morphologically clearly associated with the bipolar nebula. No further studies of S106 were reported throughout the 1960s and into the early 1970s, until progress in infrared detection techniques and the discovery in the infrared of the first very young stars focused attention on S106 as possibly a prototypical example of a disk accretion object. The first near-infrared observations of S106 were reported by Sibille et al. (1975) who used the name Sh 2-106. They found a bright near-infrared source and discussed that it could be the exciting star of the HII emission