Hubble Space Telescope Wide Field Planetary Camera 2 Observations of the Young Bipolar H II Region S106
We present narrowband Hubble Space Telescope Wide Field Planetary Camera 2 images of the bipolar H ii region S106 in the Hα line. The high-resolution images show unprecedented detail in the two nebular lobes, including prominent scalloped structure of the emission at the ionization front. The illuminating source of the nebula, S106 IR, is detected in Hα but not in an adjacent passband that excludes bright nebular lines. Our estimate of the extinction to the exciting source is A_V ~ 19-22, which is in agreement with past determinations. The northern lobe of S106 is seen through a nearly opaque veil of foreground material, so that its structure is hard to discern. Most of the nebular emission toward the southern lobe emanates from the scalloped ionization fronts at the surfaces of hemispherical clumps located at the back side of the H ii region. We infer the existence of a translucent layer of foreground extinction in front of the southern lobe that decreases with distance from the exciting source. Despite the presence of a cluster of 50 to 150 young stars, externally illuminated young stellar objects similar to the over 100 ``proplyds'' seen in the Orion Nebula are not found in S106. This nondetection is most likely a result of the complete absence of young stars within the nebular lobes. However, if some of the stars seen toward the nebular lobes are found to be low-mass members of the S106 cluster and embedded within the photoionized region, the lack of extended Hα emission would imply a nebular photoionization age greater than 10^5 yr, the photoevaporation timescale for circumstellar disks.