Near Infrared View on the Photo-Dissociation Regions S 255, S 257, NGC 7538, and S 140

Abstract

We performed photometric observations of the S 255, S 257, S 140, NGC 7358, and the Orion Bar photo-dissociation regions (PDRs) at 2 \(\mu\)m using narrow-band filters centered on the Br\(\gamma\), H\({}_{2}\), and [Fe II] lines, as well as the narrow-band \(K_{\textrm{cont}}\) and the broad-band \(H\) filters for continuum subtraction. The observations were done with the 2.5-m telescope of the SAI Caucasian Mountain Observatory and the near-infrared camera and spectrograph ASTRONIRCAM. We find several high-density arc-like structures in the Br\(\gamma\) and [Fe II] images of the ionized gas in NGC 7538 and extended shells and arcs visible through the H\({}_{2}\) emission. The H ionization front and H\({}_{2}\) dissociation front are merged in NGC 7538. In S 255 and S 257 we detected only Br\(\gamma\) emission from the H II regions and bright H\({}_{2}\) emission from the PDRs. The projected distance between the H ionization and H\({}_{2}\) dissociation fronts is approximately 0.3–0.4 pc, which cannot be explained using models of a uniform medium. Most probably, the ionized and neutral gas in these PDRs is clumpy. The H I-to-H\({}_{2}\) transitions in the NGC 7538, S 255, S 257 and S 140 PDRs are gradual with no sharp borders. This conclusion also agrees with the suggestion of a clumpy medium.