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.

中文翻译：

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光解离区域 S 255、S 257、NGC 7538 和 S 140 的近红外视图

摘要

我们使用以 Br \(\) 为中心的窄带滤光片，对 2 \(\mu\) m 处的S 255、S 257、S 140、NGC 7358 和 Orion Bar 光解离区域 (PDR) 进行了光度观测。gamma\)、 H \({}_{2}\)和 [Fe II] 线，以及窄带\(K_{\textrm{cont}}\)和宽带\(H \)用于连续减法的过滤器。观测是通过SAI高加索山天文台的2.5米望远镜以及近红外相机和摄谱仪ASTRONIRCAM完成的。我们在 NGC 7538 中电离气体的Br \(\gamma\)和 [Fe II] 图像中发现了几个高密度弧状结构，并且通过 H \({}_{2}\可见延伸的壳和弧）排放。H 电离前沿和 H \({}_{2}\)解离前沿在 NGC 7538 中合并。在 S 255 和 S 257 中，我们仅检测到来自 H II 区域的Br \(\gamma\)发射和明亮的 H \来自 PDR 的({}_{2}\)排放。H 电离和 H \({}_{2}\)解离前沿之间的投影距离约为 0.3–0.4 pc，这无法使用均匀介质模型来解释。最有可能的是，这些 PDR 中的电离气体和中性气体是块状的。NGC 7538、S 255、S 257 和 S 140 PDR 中的H I 到 H \({}_{2}\)跃迁是渐进的，没有尖锐的边界。这一结论也与块状介质的建议一致。