2017ApJ...849..140X

We performed a multi-wavelength study toward H II region Sh2-104. New maps of ¹²CO J = 1 - 0 and ¹³CO J = 1 - 0 were obtained from the Purple Mountain Observatory 13.7 m radio telescope. Sh2-104 displays a double-ring structure. The outer ring with a radius of 4.4 pc is dominated by 12, 500 µm, ¹²CO J = 1 - 0, and ¹³CO J = 1 - 0 emission, while the inner ring with a radius of 2.9 pc is dominated by 22 µm and 21 cm emission. We did not detect CO emission inside the outer ring. The north-east portion of the outer ring is blueshifted, while the south-west portion is redshifted. The present observations have provided evidence that the collected outer ring around Sh2-104 is a two-dimensional structure. From the column density map constructed by the Hi-GAL survey data, we extract 21 clumps. About 90% of all the clumps will form low-mass stars. A power-law fit to the clumps yields M=281 M_☉(r/pc)^1.31±0.08^. The selected YSOs are associated with the collected material on the edge of Sh2-104. The derived dynamical age of Sh2-104 is 1.6×10⁶ yr. Comparing the Sh2-104 dynamical age with the YSO timescale and the fragmentation time of the molecular ring, we further confirm that the collect-and-collapse process operates in this region, indicating positive feedback from a massive star for surrounding gas.