Two-Dimensional Molecular Gas and ongoing star formation around H II
  region Sh2-104
  We performed a multiwavelength study towards HII region Sh2-104. New maps of
12CO J=1-0 and 13CO J=1-0 were obtained from the Purple Mountain Observatory
(PMO) 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 um, 500 um, 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 um 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=281Msun(r/pc)^1.31. 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*10^6 yr. Compared the Sh2-104 dynamical age
with the YSOs timescale and the fragmentation time of the molecular ring, we
further confirm that collect-and-collapse process operates in this region,
indicating a positive feedback from a massive star for surrounding gas.
