A submillimeter study of the IR dust bubble S 21 and its environs
  Based on the molecular emission in the $^{12}$CO(2-1) and $^{13}$CO(2-1)
lines, and the continuum emission in the MIR and FIR towards the S21 IR dust
bubble, we analyze the physical characteristics of the gas and dust linked to
the nebula and the presence of young stellar objects (YSOs) in its environs.
The line emission reveals a clumpy molecular shell, 1.4 pc in radius,
encircling S21. The total molecular mass in the shell amounts to 2900 solar
masses and the original ambient density, 2.1 x 10$^3$ cm$^{-3}$, indicating
that the bubble is evolving in a high density interstellar medium. The image at
24 $\mu$m shows warm dust inside the bubble, while the emission in the range
250 to 870 $\mu$m reveal cold dust in its outskirts, coincident with the
molecular gas. The detection of radio continuun emission indicates that the
bubble is a compact HII region. A search for YSOs using photometric criteria
allowed to identify many candidates projected onto the molecular clumps. We
analize if the collect and collapse process has triggered a new generation of
stars.
