Star formation in two irradiated globules around Cygnus OB2
  We investigate the young stellar populations associated with DR 18 and ECX
6-21, which are two isolated globules irradiated by the O-type stars of the
Cygnus OB2 association. Both are HII regions containing obvious tracers of
recent and ongoing star formation. We also study smaller isolated molecular
structures in their surroundings. Both globules contain their own embedded
populations, with a higher fraction of the less-evolved classes. Masses and
temperatures are estimated under the assumption of a common age of 1 Myr, which
has been found to appropriately represent the general Cygnus OB2 YSO population
but is most probably an overestimate for both globules, especially ECX 6-21.
The early-B star responsible for the erosion of DR 18 is found to be part of a
small aggregate of intermediate-mass stars still embedded in the cloud, which
probably contains a second site of recent star formation, also with
intermediate-mass stars. We confirm the two main star forming sites embedded in
ECX 6-21 described in previous works, with the southern site being more evolved
than the northern site. We also discuss the small globule ECX 6-21-W ($=
G79.8+1.2$), and propose that its non thermal radio spectrum is due to
synchrotron emission from an embedded jet, whose existence is suggested by our
observations. The extreme youth of some of the YSOs suggests that star
formation in both globules started after they became externally irradiated. The
populations of both globules are not found to be particularly rich, but they
contain stars with estimated masses similar or above that of the Sun in numbers
that hint at some differences with respect to the star formation process taking
place in more quiescent regions where low-mass stars dominate, which deeper
observations may confirm.
