Dense gas in IRAS 20343+4129: an ultracompact HII region caught in the
  act of creating a cavity
  The intermediate- to high-mass star-forming region IRAS 20343+4129 is an
excellent laboratory to study the influence of high- and intermediate-mass
young stellar objects on nearby starless dense cores, and investigate for
possible implications in the clustered star formation process. We present 3 mm
observations of continuum and rotational transitions of several molecular
species (C2H, c-C3H2, N2H+, NH2D) obtained with the Combined Array for Research
in Millimetre-wave Astronomy, as well as 1.3 cm continuum and NH3 observations
carried out with the Very Large Array, to reveal the properties of the dense
gas. We confirm undoubtedly previous claims of an expanding cavity created by
an ultracompact HII region associated with a young B2 zero-age main sequence
(ZAMS) star. The dense gas surrounding the cavity is distributed in a filament
that seems squeezed in between the cavity and a collimated outflow associated
with an intermediate-mass protostar. We have identified 5 millimeter continuum
condensations in the filament. All of them show column densities consistent
with potentially being the birthplace of intermediate- to high-mass objects.
These cores appear different from those observed in low-mass clustered
environments in sereval observational aspects (kinematics, temperature,
chemical gradients), indicating a strong influence of the most massive and
evolved members of the protocluster. We suggest a possible scenario in which
the B2 ZAMS star driving the cavity has compressed the surrounding gas,
perturbed its properties and induced the star formation in its immediate
surroundings.
