Dense gas in IRAS 20343+4129: an ultracompact H ii 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 (C_(2)H, c-C_(3)H_(2), N_(2)H+, NH_(2)D) obtained with the Combined Array for Research in Millimetre-wave Astronomy, as well as 1.3 cm continuum and NH_3 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 H ii 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 mm 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 several 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.