APEX CO observations towards the photodissociation region of RCW 120

Abstract

Context. The edges of ionized (HII) regions are important sites for the formation of (high-mass) stars. Indeed, at least 30% of the Galactic high-mass-star formation is observed there. The radiative and compressive impact of the HII region could induce star formation at the border following different mechanisms such as the collect and collapse or the radiation-driven implosion (RDI) models and change their properties.Aims. We aim to study the properties of two zones located in the photo dissociation region (PDR) of the Galactic HII region RCW 120 and discuss them as a function of the physical conditions and young star contents found in both clumps.Methods. Using the APEX telescope, we mapped two regions of size 1.5 x 1.5 toward the most massive clump of RCW 120 hosting young massive sources and toward a clump showing a protrusion inside the HII region and hosting more evolved low-mass sources. The (CO)-C-12 (J = 3-2), (CO)-C-13 (J = 3-2) and (CO)-O-18 (J = 3-2) lines observed, together with Herschel data, are used to derive the properties and dynamics of these clumps. We discuss their relation with the hosted star formation.Results. Assuming local thermodynamic equilibrium, the increase of velocity dispersion and T-ex are found toward the center of the maps, where star-formation is observed with Herschel. Furthermore, both regions show supersonic Mach numbers (7 and 17 in average). No substantial information has been gathered about the impact of far ultraviolet radiation on (CO)-O-18 photodissociation at the edges of RCW 120. The fragmentation time needed for CC to be at work is equivalent to the dynamical age of RCW 120 and the properties of region B are in agreement with bright-rimmed clouds.Conclusions. Although conclusions from this fragmentation model should be taken with caution, it strengthens the fact that, together with evidence of compression, CC might be at work at the edges of RCW 120. Additionally, the clump located at the eastern part of the PDR is a good candidate pre-existing clump where star-formation may be induced by the RDI mechanism.