ALMA observations of RCW 120 Fragmentation at 0.01 pc scale

Journal article, 2018

Context. Little is known about how high-mass stars form. Around 30% of the young high-mass stars in the Galaxy are observed at the edges of ionized (H ii) regions. Therefore these are places of choice to study the earliest stages of high-mass star formation, especially toward the most massive condensations. High spatial resolution observations in the millimeter range might reveal how these stars form and how they assemble their mass. Aims. We want to study the fragmentation process down to the 0.01 pc scale in the most massive condensation (1700 M ) observed at the southwestern edge of the H ii region RCW 120 where the most massive Herschel cores (∼124 M in average) could form high-mass stars. Methods. Using ALMA 3 mm continuum observations toward the densest and most massive millimetric condensation (Condensation 1) of RCW 120, we used the getimages and getsources algorithms to extract the sources detected with ALMA and obtained their physical parameters. The fragmentation of the Herschel cores is discussed through their Jeans mass to understand the properties of these future stars. Results. We extracted 18 fragments from the ALMA continuum observation at 3 mm toward eight cores detected with Herschel, whose mass and deconvolved size range from 2 M to 32 M and from 1.6 mpc to 28.8 mpc, respectively. The low degree of fragmentation observed regarding thermal Jeans fragmentation suggests that the observed fragmentation is inconsistent with ideal gravitational fragmentation and other ingredients such as turbulence or magnetic fields should be added to explain this inconsistency. Finally, the range of the mass of the fragments indicates that the densest condensation of RCW 120 is a favorable place for the formation of high-mass stars with the presence of a probable UCH ii region associated with the 27 M Fragment 1 of Core 2.