HOBYS Observations of Ridges and Filaments, and the Evolution of Massive Dense Cores

Abstract

Herschel large-scale observations of close-by massive star-forming regions obtained by HOBYS provide an unbiased view on the detailed cloud structure and its population of massive dense cores – excellent candidates for high-mass star precursors. Structures like the DR21 ridge, the most massive cloud structure in Cygnus X, could be formed by the merging of filaments or flows: several connected sub-filaments are resolved with Herschel. The sub-filaments are gravitationally unstable and form cores and protostars which may become low-mass members of the forming OB star cluster(s). They show decreasing dust temperature towards the ridge, indicating the pile-up of material to high densities which cools down to a minimum of 14 K towards the Northern part. The present mass in the sub-filaments is a factor of three lower than the ridge mass, so they represent remnant flows. However, their link to the clumps around DR21 and DR21(OH) suggests that these flows have been important to build-up massive clumps inside the ridge. Extrapolating, we would expect the assembly of massive clumps towards the Northern part of the DR21 ridge where the most massive subfilaments connect, in continuation of the evolutionary sequence of star formation along the ridge from South to North. We also use the large coverage of Cygnus X obtained by HOBYS (7,000 pc²) to establish an extensive sample of compact, cold, and dense cores in the region and constrain their luminosities, dust temperatures, and envelope masses. An evolutionary diagram and simple evolutionary tracks show that the sample provides the statistics to study the formation of stars with mass up to 20 M_⊙.